diff --git a/head/schema-cjson/test/inputs/schema/mixed-additional-properties.schema/default/TopLevel.c b/head/schema-cjson/test/inputs/schema/mixed-additional-properties.schema/default/TopLevel.c
new file mode 100644
index 0000000..f9b8c13
--- /dev/null
+++ b/head/schema-cjson/test/inputs/schema/mixed-additional-properties.schema/default/TopLevel.c
@@ -0,0 +1,131 @@
+/**
+ * TopLevel.c
+ * This file has been autogenerated using quicktype https://github.com/quicktype/quicktype - DO NOT EDIT
+ */
+
+#include "TopLevel.h"
+
+struct TopLevelValue * cJSON_GetTopLevelValueValue(const cJSON * j) {
+    struct TopLevelValue * x = cJSON_malloc(sizeof(struct TopLevelValue));
+    if (NULL != x) {
+        memset(x, 0, sizeof(struct TopLevelValue));
+        if (cJSON_IsString(j)) {
+            x->type = cJSON_String;
+            x->value.string = strdup(cJSON_GetStringValue(j));
+        }
+        else if (cJSON_IsNumber(j)) {
+            x->type = cJSON_Number;
+            x->value.number = cJSON_GetNumberValue(j);
+        }
+        if (0 == x->type) { cJSON_free(x); return NULL; }
+    }
+    return x;
+}
+struct TopLevelValue * cJSON_ParseTopLevelValue(const char * s) { cJSON * j = cJSON_Parse(s); struct TopLevelValue * x = j ? cJSON_GetTopLevelValueValue(j) : NULL; cJSON_Delete(j); return x; }
+char * cJSON_PrintTopLevelValue(const struct TopLevelValue * x) { cJSON * j = cJSON_CreateTopLevelValue(x); char * s = j ? cJSON_Print(j) : NULL; cJSON_Delete(j); return s; }
+
+cJSON * cJSON_CreateTopLevelValue(const struct TopLevelValue * x) {
+    cJSON * j = NULL;
+    if (NULL != x) {
+        if (cJSON_String == x->type) {
+            j = cJSON_CreateString(x->value.string);
+        }
+        else if (cJSON_Number == x->type) {
+            j = cJSON_CreateNumber(x->value.number);
+        }
+    }
+    return j;
+}
+
+void cJSON_DeleteTopLevelValue(struct TopLevelValue * x) {
+    if (NULL != x) {
+        if (cJSON_String == x->type) {
+            cJSON_free(x->value.string);
+        }
+        else if (cJSON_Number == x->type) {
+        }
+        cJSON_free(x);
+    }
+}
+
+struct TopLevel * cJSON_ParseTopLevel(const char * s) {
+    struct TopLevel * x = NULL;
+    if (NULL != s) {
+        cJSON * j = cJSON_Parse(s);
+        if (NULL != j) {
+            x = cJSON_GetTopLevelValue(j);
+            cJSON_Delete(j);
+        }
+    }
+    return x;
+}
+
+struct TopLevel * cJSON_GetTopLevelValue(const cJSON * j) {
+    struct TopLevel * x = NULL;
+    if (NULL != j) {
+        if (NULL != (x = cJSON_malloc(sizeof(struct TopLevel)))) {
+            memset(x, 0, sizeof(struct TopLevel));
+            x->value = hashtable_create(64, false);
+            if (NULL != x->value) {
+                cJSON * e = NULL;
+                cJSON_ArrayForEach(e, j) {
+                    hashtable_add(x->value, e->string, cJSON_GetTopLevelValueValue(e), sizeof(struct TopLevelValue *));
+                }
+            }
+        }
+    }
+    return x;
+}
+
+cJSON * cJSON_CreateTopLevel(const struct TopLevel * x) {
+    cJSON * j = NULL;
+    if (NULL != x) {
+        if (NULL != x->value) {
+            j = cJSON_CreateObject();
+            if (NULL != j) {
+                char **keys = NULL;
+                size_t count = hashtable_get_keys(x->value, &keys);
+                if (NULL != keys) {
+                    for (size_t index = 0; index < count; index++) {
+                        struct TopLevelValue *x2 = hashtable_lookup(x->value, keys[index]);
+                        cJSON_AddItemToObject(j, keys[index], cJSON_CreateTopLevelValue(x2));
+                    }
+                    cJSON_free(keys);
+                }
+            }
+        }
+    }
+    return j;
+}
+
+char * cJSON_PrintTopLevel(const struct TopLevel * x) {
+    char * s = NULL;
+    if (NULL != x) {
+        cJSON * j = cJSON_CreateTopLevel(x);
+        if (NULL != j) {
+            s = cJSON_Print(j);
+            cJSON_Delete(j);
+        }
+    }
+    return s;
+}
+
+void cJSON_DeleteTopLevel(struct TopLevel * x) {
+    if (NULL != x) {
+        if (NULL != x->value) {
+            char **keys = NULL;
+            size_t count = hashtable_get_keys(x->value, &keys);
+            if (NULL != keys) {
+                for (size_t index = 0; index < count; index++) {
+                    struct TopLevelValue *x2 = hashtable_lookup(x->value, keys[index]);
+                    if (NULL != x2) {
+                        cJSON_DeleteTopLevelValue(x2);
+                    }
+                }
+                cJSON_free(keys);
+            }
+            hashtable_release(x->value);
+        }
+        cJSON_free(x);
+    }
+}
diff --git a/head/schema-cjson/test/inputs/schema/mixed-additional-properties.schema/default/TopLevel.h b/head/schema-cjson/test/inputs/schema/mixed-additional-properties.schema/default/TopLevel.h
new file mode 100644
index 0000000..1d4c59b
--- /dev/null
+++ b/head/schema-cjson/test/inputs/schema/mixed-additional-properties.schema/default/TopLevel.h
@@ -0,0 +1,70 @@
+/**
+ * TopLevel.h
+ * This file has been autogenerated using quicktype https://github.com/quicktype/quicktype - DO NOT EDIT
+ * This file depends of https://github.com/DaveGamble/cJSON, https://github.com/joelguittet/c-list and https://github.com/joelguittet/c-hashtable
+ * To parse json data from json string use the following: struct <type> * data = cJSON_Parse<type>(<string>);
+ * To get json data from cJSON object use the following: struct <type> * data = cJSON_Get<type>Value(<cjson>);
+ * To get cJSON object from json data use the following: cJSON * cjson = cJSON_Create<type>(<data>);
+ * To print json string from json data use the following: char * string = cJSON_Print<type>(<data>);
+ * To delete json data use the following: cJSON_Delete<type>(<data>);
+ */
+
+#ifndef __TOPLEVEL_H__
+#define __TOPLEVEL_H__
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#include <stdint.h>
+#include <stdbool.h>
+#include <stdlib.h>
+#include <string.h>
+#include <regex.h>
+#include <cJSON.h>
+#include <hashtable.h>
+#include <list.h>
+
+#define quicktype_cJSON_Duplicate(j) cJSON_Duplicate(j, true)
+#define cJSON_Integer (1 << 18)
+#define quicktype_cJSON_IsInteger(j) (cJSON_IsNumber(j) && (j)->valuedouble == (int64_t)(j)->valuedouble)
+#ifndef cJSON_Bool
+#define cJSON_Bool (cJSON_True | cJSON_False)
+#endif
+#ifndef cJSON_Map
+#define cJSON_Map (1 << 16)
+#endif
+#ifndef cJSON_Enum
+#define cJSON_Enum (1 << 17)
+#endif
+
+struct TopLevelValue {
+    int type;
+    union {
+        char * string;
+        double number;
+    } value;
+};
+
+struct TopLevel {
+    hashtable_t * value;
+};
+
+#define cJSON_IsTopLevelValue(j) (cJSON_IsString(j) || cJSON_IsNumber(j))
+struct TopLevelValue * cJSON_GetTopLevelValueValue(const cJSON * j);
+cJSON * cJSON_CreateTopLevelValue(const struct TopLevelValue * x);
+void cJSON_DeleteTopLevelValue(struct TopLevelValue * x);
+struct TopLevelValue * cJSON_ParseTopLevelValue(const char * s);
+char * cJSON_PrintTopLevelValue(const struct TopLevelValue * x);
+
+struct TopLevel * cJSON_ParseTopLevel(const char * s);
+struct TopLevel * cJSON_GetTopLevelValue(const cJSON * j);
+cJSON * cJSON_CreateTopLevel(const struct TopLevel * x);
+char * cJSON_PrintTopLevel(const struct TopLevel * x);
+void cJSON_DeleteTopLevel(struct TopLevel * x);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* __TOPLEVEL_H__ */
diff --git a/head/schema-cplusplus/test/inputs/schema/mixed-additional-properties.schema/default/quicktype.hpp b/head/schema-cplusplus/test/inputs/schema/mixed-additional-properties.schema/default/quicktype.hpp
new file mode 100644
index 0000000..af5ad75
--- /dev/null
+++ b/head/schema-cplusplus/test/inputs/schema/mixed-additional-properties.schema/default/quicktype.hpp
@@ -0,0 +1,122 @@
+//  To parse this JSON data, first install
+//
+//      json.hpp  https://github.com/nlohmann/json
+//
+//  Then include this file, and then do
+//
+//     TopLevel data = nlohmann::json::parse(jsonString);
+
+#pragma once
+
+#include <variant>
+#include "json.hpp"
+
+#include <optional>
+#include <stdexcept>
+#include <regex>
+
+#ifndef NLOHMANN_OPT_HELPER
+#define NLOHMANN_OPT_HELPER
+namespace nlohmann {
+    template <typename T>
+    struct adl_serializer<std::shared_ptr<T>> {
+        static void to_json(json & j, const std::shared_ptr<T> & opt) {
+            if (!opt) j = nullptr; else j = *opt;
+        }
+
+        static std::shared_ptr<T> from_json(const json & j) {
+            if (j.is_null()) return std::shared_ptr<T>(); else return std::make_shared<T>(j.get<T>());
+        }
+    };
+    template <typename T>
+    struct adl_serializer<std::optional<T>> {
+        static void to_json(json & j, const std::optional<T> & opt) {
+            if (!opt) j = nullptr; else j = *opt;
+        }
+
+        static std::optional<T> from_json(const json & j) {
+            if (j.is_null()) return std::optional<T>(); else return std::make_optional<T>(j.get<T>());
+        }
+    };
+}
+#endif
+
+namespace quicktype {
+    using nlohmann::json;
+
+    #ifndef NLOHMANN_UNTYPED_quicktype_HELPER
+    #define NLOHMANN_UNTYPED_quicktype_HELPER
+    inline json get_untyped(const json & j, const char * property) {
+        if (j.find(property) != j.end()) {
+            return j.at(property).get<json>();
+        }
+        return json();
+    }
+
+    inline json get_untyped(const json & j, std::string property) {
+        return get_untyped(j, property.data());
+    }
+    #endif
+
+    #ifndef NLOHMANN_OPTIONAL_quicktype_HELPER
+    #define NLOHMANN_OPTIONAL_quicktype_HELPER
+    template <typename T>
+    inline std::shared_ptr<T> get_heap_optional(const json & j, const char * property) {
+        auto it = j.find(property);
+        if (it != j.end() && !it->is_null()) {
+            return j.at(property).get<std::shared_ptr<T>>();
+        }
+        return std::shared_ptr<T>();
+    }
+
+    template <typename T>
+    inline std::shared_ptr<T> get_heap_optional(const json & j, std::string property) {
+        return get_heap_optional<T>(j, property.data());
+    }
+    template <typename T>
+    inline std::optional<T> get_stack_optional(const json & j, const char * property) {
+        auto it = j.find(property);
+        if (it != j.end() && !it->is_null()) {
+            return j.at(property).get<std::optional<T>>();
+        }
+        return std::optional<T>();
+    }
+
+    template <typename T>
+    inline std::optional<T> get_stack_optional(const json & j, std::string property) {
+        return get_stack_optional<T>(j, property.data());
+    }
+    #endif
+
+    using TopLevelValue = std::variant<double, std::string>;
+
+    using TopLevel = std::map<std::string, TopLevelValue>;
+}
+
+namespace nlohmann {
+    template <>
+    struct adl_serializer<std::variant<double, std::string>> {
+        static void from_json(const json & j, std::variant<double, std::string> & x);
+        static void to_json(json & j, const std::variant<double, std::string> & x);
+    };
+
+    inline void adl_serializer<std::variant<double, std::string>>::from_json(const json & j, std::variant<double, std::string> & x) {
+        if (j.is_number())
+            x = j.get<double>();
+        else if (j.is_string())
+            x = j.get<std::string>();
+        else throw std::runtime_error("Could not deserialise!");
+    }
+
+    inline void adl_serializer<std::variant<double, std::string>>::to_json(json & j, const std::variant<double, std::string> & x) {
+        switch (x.index()) {
+            case 0:
+                j = std::get<double>(x);
+                break;
+            case 1:
+                j = std::get<std::string>(x);
+                break;
+            default: throw std::runtime_error("Input JSON does not conform to schema!");
+        }
+    }
+}
diff --git a/head/schema-crystal/test/inputs/schema/mixed-additional-properties.schema/default/TopLevel.cr b/head/schema-crystal/test/inputs/schema/mixed-additional-properties.schema/default/TopLevel.cr
new file mode 100644
index 0000000..055bf62
--- /dev/null
+++ b/head/schema-crystal/test/inputs/schema/mixed-additional-properties.schema/default/TopLevel.cr
@@ -0,0 +1,5 @@
+require "json"
+
+alias TopLevel = Hash(String, TopLevelValue)
+
+alias TopLevelValue = Float64 | String
diff --git a/head/schema-csharp/test/inputs/schema/mixed-additional-properties.schema/default/QuickType.cs b/head/schema-csharp/test/inputs/schema/mixed-additional-properties.schema/default/QuickType.cs
new file mode 100644
index 0000000..2f58f74
--- /dev/null
+++ b/head/schema-csharp/test/inputs/schema/mixed-additional-properties.schema/default/QuickType.cs
@@ -0,0 +1,104 @@
+// <auto-generated />
+//
+// To parse this JSON data, add NuGet 'Newtonsoft.Json' then do:
+//
+//    using QuickType;
+//
+//    var topLevel = TopLevel.FromJson(jsonString);
+#nullable enable
+#pragma warning disable CS8618
+#pragma warning disable CS8601
+#pragma warning disable CS8602
+#pragma warning disable CS8603
+#pragma warning disable CS8604
+#pragma warning disable CS8625
+#pragma warning disable CS8765
+
+namespace QuickType
+{
+    using System;
+    using System.Collections.Generic;
+
+    using System.Globalization;
+    using Newtonsoft.Json;
+    using Newtonsoft.Json.Converters;
+
+    public partial struct TopLevelValue
+    {
+        public double? Double;
+        public string? String;
+
+        public static implicit operator TopLevelValue(double Double) => new TopLevelValue { Double = Double };
+        public static implicit operator TopLevelValue(string String) => new TopLevelValue { String = String };
+    }
+
+    public class TopLevel
+    {
+        public static Dictionary<string, TopLevelValue> FromJson(string json) => JsonConvert.DeserializeObject<Dictionary<string, TopLevelValue>>(json, QuickType.Converter.Settings);
+    }
+
+    public static partial class Serialize
+    {
+        public static string ToJson(this Dictionary<string, TopLevelValue> self) => JsonConvert.SerializeObject(self, QuickType.Converter.Settings);
+    }
+
+    internal static partial class Converter
+    {
+        public static readonly JsonSerializerSettings Settings = new JsonSerializerSettings
+        {
+            MetadataPropertyHandling = MetadataPropertyHandling.Ignore,
+            DateParseHandling = DateParseHandling.None,
+            Converters =
+            {
+                TopLevelValueConverter.Singleton,
+                new IsoDateTimeConverter { DateTimeStyles = DateTimeStyles.AssumeUniversal }
+            },
+        };
+    }
+
+    internal class TopLevelValueConverter : JsonConverter
+    {
+        public override bool CanConvert(Type t) => t == typeof(TopLevelValue) || t == typeof(TopLevelValue?);
+
+        public override object ReadJson(JsonReader reader, Type t, object existingValue, JsonSerializer serializer)
+        {
+            switch (reader.TokenType)
+            {
+                case JsonToken.Integer:
+                case JsonToken.Float:
+                    var doubleValue = serializer.Deserialize<double>(reader);
+                    return new TopLevelValue { Double = doubleValue };
+                case JsonToken.String:
+                case JsonToken.Date:
+                    var stringValue = serializer.Deserialize<string>(reader);
+                    return new TopLevelValue { String = stringValue };
+            }
+            throw new Exception("Cannot unmarshal type TopLevelValue");
+        }
+
+        public override void WriteJson(JsonWriter writer, object untypedValue, JsonSerializer serializer)
+        {
+            var value = (TopLevelValue)untypedValue;
+            if (value.Double != null)
+            {
+                serializer.Serialize(writer, value.Double.Value);
+                return;
+            }
+            if (value.String != null)
+            {
+                serializer.Serialize(writer, value.String);
+                return;
+            }
+            throw new Exception("Cannot marshal type TopLevelValue");
+        }
+
+        public static readonly TopLevelValueConverter Singleton = new TopLevelValueConverter();
+    }
+}
+#pragma warning restore CS8618
+#pragma warning restore CS8601
+#pragma warning restore CS8602
+#pragma warning restore CS8603
+#pragma warning restore CS8604
+#pragma warning restore CS8625
+#pragma warning restore CS8765
diff --git a/head/schema-csharp-SystemTextJson/test/inputs/schema/mixed-additional-properties.schema/default/QuickType.cs b/head/schema-csharp-SystemTextJson/test/inputs/schema/mixed-additional-properties.schema/default/QuickType.cs
new file mode 100644
index 0000000..c585230
--- /dev/null
+++ b/head/schema-csharp-SystemTextJson/test/inputs/schema/mixed-additional-properties.schema/default/QuickType.cs
@@ -0,0 +1,205 @@
+// <auto-generated />
+//
+// To parse this JSON data, add NuGet 'System.Text.Json' then do:
+//
+//    using QuickType;
+//
+//    var topLevel = TopLevel.FromJson(jsonString);
+#nullable enable
+#pragma warning disable CS8618
+#pragma warning disable CS8601
+#pragma warning disable CS8602
+#pragma warning disable CS8603
+
+namespace QuickType
+{
+    using System;
+    using System.Collections.Generic;
+
+    using System.Text.Json;
+    using System.Text.Json.Serialization;
+    using System.Globalization;
+
+    public partial struct TopLevelValue
+    {
+        public double? Double;
+        public string? String;
+
+        public static implicit operator TopLevelValue(double Double) => new TopLevelValue { Double = Double };
+        public static implicit operator TopLevelValue(string String) => new TopLevelValue { String = String };
+    }
+
+    public class TopLevel
+    {
+        public static Dictionary<string, TopLevelValue> FromJson(string json) => global::System.Text.Json.JsonSerializer.Deserialize<Dictionary<string, TopLevelValue>>(json, QuickType.Converter.Settings);
+    }
+
+    public static partial class Serialize
+    {
+        public static string ToJson(this Dictionary<string, TopLevelValue> self) => JsonSerializer.Serialize(self, QuickType.Converter.Settings);
+    }
+
+    internal static partial class Converter
+    {
+        public static readonly JsonSerializerOptions Settings = new(JsonSerializerDefaults.General)
+        {
+            Converters =
+            {
+                TopLevelValueConverter.Singleton,
+                new DateOnlyConverter(),
+                new TimeOnlyConverter(),
+                IsoDateTimeOffsetConverter.Singleton
+            },
+        };
+    }
+
+    internal class TopLevelValueConverter : JsonConverter<TopLevelValue>
+    {
+        public override bool CanConvert(Type t) => t == typeof(TopLevelValue);
+
+        public override TopLevelValue Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
+        {
+            switch (reader.TokenType)
+            {
+                case JsonTokenType.Number:
+                    var doubleValue1 = reader.GetDouble();
+                    return new TopLevelValue { Double = doubleValue1 };
+                case JsonTokenType.String:
+                    var stringValue = reader.GetString();
+                    return new TopLevelValue { String = stringValue };
+            }
+            throw new JsonException("Cannot unmarshal type TopLevelValue");
+        }
+
+        public override void Write(Utf8JsonWriter writer, TopLevelValue value, JsonSerializerOptions options)
+        {
+            if (value.Double != null)
+            {
+                JsonSerializer.Serialize(writer, value.Double.Value, options);
+                return;
+            }
+            if (value.String != null)
+            {
+                JsonSerializer.Serialize(writer, value.String, options);
+                return;
+            }
+            throw new NotSupportedException("Cannot marshal type TopLevelValue");
+        }
+
+        public static readonly TopLevelValueConverter Singleton = new TopLevelValueConverter();
+    }
+    
+    public class DateOnlyConverter : JsonConverter<DateOnly>
+    {
+        private readonly string serializationFormat;
+        public DateOnlyConverter() : this(null) { }
+
+        public DateOnlyConverter(string? serializationFormat)
+        {
+                this.serializationFormat = serializationFormat ?? "yyyy-MM-dd";
+        }
+
+        public override DateOnly Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
+        {
+                var value = reader.GetString();
+                return DateOnly.Parse(value!);
+        }
+
+        public override void Write(Utf8JsonWriter writer, DateOnly value, JsonSerializerOptions options)
+                => writer.WriteStringValue(value.ToString(serializationFormat));
+    }
+
+    public class TimeOnlyConverter : JsonConverter<TimeOnly>
+    {
+        private readonly string serializationFormat;
+
+        public TimeOnlyConverter() : this(null) { }
+
+        public TimeOnlyConverter(string? serializationFormat)
+        {
+                this.serializationFormat = serializationFormat ?? "HH:mm:ss.fff";
+        }
+
+        public override TimeOnly Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
+        {
+                var value = reader.GetString();
+                return TimeOnly.Parse(value!);
+        }
+
+        public override void Write(Utf8JsonWriter writer, TimeOnly value, JsonSerializerOptions options)
+                => writer.WriteStringValue(value.ToString(serializationFormat));
+    }
+
+    internal class IsoDateTimeOffsetConverter : JsonConverter<DateTimeOffset>
+    {
+        public override bool CanConvert(Type t) => t == typeof(DateTimeOffset);
+
+        private const string DefaultDateTimeFormat = "yyyy'-'MM'-'dd'T'HH':'mm':'ss.FFFFFFFK";
+
+        private DateTimeStyles _dateTimeStyles = DateTimeStyles.RoundtripKind;
+        private string? _dateTimeFormat;
+        private CultureInfo? _culture;
+
+        public DateTimeStyles DateTimeStyles
+        {
+                get => _dateTimeStyles;
+                set => _dateTimeStyles = value;
+        }
+
+        public string? DateTimeFormat
+        {
+                get => _dateTimeFormat ?? string.Empty;
+                set => _dateTimeFormat = (string.IsNullOrEmpty(value)) ? null : value;
+        }
+
+        public CultureInfo Culture
+        {
+                get => _culture ?? CultureInfo.CurrentCulture;
+                set => _culture = value;
+        }
+
+        public override void Write(Utf8JsonWriter writer, DateTimeOffset value, JsonSerializerOptions options)
+        {
+                string text;
+
+
+                if ((_dateTimeStyles & DateTimeStyles.AdjustToUniversal) == DateTimeStyles.AdjustToUniversal
+                        || (_dateTimeStyles & DateTimeStyles.AssumeUniversal) == DateTimeStyles.AssumeUniversal)
+                {
+                        value = value.ToUniversalTime();
+                }
+
+                text = value.ToString(_dateTimeFormat ?? DefaultDateTimeFormat, Culture);
+
+                writer.WriteStringValue(text);
+        }
+
+        public override DateTimeOffset Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
+        {
+                string? dateText = reader.GetString();
+
+                if (string.IsNullOrEmpty(dateText) == false)
+                {
+                        if (!string.IsNullOrEmpty(_dateTimeFormat))
+                        {
+                                return DateTimeOffset.ParseExact(dateText, _dateTimeFormat, Culture, _dateTimeStyles);
+                        }
+                        else
+                        {
+                                return DateTimeOffset.Parse(dateText, Culture, _dateTimeStyles);
+                        }
+                }
+                else
+                {
+                        throw new JsonException("Cannot unmarshal date-time");
+                }
+        }
+
+
+        public static readonly IsoDateTimeOffsetConverter Singleton = new IsoDateTimeOffsetConverter();
+    }
+}
+#pragma warning restore CS8618
+#pragma warning restore CS8601
+#pragma warning restore CS8602
+#pragma warning restore CS8603
diff --git a/head/schema-csharp-records/test/inputs/schema/mixed-additional-properties.schema/default/QuickType.cs b/head/schema-csharp-records/test/inputs/schema/mixed-additional-properties.schema/default/QuickType.cs
new file mode 100644
index 0000000..2f58f74
--- /dev/null
+++ b/head/schema-csharp-records/test/inputs/schema/mixed-additional-properties.schema/default/QuickType.cs
@@ -0,0 +1,104 @@
+// <auto-generated />
+//
+// To parse this JSON data, add NuGet 'Newtonsoft.Json' then do:
+//
+//    using QuickType;
+//
+//    var topLevel = TopLevel.FromJson(jsonString);
+#nullable enable
+#pragma warning disable CS8618
+#pragma warning disable CS8601
+#pragma warning disable CS8602
+#pragma warning disable CS8603
+#pragma warning disable CS8604
+#pragma warning disable CS8625
+#pragma warning disable CS8765
+
+namespace QuickType
+{
+    using System;
+    using System.Collections.Generic;
+
+    using System.Globalization;
+    using Newtonsoft.Json;
+    using Newtonsoft.Json.Converters;
+
+    public partial struct TopLevelValue
+    {
+        public double? Double;
+        public string? String;
+
+        public static implicit operator TopLevelValue(double Double) => new TopLevelValue { Double = Double };
+        public static implicit operator TopLevelValue(string String) => new TopLevelValue { String = String };
+    }
+
+    public class TopLevel
+    {
+        public static Dictionary<string, TopLevelValue> FromJson(string json) => JsonConvert.DeserializeObject<Dictionary<string, TopLevelValue>>(json, QuickType.Converter.Settings);
+    }
+
+    public static partial class Serialize
+    {
+        public static string ToJson(this Dictionary<string, TopLevelValue> self) => JsonConvert.SerializeObject(self, QuickType.Converter.Settings);
+    }
+
+    internal static partial class Converter
+    {
+        public static readonly JsonSerializerSettings Settings = new JsonSerializerSettings
+        {
+            MetadataPropertyHandling = MetadataPropertyHandling.Ignore,
+            DateParseHandling = DateParseHandling.None,
+            Converters =
+            {
+                TopLevelValueConverter.Singleton,
+                new IsoDateTimeConverter { DateTimeStyles = DateTimeStyles.AssumeUniversal }
+            },
+        };
+    }
+
+    internal class TopLevelValueConverter : JsonConverter
+    {
+        public override bool CanConvert(Type t) => t == typeof(TopLevelValue) || t == typeof(TopLevelValue?);
+
+        public override object ReadJson(JsonReader reader, Type t, object existingValue, JsonSerializer serializer)
+        {
+            switch (reader.TokenType)
+            {
+                case JsonToken.Integer:
+                case JsonToken.Float:
+                    var doubleValue = serializer.Deserialize<double>(reader);
+                    return new TopLevelValue { Double = doubleValue };
+                case JsonToken.String:
+                case JsonToken.Date:
+                    var stringValue = serializer.Deserialize<string>(reader);
+                    return new TopLevelValue { String = stringValue };
+            }
+            throw new Exception("Cannot unmarshal type TopLevelValue");
+        }
+
+        public override void WriteJson(JsonWriter writer, object untypedValue, JsonSerializer serializer)
+        {
+            var value = (TopLevelValue)untypedValue;
+            if (value.Double != null)
+            {
+                serializer.Serialize(writer, value.Double.Value);
+                return;
+            }
+            if (value.String != null)
+            {
+                serializer.Serialize(writer, value.String);
+                return;
+            }
+            throw new Exception("Cannot marshal type TopLevelValue");
+        }
+
+        public static readonly TopLevelValueConverter Singleton = new TopLevelValueConverter();
+    }
+}
+#pragma warning restore CS8618
+#pragma warning restore CS8601
+#pragma warning restore CS8602
+#pragma warning restore CS8603
+#pragma warning restore CS8604
+#pragma warning restore CS8625
+#pragma warning restore CS8765
diff --git a/head/schema-dart/test/inputs/schema/mixed-additional-properties.schema/default/TopLevel.dart b/head/schema-dart/test/inputs/schema/mixed-additional-properties.schema/default/TopLevel.dart
new file mode 100644
index 0000000..fb45db8
--- /dev/null
+++ b/head/schema-dart/test/inputs/schema/mixed-additional-properties.schema/default/TopLevel.dart
@@ -0,0 +1,9 @@
+// To parse this JSON data, do
+//
+//     final topLevel = topLevelFromJson(jsonString);
+
+import 'dart:convert';
+
+Map<String, dynamic> topLevelFromJson(String str) => Map.from(json.decode(str)).map((k, v) => MapEntry<String, dynamic>(k, v));
+
+String topLevelToJson(Map<String, dynamic> data) => json.encode(Map.from(data).map((k, v) => MapEntry<String, dynamic>(k, v)));
diff --git a/head/schema-elixir/test/inputs/schema/mixed-additional-properties.schema/default/QuickType.ex b/head/schema-elixir/test/inputs/schema/mixed-additional-properties.schema/default/QuickType.ex
new file mode 100644
index 0000000..10f0eee
--- /dev/null
+++ b/head/schema-elixir/test/inputs/schema/mixed-additional-properties.schema/default/QuickType.ex
@@ -0,0 +1,20 @@
+# This file was autogenerated using quicktype https://github.com/quicktype/quicktype
+#
+# Add Jason to your mix.exs
+#
+# Decode a JSON string: TopLevel.from_json(data)
+# Encode into a JSON string: TopLevel.to_json(struct)
+
+defmodule TopLevel do
+  def decode_value(value) when is_map(value), do: value
+
+  def from_json(json) do
+    json
+    |> Jason.decode!()
+    |> decode_value()
+  end
+
+  def to_json(data) do
+    Jason.encode!(data)
+  end
+end
diff --git a/head/schema-elm/test/inputs/schema/mixed-additional-properties.schema/default/QuickType.elm b/head/schema-elm/test/inputs/schema/mixed-additional-properties.schema/default/QuickType.elm
new file mode 100644
index 0000000..2893d8b
--- /dev/null
+++ b/head/schema-elm/test/inputs/schema/mixed-additional-properties.schema/default/QuickType.elm
@@ -0,0 +1,64 @@
+-- To decode the JSON data, add this file to your project, run
+--
+--     elm install NoRedInk/elm-json-decode-pipeline
+--
+-- add these imports
+--
+--     import Json.Decode exposing (decodeString)
+--     import QuickType exposing (quickType)
+--
+-- and you're off to the races with
+--
+--     decodeString quickType myJsonString
+
+module QuickType exposing
+    ( QuickType
+    , quickTypeToString
+    , quickType
+    , QuickTypeValue(..)
+    )
+
+import Json.Decode as Jdec
+import Json.Decode.Pipeline as Jpipe
+import Json.Encode as Jenc
+import Dict exposing (Dict)
+
+type alias QuickType = Dict String QuickTypeValue
+
+type QuickTypeValue
+    = DoubleInQuickTypeValue Float
+    | StringInQuickTypeValue String
+
+-- decoders and encoders
+optionalField key decoder fallback =
+    Jdec.dict Jdec.value
+        |> Jdec.andThen (\m ->
+            case Dict.get key m of
+                Nothing -> Jdec.succeed fallback
+                Just x -> Jdec.decodeValue decoder x |> Result.map Jdec.succeed |> Result.withDefault (Jdec.fail ("Invalid " ++ key)))
+
+quickType : Jdec.Decoder QuickType
+quickType = Jdec.dict quickTypeValue
+
+quickTypeToString : QuickType -> String
+quickTypeToString r = Jenc.encode 0 (Jenc.dict identity encodeQuickTypeValue r)
+
+quickTypeValue : Jdec.Decoder QuickTypeValue
+quickTypeValue =
+    Jdec.oneOf
+        [ Jdec.map StringInQuickTypeValue Jdec.string
+        , Jdec.map DoubleInQuickTypeValue Jdec.float
+        ]
+
+encodeQuickTypeValue : QuickTypeValue -> Jenc.Value
+encodeQuickTypeValue x = case x of
+    StringInQuickTypeValue y -> Jenc.string y
+    DoubleInQuickTypeValue y -> Jenc.float y
+
+--- encoder helpers
+
+makeNullableEncoder : (a -> Jenc.Value) -> Maybe a -> Jenc.Value
+makeNullableEncoder f m =
+    case m of
+    Just x -> f x
+    Nothing -> Jenc.null
diff --git a/base/schema-flow/test/inputs/schema/any.schema/default/TopLevel.js b/head/schema-flow/test/inputs/schema/any.schema/default/TopLevel.js
index dbe7769..9b41bf2 100644
--- a/base/schema-flow/test/inputs/schema/any.schema/default/TopLevel.js
+++ b/head/schema-flow/test/inputs/schema/any.schema/default/TopLevel.js
@@ -12,7 +12,7 @@
 export type TopLevel = {
     foo:    mixed;
     values: { [key: string]: mixed };
-    [property: string]: mixed | mixed | { [key: string]: mixed };
+    [property: string]: mixed;
 };
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-flow/test/inputs/schema/bool-string.schema/default/TopLevel.js b/head/schema-flow/test/inputs/schema/bool-string.schema/default/TopLevel.js
index 538997b..de3bb1e 100644
--- a/base/schema-flow/test/inputs/schema/bool-string.schema/default/TopLevel.js
+++ b/head/schema-flow/test/inputs/schema/bool-string.schema/default/TopLevel.js
@@ -17,7 +17,7 @@ export type TopLevel = {
     optional?:            string;
     unionWithBool:        UnionWithBool;
     unionWithBoolAndEnum: UnionWithBool;
-    [property: string]: mixed | (null | string)[] | string[] | null | string | string | string | UnionWithBool | UnionWithBool;
+    [property: string]: mixed;
 };
 
 export type UnionWithBool = boolean | string;
diff --git a/base/schema-flow/test/inputs/schema/camelCase.schema/default/TopLevel.js b/head/schema-flow/test/inputs/schema/camelCase.schema/default/TopLevel.js
index 759cf7f..e050014 100644
--- a/base/schema-flow/test/inputs/schema/camelCase.schema/default/TopLevel.js
+++ b/head/schema-flow/test/inputs/schema/camelCase.schema/default/TopLevel.js
@@ -12,7 +12,7 @@
 export type TopLevel = {
     myProperty?:     string;
     secondProperty?: string;
-    [property: string]: mixed | string | string;
+    [property: string]: mixed;
 };
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-flow/test/inputs/schema/class-map-union.schema/default/TopLevel.js b/head/schema-flow/test/inputs/schema/class-map-union.schema/default/TopLevel.js
index c3c17ad..f8747ce 100644
--- a/base/schema-flow/test/inputs/schema/class-map-union.schema/default/TopLevel.js
+++ b/head/schema-flow/test/inputs/schema/class-map-union.schema/default/TopLevel.js
@@ -16,14 +16,14 @@ export type TopLevel = {
 export type TopLevelUnion = {
     foo?: Foo;
     bar?: Bar;
-    [property: string]: UnionValue | Foo | Bar;
+    [property: string]: UnionValue | Foo | Bar | void;
 };
 
 export type Bar = boolean | BarObject | string;
 
 export type BarObject = {
     quux?: number;
-    [property: string]: mixed | number;
+    [property: string]: mixed;
 };
 
 export type Foo = boolean | number | BarObject;
diff --git a/base/schema-flow/test/inputs/schema/class-with-additional.schema/default/TopLevel.js b/head/schema-flow/test/inputs/schema/class-with-additional.schema/default/TopLevel.js
index 25c9f2e..319b5d0 100644
--- a/base/schema-flow/test/inputs/schema/class-with-additional.schema/default/TopLevel.js
+++ b/head/schema-flow/test/inputs/schema/class-with-additional.schema/default/TopLevel.js
@@ -15,7 +15,7 @@ export type TopLevel = {
 
 export type Map = {
     foo?: number;
-    [property: string]: boolean | number;
+    [property: string]: boolean | number | void;
 };
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-flow/test/inputs/schema/const-non-string.schema/default/TopLevel.js b/head/schema-flow/test/inputs/schema/const-non-string.schema/default/TopLevel.js
index 98038cd..bfdefce 100644
--- a/base/schema-flow/test/inputs/schema/const-non-string.schema/default/TopLevel.js
+++ b/head/schema-flow/test/inputs/schema/const-non-string.schema/default/TopLevel.js
@@ -15,7 +15,7 @@ export type TopLevel = {
     kind:    Kind;
     ratio:   number;
     version: number;
-    [property: string]: mixed | number | boolean | Kind | number | number;
+    [property: string]: mixed;
 };
 
 export type Kind =
diff --git a/base/schema-flow/test/inputs/schema/constructor.schema/default/TopLevel.js b/head/schema-flow/test/inputs/schema/constructor.schema/default/TopLevel.js
index 3e47f3a..461857e 100644
--- a/base/schema-flow/test/inputs/schema/constructor.schema/default/TopLevel.js
+++ b/head/schema-flow/test/inputs/schema/constructor.schema/default/TopLevel.js
@@ -11,7 +11,7 @@
 
 export type TopLevel = {
     constructor?: UnionConstructor;
-    [property: string]: mixed | UnionConstructor;
+    [property: string]: mixed;
 };
 
 export type UnionConstructor = Constructor | number;
diff --git a/base/schema-flow/test/inputs/schema/cut-enum.schema/default/TopLevel.js b/head/schema-flow/test/inputs/schema/cut-enum.schema/default/TopLevel.js
index f614921..fd3804b 100644
--- a/base/schema-flow/test/inputs/schema/cut-enum.schema/default/TopLevel.js
+++ b/head/schema-flow/test/inputs/schema/cut-enum.schema/default/TopLevel.js
@@ -11,7 +11,7 @@
 
 export type TopLevel = {
     foo: number;
-    [property: string]: mixed | number;
+    [property: string]: mixed;
 };
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-flow/test/inputs/schema/date-time-or-string.schema/default/TopLevel.js b/head/schema-flow/test/inputs/schema/date-time-or-string.schema/default/TopLevel.js
index aedb4eb..159b882 100644
--- a/base/schema-flow/test/inputs/schema/date-time-or-string.schema/default/TopLevel.js
+++ b/head/schema-flow/test/inputs/schema/date-time-or-string.schema/default/TopLevel.js
@@ -12,7 +12,7 @@
 export type TopLevel = {
     bar: BarUnion;
     foo: string;
-    [property: string]: mixed | BarUnion | string;
+    [property: string]: mixed;
 };
 
 export type BarUnion = Date | BarEnum;
diff --git a/base/schema-flow/test/inputs/schema/description-with-double-quotes.schema/default/TopLevel.js b/head/schema-flow/test/inputs/schema/description-with-double-quotes.schema/default/TopLevel.js
index 0f1973b..460c470 100644
--- a/base/schema-flow/test/inputs/schema/description-with-double-quotes.schema/default/TopLevel.js
+++ b/head/schema-flow/test/inputs/schema/description-with-double-quotes.schema/default/TopLevel.js
@@ -14,7 +14,7 @@
  */
 export type TopLevel = {
     foo?: boolean;
-    [property: string]: mixed | boolean;
+    [property: string]: mixed;
 };
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-flow/test/inputs/schema/description.schema/default/TopLevel.js b/head/schema-flow/test/inputs/schema/description.schema/default/TopLevel.js
index 7bc136e..8a80a17 100644
--- a/base/schema-flow/test/inputs/schema/description.schema/default/TopLevel.js
+++ b/head/schema-flow/test/inputs/schema/description.schema/default/TopLevel.js
@@ -44,7 +44,7 @@ export type ObjectOrStringObject = {
      * This must not get lost
      */
     prop: number;
-    [property: string]: mixed | number;
+    [property: string]: mixed;
 };
 
 /**
diff --git a/base/schema-flow/test/inputs/schema/enum-large.schema/default/TopLevel.js b/head/schema-flow/test/inputs/schema/enum-large.schema/default/TopLevel.js
index 831aaed..da2bc16 100644
--- a/base/schema-flow/test/inputs/schema/enum-large.schema/default/TopLevel.js
+++ b/head/schema-flow/test/inputs/schema/enum-large.schema/default/TopLevel.js
@@ -12,7 +12,7 @@
 export type TopLevel = {
     callsign: Callsign;
     priority: Priority;
-    [property: string]: mixed | Callsign | Priority;
+    [property: string]: mixed;
 };
 
 export type Callsign =
diff --git a/base/schema-flow/test/inputs/schema/enum-with-values.schema/default/TopLevel.js b/head/schema-flow/test/inputs/schema/enum-with-values.schema/default/TopLevel.js
index da7585e..b984eb2 100644
--- a/base/schema-flow/test/inputs/schema/enum-with-values.schema/default/TopLevel.js
+++ b/head/schema-flow/test/inputs/schema/enum-with-values.schema/default/TopLevel.js
@@ -11,7 +11,7 @@
 
 export type TopLevel = {
     weekdays: Weekdays;
-    [property: string]: mixed | Weekdays;
+    [property: string]: mixed;
 };
 
 export type Weekdays =
diff --git a/base/schema-flow/test/inputs/schema/enum.schema/default/TopLevel.js b/head/schema-flow/test/inputs/schema/enum.schema/default/TopLevel.js
index 1761980..6fe5b6c 100644
--- a/base/schema-flow/test/inputs/schema/enum.schema/default/TopLevel.js
+++ b/head/schema-flow/test/inputs/schema/enum.schema/default/TopLevel.js
@@ -15,7 +15,7 @@ export type TopLevel = {
     gve:       Gve;
     lvc?:      Lvc;
     otherArr?: OtherArr[];
-    [property: string]: mixed | Arr[] | string | Gve | Lvc | OtherArr[];
+    [property: string]: mixed;
 };
 
 export type Arr = OtherArr | number;
diff --git a/base/schema-flow/test/inputs/schema/fractional-bounds.schema/default/TopLevel.js b/head/schema-flow/test/inputs/schema/fractional-bounds.schema/default/TopLevel.js
index 706aa33..022ded2 100644
--- a/base/schema-flow/test/inputs/schema/fractional-bounds.schema/default/TopLevel.js
+++ b/head/schema-flow/test/inputs/schema/fractional-bounds.schema/default/TopLevel.js
@@ -11,7 +11,7 @@
 
 export type TopLevel = {
     value: number;
-    [property: string]: mixed | number;
+    [property: string]: mixed;
 };
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-flow/test/inputs/schema/go-schema-pattern-properties.schema/default/TopLevel.js b/head/schema-flow/test/inputs/schema/go-schema-pattern-properties.schema/default/TopLevel.js
index 134dbde..35ad8a1 100644
--- a/base/schema-flow/test/inputs/schema/go-schema-pattern-properties.schema/default/TopLevel.js
+++ b/head/schema-flow/test/inputs/schema/go-schema-pattern-properties.schema/default/TopLevel.js
@@ -11,7 +11,7 @@
 
 export type TopLevel = {
     map: { [key: string]: number };
-    [property: string]: mixed | { [key: string]: number };
+    [property: string]: mixed;
 };
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-flow/test/inputs/schema/haskell-enum-forbidden.schema/default/TopLevel.js b/head/schema-flow/test/inputs/schema/haskell-enum-forbidden.schema/default/TopLevel.js
index b245d3e..4142df5 100644
--- a/base/schema-flow/test/inputs/schema/haskell-enum-forbidden.schema/default/TopLevel.js
+++ b/head/schema-flow/test/inputs/schema/haskell-enum-forbidden.schema/default/TopLevel.js
@@ -11,7 +11,7 @@
 
 export type TopLevel = {
     health: Health;
-    [property: string]: mixed | Health;
+    [property: string]: mixed;
 };
 
 export type Health =
diff --git a/base/schema-flow/test/inputs/schema/id-no-address.schema/default/TopLevel.js b/head/schema-flow/test/inputs/schema/id-no-address.schema/default/TopLevel.js
index c94b330..94dd602 100644
--- a/base/schema-flow/test/inputs/schema/id-no-address.schema/default/TopLevel.js
+++ b/head/schema-flow/test/inputs/schema/id-no-address.schema/default/TopLevel.js
@@ -11,7 +11,7 @@
 
 export type TopLevel = {
     item: number;
-    [property: string]: mixed | number;
+    [property: string]: mixed;
 };
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-flow/test/inputs/schema/id-root.schema/default/TopLevel.js b/head/schema-flow/test/inputs/schema/id-root.schema/default/TopLevel.js
index ab8f050..0da7b3c 100644
--- a/base/schema-flow/test/inputs/schema/id-root.schema/default/TopLevel.js
+++ b/head/schema-flow/test/inputs/schema/id-root.schema/default/TopLevel.js
@@ -11,7 +11,7 @@
 
 export type TopLevel = {
     bar: number;
-    [property: string]: mixed | number;
+    [property: string]: mixed;
 };
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-flow/test/inputs/schema/ie-suffix-singularization.schema/default/TopLevel.js b/head/schema-flow/test/inputs/schema/ie-suffix-singularization.schema/default/TopLevel.js
index 0ed6a5b..282e724 100644
--- a/base/schema-flow/test/inputs/schema/ie-suffix-singularization.schema/default/TopLevel.js
+++ b/head/schema-flow/test/inputs/schema/ie-suffix-singularization.schema/default/TopLevel.js
@@ -11,13 +11,13 @@
 
 export type TopLevel = {
     cookies: Cookie[];
-    [property: string]: mixed | Cookie[];
+    [property: string]: mixed;
 };
 
 export type Cookie = {
     name:  string;
     value: string;
-    [property: string]: mixed | string | string;
+    [property: string]: mixed;
 };
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-flow/test/inputs/schema/implicit-all-of.schema/default/TopLevel.js b/head/schema-flow/test/inputs/schema/implicit-all-of.schema/default/TopLevel.js
index 44fe2af..3e5b1af 100644
--- a/base/schema-flow/test/inputs/schema/implicit-all-of.schema/default/TopLevel.js
+++ b/head/schema-flow/test/inputs/schema/implicit-all-of.schema/default/TopLevel.js
@@ -13,7 +13,7 @@ export type TopLevel = {
     foo:  number;
     bar:  boolean;
     quux: string;
-    [property: string]: mixed | number | boolean | string;
+    [property: string]: mixed;
 };
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-flow/test/inputs/schema/implicit-class-array-union.schema/default/TopLevel.js b/head/schema-flow/test/inputs/schema/implicit-class-array-union.schema/default/TopLevel.js
index 1f2949a..7c09067 100644
--- a/base/schema-flow/test/inputs/schema/implicit-class-array-union.schema/default/TopLevel.js
+++ b/head/schema-flow/test/inputs/schema/implicit-class-array-union.schema/default/TopLevel.js
@@ -11,14 +11,14 @@
 
 export type TopLevel = {
     foo: FooUnion;
-    [property: string]: mixed | FooUnion;
+    [property: string]: mixed;
 };
 
 export type FooUnion = number[] | boolean | number | number | null | FooObject | string;
 
 export type FooObject = {
     bar: number;
-    [property: string]: mixed | number;
+    [property: string]: mixed;
 };
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-flow/test/inputs/schema/implicit-one-of.schema/default/TopLevel.js b/head/schema-flow/test/inputs/schema/implicit-one-of.schema/default/TopLevel.js
index 6c3103c..028af63 100644
--- a/base/schema-flow/test/inputs/schema/implicit-one-of.schema/default/TopLevel.js
+++ b/head/schema-flow/test/inputs/schema/implicit-one-of.schema/default/TopLevel.js
@@ -13,7 +13,7 @@ export type TopLevel = {
     foo:   number;
     bar?:  boolean;
     quux?: string;
-    [property: string]: mixed | number | boolean | string;
+    [property: string]: mixed;
 };
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-flow/test/inputs/schema/integer-float-union.schema/default/TopLevel.js b/head/schema-flow/test/inputs/schema/integer-float-union.schema/default/TopLevel.js
index 523af06..c434c99 100644
--- a/base/schema-flow/test/inputs/schema/integer-float-union.schema/default/TopLevel.js
+++ b/head/schema-flow/test/inputs/schema/integer-float-union.schema/default/TopLevel.js
@@ -11,7 +11,7 @@
 
 export type TopLevel = {
     number: number;
-    [property: string]: mixed | number;
+    [property: string]: mixed;
 };
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-flow/test/inputs/schema/integer-string.schema/default/TopLevel.js b/head/schema-flow/test/inputs/schema/integer-string.schema/default/TopLevel.js
index b99a3c1..ef28a5e 100644
--- a/base/schema-flow/test/inputs/schema/integer-string.schema/default/TopLevel.js
+++ b/head/schema-flow/test/inputs/schema/integer-string.schema/default/TopLevel.js
@@ -17,7 +17,7 @@ export type TopLevel = {
     optional?:           string;
     unionWithInt:        UnionWithInt;
     unionWithIntAndEnum: UnionWithInt;
-    [property: string]: mixed | (null | string)[] | string[] | null | string | string | string | UnionWithInt | UnionWithInt;
+    [property: string]: mixed;
 };
 
 export type UnionWithInt = number | string;
diff --git a/base/schema-flow/test/inputs/schema/integer-type.schema/default/TopLevel.js b/head/schema-flow/test/inputs/schema/integer-type.schema/default/TopLevel.js
index 1d1eaad..51b83f3 100644
--- a/base/schema-flow/test/inputs/schema/integer-type.schema/default/TopLevel.js
+++ b/head/schema-flow/test/inputs/schema/integer-type.schema/default/TopLevel.js
@@ -19,7 +19,7 @@ export type TopLevel = {
     small_negative: number;
     small_positive: number;
     unbounded:      number;
-    [property: string]: mixed | number | number | number | number | number | number | number | number | number;
+    [property: string]: mixed;
 };
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-flow/test/inputs/schema/intersection.schema/default/TopLevel.js b/head/schema-flow/test/inputs/schema/intersection.schema/default/TopLevel.js
index ba2297d..7ff9370 100644
--- a/base/schema-flow/test/inputs/schema/intersection.schema/default/TopLevel.js
+++ b/head/schema-flow/test/inputs/schema/intersection.schema/default/TopLevel.js
@@ -16,7 +16,7 @@ export type TopLevel = {
 export type Intersection = {
     foo:  number;
     bar?: string;
-    [property: string]: mixed | number | string;
+    [property: string]: mixed;
 };
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-flow/test/inputs/schema/keyword-enum.schema/default/TopLevel.js b/head/schema-flow/test/inputs/schema/keyword-enum.schema/default/TopLevel.js
index 0bd422e..3fcb534 100644
--- a/base/schema-flow/test/inputs/schema/keyword-enum.schema/default/TopLevel.js
+++ b/head/schema-flow/test/inputs/schema/keyword-enum.schema/default/TopLevel.js
@@ -11,7 +11,7 @@
 
 export type TopLevel = {
     enum?: Enum;
-    [property: string]: mixed | Enum;
+    [property: string]: mixed;
 };
 
 export type Enum =
diff --git a/base/schema-flow/test/inputs/schema/keyword-unions.schema/default/TopLevel.js b/head/schema-flow/test/inputs/schema/keyword-unions.schema/default/TopLevel.js
index dd0f872..381a90c 100644
--- a/base/schema-flow/test/inputs/schema/keyword-unions.schema/default/TopLevel.js
+++ b/head/schema-flow/test/inputs/schema/keyword-unions.schema/default/TopLevel.js
@@ -287,7 +287,7 @@ export type TopLevel = {
     xor_eq?:                     UnionXorEq;
     YES?:                        UnionYES;
     yield?:                      UnionYield;
-    [property: string]: mixed | UnionUnion | UnionBool | UnionComplex | UnionImaginery | UnionAbstract | UnionAlignas | UnionAlignof | UnionAnd | UnionAndEq | UnionAnyUnion | UnionAny | UnionArray | UnionAs | UnionASM | UnionAssert | UnionAssociatedtype | UnionAssociativity | UnionAsync | UnionAtomic | UnionAtomicCancel | UnionAtomicCommit | UnionAtomicNoexcept | UnionAuto | UnionAwait | UnionBase | UnionBitand | UnionBitor | UnionBOOL | UnionBoolUnion | UnionBoolean | UnionBreak | UnionBycopy | UnionByref | UnionByte | UnionCase | UnionCatch | UnionChan | UnionChar | UnionChar16T | UnionChar32T | UnionChecked | UnionClassUnion | UnionClass | UnionCoAwait | UnionCoReturn | UnionCoYield | UnionCompl | UnionConcept | UnionConsole | UnionConst | UnionConstCast | UnionConstexpr | UnionConstructor | UnionContinue | UnionConvenience | UnionConvert | UnionConverter | UnionDate | UnionDateParseHandling | UnionDebugger | UnionDecimal | UnionDeclare | UnionDecltype | UnionDecodeString | UnionDef | UnionDefault | UnionDefer | UnionDeinit | UnionDel | UnionDelegate | UnionDelete | UnionDict | UnionDictionary | UnionDidSet | UnionDo | UnionDouble | number | UnionDynamic | UnionDynamicCast | UnionElif | UnionElse | UnionEncodeQuickType | UnionEnum | UnionEvent | UnionExcept | UnionException | UnionExplicit | UnionExport | UnionExposing | UnionExtends | UnionExtension | UnionExtern | UnionFallthrough | UnionFalseUnion | UnionFalse | UnionFileprivate | UnionFinal | UnionFinally | UnionFixed | UnionFloat | UnionFor | UnionForeach | UnionFriend | UnionFrom | UnionFromJSON | UnionFunc | UnionFunction | UnionGet | UnionGlobal | UnionGo | UnionGoto | UnionGuard | UnionHasOwnProperty | UnionID | UnionIf | UnionIMP | UnionImplements | UnionImplicit | UnionImport | UnionIn | UnionIndirect | UnionInfix | UnionInit | UnionInline | UnionInout | UnionInstanceof | UnionInt | UnionInterface | UnionInternal | UnionIs | UnionIterable | UnionJdec | UnionJenc | UnionJpipe | UnionJSON | UnionJSONConverter | UnionJSONSerializer | UnionJSONToken | UnionJSONWriter | UnionLambda | UnionLazy | UnionLeft | UnionLet | UnionList | UnionLock | UnionLong | UnionMap | UnionMetadataPropertyHandling | UnionModule | UnionMutable | UnionMutating | UnionNamespace | UnionNative | UnionNew | UnionNewtonsoft | UnionNil | UnionNO | UnionNoexcept | UnionNonatomic | UnionNoneUnion | UnionNone | UnionNonlocal | UnionNonmutating | UnionNot | UnionNotEq | UnionNSString | UnionNULL | UnionNull | UnionNullptr | UnionNumber | UnionObject | UnionOf | UnionOneway | UnionOpen | UnionOperator | UnionOptional | UnionOr | UnionOrEq | UnionOut | UnionOverride | UnionPackage | UnionParams | UnionPass | UnionPort | UnionPostfix | UnionPrecedence | UnionPrefix | UnionPrint | UnionPrintf | UnionPrivate | UnionProtected | UnionProtocol | UnionProtocolUnion | UnionPublic | UnionQuicktype | UnionRaise | UnionRange | UnionReadonly | UnionRef | UnionRegister | UnionReinterpretCast | UnionRepeat | UnionRequire | UnionRequired | UnionRequires | UnionRestrict | UnionRetain | UnionRethrows | UnionReturn | UnionRight | UnionSbyte | UnionSealed | UnionSEL | UnionSelect | UnionSelf | UnionSelfUnion | UnionSerialize | UnionSet | UnionShort | UnionSigned | UnionSizeof | UnionStackalloc | UnionStatic | UnionStaticAssert | UnionStaticCast | UnionStrictfp | UnionString | UnionStruct | UnionSubscript | UnionSuper | UnionSwitch | UnionSymbol | UnionSynchronized | UnionSystem | UnionTemplate | UnionThen | UnionThis | UnionThreadLocal | UnionThrow | UnionThrows | UnionToJSON | UnionTopLevel | UnionTransient | UnionTrue | UnionTrueUnion | UnionTry | UnionType | UnionTypeUnion | UnionTypealias | UnionTypedef | UnionTypeid | UnionTypename | UnionTypeof | UnionUint | UnionUlong | UnionUnchecked | UnionUndefined | UnionUnionUnion | UnionUnowned | UnionUnsafe | UnionUnsigned | UnionUshort | UnionUsing | UnionVar | UnionVirtual | UnionVoid | UnionVolatile | UnionWcharT | UnionWeak | UnionWhere | UnionWhile | UnionWillSet | UnionWith | UnionXor | UnionXorEq | UnionYES | UnionYield;
+    [property: string]: mixed;
 };
 
 export type UnionAny = Any | number;
diff --git a/base/schema-flow/test/inputs/schema/list.schema/default/TopLevel.js b/head/schema-flow/test/inputs/schema/list.schema/default/TopLevel.js
index 76e467f..17c0e10 100644
--- a/base/schema-flow/test/inputs/schema/list.schema/default/TopLevel.js
+++ b/head/schema-flow/test/inputs/schema/list.schema/default/TopLevel.js
@@ -14,7 +14,7 @@
  */
 export type TopLevel = {
     next?: TopLevel;
-    [property: string]: mixed | TopLevel;
+    [property: string]: mixed;
 };
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-flow/test/inputs/schema/min-max-items.schema/default/TopLevel.js b/head/schema-flow/test/inputs/schema/min-max-items.schema/default/TopLevel.js
index f96b897..669266c 100644
--- a/base/schema-flow/test/inputs/schema/min-max-items.schema/default/TopLevel.js
+++ b/head/schema-flow/test/inputs/schema/min-max-items.schema/default/TopLevel.js
@@ -15,7 +15,7 @@ export type TopLevel = {
     minOnly:    string[];
     plain:      string[];
     unionItems: UnionItem[];
-    [property: string]: mixed | number[] | number[] | string[] | string[] | UnionItem[];
+    [property: string]: mixed;
 };
 
 export type UnionItem = number | string;
diff --git a/base/schema-flow/test/inputs/schema/minmax-integer.schema/default/TopLevel.js b/head/schema-flow/test/inputs/schema/minmax-integer.schema/default/TopLevel.js
index c8e4323..60be51d 100644
--- a/base/schema-flow/test/inputs/schema/minmax-integer.schema/default/TopLevel.js
+++ b/head/schema-flow/test/inputs/schema/minmax-integer.schema/default/TopLevel.js
@@ -18,7 +18,7 @@ export type TopLevel = {
     minMaxIntersection: number;
     minMaxUnion:        number;
     union:              number;
-    [property: string]: mixed | number | number | number | number | number | number | number | number;
+    [property: string]: mixed;
 };
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-flow/test/inputs/schema/minmax.schema/default/TopLevel.js b/head/schema-flow/test/inputs/schema/minmax.schema/default/TopLevel.js
index 5a1e1b8..d58225c 100644
--- a/base/schema-flow/test/inputs/schema/minmax.schema/default/TopLevel.js
+++ b/head/schema-flow/test/inputs/schema/minmax.schema/default/TopLevel.js
@@ -18,7 +18,7 @@ export type TopLevel = {
     minMaxIntersection: number;
     minMaxUnion:        number;
     union:              number;
-    [property: string]: mixed | number | number | number | number | number | number | number | number;
+    [property: string]: mixed;
 };
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-flow/test/inputs/schema/minmaxlength.schema/default/TopLevel.js b/head/schema-flow/test/inputs/schema/minmaxlength.schema/default/TopLevel.js
index 6f68f75..bd4537b 100644
--- a/base/schema-flow/test/inputs/schema/minmaxlength.schema/default/TopLevel.js
+++ b/head/schema-flow/test/inputs/schema/minmaxlength.schema/default/TopLevel.js
@@ -18,7 +18,7 @@ export type TopLevel = {
     minmaxlength:       string;
     minMaxUnion:        string;
     union:              string;
-    [property: string]: mixed | string | InUnion | string | string | string | string | string | string;
+    [property: string]: mixed;
 };
 
 export type InUnion = number | string;
diff --git a/head/schema-flow/test/inputs/schema/mixed-additional-properties.schema/default/TopLevel.js b/head/schema-flow/test/inputs/schema/mixed-additional-properties.schema/default/TopLevel.js
new file mode 100644
index 0000000..cebc9d4
--- /dev/null
+++ b/head/schema-flow/test/inputs/schema/mixed-additional-properties.schema/default/TopLevel.js
@@ -0,0 +1,210 @@
+// @flow
+
+// To parse this data:
+//
+//   const Convert = require("./TopLevel");
+//
+//   const topLevel = Convert.toTopLevel(json);
+//
+// These functions will throw an error if the JSON doesn't
+// match the expected interface, even if the JSON is valid.
+
+export type TopLevel = {
+    id?: string;
+    [property: string]: number | string | void;
+};
+
+// Converts JSON strings to/from your types
+// and asserts the results of JSON.parse at runtime
+function toTopLevel(json: string): TopLevel {
+    return cast(JSON.parse(json), r("TopLevel"));
+}
+
+function topLevelToJson(value: TopLevel): string {
+    return JSON.stringify(uncast(value, r("TopLevel")), null, 2);
+}
+
+function invalidValue(typ: any, val: any, key: any, parent: any = '') {
+    const prettyTyp = prettyTypeName(typ);
+    const parentText = parent ? ` on ${parent}` : '';
+    const keyText = key ? ` for key "${key}"` : '';
+    throw Error(`Invalid value${keyText}${parentText}. Expected ${prettyTyp} but got ${JSON.stringify(val)}`);
+}
+
+function prettyTypeName(typ: any): string {
+    if (Array.isArray(typ)) {
+        if (typ.length === 2 && typ[0] === undefined) {
+            return `an optional ${prettyTypeName(typ[1])}`;
+        } else {
+            return `one of [${typ.map(a => { return prettyTypeName(a); }).join(", ")}]`;
+        }
+    } else if (typeof typ === "object" && typ.literal !== undefined) {
+        return typ.literal;
+    } else {
+        return typeof typ;
+    }
+}
+
+function jsonToJSProps(typ: any): any {
+    if (typ.jsonToJS === undefined) {
+        const map: any = {};
+        typ.props.forEach((p: any) => map[p.json] = { key: p.js, typ: p.typ });
+        typ.jsonToJS = map;
+    }
+    return typ.jsonToJS;
+}
+
+function jsToJSONProps(typ: any): any {
+    if (typ.jsToJSON === undefined) {
+        const map: any = {};
+        typ.props.forEach((p: any) => map[p.js] = { key: p.json, typ: p.typ });
+        typ.jsToJSON = map;
+    }
+    return typ.jsToJSON;
+}
+
+function transform(val: any, typ: any, getProps: any, key: any = '', parent: any = ''): any {
+    function transformPrimitive(typ: string, val: any): any {
+        if (typeof typ === typeof val) return val;
+        return invalidValue(typ, val, key, parent);
+    }
+
+    function transformUnion(typs: any[], val: any): any {
+        // val must validate against one typ in typs
+        const l = typs.length;
+        for (let i = 0; i < l; i++) {
+            const typ = typs[i];
+            try {
+                return transform(val, typ, getProps);
+            } catch (_) {}
+        }
+        return invalidValue(typs, val, key, parent);
+    }
+
+    function transformEnum(cases: string[], val: any): any {
+        if (cases.indexOf(val) !== -1) return val;
+        return invalidValue(cases.map(a => { return l(a); }), val, key, parent);
+    }
+
+    function transformArray(typ: any, val: any): any {
+        // val must be an array with no invalid elements
+        if (!Array.isArray(val)) return invalidValue(l("array"), val, key, parent);
+
+        return val.map(el => transform(el, typ, getProps));
+    }
+
+    function transformDate(val: any): any {
+        if (val === null) {
+            return null;
+        }
+        const d = new Date(val);
+        if (isNaN(d.valueOf())) {
+            return invalidValue(l("Date"), val, key, parent);
+        }
+        return d;
+    }
+
+    function transformObject(props: { [k: string]: any }, additional: any, val: any): any {
+        if (val === null || typeof val !== "object" || Array.isArray(val)) {
+            return invalidValue(l(ref || "object"), val, key, parent);
+        }
+        const result: any = {};
+        Object.getOwnPropertyNames(props).forEach(key => {
+            const prop = props[key];
+            const v = Object.prototype.hasOwnProperty.call(val, key) ? val[key] : undefined;
+            result[prop.key] = transform(v, prop.typ, getProps, key, ref);
+        });
+        Object.getOwnPropertyNames(val).forEach(key => {
+            if (!Object.prototype.hasOwnProperty.call(props, key)) {
+                result[key] = transform(val[key], additional, getProps, key, ref);
+            }
+        });
+        return result;
+    }
+
+    if (typ === "any") return val;
+    if (typ === null) {
+        if (val === null) return val;
+        return invalidValue(typ, val, key, parent);
+    }
+    if (typ === false) return invalidValue(typ, val, key, parent);
+    let ref: any = undefined;
+    while (typeof typ === "object" && typ.ref !== undefined) {
+        ref = typ.ref;
+        typ = typeMap[typ.ref];
+    }
+    if (Array.isArray(typ)) return transformEnum(typ, val);
+    if (typeof typ === "object") {
+        return typ.hasOwnProperty("pattern")      ? typeof val === "string" && new RegExp(typ.pattern).test(val) ? val : invalidValue(typ, val, key, parent)
+            : typ.hasOwnProperty("string")       ? typeof val === "string" && (typ.min === undefined || Array.from(val).length >= typ.min) && (typ.max === undefined || Array.from(val).length <= typ.max) ? transform(val, typ.string, getProps, key, parent) : invalidValue(typ, val, key, parent)
+            : typ.hasOwnProperty("number")       ? typeof val === "number" && (typ.min === undefined || val >= typ.min) && (typ.max === undefined || val <= typ.max) ? transform(val, typ.number, getProps, key, parent) : invalidValue(typ, val, key, parent)
+            : typ.hasOwnProperty("integer")      ? typeof val === "number" && val % 1 === 0 ? val : invalidValue(l("integer"), val, key, parent)
+            : typ.hasOwnProperty("unionMembers") ? transformUnion(typ.unionMembers, val)
+            : typ.hasOwnProperty("arrayItems")    ? transformArray(typ.arrayItems, val)
+            : typ.hasOwnProperty("props")         ? transformObject(getProps(typ), typ.additional, val)
+            : invalidValue(typ, val, key, parent);
+    }
+    // Numbers can be parsed by Date but shouldn't be.
+    if (typ === Date && (typeof val === "string" || val instanceof Date)) return transformDate(val);
+    return transformPrimitive(typ, val);
+}
+
+function cast<T>(val: any, typ: any): T {
+    return transform(val, typ, jsonToJSProps);
+}
+
+function uncast<T>(val: T, typ: any): any {
+    return transform(val, typ, jsToJSONProps);
+}
+
+function l(typ: any) {
+    return { literal: typ };
+}
+
+function a(typ: any) {
+    return { arrayItems: typ };
+}
+
+function i(typ: any) {
+    return { integer: typ };
+}
+
+function p(pattern: any) {
+    return { pattern };
+}
+
+function s(typ: any, min: any, max: any) {
+    return { string: typ, min, max };
+}
+
+function n(typ: any, min: any, max: any) {
+    return { number: typ, min, max };
+}
+
+function u(...typs: any[]) {
+    return { unionMembers: typs };
+}
+
+function o(props: any[], additional: any) {
+    return { props, additional };
+}
+
+function m(additional: any) {
+    const props: any[] = [];
+    return { props, additional };
+}
+
+function r(name: string) {
+    return { ref: name };
+}
+
+const typeMap: any = {
+    "TopLevel": o([
+        { json: "id", js: "id", typ: u(undefined, "") },
+    ], 3.14),
+};
+
+module.exports = {
+    "topLevelToJson": topLevelToJson,
+    "toTopLevel": toTopLevel,
+};
diff --git a/base/schema-flow/test/inputs/schema/multi-type-enum.schema/default/TopLevel.js b/head/schema-flow/test/inputs/schema/multi-type-enum.schema/default/TopLevel.js
index e55c6cb..61a8238 100644
--- a/base/schema-flow/test/inputs/schema/multi-type-enum.schema/default/TopLevel.js
+++ b/head/schema-flow/test/inputs/schema/multi-type-enum.schema/default/TopLevel.js
@@ -11,7 +11,7 @@
 
 export type TopLevel = {
     foo: FooUnion;
-    [property: string]: mixed | FooUnion;
+    [property: string]: mixed;
 };
 
 export type FooUnion = boolean | number | FooEnum;
diff --git a/base/schema-flow/test/inputs/schema/mutually-recursive.schema/default/TopLevel.js b/head/schema-flow/test/inputs/schema/mutually-recursive.schema/default/TopLevel.js
index 19fa98b..cbf3645 100644
--- a/base/schema-flow/test/inputs/schema/mutually-recursive.schema/default/TopLevel.js
+++ b/head/schema-flow/test/inputs/schema/mutually-recursive.schema/default/TopLevel.js
@@ -13,12 +13,12 @@ export type Foo = TopLevel[] | TopLevel;
 
 export type Bar = {
     foo: Foo;
-    [property: string]: mixed | Foo;
+    [property: string]: mixed;
 };
 
 export type TopLevel = {
     bar?: Bar;
-    [property: string]: mixed | Bar;
+    [property: string]: mixed;
 };
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-flow/test/inputs/schema/nested-intersection-union.schema/default/TopLevel.js b/head/schema-flow/test/inputs/schema/nested-intersection-union.schema/default/TopLevel.js
index 7f7fdbe..11caa97 100644
--- a/base/schema-flow/test/inputs/schema/nested-intersection-union.schema/default/TopLevel.js
+++ b/head/schema-flow/test/inputs/schema/nested-intersection-union.schema/default/TopLevel.js
@@ -18,7 +18,7 @@ export type Item = {
     id?:      string;
     output?:  string;
     summary?: string;
-    [property: string]: mixed | string | string | string | string;
+    [property: string]: mixed;
 };
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-flow/test/inputs/schema/non-standard-ref.schema/default/TopLevel.js b/head/schema-flow/test/inputs/schema/non-standard-ref.schema/default/TopLevel.js
index a19290b..63ee252 100644
--- a/base/schema-flow/test/inputs/schema/non-standard-ref.schema/default/TopLevel.js
+++ b/head/schema-flow/test/inputs/schema/non-standard-ref.schema/default/TopLevel.js
@@ -13,7 +13,7 @@ export type TopLevel = {
     bar:  number;
     foo:  number;
     quux: boolean;
-    [property: string]: mixed | number | number | boolean;
+    [property: string]: mixed;
 };
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-flow/test/inputs/schema/nullable-optional-one-of.schema/default/TopLevel.js b/head/schema-flow/test/inputs/schema/nullable-optional-one-of.schema/default/TopLevel.js
index 73d3e10..b682aea 100644
--- a/base/schema-flow/test/inputs/schema/nullable-optional-one-of.schema/default/TopLevel.js
+++ b/head/schema-flow/test/inputs/schema/nullable-optional-one-of.schema/default/TopLevel.js
@@ -12,7 +12,7 @@
 export type TopLevel = {
     b?:   null | string;
     kind: Kind;
-    [property: string]: mixed | null | string | Kind;
+    [property: string]: mixed;
 };
 
 export type Kind =
diff --git a/base/schema-flow/test/inputs/schema/object-type-required.schema/default/TopLevel.js b/head/schema-flow/test/inputs/schema/object-type-required.schema/default/TopLevel.js
index 0452054..695aabc 100644
--- a/base/schema-flow/test/inputs/schema/object-type-required.schema/default/TopLevel.js
+++ b/head/schema-flow/test/inputs/schema/object-type-required.schema/default/TopLevel.js
@@ -13,7 +13,7 @@ export type TopLevel = {
     empty: Empty;
     foo:   string;
     bar?:  string;
-    [property: string]: mixed | Empty | string | string;
+    [property: string]: mixed;
 };
 
 export type Empty = {
diff --git a/base/schema-flow/test/inputs/schema/optional-constraints.schema/default/TopLevel.js b/head/schema-flow/test/inputs/schema/optional-constraints.schema/default/TopLevel.js
index 0383e05..3afc5fb 100644
--- a/base/schema-flow/test/inputs/schema/optional-constraints.schema/default/TopLevel.js
+++ b/head/schema-flow/test/inputs/schema/optional-constraints.schema/default/TopLevel.js
@@ -15,7 +15,7 @@ export type TopLevel = {
     optPattern?: string;
     optString?:  string;
     reqZeroMin:  number;
-    [property: string]: mixed | number | number | string | string | number;
+    [property: string]: mixed;
 };
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-flow/test/inputs/schema/pattern.schema/default/TopLevel.js b/head/schema-flow/test/inputs/schema/pattern.schema/default/TopLevel.js
index 8a37f20..90e6c9d 100644
--- a/base/schema-flow/test/inputs/schema/pattern.schema/default/TopLevel.js
+++ b/head/schema-flow/test/inputs/schema/pattern.schema/default/TopLevel.js
@@ -13,7 +13,7 @@ export type TopLevel = {
     pattern1: string;
     pattern2: string;
     union:    string;
-    [property: string]: mixed | string | string | string;
+    [property: string]: mixed;
 };
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-flow/test/inputs/schema/postman-collection.schema/default/TopLevel.js b/head/schema-flow/test/inputs/schema/postman-collection.schema/default/TopLevel.js
index d607dda..8139ed9 100644
--- a/base/schema-flow/test/inputs/schema/postman-collection.schema/default/TopLevel.js
+++ b/head/schema-flow/test/inputs/schema/postman-collection.schema/default/TopLevel.js
@@ -16,12 +16,12 @@ export type TopLevel = {
     item?:     TopLevel[];
     name?:     string;
     response?: Response[];
-    [property: string]: mixed | TopLevel[] | string | Response[];
+    [property: string]: mixed;
 };
 
 export type Response = {
     body?: string;
-    [property: string]: mixed | string;
+    [property: string]: mixed;
 };
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-flow/test/inputs/schema/prefix-items.schema/default/TopLevel.js b/head/schema-flow/test/inputs/schema/prefix-items.schema/default/TopLevel.js
index 072220d..de153a7 100644
--- a/base/schema-flow/test/inputs/schema/prefix-items.schema/default/TopLevel.js
+++ b/head/schema-flow/test/inputs/schema/prefix-items.schema/default/TopLevel.js
@@ -12,7 +12,7 @@
 export type TopLevel = {
     open:  Open[];
     tuple: Open[];
-    [property: string]: mixed | Open[] | Open[];
+    [property: string]: mixed;
 };
 
 export type Open = boolean | number;
diff --git a/base/schema-flow/test/inputs/schema/recursive-union-flattening.schema/default/TopLevel.js b/head/schema-flow/test/inputs/schema/recursive-union-flattening.schema/default/TopLevel.js
index 78a190d..b1766ee 100644
--- a/base/schema-flow/test/inputs/schema/recursive-union-flattening.schema/default/TopLevel.js
+++ b/head/schema-flow/test/inputs/schema/recursive-union-flattening.schema/default/TopLevel.js
@@ -15,12 +15,12 @@ export type TopLevelElement = mixed[] | boolean | number | null | TopLevelObject
 
 export type TopLevelObject = {
     x?: X;
-    [property: string]: mixed | X;
+    [property: string]: mixed;
 };
 
 export type XObject = {
     x?: X;
-    [property: string]: mixed | X;
+    [property: string]: mixed;
 };
 
 export type XElement = mixed[] | boolean | number | null | XObject | string;
diff --git a/base/schema-flow/test/inputs/schema/ref-id-files.schema/default/TopLevel.js b/head/schema-flow/test/inputs/schema/ref-id-files.schema/default/TopLevel.js
index f614921..fd3804b 100644
--- a/base/schema-flow/test/inputs/schema/ref-id-files.schema/default/TopLevel.js
+++ b/head/schema-flow/test/inputs/schema/ref-id-files.schema/default/TopLevel.js
@@ -11,7 +11,7 @@
 
 export type TopLevel = {
     foo: number;
-    [property: string]: mixed | number;
+    [property: string]: mixed;
 };
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-flow/test/inputs/schema/ref-remote.schema/default/TopLevel.js b/head/schema-flow/test/inputs/schema/ref-remote.schema/default/TopLevel.js
index 76e467f..17c0e10 100644
--- a/base/schema-flow/test/inputs/schema/ref-remote.schema/default/TopLevel.js
+++ b/head/schema-flow/test/inputs/schema/ref-remote.schema/default/TopLevel.js
@@ -14,7 +14,7 @@
  */
 export type TopLevel = {
     next?: TopLevel;
-    [property: string]: mixed | TopLevel;
+    [property: string]: mixed;
 };
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-flow/test/inputs/schema/renaming-bug.schema/default/TopLevel.js b/head/schema-flow/test/inputs/schema/renaming-bug.schema/default/TopLevel.js
index 480f43f..bfa4e9f 100644
--- a/base/schema-flow/test/inputs/schema/renaming-bug.schema/default/TopLevel.js
+++ b/head/schema-flow/test/inputs/schema/renaming-bug.schema/default/TopLevel.js
@@ -13,60 +13,60 @@ export type TopLevel = {
     version?:  string;
     fruits?:   Fruit[];
     vehicles?: Vehicle[];
-    [property: string]: mixed | string | Fruit[] | Vehicle[];
+    [property: string]: mixed;
 };
 
 export type Fruit = {
     apple?:   boolean;
     berries?: Berry[];
     orange?:  boolean;
-    [property: string]: mixed | boolean | Berry[] | boolean;
+    [property: string]: mixed;
 };
 
 export type Berry = {
     color?:  Color;
     name?:   string;
     shapes?: Shape[];
-    [property: string]: mixed | Color | string | Shape[];
+    [property: string]: mixed;
 };
 
 export type Color = {
     rgb?: number;
-    [property: string]: mixed | number;
+    [property: string]: mixed;
 };
 
 export type Shape = {
     geometry?: Geometry;
     history?:  History;
-    [property: string]: mixed | Geometry | History;
+    [property: string]: mixed;
 };
 
 export type Geometry = {
     rectShape?:     RectShape;
     circularShape?: CircularShape[];
-    [property: string]: mixed | RectShape | CircularShape[];
+    [property: string]: mixed;
 };
 
 export type CircularShape = {
     shapeName?: string;
-    [property: string]: mixed | string;
+    [property: string]: mixed;
 };
 
 export type RectShape = {
     parts?: Part[];
-    [property: string]: mixed | Part[];
+    [property: string]: mixed;
 };
 
 export type Part = {
     depth?:  string;
     length?: string;
     width?:  string;
-    [property: string]: mixed | string | string | string;
+    [property: string]: mixed;
 };
 
 export type History = {
     class?: string;
-    [property: string]: mixed | string;
+    [property: string]: mixed;
 };
 
 export type Vehicle = {
@@ -76,25 +76,25 @@ export type Vehicle = {
     subModule?: boolean;
     type?:      VehicleType;
     year?:      string;
-    [property: string]: mixed | string | string | Speed | boolean | VehicleType | string;
+    [property: string]: mixed;
 };
 
 export type Speed = {
     velocity?: Limit;
-    [property: string]: mixed | Limit;
+    [property: string]: mixed;
 };
 
 export type Limit = {
     maximum?: number;
     minimum?: number;
-    [property: string]: mixed | number | number;
+    [property: string]: mixed;
 };
 
 export type VehicleType = {
     name?:   Name;
     width?:  Axis;
     length?: Axis;
-    [property: string]: mixed | Name | Axis | Axis;
+    [property: string]: mixed;
 };
 
 export type Axis =
diff --git a/base/schema-flow/test/inputs/schema/required-draft3.schema/default/TopLevel.js b/head/schema-flow/test/inputs/schema/required-draft3.schema/default/TopLevel.js
index 3f5061b..d35b9e7 100644
--- a/base/schema-flow/test/inputs/schema/required-draft3.schema/default/TopLevel.js
+++ b/head/schema-flow/test/inputs/schema/required-draft3.schema/default/TopLevel.js
@@ -11,7 +11,7 @@
 
 export type TopLevel = {
     longitude: number;
-    [property: string]: mixed | number;
+    [property: string]: mixed;
 };
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-flow/test/inputs/schema/required-non-properties.schema/default/TopLevel.js b/head/schema-flow/test/inputs/schema/required-non-properties.schema/default/TopLevel.js
index 58975ad..99c86ad 100644
--- a/base/schema-flow/test/inputs/schema/required-non-properties.schema/default/TopLevel.js
+++ b/head/schema-flow/test/inputs/schema/required-non-properties.schema/default/TopLevel.js
@@ -12,7 +12,7 @@
 export type TopLevel = {
     foo: number;
     bar: mixed;
-    [property: string]: mixed | number | mixed;
+    [property: string]: mixed;
 };
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-flow/test/inputs/schema/required.schema/default/TopLevel.js b/head/schema-flow/test/inputs/schema/required.schema/default/TopLevel.js
index 3f5061b..d35b9e7 100644
--- a/base/schema-flow/test/inputs/schema/required.schema/default/TopLevel.js
+++ b/head/schema-flow/test/inputs/schema/required.schema/default/TopLevel.js
@@ -11,7 +11,7 @@
 
 export type TopLevel = {
     longitude: number;
-    [property: string]: mixed | number;
+    [property: string]: mixed;
 };
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-flow/test/inputs/schema/rust-cycle-breaker-union.schema/default/TopLevel.js b/head/schema-flow/test/inputs/schema/rust-cycle-breaker-union.schema/default/TopLevel.js
index 09415d2..9868a9b 100644
--- a/base/schema-flow/test/inputs/schema/rust-cycle-breaker-union.schema/default/TopLevel.js
+++ b/head/schema-flow/test/inputs/schema/rust-cycle-breaker-union.schema/default/TopLevel.js
@@ -12,13 +12,13 @@
 export type TopLevel = {
     next?: Next;
     value: string;
-    [property: string]: mixed | Next | string;
+    [property: string]: mixed;
 };
 
 export type Node = {
     next?: Next;
     value: string;
-    [property: string]: mixed | Next | string;
+    [property: string]: mixed;
 };
 
 export type Next = null | Node | string;
diff --git a/base/schema-flow/test/inputs/schema/schema-constraints.schema/default/TopLevel.js b/head/schema-flow/test/inputs/schema/schema-constraints.schema/default/TopLevel.js
index 1a7d30e..2f38303 100644
--- a/base/schema-flow/test/inputs/schema/schema-constraints.schema/default/TopLevel.js
+++ b/head/schema-flow/test/inputs/schema/schema-constraints.schema/default/TopLevel.js
@@ -12,7 +12,7 @@
 export type TopLevel = {
     minMaxLength: string;
     percent:      number;
-    [property: string]: mixed | string | number;
+    [property: string]: mixed;
 };
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-flow/test/inputs/schema/simple-ref.schema/default/TopLevel.js b/head/schema-flow/test/inputs/schema/simple-ref.schema/default/TopLevel.js
index 76e467f..17c0e10 100644
--- a/base/schema-flow/test/inputs/schema/simple-ref.schema/default/TopLevel.js
+++ b/head/schema-flow/test/inputs/schema/simple-ref.schema/default/TopLevel.js
@@ -14,7 +14,7 @@
  */
 export type TopLevel = {
     next?: TopLevel;
-    [property: string]: mixed | TopLevel;
+    [property: string]: mixed;
 };
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-flow/test/inputs/schema/strict-optional.schema/default/TopLevel.js b/head/schema-flow/test/inputs/schema/strict-optional.schema/default/TopLevel.js
index 87124b1..f385a6f 100644
--- a/base/schema-flow/test/inputs/schema/strict-optional.schema/default/TopLevel.js
+++ b/head/schema-flow/test/inputs/schema/strict-optional.schema/default/TopLevel.js
@@ -11,7 +11,7 @@
 
 export type TopLevel = {
     foo: number | null;
-    [property: string]: mixed | number | null;
+    [property: string]: mixed;
 };
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-flow/test/inputs/schema/top-level-array.schema/default/TopLevel.js b/head/schema-flow/test/inputs/schema/top-level-array.schema/default/TopLevel.js
index 841d7aa..6db6cca 100644
--- a/base/schema-flow/test/inputs/schema/top-level-array.schema/default/TopLevel.js
+++ b/head/schema-flow/test/inputs/schema/top-level-array.schema/default/TopLevel.js
@@ -14,7 +14,7 @@ export type TopLevel = TextClassificationOutputElement[];
 export type TextClassificationOutputElement = {
     label: string;
     score: number;
-    [property: string]: mixed | string | number;
+    [property: string]: mixed;
 };
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-flow/test/inputs/schema/tuple.schema/default/TopLevel.js b/head/schema-flow/test/inputs/schema/tuple.schema/default/TopLevel.js
index 3a44696..0a3be23 100644
--- a/base/schema-flow/test/inputs/schema/tuple.schema/default/TopLevel.js
+++ b/head/schema-flow/test/inputs/schema/tuple.schema/default/TopLevel.js
@@ -11,7 +11,7 @@
 
 export type TopLevel = {
     tuple: Tuple[];
-    [property: string]: mixed | Tuple[];
+    [property: string]: mixed;
 };
 
 export type Tuple = boolean | number;
diff --git a/base/schema-flow/test/inputs/schema/unevaluated-properties.schema/default/TopLevel.js b/head/schema-flow/test/inputs/schema/unevaluated-properties.schema/default/TopLevel.js
index 98c3a85..8c9f7fb 100644
--- a/base/schema-flow/test/inputs/schema/unevaluated-properties.schema/default/TopLevel.js
+++ b/head/schema-flow/test/inputs/schema/unevaluated-properties.schema/default/TopLevel.js
@@ -11,14 +11,14 @@
 
 export type TopLevel = {
     config?: Config;
-    [property: string]: mixed | Config;
+    [property: string]: mixed;
 };
 
 export type Config = {
     closed?:   Closed;
     name?:     string;
     settings?: { [key: string]: Item[] };
-    [property: string]: mixed | Closed | string | { [key: string]: Item[] };
+    [property: string]: mixed;
 };
 
 export type Closed = mixed[] | boolean | ClosedClass | number | number | null | string;
@@ -29,7 +29,7 @@ export type ClosedClass = {
 export type Item = {
     key:   string;
     value: string;
-    [property: string]: mixed | string | string;
+    [property: string]: mixed;
 };
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-flow/test/inputs/schema/union-int-double.schema/default/TopLevel.js b/head/schema-flow/test/inputs/schema/union-int-double.schema/default/TopLevel.js
index e5f6ec3..0e237ee 100644
--- a/base/schema-flow/test/inputs/schema/union-int-double.schema/default/TopLevel.js
+++ b/head/schema-flow/test/inputs/schema/union-int-double.schema/default/TopLevel.js
@@ -11,7 +11,7 @@
 
 export type TopLevel = {
     value: Value;
-    [property: string]: mixed | Value;
+    [property: string]: mixed;
 };
 
 export type Value = number | string;
diff --git a/base/schema-flow/test/inputs/schema/union.schema/default/TopLevel.js b/head/schema-flow/test/inputs/schema/union.schema/default/TopLevel.js
index ef8f1b5..af3f6dc 100644
--- a/base/schema-flow/test/inputs/schema/union.schema/default/TopLevel.js
+++ b/head/schema-flow/test/inputs/schema/union.schema/default/TopLevel.js
@@ -15,7 +15,7 @@ export type TopLevelElement = {
     one?:   number;
     two:    boolean;
     three?: number;
-    [property: string]: mixed | number | boolean | number;
+    [property: string]: mixed;
 };
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-flow/test/inputs/schema/uuid.schema/default/TopLevel.js b/head/schema-flow/test/inputs/schema/uuid.schema/default/TopLevel.js
index e7082f4..b30ae68 100644
--- a/base/schema-flow/test/inputs/schema/uuid.schema/default/TopLevel.js
+++ b/head/schema-flow/test/inputs/schema/uuid.schema/default/TopLevel.js
@@ -16,7 +16,7 @@ export type TopLevel = {
     one:           string;
     optional?:     string;
     unionWithEnum: UnionWithEnumUnion;
-    [property: string]: mixed | (null | string)[] | string[] | null | string | string | string | UnionWithEnumUnion;
+    [property: string]: mixed;
 };
 
 export type UnionWithEnumUnion = UnionWithEnumEnum | string;
diff --git a/base/schema-flow/test/inputs/schema/vega-lite.schema/default/TopLevel.js b/head/schema-flow/test/inputs/schema/vega-lite.schema/default/TopLevel.js
index fa4f24a..e870aef 100644
--- a/base/schema-flow/test/inputs/schema/vega-lite.schema/default/TopLevel.js
+++ b/head/schema-flow/test/inputs/schema/vega-lite.schema/default/TopLevel.js
@@ -1800,7 +1800,7 @@ export type RangeConfig = {
      * Default range palette for the `shape` channel.
      */
     symbol?: string[];
-    [property: string]: RangeConfigValue | Category | Category | Category | Category | Category | string[];
+    [property: string]: RangeConfigValue | Category | string[] | void;
 };
 
 /**
diff --git a/head/schema-golang/test/inputs/schema/mixed-additional-properties.schema/default/quicktype.go b/head/schema-golang/test/inputs/schema/mixed-additional-properties.schema/default/quicktype.go
new file mode 100644
index 0000000..58012ae
--- /dev/null
+++ b/head/schema-golang/test/inputs/schema/mixed-additional-properties.schema/default/quicktype.go
@@ -0,0 +1,156 @@
+// Code generated from JSON Schema using quicktype. DO NOT EDIT.
+// To parse and unparse this JSON data, add this code to your project and do:
+//
+//    topLevel, err := UnmarshalTopLevel(bytes)
+//    bytes, err = topLevel.Marshal()
+
+package main
+
+import "bytes"
+import "errors"
+
+import "encoding/json"
+
+type TopLevel map[string]*TopLevelValue
+
+func UnmarshalTopLevel(data []byte) (TopLevel, error) {
+	var r TopLevel
+	err := json.Unmarshal(data, &r)
+	return r, err
+}
+
+func (r *TopLevel) Marshal() ([]byte, error) {
+	return json.Marshal(r)
+}
+
+type TopLevelValue struct {
+	Double *float64
+	String *string
+}
+
+func (x *TopLevelValue) UnmarshalJSON(data []byte) error {
+	object, err := unmarshalUnion(data, nil, &x.Double, nil, &x.String, false, nil, false, nil, false, nil, false, nil, false)
+	if err != nil {
+		return err
+	}
+	if object {
+	}
+	return nil
+}
+
+func (x *TopLevelValue) MarshalJSON() ([]byte, error) {
+	return marshalUnion(nil, x.Double, nil, x.String, false, nil, false, nil, false, nil, false, nil, false)
+}
+
+func unmarshalUnion(data []byte, pi **int64, pf **float64, pb **bool, ps **string, haveArray bool, pa interface{}, haveObject bool, pc interface{}, haveMap bool, pm interface{}, haveEnum bool, pe interface{}, nullable bool) (bool, error) {
+	if pi != nil {
+			*pi = nil
+	}
+	if pf != nil {
+			*pf = nil
+	}
+	if pb != nil {
+			*pb = nil
+	}
+	if ps != nil {
+			*ps = nil
+	}
+
+	dec := json.NewDecoder(bytes.NewReader(data))
+	dec.UseNumber()
+	tok, err := dec.Token()
+	if err != nil {
+			return false, err
+	}
+
+	switch v := tok.(type) {
+	case json.Number:
+			if pi != nil {
+					i, err := v.Int64()
+					if err == nil {
+							*pi = &i
+							return false, nil
+					}
+			}
+			if pf != nil {
+					f, err := v.Float64()
+					if err == nil {
+							*pf = &f
+							return false, nil
+					}
+					return false, errors.New("Unparsable number")
+			}
+			return false, errors.New("Union does not contain number")
+	case float64:
+			return false, errors.New("Decoder should not return float64")
+	case bool:
+			if pb != nil {
+					*pb = &v
+					return false, nil
+			}
+			return false, errors.New("Union does not contain bool")
+	case string:
+			if haveEnum {
+					return false, json.Unmarshal(data, pe)
+			}
+			if ps != nil {
+					*ps = &v
+					return false, nil
+			}
+			return false, errors.New("Union does not contain string")
+	case nil:
+			if nullable {
+					return false, nil
+			}
+			return false, errors.New("Union does not contain null")
+	case json.Delim:
+			if v == '{' {
+					if haveObject {
+							return true, json.Unmarshal(data, pc)
+					}
+					if haveMap {
+							return false, json.Unmarshal(data, pm)
+					}
+					return false, errors.New("Union does not contain object")
+			}
+			if v == '[' {
+					if haveArray {
+							return false, json.Unmarshal(data, pa)
+					}
+					return false, errors.New("Union does not contain array")
+			}
+			return false, errors.New("Cannot handle delimiter")
+	}
+	return false, errors.New("Cannot unmarshal union")
+}
+
+func marshalUnion(pi *int64, pf *float64, pb *bool, ps *string, haveArray bool, pa interface{}, haveObject bool, pc interface{}, haveMap bool, pm interface{}, haveEnum bool, pe interface{}, nullable bool) ([]byte, error) {
+	if pi != nil {
+			return json.Marshal(*pi)
+	}
+	if pf != nil {
+			return json.Marshal(*pf)
+	}
+	if pb != nil {
+			return json.Marshal(*pb)
+	}
+	if ps != nil {
+			return json.Marshal(*ps)
+	}
+	if haveArray {
+			return json.Marshal(pa)
+	}
+	if haveObject {
+			return json.Marshal(pc)
+	}
+	if haveMap {
+			return json.Marshal(pm)
+	}
+	if haveEnum {
+			return json.Marshal(pe)
+	}
+	if nullable {
+			return json.Marshal(nil)
+	}
+	return nil, errors.New("Union must not be null")
+}
diff --git a/head/schema-haskell/test/inputs/schema/mixed-additional-properties.schema/default/QuickType.hs b/head/schema-haskell/test/inputs/schema/mixed-additional-properties.schema/default/QuickType.hs
new file mode 100644
index 0000000..2b5677d
--- /dev/null
+++ b/head/schema-haskell/test/inputs/schema/mixed-additional-properties.schema/default/QuickType.hs
@@ -0,0 +1,32 @@
+{-# LANGUAGE StrictData #-}
+{-# LANGUAGE OverloadedStrings #-}
+
+module QuickType
+    ( QuickType (..)
+    , QuickTypeValue (..)
+    , decodeTopLevel
+    ) where
+
+import Data.Aeson
+import Data.Aeson.Types (emptyObject)
+import Data.ByteString.Lazy (ByteString)
+import Data.HashMap.Strict (HashMap)
+import Data.Text (Text)
+
+type QuickType = HashMap Text QuickTypeValue
+
+data QuickTypeValue
+    = DoubleInQuickTypeValue Double
+    | StringInQuickTypeValue Text
+    deriving (Show)
+
+decodeTopLevel :: ByteString -> Maybe QuickType
+decodeTopLevel = decode
+
+instance ToJSON QuickTypeValue where
+    toJSON (DoubleInQuickTypeValue x) = toJSON x
+    toJSON (StringInQuickTypeValue x) = toJSON x
+
+instance FromJSON QuickTypeValue where
+    parseJSON xs@(Number _) = (fmap DoubleInQuickTypeValue . parseJSON) xs
+    parseJSON xs@(String _) = (fmap StringInQuickTypeValue . parseJSON) xs
diff --git a/head/schema-java/test/inputs/schema/mixed-additional-properties.schema/default/src/main/java/io/quicktype/Converter.java b/head/schema-java/test/inputs/schema/mixed-additional-properties.schema/default/src/main/java/io/quicktype/Converter.java
new file mode 100644
index 0000000..d7168c5
--- /dev/null
+++ b/head/schema-java/test/inputs/schema/mixed-additional-properties.schema/default/src/main/java/io/quicktype/Converter.java
@@ -0,0 +1,102 @@
+// To use this code, add the following Maven dependency to your project:
+//
+//
+//     com.fasterxml.jackson.core     : jackson-databind          : 2.9.0
+//     com.fasterxml.jackson.datatype : jackson-datatype-jsr310   : 2.9.0
+//
+// Import this package:
+//
+//     import io.quicktype.Converter;
+//
+// Then you can deserialize a JSON string with
+//
+//     Map<String, TopLevel> data = Converter.fromJsonString(jsonString);
+
+package io.quicktype;
+
+import java.io.IOException;
+import com.fasterxml.jackson.databind.*;
+import com.fasterxml.jackson.databind.module.SimpleModule;
+import com.fasterxml.jackson.core.JsonParser;
+import com.fasterxml.jackson.core.JsonProcessingException;
+import com.fasterxml.jackson.core.type.TypeReference;
+import java.util.*;
+import java.time.LocalDate;
+import java.time.OffsetDateTime;
+import java.time.OffsetTime;
+import java.time.ZoneOffset;
+import java.time.ZonedDateTime;
+import java.time.format.DateTimeFormatter;
+import java.time.format.DateTimeFormatterBuilder;
+import java.time.temporal.ChronoField;
+
+public class Converter {
+    // Date-time helpers
+
+    private static final DateTimeFormatter DATE_TIME_FORMATTER = new DateTimeFormatterBuilder()
+            .appendOptional(DateTimeFormatter.ISO_DATE_TIME)
+            .appendOptional(DateTimeFormatter.ISO_OFFSET_DATE_TIME)
+            .appendOptional(DateTimeFormatter.ISO_INSTANT)
+            .appendOptional(DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss.SX"))
+            .appendOptional(DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ssX"))
+            .appendOptional(DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss"))
+            .toFormatter()
+            .withZone(ZoneOffset.UTC);
+
+    public static OffsetDateTime parseDateTimeString(String str) {
+        return ZonedDateTime.from(Converter.DATE_TIME_FORMATTER.parse(str)).toOffsetDateTime();
+    }
+
+    private static final DateTimeFormatter TIME_FORMATTER = new DateTimeFormatterBuilder()
+            .appendOptional(DateTimeFormatter.ISO_TIME)
+            .appendOptional(DateTimeFormatter.ISO_OFFSET_TIME)
+            .parseDefaulting(ChronoField.YEAR, 2020)
+            .parseDefaulting(ChronoField.MONTH_OF_YEAR, 1)
+            .parseDefaulting(ChronoField.DAY_OF_MONTH, 1)
+            .toFormatter()
+            .withZone(ZoneOffset.UTC);
+
+    public static OffsetTime parseTimeString(String str) {
+        return ZonedDateTime.from(Converter.TIME_FORMATTER.parse(str)).toOffsetDateTime().toOffsetTime();
+    }
+    // Serialize/deserialize helpers
+
+    public static Map<String, TopLevel> fromJsonString(String json) throws IOException {
+        return getObjectReader().readValue(json);
+    }
+
+    public static String toJsonString(Map<String, TopLevel> obj) throws JsonProcessingException {
+        return getObjectWriter().writeValueAsString(obj);
+    }
+
+    private static ObjectReader reader;
+    private static ObjectWriter writer;
+
+    private static void instantiateMapper() {
+        ObjectMapper mapper = new ObjectMapper();
+        mapper.findAndRegisterModules();
+        mapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
+        mapper.configure(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS, false);
+        SimpleModule module = new SimpleModule();
+        module.addDeserializer(OffsetDateTime.class, new JsonDeserializer<OffsetDateTime>() {
+            @Override
+            public OffsetDateTime deserialize(JsonParser jsonParser, DeserializationContext deserializationContext) throws IOException, JsonProcessingException {
+                String value = jsonParser.getText();
+                return Converter.parseDateTimeString(value);
+            }
+        });
+        mapper.registerModule(module);
+        reader = mapper.readerFor(Map.class);
+        writer = mapper.writerFor(Map.class);
+    }
+
+    private static ObjectReader getObjectReader() {
+        if (reader == null) instantiateMapper();
+        return reader;
+    }
+
+    private static ObjectWriter getObjectWriter() {
+        if (writer == null) instantiateMapper();
+        return writer;
+    }
+}
diff --git a/head/schema-java/test/inputs/schema/mixed-additional-properties.schema/default/src/main/java/io/quicktype/TopLevel.java b/head/schema-java/test/inputs/schema/mixed-additional-properties.schema/default/src/main/java/io/quicktype/TopLevel.java
new file mode 100644
index 0000000..2a8322b
--- /dev/null
+++ b/head/schema-java/test/inputs/schema/mixed-additional-properties.schema/default/src/main/java/io/quicktype/TopLevel.java
@@ -0,0 +1,49 @@
+package io.quicktype;
+
+import java.io.IOException;
+import java.io.IOException;
+import com.fasterxml.jackson.core.*;
+import com.fasterxml.jackson.databind.*;
+import com.fasterxml.jackson.databind.annotation.*;
+import com.fasterxml.jackson.core.type.*;
+
+@JsonDeserialize(using = TopLevel.Deserializer.class)
+@JsonSerialize(using = TopLevel.Serializer.class)
+public class TopLevel {
+    public String stringValue;
+    public Double doubleValue;
+
+    static class Deserializer extends JsonDeserializer<TopLevel> {
+        @Override
+        public TopLevel deserialize(JsonParser jsonParser, DeserializationContext deserializationContext) throws IOException, JsonProcessingException {
+            TopLevel value = new TopLevel();
+            switch (jsonParser.currentToken()) {
+                case VALUE_NUMBER_INT:
+                case VALUE_NUMBER_FLOAT:
+                    value.doubleValue = jsonParser.readValueAs(Double.class);
+                    break;
+                case VALUE_STRING:
+                    String string = jsonParser.readValueAs(String.class);
+                    value.stringValue = string;
+                    break;
+                default: throw new IOException("Cannot deserialize TopLevel");
+            }
+            return value;
+        }
+    }
+
+    static class Serializer extends JsonSerializer<TopLevel> {
+        @Override
+        public void serialize(TopLevel obj, JsonGenerator jsonGenerator, SerializerProvider serializerProvider) throws IOException {
+            if (obj.stringValue != null) {
+                jsonGenerator.writeObject(obj.stringValue);
+                return;
+            }
+            if (obj.doubleValue != null) {
+                jsonGenerator.writeObject(obj.doubleValue);
+                return;
+            }
+            throw new IOException("TopLevel must not be null");
+        }
+    }
+}
diff --git a/head/schema-java-datetime-legacy/test/inputs/schema/mixed-additional-properties.schema/default/src/main/java/io/quicktype/Converter.java b/head/schema-java-datetime-legacy/test/inputs/schema/mixed-additional-properties.schema/default/src/main/java/io/quicktype/Converter.java
new file mode 100644
index 0000000..0d0c469
--- /dev/null
+++ b/head/schema-java-datetime-legacy/test/inputs/schema/mixed-additional-properties.schema/default/src/main/java/io/quicktype/Converter.java
@@ -0,0 +1,123 @@
+// To use this code, add the following Maven dependency to your project:
+//
+//
+//     com.fasterxml.jackson.core     : jackson-databind          : 2.9.0
+//
+//
+// Import this package:
+//
+//     import io.quicktype.Converter;
+//
+// Then you can deserialize a JSON string with
+//
+//     Map<String, TopLevel> data = Converter.fromJsonString(jsonString);
+
+package io.quicktype;
+
+import java.io.IOException;
+import com.fasterxml.jackson.databind.*;
+import com.fasterxml.jackson.databind.module.SimpleModule;
+import com.fasterxml.jackson.core.JsonParser;
+import com.fasterxml.jackson.core.JsonProcessingException;
+import com.fasterxml.jackson.core.type.TypeReference;
+import java.util.*;
+import java.util.Date;
+import java.text.SimpleDateFormat;
+
+public class Converter {
+    // Date-time helpers
+
+    private static final String[] DATE_TIME_FORMATS = {
+            "yyyy-MM-dd'T'HH:mm:ss.SX",
+            "yyyy-MM-dd'T'HH:mm:ss.S",
+            "yyyy-MM-dd'T'HH:mm:ssX",
+            "yyyy-MM-dd'T'HH:mm:ss",
+            "yyyy-MM-dd HH:mm:ss.SX",
+            "yyyy-MM-dd HH:mm:ss.S",
+            "yyyy-MM-dd HH:mm:ssX",
+            "yyyy-MM-dd HH:mm:ss",
+            "HH:mm:ss.SZ",
+            "HH:mm:ss.S",
+            "HH:mm:ssZ",
+            "HH:mm:ss",
+            "yyyy-MM-dd",
+    };
+
+    public static Date parseAllDateTimeString(String str) {
+        str = str.replaceFirst("(\\.\\d{3})\\d+", "$1");
+        for (String format : DATE_TIME_FORMATS) {
+            try {
+                return new SimpleDateFormat(format).parse(str);
+            } catch (Exception ex) {
+                // Ignored
+            }
+        }
+        return null;
+    }
+
+    public static String serializeDateTime(Date datetime) {
+        return new SimpleDateFormat("yyyy-MM-dd'T'hh:mm:ssZ").format(datetime);
+    }
+
+    public static String serializeDate(Date datetime) {
+        return new SimpleDateFormat("yyyy-MM-dd").format(datetime);
+    }
+
+    public static String serializeTime(Date datetime) {
+        return new SimpleDateFormat("hh:mm:ssZ").format(datetime);
+    }
+    // Serialize/deserialize helpers
+
+    public static Map<String, TopLevel> fromJsonString(String json) throws IOException {
+        return getObjectReader().readValue(json);
+    }
+
+    public static String toJsonString(Map<String, TopLevel> obj) throws JsonProcessingException {
+        return getObjectWriter().writeValueAsString(obj);
+    }
+
+    private static ObjectReader reader;
+    private static ObjectWriter writer;
+
+    private static void instantiateMapper() {
+        ObjectMapper mapper = new ObjectMapper();
+        mapper.findAndRegisterModules();
+        mapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
+        mapper.configure(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS, false);
+        SimpleModule module = new SimpleModule();
+        module.addDeserializer(Date.class, new JsonDeserializer<Date>() {
+            @Override
+            public Date deserialize(JsonParser jsonParser, DeserializationContext deserializationContext) throws IOException, JsonProcessingException {
+                String value = jsonParser.getText();
+                return Converter.parseAllDateTimeString(value);
+            }
+        });
+        module.addDeserializer(Date.class, new JsonDeserializer<Date>() {
+            @Override
+            public Date deserialize(JsonParser jsonParser, DeserializationContext deserializationContext) throws IOException, JsonProcessingException {
+                String value = jsonParser.getText();
+                return Converter.parseAllDateTimeString(value);
+            }
+        });
+        module.addDeserializer(Date.class, new JsonDeserializer<Date>() {
+            @Override
+            public Date deserialize(JsonParser jsonParser, DeserializationContext deserializationContext) throws IOException, JsonProcessingException {
+                String value = jsonParser.getText();
+                return Converter.parseAllDateTimeString(value);
+            }
+        });
+        mapper.registerModule(module);
+        reader = mapper.readerFor(Map.class);
+        writer = mapper.writerFor(Map.class);
+    }
+
+    private static ObjectReader getObjectReader() {
+        if (reader == null) instantiateMapper();
+        return reader;
+    }
+
+    private static ObjectWriter getObjectWriter() {
+        if (writer == null) instantiateMapper();
+        return writer;
+    }
+}
diff --git a/head/schema-java-datetime-legacy/test/inputs/schema/mixed-additional-properties.schema/default/src/main/java/io/quicktype/TopLevel.java b/head/schema-java-datetime-legacy/test/inputs/schema/mixed-additional-properties.schema/default/src/main/java/io/quicktype/TopLevel.java
new file mode 100644
index 0000000..2a8322b
--- /dev/null
+++ b/head/schema-java-datetime-legacy/test/inputs/schema/mixed-additional-properties.schema/default/src/main/java/io/quicktype/TopLevel.java
@@ -0,0 +1,49 @@
+package io.quicktype;
+
+import java.io.IOException;
+import java.io.IOException;
+import com.fasterxml.jackson.core.*;
+import com.fasterxml.jackson.databind.*;
+import com.fasterxml.jackson.databind.annotation.*;
+import com.fasterxml.jackson.core.type.*;
+
+@JsonDeserialize(using = TopLevel.Deserializer.class)
+@JsonSerialize(using = TopLevel.Serializer.class)
+public class TopLevel {
+    public String stringValue;
+    public Double doubleValue;
+
+    static class Deserializer extends JsonDeserializer<TopLevel> {
+        @Override
+        public TopLevel deserialize(JsonParser jsonParser, DeserializationContext deserializationContext) throws IOException, JsonProcessingException {
+            TopLevel value = new TopLevel();
+            switch (jsonParser.currentToken()) {
+                case VALUE_NUMBER_INT:
+                case VALUE_NUMBER_FLOAT:
+                    value.doubleValue = jsonParser.readValueAs(Double.class);
+                    break;
+                case VALUE_STRING:
+                    String string = jsonParser.readValueAs(String.class);
+                    value.stringValue = string;
+                    break;
+                default: throw new IOException("Cannot deserialize TopLevel");
+            }
+            return value;
+        }
+    }
+
+    static class Serializer extends JsonSerializer<TopLevel> {
+        @Override
+        public void serialize(TopLevel obj, JsonGenerator jsonGenerator, SerializerProvider serializerProvider) throws IOException {
+            if (obj.stringValue != null) {
+                jsonGenerator.writeObject(obj.stringValue);
+                return;
+            }
+            if (obj.doubleValue != null) {
+                jsonGenerator.writeObject(obj.doubleValue);
+                return;
+            }
+            throw new IOException("TopLevel must not be null");
+        }
+    }
+}
diff --git a/head/schema-java-lombok/test/inputs/schema/mixed-additional-properties.schema/default/src/main/java/io/quicktype/Converter.java b/head/schema-java-lombok/test/inputs/schema/mixed-additional-properties.schema/default/src/main/java/io/quicktype/Converter.java
new file mode 100644
index 0000000..d7168c5
--- /dev/null
+++ b/head/schema-java-lombok/test/inputs/schema/mixed-additional-properties.schema/default/src/main/java/io/quicktype/Converter.java
@@ -0,0 +1,102 @@
+// To use this code, add the following Maven dependency to your project:
+//
+//
+//     com.fasterxml.jackson.core     : jackson-databind          : 2.9.0
+//     com.fasterxml.jackson.datatype : jackson-datatype-jsr310   : 2.9.0
+//
+// Import this package:
+//
+//     import io.quicktype.Converter;
+//
+// Then you can deserialize a JSON string with
+//
+//     Map<String, TopLevel> data = Converter.fromJsonString(jsonString);
+
+package io.quicktype;
+
+import java.io.IOException;
+import com.fasterxml.jackson.databind.*;
+import com.fasterxml.jackson.databind.module.SimpleModule;
+import com.fasterxml.jackson.core.JsonParser;
+import com.fasterxml.jackson.core.JsonProcessingException;
+import com.fasterxml.jackson.core.type.TypeReference;
+import java.util.*;
+import java.time.LocalDate;
+import java.time.OffsetDateTime;
+import java.time.OffsetTime;
+import java.time.ZoneOffset;
+import java.time.ZonedDateTime;
+import java.time.format.DateTimeFormatter;
+import java.time.format.DateTimeFormatterBuilder;
+import java.time.temporal.ChronoField;
+
+public class Converter {
+    // Date-time helpers
+
+    private static final DateTimeFormatter DATE_TIME_FORMATTER = new DateTimeFormatterBuilder()
+            .appendOptional(DateTimeFormatter.ISO_DATE_TIME)
+            .appendOptional(DateTimeFormatter.ISO_OFFSET_DATE_TIME)
+            .appendOptional(DateTimeFormatter.ISO_INSTANT)
+            .appendOptional(DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss.SX"))
+            .appendOptional(DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ssX"))
+            .appendOptional(DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss"))
+            .toFormatter()
+            .withZone(ZoneOffset.UTC);
+
+    public static OffsetDateTime parseDateTimeString(String str) {
+        return ZonedDateTime.from(Converter.DATE_TIME_FORMATTER.parse(str)).toOffsetDateTime();
+    }
+
+    private static final DateTimeFormatter TIME_FORMATTER = new DateTimeFormatterBuilder()
+            .appendOptional(DateTimeFormatter.ISO_TIME)
+            .appendOptional(DateTimeFormatter.ISO_OFFSET_TIME)
+            .parseDefaulting(ChronoField.YEAR, 2020)
+            .parseDefaulting(ChronoField.MONTH_OF_YEAR, 1)
+            .parseDefaulting(ChronoField.DAY_OF_MONTH, 1)
+            .toFormatter()
+            .withZone(ZoneOffset.UTC);
+
+    public static OffsetTime parseTimeString(String str) {
+        return ZonedDateTime.from(Converter.TIME_FORMATTER.parse(str)).toOffsetDateTime().toOffsetTime();
+    }
+    // Serialize/deserialize helpers
+
+    public static Map<String, TopLevel> fromJsonString(String json) throws IOException {
+        return getObjectReader().readValue(json);
+    }
+
+    public static String toJsonString(Map<String, TopLevel> obj) throws JsonProcessingException {
+        return getObjectWriter().writeValueAsString(obj);
+    }
+
+    private static ObjectReader reader;
+    private static ObjectWriter writer;
+
+    private static void instantiateMapper() {
+        ObjectMapper mapper = new ObjectMapper();
+        mapper.findAndRegisterModules();
+        mapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
+        mapper.configure(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS, false);
+        SimpleModule module = new SimpleModule();
+        module.addDeserializer(OffsetDateTime.class, new JsonDeserializer<OffsetDateTime>() {
+            @Override
+            public OffsetDateTime deserialize(JsonParser jsonParser, DeserializationContext deserializationContext) throws IOException, JsonProcessingException {
+                String value = jsonParser.getText();
+                return Converter.parseDateTimeString(value);
+            }
+        });
+        mapper.registerModule(module);
+        reader = mapper.readerFor(Map.class);
+        writer = mapper.writerFor(Map.class);
+    }
+
+    private static ObjectReader getObjectReader() {
+        if (reader == null) instantiateMapper();
+        return reader;
+    }
+
+    private static ObjectWriter getObjectWriter() {
+        if (writer == null) instantiateMapper();
+        return writer;
+    }
+}
diff --git a/head/schema-java-lombok/test/inputs/schema/mixed-additional-properties.schema/default/src/main/java/io/quicktype/TopLevel.java b/head/schema-java-lombok/test/inputs/schema/mixed-additional-properties.schema/default/src/main/java/io/quicktype/TopLevel.java
new file mode 100644
index 0000000..2a8322b
--- /dev/null
+++ b/head/schema-java-lombok/test/inputs/schema/mixed-additional-properties.schema/default/src/main/java/io/quicktype/TopLevel.java
@@ -0,0 +1,49 @@
+package io.quicktype;
+
+import java.io.IOException;
+import java.io.IOException;
+import com.fasterxml.jackson.core.*;
+import com.fasterxml.jackson.databind.*;
+import com.fasterxml.jackson.databind.annotation.*;
+import com.fasterxml.jackson.core.type.*;
+
+@JsonDeserialize(using = TopLevel.Deserializer.class)
+@JsonSerialize(using = TopLevel.Serializer.class)
+public class TopLevel {
+    public String stringValue;
+    public Double doubleValue;
+
+    static class Deserializer extends JsonDeserializer<TopLevel> {
+        @Override
+        public TopLevel deserialize(JsonParser jsonParser, DeserializationContext deserializationContext) throws IOException, JsonProcessingException {
+            TopLevel value = new TopLevel();
+            switch (jsonParser.currentToken()) {
+                case VALUE_NUMBER_INT:
+                case VALUE_NUMBER_FLOAT:
+                    value.doubleValue = jsonParser.readValueAs(Double.class);
+                    break;
+                case VALUE_STRING:
+                    String string = jsonParser.readValueAs(String.class);
+                    value.stringValue = string;
+                    break;
+                default: throw new IOException("Cannot deserialize TopLevel");
+            }
+            return value;
+        }
+    }
+
+    static class Serializer extends JsonSerializer<TopLevel> {
+        @Override
+        public void serialize(TopLevel obj, JsonGenerator jsonGenerator, SerializerProvider serializerProvider) throws IOException {
+            if (obj.stringValue != null) {
+                jsonGenerator.writeObject(obj.stringValue);
+                return;
+            }
+            if (obj.doubleValue != null) {
+                jsonGenerator.writeObject(obj.doubleValue);
+                return;
+            }
+            throw new IOException("TopLevel must not be null");
+        }
+    }
+}
diff --git a/head/schema-javascript/test/inputs/schema/mixed-additional-properties.schema/default/TopLevel.js b/head/schema-javascript/test/inputs/schema/mixed-additional-properties.schema/default/TopLevel.js
new file mode 100644
index 0000000..84c7ebb
--- /dev/null
+++ b/head/schema-javascript/test/inputs/schema/mixed-additional-properties.schema/default/TopLevel.js
@@ -0,0 +1,203 @@
+// To parse this data:
+//
+//   const Convert = require("./TopLevel");
+//
+//   const topLevel = Convert.toTopLevel(json);
+//
+// These functions will throw an error if the JSON doesn't
+// match the expected interface, even if the JSON is valid.
+
+// Converts JSON strings to/from your types
+// and asserts the results of JSON.parse at runtime
+function toTopLevel(json) {
+    return cast(JSON.parse(json), r("TopLevel"));
+}
+
+function topLevelToJson(value) {
+    return JSON.stringify(uncast(value, r("TopLevel")), null, 2);
+}
+
+function invalidValue(typ, val, key, parent = '') {
+    const prettyTyp = prettyTypeName(typ);
+    const parentText = parent ? ` on ${parent}` : '';
+    const keyText = key ? ` for key "${key}"` : '';
+    throw Error(`Invalid value${keyText}${parentText}. Expected ${prettyTyp} but got ${JSON.stringify(val)}`);
+}
+
+function prettyTypeName(typ) {
+    if (Array.isArray(typ)) {
+        if (typ.length === 2 && typ[0] === undefined) {
+            return `an optional ${prettyTypeName(typ[1])}`;
+        } else {
+            return `one of [${typ.map(a => { return prettyTypeName(a); }).join(", ")}]`;
+        }
+    } else if (typeof typ === "object" && typ.literal !== undefined) {
+        return typ.literal;
+    } else {
+        return typeof typ;
+    }
+}
+
+function jsonToJSProps(typ) {
+    if (typ.jsonToJS === undefined) {
+        const map = {};
+        typ.props.forEach((p) => map[p.json] = { key: p.js, typ: p.typ });
+        typ.jsonToJS = map;
+    }
+    return typ.jsonToJS;
+}
+
+function jsToJSONProps(typ) {
+    if (typ.jsToJSON === undefined) {
+        const map = {};
+        typ.props.forEach((p) => map[p.js] = { key: p.json, typ: p.typ });
+        typ.jsToJSON = map;
+    }
+    return typ.jsToJSON;
+}
+
+function transform(val, typ, getProps, key = '', parent = '') {
+    function transformPrimitive(typ, val) {
+        if (typeof typ === typeof val) return val;
+        return invalidValue(typ, val, key, parent);
+    }
+
+    function transformUnion(typs, val) {
+        // val must validate against one typ in typs
+        const l = typs.length;
+        for (let i = 0; i < l; i++) {
+            const typ = typs[i];
+            try {
+                return transform(val, typ, getProps);
+            } catch (_) {}
+        }
+        return invalidValue(typs, val, key, parent);
+    }
+
+    function transformEnum(cases, val) {
+        if (cases.indexOf(val) !== -1) return val;
+        return invalidValue(cases.map(a => { return l(a); }), val, key, parent);
+    }
+
+    function transformArray(typ, val) {
+        // val must be an array with no invalid elements
+        if (!Array.isArray(val)) return invalidValue(l("array"), val, key, parent);
+
+        return val.map(el => transform(el, typ, getProps));
+    }
+
+    function transformDate(val) {
+        if (val === null) {
+            return null;
+        }
+        const d = new Date(val);
+        if (isNaN(d.valueOf())) {
+            return invalidValue(l("Date"), val, key, parent);
+        }
+        return d;
+    }
+
+    function transformObject(props, additional, val) {
+        if (val === null || typeof val !== "object" || Array.isArray(val)) {
+            return invalidValue(l(ref || "object"), val, key, parent);
+        }
+        const result = {};
+        Object.getOwnPropertyNames(props).forEach(key => {
+            const prop = props[key];
+            const v = Object.prototype.hasOwnProperty.call(val, key) ? val[key] : undefined;
+            result[prop.key] = transform(v, prop.typ, getProps, key, ref);
+        });
+        Object.getOwnPropertyNames(val).forEach(key => {
+            if (!Object.prototype.hasOwnProperty.call(props, key)) {
+                result[key] = transform(val[key], additional, getProps, key, ref);
+            }
+        });
+        return result;
+    }
+
+    if (typ === "any") return val;
+    if (typ === null) {
+        if (val === null) return val;
+        return invalidValue(typ, val, key, parent);
+    }
+    if (typ === false) return invalidValue(typ, val, key, parent);
+    let ref = undefined;
+    while (typeof typ === "object" && typ.ref !== undefined) {
+        ref = typ.ref;
+        typ = typeMap[typ.ref];
+    }
+    if (Array.isArray(typ)) return transformEnum(typ, val);
+    if (typeof typ === "object") {
+        return typ.hasOwnProperty("pattern")      ? typeof val === "string" && new RegExp(typ.pattern).test(val) ? val : invalidValue(typ, val, key, parent)
+            : typ.hasOwnProperty("string")       ? typeof val === "string" && (typ.min === undefined || Array.from(val).length >= typ.min) && (typ.max === undefined || Array.from(val).length <= typ.max) ? transform(val, typ.string, getProps, key, parent) : invalidValue(typ, val, key, parent)
+            : typ.hasOwnProperty("number")       ? typeof val === "number" && (typ.min === undefined || val >= typ.min) && (typ.max === undefined || val <= typ.max) ? transform(val, typ.number, getProps, key, parent) : invalidValue(typ, val, key, parent)
+            : typ.hasOwnProperty("integer")      ? typeof val === "number" && val % 1 === 0 ? val : invalidValue(l("integer"), val, key, parent)
+            : typ.hasOwnProperty("unionMembers") ? transformUnion(typ.unionMembers, val)
+            : typ.hasOwnProperty("arrayItems")    ? transformArray(typ.arrayItems, val)
+            : typ.hasOwnProperty("props")         ? transformObject(getProps(typ), typ.additional, val)
+            : invalidValue(typ, val, key, parent);
+    }
+    // Numbers can be parsed by Date but shouldn't be.
+    if (typ === Date && (typeof val === "string" || val instanceof Date)) return transformDate(val);
+    return transformPrimitive(typ, val);
+}
+
+function cast(val, typ) {
+    return transform(val, typ, jsonToJSProps);
+}
+
+function uncast(val, typ) {
+    return transform(val, typ, jsToJSONProps);
+}
+
+function l(typ) {
+    return { literal: typ };
+}
+
+function a(typ) {
+    return { arrayItems: typ };
+}
+
+function i(typ) {
+    return { integer: typ };
+}
+
+function p(pattern) {
+    return { pattern };
+}
+
+function s(typ, min, max) {
+    return { string: typ, min, max };
+}
+
+function n(typ, min, max) {
+    return { number: typ, min, max };
+}
+
+function u(...typs) {
+    return { unionMembers: typs };
+}
+
+function o(props, additional) {
+    return { props, additional };
+}
+
+function m(additional) {
+    const props = [];
+    return { props, additional };
+}
+
+function r(name) {
+    return { ref: name };
+}
+
+const typeMap = {
+    "TopLevel": o([
+        { json: "id", js: "id", typ: u(undefined, "") },
+    ], 3.14),
+};
+
+module.exports = {
+    "topLevelToJson": topLevelToJson,
+    "toTopLevel": toTopLevel,
+};
diff --git a/head/schema-javascript-prop-types/test/inputs/schema/mixed-additional-properties.schema/default/toplevel.js b/head/schema-javascript-prop-types/test/inputs/schema/mixed-additional-properties.schema/default/toplevel.js
new file mode 100644
index 0000000..cea18a2
--- /dev/null
+++ b/head/schema-javascript-prop-types/test/inputs/schema/mixed-additional-properties.schema/default/toplevel.js
@@ -0,0 +1,15 @@
+// Example usage:
+//
+// import { MyShape } from ./myShape.js;
+//
+// class MyComponent extends React.Component {
+//   //
+// }
+//
+// MyComponent.propTypes = {
+//   input: MyShape
+// };
+
+import PropTypes from "prop-types";
+
+export const TopLevel = PropTypes.oneOfType([PropTypes.objectOf(PropTypes.oneOfType([PropTypes.number, PropTypes.string]))]).isRequired;
diff --git a/head/schema-kotlin-jackson/test/inputs/schema/mixed-additional-properties.schema/default/TopLevel.kt b/head/schema-kotlin-jackson/test/inputs/schema/mixed-additional-properties.schema/default/TopLevel.kt
new file mode 100644
index 0000000..4ec88a6
--- /dev/null
+++ b/head/schema-kotlin-jackson/test/inputs/schema/mixed-additional-properties.schema/default/TopLevel.kt
@@ -0,0 +1,57 @@
+// To parse the JSON, install jackson-module-kotlin and do:
+//
+//   val topLevel = TopLevel.fromJson(jsonString)
+
+package quicktype
+
+import com.fasterxml.jackson.annotation.*
+import com.fasterxml.jackson.core.*
+import com.fasterxml.jackson.databind.*
+import com.fasterxml.jackson.databind.deser.std.StdDeserializer
+import com.fasterxml.jackson.databind.module.SimpleModule
+import com.fasterxml.jackson.databind.node.*
+import com.fasterxml.jackson.databind.ser.std.StdSerializer
+import com.fasterxml.jackson.module.kotlin.*
+
+
+@Suppress("UNCHECKED_CAST")
+private fun <T> ObjectMapper.convert(k: kotlin.reflect.KClass<*>, fromJson: (JsonNode) -> T, toJson: (T) -> String, isUnion: Boolean = false) = registerModule(SimpleModule().apply {
+    addSerializer(k.java as Class<T>, object : StdSerializer<T>(k.java as Class<T>) {
+            override fun serialize(value: T, gen: JsonGenerator, provider: SerializerProvider) = gen.writeRawValue(toJson(value))
+    })
+    addDeserializer(k.java as Class<T>, object : StdDeserializer<T>(k.java as Class<T>) {
+            override fun deserialize(p: JsonParser, ctxt: DeserializationContext) = fromJson(p.readValueAsTree())
+    })
+})
+
+val mapper = jacksonObjectMapper().apply {
+    propertyNamingStrategy = object : PropertyNamingStrategy.PropertyNamingStrategyBase() { override fun translate(name: String) = if (name == "empty") "" else name }
+    setSerializationInclusion(JsonInclude.Include.NON_NULL)
+    convert(TopLevelValue::class, { TopLevelValue.fromJson(it) }, { it.toJson() }, true)
+}
+
+class TopLevel(elements: Map<String, TopLevelValue>) : HashMap<String, TopLevelValue>(elements) {
+    fun toJson() = mapper.writeValueAsString(this)
+
+    companion object {
+        fun fromJson(json: String) = mapper.readValue<TopLevel>(json)
+    }
+}
+
+sealed class TopLevelValue {
+    class DoubleValue(val value: Double) : TopLevelValue()
+    class StringValue(val value: String) : TopLevelValue()
+
+    fun toJson(): String = mapper.writeValueAsString(when (this) {
+        is DoubleValue -> this.value
+        is StringValue -> this.value
+    })
+
+    companion object {
+        fun fromJson(jn: JsonNode): TopLevelValue = when (jn) {
+            is IntNode, is LongNode, is DoubleNode -> DoubleValue(mapper.convertValue(jn))
+            is TextNode                            -> StringValue(mapper.convertValue(jn))
+            else                                   -> throw IllegalArgumentException()
+        }
+    }
+}
diff --git a/head/schema-kotlinx/test/inputs/schema/mixed-additional-properties.schema/default/TopLevel.kt b/head/schema-kotlinx/test/inputs/schema/mixed-additional-properties.schema/default/TopLevel.kt
new file mode 100644
index 0000000..8ffa212
--- /dev/null
+++ b/head/schema-kotlinx/test/inputs/schema/mixed-additional-properties.schema/default/TopLevel.kt
@@ -0,0 +1,15 @@
+// To parse the JSON, install kotlin's serialization plugin and do:
+//
+// val json     = Json { allowStructuredMapKeys = true }
+// val topLevel = json.parse(TopLevel.serializer(), jsonString)
+
+package quicktype
+
+import kotlinx.serialization.*
+import kotlinx.serialization.json.*
+import kotlinx.serialization.descriptors.*
+import kotlinx.serialization.encoding.*
+
+typealias TopLevel = HashMap<String, JsonElement>
+
+typealias TopLevelValue = JsonElement
diff --git a/head/schema-objective-c/test/inputs/schema/mixed-additional-properties.schema/default/QTTopLevel.h b/head/schema-objective-c/test/inputs/schema/mixed-additional-properties.schema/default/QTTopLevel.h
new file mode 100644
index 0000000..94ad13c
--- /dev/null
+++ b/head/schema-objective-c/test/inputs/schema/mixed-additional-properties.schema/default/QTTopLevel.h
@@ -0,0 +1,21 @@
+// To parse this JSON:
+//
+//   NSError *error;
+//   QTTopLevel *topLevel = QTTopLevelFromJSON(json, NSUTF8Encoding, &error);
+
+#import <Foundation/Foundation.h>
+
+NS_ASSUME_NONNULL_BEGIN
+
+typedef NSDictionary<NSString *, id> QTTopLevel;
+
+#pragma mark - Top-level marshaling functions
+
+QTTopLevel *_Nullable QTTopLevelFromData(NSData *data, NSError **error);
+QTTopLevel *_Nullable QTTopLevelFromJSON(NSString *json, NSStringEncoding encoding, NSError **error);
+NSData     *_Nullable QTTopLevelToData(QTTopLevel *topLevel, NSError **error);
+NSString   *_Nullable QTTopLevelToJSON(QTTopLevel *topLevel, NSStringEncoding encoding, NSError **error);
+
+#pragma mark - Object interfaces
+
+NS_ASSUME_NONNULL_END
diff --git a/head/schema-objective-c/test/inputs/schema/mixed-additional-properties.schema/default/QTTopLevel.m b/head/schema-objective-c/test/inputs/schema/mixed-additional-properties.schema/default/QTTopLevel.m
new file mode 100644
index 0000000..e853798
--- /dev/null
+++ b/head/schema-objective-c/test/inputs/schema/mixed-additional-properties.schema/default/QTTopLevel.m
@@ -0,0 +1,60 @@
+#import "QTTopLevel.h"
+
+#define λ(decl, expr) (^(decl) { return (expr); })
+
+static id NSNullify(id _Nullable x) {
+    return (x == nil || x == NSNull.null) ? NSNull.null : x;
+}
+
+NS_ASSUME_NONNULL_BEGIN
+
+static id map(id collection, id (^f)(id value)) {
+    id result = nil;
+    if ([collection isKindOfClass:NSArray.class]) {
+            result = [NSMutableArray arrayWithCapacity:[(NSArray *)collection count]];
+            for (id x in collection) [result addObject:NSNullify(f(x))];
+    } else if ([collection isKindOfClass:NSDictionary.class]) {
+            result = [NSMutableDictionary dictionaryWithCapacity:[(NSDictionary *)collection count]];
+            for (id key in collection) [result setObject:f([collection objectForKey:key]) forKey:key];
+    }
+    return result;
+}
+
+#pragma mark - JSON serialization
+
+QTTopLevel *_Nullable QTTopLevelFromData(NSData *data, NSError **error)
+{
+    @try {
+        id json = [NSJSONSerialization JSONObjectWithData:data options:NSJSONReadingAllowFragments error:error];
+        if (![json isKindOfClass:NSDictionary.class]) [NSException raise:@"Invalid JSON" format:@"Expected NSDictionary."];
+        return *error ? nil : map(json, λ(id x, x));
+    } @catch (NSException *exception) {
+        *error = [NSError errorWithDomain:@"JSONSerialization" code:-1 userInfo:@{ @"exception": exception }];
+        return nil;
+    }
+}
+
+QTTopLevel *_Nullable QTTopLevelFromJSON(NSString *json, NSStringEncoding encoding, NSError **error)
+{
+    return QTTopLevelFromData([json dataUsingEncoding:encoding], error);
+}
+
+NSData *_Nullable QTTopLevelToData(QTTopLevel *topLevel, NSError **error)
+{
+    @try {
+        id json = topLevel;
+        NSData *data = [NSJSONSerialization dataWithJSONObject:json options:NSJSONWritingFragmentsAllowed error:error];
+        return *error ? nil : data;
+    } @catch (NSException *exception) {
+        *error = [NSError errorWithDomain:@"JSONSerialization" code:-1 userInfo:@{ @"exception": exception }];
+        return nil;
+    }
+}
+
+NSString *_Nullable QTTopLevelToJSON(QTTopLevel *topLevel, NSStringEncoding encoding, NSError **error)
+{
+    NSData *data = QTTopLevelToData(topLevel, error);
+    return data ? [[NSString alloc] initWithData:data encoding:encoding] : nil;
+}
+
+NS_ASSUME_NONNULL_END
diff --git a/head/schema-php/test/inputs/schema/mixed-additional-properties.schema/default/TopLevel.php b/head/schema-php/test/inputs/schema/mixed-additional-properties.schema/default/TopLevel.php
new file mode 100644
index 0000000..a766f11
--- /dev/null
+++ b/head/schema-php/test/inputs/schema/mixed-additional-properties.schema/default/TopLevel.php
@@ -0,0 +1,2 @@
+<?php
+declare(strict_types=1);
diff --git a/head/schema-pike/test/inputs/schema/mixed-additional-properties.schema/default/TopLevel.pmod b/head/schema-pike/test/inputs/schema/mixed-additional-properties.schema/default/TopLevel.pmod
new file mode 100644
index 0000000..b66d134
--- /dev/null
+++ b/head/schema-pike/test/inputs/schema/mixed-additional-properties.schema/default/TopLevel.pmod
@@ -0,0 +1,29 @@
+// This source has been automatically generated by quicktype.
+// ( https://github.com/quicktype/quicktype )
+//
+// To use this code, simply import it into your project as a Pike module.
+// To JSON-encode your object, you can pass it to `Standards.JSON.encode`
+// or call `encode_json` on it.
+//
+// To decode a JSON string, first pass it to `Standards.JSON.decode`,
+// and then pass the result to `<YourClass>_from_JSON`.
+// It will return an instance of <YourClass>.
+// Bear in mind that these functions have unexpected behavior,
+// and will likely throw an error, if the JSON string does not
+// match the expected interface, even if the JSON itself is valid.
+
+typedef mapping(string:TopLevelValue) TopLevel;
+
+TopLevel TopLevel_from_JSON(mixed json) {
+    mapping(string:TopLevelValue) retval = ([]);
+    foreach (json; string k; mixed v) {
+        retval[k] = TopLevelValue_from_JSON(v);
+    }
+    return retval;
+}
+
+typedef float|string TopLevelValue;
+
+TopLevelValue TopLevelValue_from_JSON(mixed json) {
+    return json;
+}
diff --git a/head/schema-python/test/inputs/schema/mixed-additional-properties.schema/default/quicktype.py b/head/schema-python/test/inputs/schema/mixed-additional-properties.schema/default/quicktype.py
new file mode 100644
index 0000000..3c679df
--- /dev/null
+++ b/head/schema-python/test/inputs/schema/mixed-additional-properties.schema/default/quicktype.py
@@ -0,0 +1,41 @@
+from typing import Any, TypeVar, Callable
+
+
+T = TypeVar("T")
+
+
+def from_dict(f: Callable[[Any], T], x: Any) -> dict[str, T]:
+    assert isinstance(x, dict)
+    return { k: f(v) for (k, v) in x.items() }
+
+
+def from_str(x: Any) -> str:
+    assert isinstance(x, str)
+    return x
+
+
+def from_float(x: Any) -> float:
+    assert isinstance(x, (float, int)) and not isinstance(x, bool)
+    return float(x)
+
+
+def from_union(fs, x):
+    for f in fs:
+        try:
+            return f(x)
+        except:
+            pass
+    assert False
+
+
+def to_float(x: Any) -> float:
+    assert isinstance(x, (int, float))
+    return x
+
+
+def top_level_from_dict(s: Any) -> dict[str, str | float]:
+    return from_dict(lambda x: from_union([from_str, from_float], x), s)
+
+
+def top_level_to_dict(x: dict[str, str | float]) -> Any:
+    return from_dict(lambda x: from_union([from_str, to_float], x), x)
diff --git a/head/schema-ruby/test/inputs/schema/mixed-additional-properties.schema/default/TopLevel.rb b/head/schema-ruby/test/inputs/schema/mixed-additional-properties.schema/default/TopLevel.rb
new file mode 100644
index 0000000..12e9ee5
--- /dev/null
+++ b/head/schema-ruby/test/inputs/schema/mixed-additional-properties.schema/default/TopLevel.rb
@@ -0,0 +1,58 @@
+# This code may look unusually verbose for Ruby (and it is), but
+# it performs some subtle and complex validation of JSON data.
+#
+# To parse this JSON, add 'dry-struct' and 'dry-types' gems, then do:
+#
+#   top_level = TopLevel.from_json! "{…}"
+#   puts top_level["…"]
+#
+# If from_json! succeeds, the value returned matches the schema.
+
+require 'json'
+require 'dry-types'
+require 'dry-struct'
+
+module Types
+  include Dry.Types(default: :nominal)
+
+  Hash   = Strict::Hash
+  String = Strict::String
+  Double = Strict::Float | Strict::Integer
+end
+
+class TopLevelValue < Dry::Struct
+  attribute :double, Types::Double.optional
+  attribute :string, Types::String.optional
+
+  def self.from_dynamic!(d)
+    if schema.key(:double).type.right.valid? d
+      return new(double: d, string: nil)
+    end
+    if schema.key(:string).type.right.valid? d
+      return new(string: d, double: nil)
+    end
+    raise "Invalid union"
+  end
+
+  def self.from_json!(json)
+    from_dynamic!(JSON.parse(json))
+  end
+
+  def to_dynamic
+    if double != nil
+      double
+    elsif string != nil
+      string
+    end
+  end
+
+  def to_json(options = nil)
+    JSON.generate(to_dynamic, options)
+  end
+end
+
+class TopLevel
+  def self.from_json!(json)
+    Types::Hash[JSON.parse(json, quirks_mode: true)].map { |k, v| [k, TopLevelValue.from_dynamic!(v)] }.to_h
+  end
+end
diff --git a/head/schema-rust/test/inputs/schema/mixed-additional-properties.schema/default/module_under_test.rs b/head/schema-rust/test/inputs/schema/mixed-additional-properties.schema/default/module_under_test.rs
new file mode 100644
index 0000000..093dc80
--- /dev/null
+++ b/head/schema-rust/test/inputs/schema/mixed-additional-properties.schema/default/module_under_test.rs
@@ -0,0 +1,25 @@
+// Example code that deserializes and serializes the model.
+// extern crate serde;
+// #[macro_use]
+// extern crate serde_derive;
+// extern crate serde_json;
+//
+// use generated_module::TopLevel;
+//
+// fn main() {
+//     let json = r#"{"answer": 42}"#;
+//     let model: TopLevel = serde_json::from_str(&json).unwrap();
+// }
+
+use serde::{Serialize, Deserialize};
+use std::collections::HashMap;
+
+pub type TopLevel = HashMap<String, TopLevelValue>;
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+#[serde(untagged)]
+pub enum TopLevelValue {
+    Double(f64),
+
+    PurpleString(String),
+}
diff --git a/head/schema-scala3/test/inputs/schema/mixed-additional-properties.schema/default/TopLevel.scala b/head/schema-scala3/test/inputs/schema/mixed-additional-properties.schema/default/TopLevel.scala
new file mode 100644
index 0000000..34f7359
--- /dev/null
+++ b/head/schema-scala3/test/inputs/schema/mixed-additional-properties.schema/default/TopLevel.scala
@@ -0,0 +1,25 @@
+package quicktype
+
+import io.circe.syntax._
+import io.circe._
+import cats.syntax.functor._
+
+// If a union has a null in, then we'll need this too... 
+type NullValue = None.type
+
+type TopLevel = Map[String, TopLevelValue]
+
+given (using ev : TopLevelValue): Encoder[Map[String, TopLevelValue]] = Encoder.encodeMap[String, TopLevelValue]
+
+type TopLevelValue = Double | String
+given Decoder[TopLevelValue] = {
+    List[Decoder[TopLevelValue]](
+        Decoder[String].widen,
+        Decoder[Double].widen,
+    ).reduceLeft(_ or _)
+}
+
+given Encoder[TopLevelValue] = Encoder.instance {
+    case enc0 : String => Encoder.encodeString(enc0)
+    case enc1 : Double => Encoder.encodeDouble(enc1)
+}
diff --git a/head/schema-scala3-upickle/test/inputs/schema/mixed-additional-properties.schema/default/TopLevel.scala b/head/schema-scala3-upickle/test/inputs/schema/mixed-additional-properties.schema/default/TopLevel.scala
new file mode 100644
index 0000000..3a4fbdc
--- /dev/null
+++ b/head/schema-scala3-upickle/test/inputs/schema/mixed-additional-properties.schema/default/TopLevel.scala
@@ -0,0 +1,93 @@
+package quicktype
+
+// Custom pickler so that missing keys and JSON nulls both read as None,
+// and None is left out when writing (upickle's default for Option is a
+// JSON array).
+object OptionPickler extends upickle.AttributeTagged:
+    import upickle.default.Writer
+    import upickle.default.Reader
+    override implicit def OptionWriter[T: Writer]: Writer[Option[T]] =
+        implicitly[Writer[T]].comap[Option[T]] {
+            case None => null.asInstanceOf[T]
+            case Some(x) => x
+        }
+
+    override implicit def OptionReader[T: Reader]: Reader[Option[T]] = {
+        new Reader.Delegate[Any, Option[T]](implicitly[Reader[T]].map(Some(_))){
+        override def visitNull(index: Int) = None
+        }
+    }
+end OptionPickler
+
+// If a union has a null in, then we'll need this too...
+type NullValue = None.type
+given OptionPickler.ReadWriter[NullValue] = OptionPickler.readwriter[ujson.Value].bimap[NullValue](
+    _ => ujson.Null,
+    json => if json.isNull then None else throw new upickle.core.Abort("not null")
+)
+given OptionPickler.ReadWriter[java.time.Instant] = OptionPickler.readwriter[ujson.Value].bimap(
+    value => ujson.Str(value.toString),
+    json => json match
+        case ujson.Str(value) => java.time.Instant.parse(value)
+        case other => throw new upickle.core.Abort("expected date-time, got " + other)
+)
+given OptionPickler.ReadWriter[java.util.UUID] = OptionPickler.readwriter[String].bimap(
+    _.toString,
+    value =>
+        val uuid = java.util.UUID.fromString(value)
+        if uuid.toString.equalsIgnoreCase(value) then uuid else throw new upickle.core.Abort("invalid UUID")
+)
+
+object JsonExt:
+    val valueReader = OptionPickler.readwriter[ujson.Value]
+
+    // upickle's built-in primitive readers are lenient -- the numeric and
+    // boolean readers accept strings, and the string reader accepts
+    // numbers and booleans -- so untagged unions need strict readers to
+    // pick the right member.
+    val strictString: OptionPickler.Reader[String] = valueReader.map {
+        case ujson.Str(s) => s
+        case json => throw new upickle.core.Abort("expected string, got " + json)
+    }
+    val strictLong: OptionPickler.Reader[Long] = valueReader.map {
+        case ujson.Num(n) if n.isWhole => n.toLong
+        case json => throw new upickle.core.Abort("expected integer, got " + json)
+    }
+    val strictDouble: OptionPickler.Reader[Double] = valueReader.map {
+        case ujson.Num(n) => n
+        case json => throw new upickle.core.Abort("expected number, got " + json)
+    }
+    val strictBoolean: OptionPickler.Reader[Boolean] = valueReader.map {
+        case ujson.Bool(b) => b
+        case json => throw new upickle.core.Abort("expected boolean, got " + json)
+    }
+
+    def badMerge[T](r1: => OptionPickler.Reader[?], rest: OptionPickler.Reader[?]*): OptionPickler.Reader[T] = valueReader.map { json =>
+        var t: T | Null = null
+        val stack       = Vector.newBuilder[Throwable]
+        (r1 +: rest).foreach { reader =>
+            if t == null then
+            try
+                t = OptionPickler.read[T](json, trace = true)(using reader.asInstanceOf[OptionPickler.Reader[T]])
+            catch
+                case exc => stack += exc
+        }
+        if t != null then t.nn else throw new Exception(json.toString(), stack.result().headOption.getOrElse(null))
+    }
+end JsonExt
+given OptionPickler.Reader[Long] = JsonExt.strictLong
+
+
+type TopLevel = Map[String, TopLevelValue]
+
+type TopLevelValue = Double | String
+given unionReaderTopLevelValue: OptionPickler.Reader[TopLevelValue] = JsonExt.badMerge[TopLevelValue](
+    JsonExt.strictString,
+    JsonExt.strictDouble,
+    )
+
+given unionWriterTopLevelValue: OptionPickler.Writer[TopLevelValue] = OptionPickler.writer[ujson.Value].comap[TopLevelValue]{ _v =>
+    (_v: @unchecked) match 
+        case v: String => OptionPickler.writeJs[String](v)
+        case v: Double => OptionPickler.writeJs[Double](v)
+}
diff --git a/head/schema-schema/test/inputs/schema/mixed-additional-properties.schema/default/TopLevel.schema b/head/schema-schema/test/inputs/schema/mixed-additional-properties.schema/default/TopLevel.schema
new file mode 100644
index 0000000..18c814f
--- /dev/null
+++ b/head/schema-schema/test/inputs/schema/mixed-additional-properties.schema/default/TopLevel.schema
@@ -0,0 +1,19 @@
+{
+    "$schema": "http://json-schema.org/draft-06/schema#",
+    "$ref": "#/definitions/TopLevel",
+    "definitions": {
+        "TopLevel": {
+            "type": "object",
+            "additionalProperties": {
+                "type": "number"
+            },
+            "properties": {
+                "id": {
+                    "type": "string"
+                }
+            },
+            "required": [],
+            "title": "TopLevel"
+        }
+    }
+}
diff --git a/head/schema-swift/test/inputs/schema/mixed-additional-properties.schema/default/quicktype.swift b/head/schema-swift/test/inputs/schema/mixed-additional-properties.schema/default/quicktype.swift
new file mode 100644
index 0000000..2dc8426
--- /dev/null
+++ b/head/schema-swift/test/inputs/schema/mixed-additional-properties.schema/default/quicktype.swift
@@ -0,0 +1,97 @@
+// This file was generated from JSON Schema using quicktype, do not modify it directly.
+// To parse the JSON, add this file to your project and do:
+//
+//   let topLevel = try TopLevel(json)
+
+import Foundation
+
+enum TopLevelValue: Codable {
+    case double(Double)
+    case string(String)
+
+    init(from decoder: Decoder) throws {
+        let container = try decoder.singleValueContainer()
+        if let x = try? container.decode(Double.self) {
+            self = .double(x)
+            return
+        }
+        if let x = try? container.decode(String.self) {
+            self = .string(x)
+            return
+        }
+        throw DecodingError.typeMismatch(TopLevelValue.self, DecodingError.Context(codingPath: decoder.codingPath, debugDescription: "Wrong type for TopLevelValue"))
+    }
+
+    func encode(to encoder: Encoder) throws {
+        var container = encoder.singleValueContainer()
+        switch self {
+        case .double(let x):
+            try container.encode(x)
+        case .string(let x):
+            try container.encode(x)
+        }
+    }
+}
+
+typealias TopLevel = [String: TopLevelValue]
+
+extension Dictionary where Key == String, Value == TopLevelValue {
+    init(data: Data) throws {
+        self = try newJSONDecoder().decode(TopLevel.self, from: data)
+    }
+
+    init(_ json: String, using encoding: String.Encoding = .utf8) throws {
+        guard let data = json.data(using: encoding) else {
+            throw NSError(domain: "JSONDecoding", code: 0, userInfo: nil)
+        }
+        try self.init(data: data)
+    }
+
+    init(fromURL url: URL) throws {
+        try self.init(data: try Data(contentsOf: url))
+    }
+
+    func jsonData() throws -> Data {
+        return try newJSONEncoder().encode(self)
+    }
+
+    func jsonString(encoding: String.Encoding = .utf8) throws -> String? {
+        return String(data: try self.jsonData(), encoding: encoding)
+    }
+}
+
+// MARK: - Helper functions for creating encoders and decoders
+
+func newJSONDecoder() -> JSONDecoder {
+    let decoder = JSONDecoder()
+    decoder.dateDecodingStrategy = .custom({ (decoder) -> Date in
+        let container = try decoder.singleValueContainer()
+        let dateStr = try container.decode(String.self)
+
+        let formatter = DateFormatter()
+        formatter.calendar = Calendar(identifier: .iso8601)
+        formatter.locale = Locale(identifier: "en_US_POSIX")
+        formatter.timeZone = TimeZone(secondsFromGMT: 0)
+        formatter.dateFormat = "yyyy-MM-dd'T'HH:mm:ss.SSSXXXXX"
+        if let date = formatter.date(from: dateStr) {
+            return date
+        }
+        formatter.dateFormat = "yyyy-MM-dd'T'HH:mm:ssXXXXX"
+        if let date = formatter.date(from: dateStr) {
+            return date
+        }
+        throw DecodingError.typeMismatch(Date.self, DecodingError.Context(codingPath: decoder.codingPath, debugDescription: "Could not decode date"))
+    })
+    return decoder
+}
+
+func newJSONEncoder() -> JSONEncoder {
+    let encoder = JSONEncoder()
+    let formatter = DateFormatter()
+    formatter.calendar = Calendar(identifier: .iso8601)
+    formatter.locale = Locale(identifier: "en_US_POSIX")
+    formatter.timeZone = TimeZone(secondsFromGMT: 0)
+    formatter.dateFormat = "yyyy-MM-dd'T'HH:mm:ss.SSSSSSXXXXX"
+    encoder.dateEncodingStrategy = .formatted(formatter)
+    return encoder
+}
diff --git a/base/schema-typescript/test/inputs/regressions/unicode-codepoint-length.schema/default/TopLevel.ts b/head/schema-typescript/test/inputs/regressions/unicode-codepoint-length.schema/default/TopLevel.ts
index b7e83e6..3da4931 100644
--- a/base/schema-typescript/test/inputs/regressions/unicode-codepoint-length.schema/default/TopLevel.ts
+++ b/head/schema-typescript/test/inputs/regressions/unicode-codepoint-length.schema/default/TopLevel.ts
@@ -11,7 +11,7 @@ export interface TopLevel {
     exact:   string;
     maximum: string;
     minimum: string;
-    [property: string]: unknown | string | string | string;
+    [property: string]: unknown;
 }
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-typescript/test/inputs/schema/any.schema/default/TopLevel.ts b/head/schema-typescript/test/inputs/schema/any.schema/default/TopLevel.ts
index c87f146..293a64c 100644
--- a/base/schema-typescript/test/inputs/schema/any.schema/default/TopLevel.ts
+++ b/head/schema-typescript/test/inputs/schema/any.schema/default/TopLevel.ts
@@ -10,7 +10,7 @@
 export interface TopLevel {
     foo:    unknown;
     values: { [key: string]: unknown };
-    [property: string]: unknown | unknown | { [key: string]: unknown };
+    [property: string]: unknown;
 }
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-typescript/test/inputs/schema/bool-string.schema/default/TopLevel.ts b/head/schema-typescript/test/inputs/schema/bool-string.schema/default/TopLevel.ts
index 68a9462..8604379 100644
--- a/base/schema-typescript/test/inputs/schema/bool-string.schema/default/TopLevel.ts
+++ b/head/schema-typescript/test/inputs/schema/bool-string.schema/default/TopLevel.ts
@@ -15,7 +15,7 @@ export interface TopLevel {
     optional?:            string;
     unionWithBool:        UnionWithBool;
     unionWithBoolAndEnum: UnionWithBool;
-    [property: string]: unknown | (null | string)[] | string[] | null | string | string | string | UnionWithBool | UnionWithBool;
+    [property: string]: unknown;
 }
 
 export type UnionWithBool = boolean | string;
diff --git a/base/schema-typescript/test/inputs/schema/camelCase.schema/default/TopLevel.ts b/head/schema-typescript/test/inputs/schema/camelCase.schema/default/TopLevel.ts
index fb0fae0..1c2f56a 100644
--- a/base/schema-typescript/test/inputs/schema/camelCase.schema/default/TopLevel.ts
+++ b/head/schema-typescript/test/inputs/schema/camelCase.schema/default/TopLevel.ts
@@ -10,7 +10,7 @@
 export interface TopLevel {
     myProperty?:     string;
     secondProperty?: string;
-    [property: string]: unknown | string | string;
+    [property: string]: unknown;
 }
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-typescript/test/inputs/schema/class-map-union.schema/default/TopLevel.ts b/head/schema-typescript/test/inputs/schema/class-map-union.schema/default/TopLevel.ts
index c35a2aa..dedacb8 100644
--- a/base/schema-typescript/test/inputs/schema/class-map-union.schema/default/TopLevel.ts
+++ b/head/schema-typescript/test/inputs/schema/class-map-union.schema/default/TopLevel.ts
@@ -14,14 +14,14 @@ export interface TopLevel {
 export interface TopLevelUnion {
     foo?: Foo;
     bar?: Bar;
-    [property: string]: UnionValue | Foo | Bar;
+    [property: string]: UnionValue | Foo | Bar | undefined;
 }
 
 export type Bar = boolean | BarObject | string;
 
 export interface BarObject {
     quux?: number;
-    [property: string]: unknown | number;
+    [property: string]: unknown;
 }
 
 export type Foo = boolean | number | BarObject;
diff --git a/base/schema-typescript/test/inputs/schema/class-with-additional.schema/default/TopLevel.ts b/head/schema-typescript/test/inputs/schema/class-with-additional.schema/default/TopLevel.ts
index 7042dba..15df75c 100644
--- a/base/schema-typescript/test/inputs/schema/class-with-additional.schema/default/TopLevel.ts
+++ b/head/schema-typescript/test/inputs/schema/class-with-additional.schema/default/TopLevel.ts
@@ -13,7 +13,7 @@ export interface TopLevel {
 
 export interface Map {
     foo?: number;
-    [property: string]: boolean | number;
+    [property: string]: boolean | number | undefined;
 }
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-typescript/test/inputs/schema/const-non-string.schema/default/TopLevel.ts b/head/schema-typescript/test/inputs/schema/const-non-string.schema/default/TopLevel.ts
index cdf8c26..471597a 100644
--- a/base/schema-typescript/test/inputs/schema/const-non-string.schema/default/TopLevel.ts
+++ b/head/schema-typescript/test/inputs/schema/const-non-string.schema/default/TopLevel.ts
@@ -13,7 +13,7 @@ export interface TopLevel {
     kind:    Kind;
     ratio:   number;
     version: number;
-    [property: string]: unknown | number | boolean | Kind | number | number;
+    [property: string]: unknown;
 }
 
 export type Kind = "widget";
diff --git a/base/schema-typescript/test/inputs/schema/constructor.schema/default/TopLevel.ts b/head/schema-typescript/test/inputs/schema/constructor.schema/default/TopLevel.ts
index ee3ba81..213c0ae 100644
--- a/base/schema-typescript/test/inputs/schema/constructor.schema/default/TopLevel.ts
+++ b/head/schema-typescript/test/inputs/schema/constructor.schema/default/TopLevel.ts
@@ -9,7 +9,7 @@
 
 export interface TopLevel {
     constructor?: UnionConstructor;
-    [property: string]: unknown | UnionConstructor;
+    [property: string]: unknown;
 }
 
 export type UnionConstructor = object | number;
diff --git a/base/schema-typescript/test/inputs/schema/cut-enum.schema/default/TopLevel.ts b/head/schema-typescript/test/inputs/schema/cut-enum.schema/default/TopLevel.ts
index 7a1dd40..4b340a1 100644
--- a/base/schema-typescript/test/inputs/schema/cut-enum.schema/default/TopLevel.ts
+++ b/head/schema-typescript/test/inputs/schema/cut-enum.schema/default/TopLevel.ts
@@ -9,7 +9,7 @@
 
 export interface TopLevel {
     foo: number;
-    [property: string]: unknown | number;
+    [property: string]: unknown;
 }
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-typescript/test/inputs/schema/date-time-or-string.schema/default/TopLevel.ts b/head/schema-typescript/test/inputs/schema/date-time-or-string.schema/default/TopLevel.ts
index 3cac887..f2f2b10 100644
--- a/base/schema-typescript/test/inputs/schema/date-time-or-string.schema/default/TopLevel.ts
+++ b/head/schema-typescript/test/inputs/schema/date-time-or-string.schema/default/TopLevel.ts
@@ -10,7 +10,7 @@
 export interface TopLevel {
     bar: BarUnion;
     foo: string;
-    [property: string]: unknown | BarUnion | string;
+    [property: string]: unknown;
 }
 
 export type BarUnion = Date | BarEnum;
diff --git a/base/schema-typescript/test/inputs/schema/description-with-double-quotes.schema/default/TopLevel.ts b/head/schema-typescript/test/inputs/schema/description-with-double-quotes.schema/default/TopLevel.ts
index 5aafef8..5fa5b81 100644
--- a/base/schema-typescript/test/inputs/schema/description-with-double-quotes.schema/default/TopLevel.ts
+++ b/head/schema-typescript/test/inputs/schema/description-with-double-quotes.schema/default/TopLevel.ts
@@ -12,7 +12,7 @@
  */
 export interface TopLevel {
     foo?: boolean;
-    [property: string]: unknown | boolean;
+    [property: string]: unknown;
 }
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-typescript/test/inputs/schema/description.schema/default/TopLevel.ts b/head/schema-typescript/test/inputs/schema/description.schema/default/TopLevel.ts
index b9d91c2..239362c 100644
--- a/base/schema-typescript/test/inputs/schema/description.schema/default/TopLevel.ts
+++ b/head/schema-typescript/test/inputs/schema/description.schema/default/TopLevel.ts
@@ -40,7 +40,7 @@ export interface ObjectOrStringObject {
      * This must not get lost
      */
     prop: number;
-    [property: string]: unknown | number;
+    [property: string]: unknown;
 }
 
 /**
diff --git a/base/schema-typescript/test/inputs/schema/enum-large.schema/default/TopLevel.ts b/head/schema-typescript/test/inputs/schema/enum-large.schema/default/TopLevel.ts
index f33915c..12c52ac 100644
--- a/base/schema-typescript/test/inputs/schema/enum-large.schema/default/TopLevel.ts
+++ b/head/schema-typescript/test/inputs/schema/enum-large.schema/default/TopLevel.ts
@@ -10,7 +10,7 @@
 export interface TopLevel {
     callsign: Callsign;
     priority: Priority;
-    [property: string]: unknown | Callsign | Priority;
+    [property: string]: unknown;
 }
 
 export type Callsign = "alpha" | "bravo" | "charlie" | "delta" | "echo" | "foxtrot" | "golf" | "hotel" | "india" | "juliett" | "kilo" | "lima" | "mike" | "november" | "oscar" | "papa" | "quebec" | "romeo" | "sierra" | "tango";
diff --git a/base/schema-typescript/test/inputs/schema/enum-with-values.schema/default/TopLevel.ts b/head/schema-typescript/test/inputs/schema/enum-with-values.schema/default/TopLevel.ts
index df0714e..0d0c447 100644
--- a/base/schema-typescript/test/inputs/schema/enum-with-values.schema/default/TopLevel.ts
+++ b/head/schema-typescript/test/inputs/schema/enum-with-values.schema/default/TopLevel.ts
@@ -9,7 +9,7 @@
 
 export interface TopLevel {
     weekdays: Weekdays;
-    [property: string]: unknown | Weekdays;
+    [property: string]: unknown;
 }
 
 export type Weekdays = "Monday" | "Tuesday" | "Wednesday" | "Thursday" | "Friday" | "Saturday" | "Sunday";
diff --git a/base/schema-typescript/test/inputs/schema/enum.schema/default/TopLevel.ts b/head/schema-typescript/test/inputs/schema/enum.schema/default/TopLevel.ts
index a863c1f..368ac2b 100644
--- a/base/schema-typescript/test/inputs/schema/enum.schema/default/TopLevel.ts
+++ b/head/schema-typescript/test/inputs/schema/enum.schema/default/TopLevel.ts
@@ -13,7 +13,7 @@ export interface TopLevel {
     gve:       Gve;
     lvc?:      Lvc;
     otherArr?: OtherArr[];
-    [property: string]: unknown | Arr[] | string | Gve | Lvc | OtherArr[];
+    [property: string]: unknown;
 }
 
 export type Arr = OtherArr | number;
diff --git a/base/schema-typescript/test/inputs/schema/fractional-bounds.schema/default/TopLevel.ts b/head/schema-typescript/test/inputs/schema/fractional-bounds.schema/default/TopLevel.ts
index 80cbe36..8ea70f3 100644
--- a/base/schema-typescript/test/inputs/schema/fractional-bounds.schema/default/TopLevel.ts
+++ b/head/schema-typescript/test/inputs/schema/fractional-bounds.schema/default/TopLevel.ts
@@ -9,7 +9,7 @@
 
 export interface TopLevel {
     value: number;
-    [property: string]: unknown | number;
+    [property: string]: unknown;
 }
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-typescript/test/inputs/schema/go-schema-pattern-properties.schema/default/TopLevel.ts b/head/schema-typescript/test/inputs/schema/go-schema-pattern-properties.schema/default/TopLevel.ts
index 1a22d87..fc206e1 100644
--- a/base/schema-typescript/test/inputs/schema/go-schema-pattern-properties.schema/default/TopLevel.ts
+++ b/head/schema-typescript/test/inputs/schema/go-schema-pattern-properties.schema/default/TopLevel.ts
@@ -9,7 +9,7 @@
 
 export interface TopLevel {
     map: { [key: string]: number };
-    [property: string]: unknown | { [key: string]: number };
+    [property: string]: unknown;
 }
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-typescript/test/inputs/schema/haskell-enum-forbidden.schema/default/TopLevel.ts b/head/schema-typescript/test/inputs/schema/haskell-enum-forbidden.schema/default/TopLevel.ts
index d421069..e07edf0 100644
--- a/base/schema-typescript/test/inputs/schema/haskell-enum-forbidden.schema/default/TopLevel.ts
+++ b/head/schema-typescript/test/inputs/schema/haskell-enum-forbidden.schema/default/TopLevel.ts
@@ -9,7 +9,7 @@
 
 export interface TopLevel {
     health: Health;
-    [property: string]: unknown | Health;
+    [property: string]: unknown;
 }
 
 export type Health = "ok" | "error";
diff --git a/base/schema-typescript/test/inputs/schema/id-no-address.schema/default/TopLevel.ts b/head/schema-typescript/test/inputs/schema/id-no-address.schema/default/TopLevel.ts
index 00c6590..29bc994 100644
--- a/base/schema-typescript/test/inputs/schema/id-no-address.schema/default/TopLevel.ts
+++ b/head/schema-typescript/test/inputs/schema/id-no-address.schema/default/TopLevel.ts
@@ -9,7 +9,7 @@
 
 export interface TopLevel {
     item: number;
-    [property: string]: unknown | number;
+    [property: string]: unknown;
 }
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-typescript/test/inputs/schema/id-root.schema/default/TopLevel.ts b/head/schema-typescript/test/inputs/schema/id-root.schema/default/TopLevel.ts
index 4703039..444933f 100644
--- a/base/schema-typescript/test/inputs/schema/id-root.schema/default/TopLevel.ts
+++ b/head/schema-typescript/test/inputs/schema/id-root.schema/default/TopLevel.ts
@@ -9,7 +9,7 @@
 
 export interface TopLevel {
     bar: number;
-    [property: string]: unknown | number;
+    [property: string]: unknown;
 }
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-typescript/test/inputs/schema/ie-suffix-singularization.schema/default/TopLevel.ts b/head/schema-typescript/test/inputs/schema/ie-suffix-singularization.schema/default/TopLevel.ts
index a14bd43..9d2df90 100644
--- a/base/schema-typescript/test/inputs/schema/ie-suffix-singularization.schema/default/TopLevel.ts
+++ b/head/schema-typescript/test/inputs/schema/ie-suffix-singularization.schema/default/TopLevel.ts
@@ -9,13 +9,13 @@
 
 export interface TopLevel {
     cookies: Cookie[];
-    [property: string]: unknown | Cookie[];
+    [property: string]: unknown;
 }
 
 export interface Cookie {
     name:  string;
     value: string;
-    [property: string]: unknown | string | string;
+    [property: string]: unknown;
 }
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-typescript/test/inputs/schema/implicit-all-of.schema/default/TopLevel.ts b/head/schema-typescript/test/inputs/schema/implicit-all-of.schema/default/TopLevel.ts
index d151566..9f7b670 100644
--- a/base/schema-typescript/test/inputs/schema/implicit-all-of.schema/default/TopLevel.ts
+++ b/head/schema-typescript/test/inputs/schema/implicit-all-of.schema/default/TopLevel.ts
@@ -11,7 +11,7 @@ export interface TopLevel {
     foo:  number;
     bar:  boolean;
     quux: string;
-    [property: string]: unknown | number | boolean | string;
+    [property: string]: unknown;
 }
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-typescript/test/inputs/schema/implicit-class-array-union.schema/default/TopLevel.ts b/head/schema-typescript/test/inputs/schema/implicit-class-array-union.schema/default/TopLevel.ts
index b864dcb..887b34d 100644
--- a/base/schema-typescript/test/inputs/schema/implicit-class-array-union.schema/default/TopLevel.ts
+++ b/head/schema-typescript/test/inputs/schema/implicit-class-array-union.schema/default/TopLevel.ts
@@ -9,14 +9,14 @@
 
 export interface TopLevel {
     foo: FooUnion;
-    [property: string]: unknown | FooUnion;
+    [property: string]: unknown;
 }
 
 export type FooUnion = number[] | boolean | number | number | null | FooObject | string;
 
 export interface FooObject {
     bar: number;
-    [property: string]: unknown | number;
+    [property: string]: unknown;
 }
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-typescript/test/inputs/schema/implicit-one-of.schema/default/TopLevel.ts b/head/schema-typescript/test/inputs/schema/implicit-one-of.schema/default/TopLevel.ts
index 51da732..8345516 100644
--- a/base/schema-typescript/test/inputs/schema/implicit-one-of.schema/default/TopLevel.ts
+++ b/head/schema-typescript/test/inputs/schema/implicit-one-of.schema/default/TopLevel.ts
@@ -11,7 +11,7 @@ export interface TopLevel {
     foo:   number;
     bar?:  boolean;
     quux?: string;
-    [property: string]: unknown | number | boolean | string;
+    [property: string]: unknown;
 }
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-typescript/test/inputs/schema/integer-float-union.schema/default/TopLevel.ts b/head/schema-typescript/test/inputs/schema/integer-float-union.schema/default/TopLevel.ts
index 525c1a4..30bbefd 100644
--- a/base/schema-typescript/test/inputs/schema/integer-float-union.schema/default/TopLevel.ts
+++ b/head/schema-typescript/test/inputs/schema/integer-float-union.schema/default/TopLevel.ts
@@ -9,7 +9,7 @@
 
 export interface TopLevel {
     number: number;
-    [property: string]: unknown | number;
+    [property: string]: unknown;
 }
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-typescript/test/inputs/schema/integer-string.schema/default/TopLevel.ts b/head/schema-typescript/test/inputs/schema/integer-string.schema/default/TopLevel.ts
index 7f1ff74..f9a36b5 100644
--- a/base/schema-typescript/test/inputs/schema/integer-string.schema/default/TopLevel.ts
+++ b/head/schema-typescript/test/inputs/schema/integer-string.schema/default/TopLevel.ts
@@ -15,7 +15,7 @@ export interface TopLevel {
     optional?:           string;
     unionWithInt:        UnionWithInt;
     unionWithIntAndEnum: UnionWithInt;
-    [property: string]: unknown | (null | string)[] | string[] | null | string | string | string | UnionWithInt | UnionWithInt;
+    [property: string]: unknown;
 }
 
 export type UnionWithInt = number | string;
diff --git a/base/schema-typescript/test/inputs/schema/integer-type.schema/default/TopLevel.ts b/head/schema-typescript/test/inputs/schema/integer-type.schema/default/TopLevel.ts
index f59a698..54caffa 100644
--- a/base/schema-typescript/test/inputs/schema/integer-type.schema/default/TopLevel.ts
+++ b/head/schema-typescript/test/inputs/schema/integer-type.schema/default/TopLevel.ts
@@ -17,7 +17,7 @@ export interface TopLevel {
     small_negative: number;
     small_positive: number;
     unbounded:      number;
-    [property: string]: unknown | number | number | number | number | number | number | number | number | number;
+    [property: string]: unknown;
 }
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-typescript/test/inputs/schema/intersection.schema/default/TopLevel.ts b/head/schema-typescript/test/inputs/schema/intersection.schema/default/TopLevel.ts
index e141117..29a519e 100644
--- a/base/schema-typescript/test/inputs/schema/intersection.schema/default/TopLevel.ts
+++ b/head/schema-typescript/test/inputs/schema/intersection.schema/default/TopLevel.ts
@@ -14,7 +14,7 @@ export interface TopLevel {
 export interface Intersection {
     foo:  number;
     bar?: string;
-    [property: string]: unknown | number | string;
+    [property: string]: unknown;
 }
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-typescript/test/inputs/schema/keyword-enum.schema/default/TopLevel.ts b/head/schema-typescript/test/inputs/schema/keyword-enum.schema/default/TopLevel.ts
index 2dd0706..78ea06e 100644
--- a/base/schema-typescript/test/inputs/schema/keyword-enum.schema/default/TopLevel.ts
+++ b/head/schema-typescript/test/inputs/schema/keyword-enum.schema/default/TopLevel.ts
@@ -9,7 +9,7 @@
 
 export interface TopLevel {
     enum?: Enum;
-    [property: string]: unknown | Enum;
+    [property: string]: unknown;
 }
 
 export type Enum = "_" | "_Bool" | "_Complex" | "_Imaginery" | "abstract" | "alignas" | "alignof" | "and" | "and_eq" | "any" | "Any" | "array" | "as" | "asm" | "assert" | "associatedtype" | "associativity" | "async" | "atomic" | "atomic_cancel" | "atomic_commit" | "atomic_noexcept" | "auto" | "await" | "base" | "bitand" | "bitor" | "BOOL" | "bool" | "boolean" | "break" | "bycopy" | "byref" | "byte" | "case" | "catch" | "chan" | "char" | "char16_t" | "char32_t" | "checked" | "class" | "Class" | "co_await" | "co_return" | "co_yield" | "compl" | "concept" | "console" | "const" | "const_cast" | "constexpr" | "constructor" | "continue" | "convenience" | "convert" | "converter" | "date" | "date_parse_handling" | "debugger" | "decimal" | "declare" | "decltype" | "decode_string" | "def" | "default" | "defer" | "deinit" | "del" | "delegate" | "delete" | "dict" | "dictionary" | "didSet" | "do" | "double" | "dynamic" | "dynamic_cast" | "elif" | "else" | "encode_quick_type" | "enum" | "event" | "except" | "exception" | "explicit" | "export" | "exposing" | "extends" | "extension" | "extern" | "fallthrough" | "false" | "False" | "fileprivate" | "final" | "finally" | "fixed" | "float" | "for" | "foreach" | "friend" | "from" | "from_json" | "func" | "function" | "get" | "global" | "go" | "goto" | "guard" | "hasOwnProperty" | "id" | "if" | "IMP" | "implements" | "implicit" | "import" | "in" | "indirect" | "infix" | "init" | "inline" | "inout" | "instanceof" | "int" | "interface" | "internal" | "iterable" | "is" | "jdec" | "jenc" | "jpipe" | "json" | "json_converter" | "json_serializer" | "json_token" | "json_writer" | "lambda" | "lazy" | "left" | "let" | "list" | "lock" | "long" | "map" | "metadata_property_handling" | "module" | "mutable" | "mutating" | "namespace" | "native" | "new" | "newtonsoft" | "nil" | "NO" | "noexcept" | "nonatomic" | "none" | "None" | "nonlocal" | "nonmutating" | "not" | "not_eq" | "NSString" | "NULL" | "null" | "nullptr" | "number" | "object" | "of" | "oneway" | "open" | "operator" | "optional" | "or" | "or_eq" | "out" | "override" | "package" | "params" | "pass" | "port" | "postfix" | "precedence" | "prefix" | "print" | "printf" | "private" | "protected" | "Protocol" | "protocol" | "public" | "quicktype" | "raise" | "range" | "readonly" | "ref" | "register" | "reinterpret_cast" | "repeat" | "require" | "required" | "requires" | "restrict" | "retain" | "rethrows" | "return" | "right" | "sbyte" | "sealed" | "SEL" | "select" | "Self" | "self" | "serialize" | "set" | "short" | "signed" | "sizeof" | "stackalloc" | "static" | "static_assert" | "static_cast" | "strictfp" | "string" | "struct" | "subscript" | "super" | "switch" | "symbol" | "synchronized" | "system" | "template" | "then" | "this" | "thread_local" | "throw" | "throws" | "to_json" | "top_level" | "transient" | "True" | "true" | "try" | "Type" | "type" | "typealias" | "typedef" | "typeid" | "typename" | "typeof" | "uint" | "ulong" | "unchecked" | "undefined" | "union" | "unowned" | "unsafe" | "unsigned" | "ushort" | "using" | "var" | "virtual" | "void" | "volatile" | "wchar_t" | "weak" | "where" | "while" | "willSet" | "with" | "xor" | "xor_eq" | "YES" | "yield" | "dummy";
diff --git a/base/schema-typescript/test/inputs/schema/keyword-unions.schema/default/TopLevel.ts b/head/schema-typescript/test/inputs/schema/keyword-unions.schema/default/TopLevel.ts
index eb29aa2..94337c1 100644
--- a/base/schema-typescript/test/inputs/schema/keyword-unions.schema/default/TopLevel.ts
+++ b/head/schema-typescript/test/inputs/schema/keyword-unions.schema/default/TopLevel.ts
@@ -285,7 +285,7 @@ export interface TopLevel {
     xor_eq?:                     UnionXorEq;
     YES?:                        UnionYES;
     yield?:                      UnionYield;
-    [property: string]: unknown | UnionUnion | UnionBool | UnionComplex | UnionImaginery | UnionAbstract | UnionAlignas | UnionAlignof | UnionAnd | UnionAndEq | UnionAnyUnion | UnionAny | UnionArray | UnionAs | UnionASM | UnionAssert | UnionAssociatedtype | UnionAssociativity | UnionAsync | UnionAtomic | UnionAtomicCancel | UnionAtomicCommit | UnionAtomicNoexcept | UnionAuto | UnionAwait | UnionBase | UnionBitand | UnionBitor | UnionBOOL | UnionBoolUnion | UnionBoolean | UnionBreak | UnionBycopy | UnionByref | UnionByte | UnionCase | UnionCatch | UnionChan | UnionChar | UnionChar16T | UnionChar32T | UnionChecked | UnionClassUnion | UnionClass | UnionCoAwait | UnionCoReturn | UnionCoYield | UnionCompl | UnionConcept | UnionConsole | UnionConst | UnionConstCast | UnionConstexpr | UnionConstructor | UnionContinue | UnionConvenience | UnionConvert | UnionConverter | UnionDate | UnionDateParseHandling | UnionDebugger | UnionDecimal | UnionDeclare | UnionDecltype | UnionDecodeString | UnionDef | UnionDefault | UnionDefer | UnionDeinit | UnionDel | UnionDelegate | UnionDelete | UnionDict | UnionDictionary | UnionDidSet | UnionDo | UnionDouble | number | UnionDynamic | UnionDynamicCast | UnionElif | UnionElse | UnionEncodeQuickType | UnionEnum | UnionEvent | UnionExcept | UnionException | UnionExplicit | UnionExport | UnionExposing | UnionExtends | UnionExtension | UnionExtern | UnionFallthrough | UnionFalseUnion | UnionFalse | UnionFileprivate | UnionFinal | UnionFinally | UnionFixed | UnionFloat | UnionFor | UnionForeach | UnionFriend | UnionFrom | UnionFromJSON | UnionFunc | UnionFunction | UnionGet | UnionGlobal | UnionGo | UnionGoto | UnionGuard | UnionHasOwnProperty | UnionID | UnionIf | UnionIMP | UnionImplements | UnionImplicit | UnionImport | UnionIn | UnionIndirect | UnionInfix | UnionInit | UnionInline | UnionInout | UnionInstanceof | UnionInt | UnionInterface | UnionInternal | UnionIs | UnionIterable | UnionJdec | UnionJenc | UnionJpipe | UnionJSON | UnionJSONConverter | UnionJSONSerializer | UnionJSONToken | UnionJSONWriter | UnionLambda | UnionLazy | UnionLeft | UnionLet | UnionList | UnionLock | UnionLong | UnionMap | UnionMetadataPropertyHandling | UnionModule | UnionMutable | UnionMutating | UnionNamespace | UnionNative | UnionNew | UnionNewtonsoft | UnionNil | UnionNO | UnionNoexcept | UnionNonatomic | UnionNoneUnion | UnionNone | UnionNonlocal | UnionNonmutating | UnionNot | UnionNotEq | UnionNSString | UnionNULL | UnionNull | UnionNullptr | UnionNumber | UnionObject | UnionOf | UnionOneway | UnionOpen | UnionOperator | UnionOptional | UnionOr | UnionOrEq | UnionOut | UnionOverride | UnionPackage | UnionParams | UnionPass | UnionPort | UnionPostfix | UnionPrecedence | UnionPrefix | UnionPrint | UnionPrintf | UnionPrivate | UnionProtected | UnionProtocol | UnionProtocolUnion | UnionPublic | UnionQuicktype | UnionRaise | UnionRange | UnionReadonly | UnionRef | UnionRegister | UnionReinterpretCast | UnionRepeat | UnionRequire | UnionRequired | UnionRequires | UnionRestrict | UnionRetain | UnionRethrows | UnionReturn | UnionRight | UnionSbyte | UnionSealed | UnionSEL | UnionSelect | UnionSelf | UnionSelfUnion | UnionSerialize | UnionSet | UnionShort | UnionSigned | UnionSizeof | UnionStackalloc | UnionStatic | UnionStaticAssert | UnionStaticCast | UnionStrictfp | UnionString | UnionStruct | UnionSubscript | UnionSuper | UnionSwitch | UnionSymbol | UnionSynchronized | UnionSystem | UnionTemplate | UnionThen | UnionThis | UnionThreadLocal | UnionThrow | UnionThrows | UnionToJSON | UnionTopLevel | UnionTransient | UnionTrue | UnionTrueUnion | UnionTry | UnionType | UnionTypeUnion | UnionTypealias | UnionTypedef | UnionTypeid | UnionTypename | UnionTypeof | UnionUint | UnionUlong | UnionUnchecked | UnionUndefined | UnionUnionUnion | UnionUnowned | UnionUnsafe | UnionUnsigned | UnionUshort | UnionUsing | UnionVar | UnionVirtual | UnionVoid | UnionVolatile | UnionWcharT | UnionWeak | UnionWhere | UnionWhile | UnionWillSet | UnionWith | UnionXor | UnionXorEq | UnionYES | UnionYield;
+    [property: string]: unknown;
 }
 
 export type UnionAny = object | number;
diff --git a/base/schema-typescript/test/inputs/schema/list.schema/default/TopLevel.ts b/head/schema-typescript/test/inputs/schema/list.schema/default/TopLevel.ts
index 9ca2844..a3e9b2b 100644
--- a/base/schema-typescript/test/inputs/schema/list.schema/default/TopLevel.ts
+++ b/head/schema-typescript/test/inputs/schema/list.schema/default/TopLevel.ts
@@ -12,7 +12,7 @@
  */
 export interface TopLevel {
     next?: TopLevel;
-    [property: string]: unknown | TopLevel;
+    [property: string]: unknown;
 }
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-typescript/test/inputs/schema/min-max-items.schema/default/TopLevel.ts b/head/schema-typescript/test/inputs/schema/min-max-items.schema/default/TopLevel.ts
index 70e7706..a88d16b 100644
--- a/base/schema-typescript/test/inputs/schema/min-max-items.schema/default/TopLevel.ts
+++ b/head/schema-typescript/test/inputs/schema/min-max-items.schema/default/TopLevel.ts
@@ -13,7 +13,7 @@ export interface TopLevel {
     minOnly:    [string, string, ...string[]];
     plain:      string[];
     unionItems: [UnionItem, UnionItem, ...UnionItem[]];
-    [property: string]: unknown | number[] | [number, ...number[]] | [string, string, ...string[]] | string[] | [UnionItem, UnionItem, ...UnionItem[]];
+    [property: string]: unknown;
 }
 
 export type UnionItem = number | string;
diff --git a/base/schema-typescript/test/inputs/schema/minmax-integer.schema/default/TopLevel.ts b/head/schema-typescript/test/inputs/schema/minmax-integer.schema/default/TopLevel.ts
index d87b58a..4d15bf4 100644
--- a/base/schema-typescript/test/inputs/schema/minmax-integer.schema/default/TopLevel.ts
+++ b/head/schema-typescript/test/inputs/schema/minmax-integer.schema/default/TopLevel.ts
@@ -16,7 +16,7 @@ export interface TopLevel {
     minMaxIntersection: number;
     minMaxUnion:        number;
     union:              number;
-    [property: string]: unknown | number | number | number | number | number | number | number | number;
+    [property: string]: unknown;
 }
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-typescript/test/inputs/schema/minmax.schema/default/TopLevel.ts b/head/schema-typescript/test/inputs/schema/minmax.schema/default/TopLevel.ts
index e322abd..74eb204 100644
--- a/base/schema-typescript/test/inputs/schema/minmax.schema/default/TopLevel.ts
+++ b/head/schema-typescript/test/inputs/schema/minmax.schema/default/TopLevel.ts
@@ -16,7 +16,7 @@ export interface TopLevel {
     minMaxIntersection: number;
     minMaxUnion:        number;
     union:              number;
-    [property: string]: unknown | number | number | number | number | number | number | number | number;
+    [property: string]: unknown;
 }
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-typescript/test/inputs/schema/minmaxlength.schema/default/TopLevel.ts b/head/schema-typescript/test/inputs/schema/minmaxlength.schema/default/TopLevel.ts
index 099bb1d..ce21e9c 100644
--- a/base/schema-typescript/test/inputs/schema/minmaxlength.schema/default/TopLevel.ts
+++ b/head/schema-typescript/test/inputs/schema/minmaxlength.schema/default/TopLevel.ts
@@ -16,7 +16,7 @@ export interface TopLevel {
     minmaxlength:       string;
     minMaxUnion:        string;
     union:              string;
-    [property: string]: unknown | string | InUnion | string | string | string | string | string | string;
+    [property: string]: unknown;
 }
 
 export type InUnion = number | string;
diff --git a/head/schema-typescript/test/inputs/schema/mixed-additional-properties.schema/default/TopLevel.ts b/head/schema-typescript/test/inputs/schema/mixed-additional-properties.schema/default/TopLevel.ts
new file mode 100644
index 0000000..edf8a6d
--- /dev/null
+++ b/head/schema-typescript/test/inputs/schema/mixed-additional-properties.schema/default/TopLevel.ts
@@ -0,0 +1,205 @@
+// To parse this data:
+//
+//   import { Convert, TopLevel } from "./TopLevel";
+//
+//   const topLevel = Convert.toTopLevel(json);
+//
+// These functions will throw an error if the JSON doesn't
+// match the expected interface, even if the JSON is valid.
+
+export interface TopLevel {
+    id?: string;
+    [property: string]: number | string | undefined;
+}
+
+// Converts JSON strings to/from your types
+// and asserts the results of JSON.parse at runtime
+export class Convert {
+    public static toTopLevel(json: string): TopLevel {
+        return cast(JSON.parse(json), r("TopLevel"));
+    }
+
+    public static topLevelToJson(value: TopLevel): string {
+        return JSON.stringify(uncast(value, r("TopLevel")), null, 2);
+    }
+}
+
+function invalidValue(typ: any, val: any, key: any, parent: any = ''): never {
+    const prettyTyp = prettyTypeName(typ);
+    const parentText = parent ? ` on ${parent}` : '';
+    const keyText = key ? ` for key "${key}"` : '';
+    throw Error(`Invalid value${keyText}${parentText}. Expected ${prettyTyp} but got ${JSON.stringify(val)}`);
+}
+
+function prettyTypeName(typ: any): string {
+    if (Array.isArray(typ)) {
+        if (typ.length === 2 && typ[0] === undefined) {
+            return `an optional ${prettyTypeName(typ[1])}`;
+        } else {
+            return `one of [${typ.map(a => { return prettyTypeName(a); }).join(", ")}]`;
+        }
+    } else if (typeof typ === "object" && typ.literal !== undefined) {
+        return typ.literal;
+    } else {
+        return typeof typ;
+    }
+}
+
+function jsonToJSProps(typ: any): any {
+    if (typ.jsonToJS === undefined) {
+        const map: any = {};
+        typ.props.forEach((p: any) => map[p.json] = { key: p.js, typ: p.typ });
+        typ.jsonToJS = map;
+    }
+    return typ.jsonToJS;
+}
+
+function jsToJSONProps(typ: any): any {
+    if (typ.jsToJSON === undefined) {
+        const map: any = {};
+        typ.props.forEach((p: any) => map[p.js] = { key: p.json, typ: p.typ });
+        typ.jsToJSON = map;
+    }
+    return typ.jsToJSON;
+}
+
+function transform(val: any, typ: any, getProps: any, key: any = '', parent: any = ''): any {
+    function transformPrimitive(typ: string, val: any): any {
+        if (typeof typ === typeof val) return val;
+        return invalidValue(typ, val, key, parent);
+    }
+
+    function transformUnion(typs: any[], val: any): any {
+        // val must validate against one typ in typs
+        const l = typs.length;
+        for (let i = 0; i < l; i++) {
+            const typ = typs[i];
+            try {
+                return transform(val, typ, getProps);
+            } catch (_) {}
+        }
+        return invalidValue(typs, val, key, parent);
+    }
+
+    function transformEnum(cases: string[], val: any): any {
+        if (cases.indexOf(val) !== -1) return val;
+        return invalidValue(cases.map(a => { return l(a); }), val, key, parent);
+    }
+
+    function transformArray(typ: any, val: any): any {
+        // val must be an array with no invalid elements
+        if (!Array.isArray(val)) return invalidValue(l("array"), val, key, parent);
+
+        return val.map(el => transform(el, typ, getProps));
+    }
+
+    function transformDate(val: any): any {
+        if (val === null) {
+            return null;
+        }
+        const d = new Date(val);
+        if (isNaN(d.valueOf())) {
+            return invalidValue(l("Date"), val, key, parent);
+        }
+        return d;
+    }
+
+    function transformObject(props: { [k: string]: any }, additional: any, val: any): any {
+        if (val === null || typeof val !== "object" || Array.isArray(val)) {
+            return invalidValue(l(ref || "object"), val, key, parent);
+        }
+        const result: any = {};
+        Object.getOwnPropertyNames(props).forEach(key => {
+            const prop = props[key];
+            const v = Object.prototype.hasOwnProperty.call(val, key) ? val[key] : undefined;
+            result[prop.key] = transform(v, prop.typ, getProps, key, ref);
+        });
+        Object.getOwnPropertyNames(val).forEach(key => {
+            if (!Object.prototype.hasOwnProperty.call(props, key)) {
+                result[key] = transform(val[key], additional, getProps, key, ref);
+            }
+        });
+        return result;
+    }
+
+    if (typ === "any") return val;
+    if (typ === null) {
+        if (val === null) return val;
+        return invalidValue(typ, val, key, parent);
+    }
+    if (typ === false) return invalidValue(typ, val, key, parent);
+    let ref: any = undefined;
+    while (typeof typ === "object" && typ.ref !== undefined) {
+        ref = typ.ref;
+        typ = typeMap[typ.ref];
+    }
+    if (Array.isArray(typ)) return transformEnum(typ, val);
+    if (typeof typ === "object") {
+        return typ.hasOwnProperty("pattern")      ? typeof val === "string" && new RegExp(typ.pattern).test(val) ? val : invalidValue(typ, val, key, parent)
+            : typ.hasOwnProperty("string")       ? typeof val === "string" && (typ.min === undefined || Array.from(val).length >= typ.min) && (typ.max === undefined || Array.from(val).length <= typ.max) ? transform(val, typ.string, getProps, key, parent) : invalidValue(typ, val, key, parent)
+            : typ.hasOwnProperty("number")       ? typeof val === "number" && (typ.min === undefined || val >= typ.min) && (typ.max === undefined || val <= typ.max) ? transform(val, typ.number, getProps, key, parent) : invalidValue(typ, val, key, parent)
+            : typ.hasOwnProperty("integer")      ? typeof val === "number" && val % 1 === 0 ? val : invalidValue(l("integer"), val, key, parent)
+            : typ.hasOwnProperty("unionMembers") ? transformUnion(typ.unionMembers, val)
+            : typ.hasOwnProperty("arrayItems")    ? transformArray(typ.arrayItems, val)
+            : typ.hasOwnProperty("props")         ? transformObject(getProps(typ), typ.additional, val)
+            : invalidValue(typ, val, key, parent);
+    }
+    // Numbers can be parsed by Date but shouldn't be.
+    if (typ === Date && (typeof val === "string" || val instanceof Date)) return transformDate(val);
+    return transformPrimitive(typ, val);
+}
+
+function cast<T>(val: any, typ: any): T {
+    return transform(val, typ, jsonToJSProps);
+}
+
+function uncast<T>(val: T, typ: any): any {
+    return transform(val, typ, jsToJSONProps);
+}
+
+function l(typ: any) {
+    return { literal: typ };
+}
+
+function a(typ: any) {
+    return { arrayItems: typ };
+}
+
+function i(typ: any) {
+    return { integer: typ };
+}
+
+function p(pattern: any) {
+    return { pattern };
+}
+
+function s(typ: any, min: any, max: any) {
+    return { string: typ, min, max };
+}
+
+function n(typ: any, min: any, max: any) {
+    return { number: typ, min, max };
+}
+
+function u(...typs: any[]) {
+    return { unionMembers: typs };
+}
+
+function o(props: any[], additional: any) {
+    return { props, additional };
+}
+
+function m(additional: any) {
+    const props: any[] = [];
+    return { props, additional };
+}
+
+function r(name: string) {
+    return { ref: name };
+}
+
+const typeMap: any = {
+    "TopLevel": o([
+        { json: "id", js: "id", typ: u(undefined, "") },
+    ], 3.14),
+};
diff --git a/base/schema-typescript/test/inputs/schema/multi-type-enum.schema/default/TopLevel.ts b/head/schema-typescript/test/inputs/schema/multi-type-enum.schema/default/TopLevel.ts
index f35c058..2ac8eea 100644
--- a/base/schema-typescript/test/inputs/schema/multi-type-enum.schema/default/TopLevel.ts
+++ b/head/schema-typescript/test/inputs/schema/multi-type-enum.schema/default/TopLevel.ts
@@ -9,7 +9,7 @@
 
 export interface TopLevel {
     foo: FooUnion;
-    [property: string]: unknown | FooUnion;
+    [property: string]: unknown;
 }
 
 export type FooUnion = boolean | number | FooEnum;
diff --git a/base/schema-typescript/test/inputs/schema/mutually-recursive.schema/default/TopLevel.ts b/head/schema-typescript/test/inputs/schema/mutually-recursive.schema/default/TopLevel.ts
index 0023b09..332f39c 100644
--- a/base/schema-typescript/test/inputs/schema/mutually-recursive.schema/default/TopLevel.ts
+++ b/head/schema-typescript/test/inputs/schema/mutually-recursive.schema/default/TopLevel.ts
@@ -11,12 +11,12 @@ export type Foo = TopLevel[] | TopLevel;
 
 export interface Bar {
     foo: Foo;
-    [property: string]: unknown | Foo;
+    [property: string]: unknown;
 }
 
 export interface TopLevel {
     bar?: Bar;
-    [property: string]: unknown | Bar;
+    [property: string]: unknown;
 }
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-typescript/test/inputs/schema/nested-intersection-union.schema/default/TopLevel.ts b/head/schema-typescript/test/inputs/schema/nested-intersection-union.schema/default/TopLevel.ts
index 9d5b3a1..8501247 100644
--- a/base/schema-typescript/test/inputs/schema/nested-intersection-union.schema/default/TopLevel.ts
+++ b/head/schema-typescript/test/inputs/schema/nested-intersection-union.schema/default/TopLevel.ts
@@ -16,7 +16,7 @@ export interface Item {
     id?:      string;
     output?:  string;
     summary?: string;
-    [property: string]: unknown | string | string | string | string;
+    [property: string]: unknown;
 }
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-typescript/test/inputs/schema/non-standard-ref.schema/default/TopLevel.ts b/head/schema-typescript/test/inputs/schema/non-standard-ref.schema/default/TopLevel.ts
index 162d815..9dd48da 100644
--- a/base/schema-typescript/test/inputs/schema/non-standard-ref.schema/default/TopLevel.ts
+++ b/head/schema-typescript/test/inputs/schema/non-standard-ref.schema/default/TopLevel.ts
@@ -11,7 +11,7 @@ export interface TopLevel {
     bar:  number;
     foo:  number;
     quux: boolean;
-    [property: string]: unknown | number | number | boolean;
+    [property: string]: unknown;
 }
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-typescript/test/inputs/schema/nullable-optional-one-of.schema/default/TopLevel.ts b/head/schema-typescript/test/inputs/schema/nullable-optional-one-of.schema/default/TopLevel.ts
index 7eaf17d..d59ca93 100644
--- a/base/schema-typescript/test/inputs/schema/nullable-optional-one-of.schema/default/TopLevel.ts
+++ b/head/schema-typescript/test/inputs/schema/nullable-optional-one-of.schema/default/TopLevel.ts
@@ -10,7 +10,7 @@
 export interface TopLevel {
     b?:   null | string;
     kind: Kind;
-    [property: string]: unknown | null | string | Kind;
+    [property: string]: unknown;
 }
 
 export type Kind = "one" | "two";
diff --git a/base/schema-typescript/test/inputs/schema/object-type-required.schema/default/TopLevel.ts b/head/schema-typescript/test/inputs/schema/object-type-required.schema/default/TopLevel.ts
index bc6571a..8a93edc 100644
--- a/base/schema-typescript/test/inputs/schema/object-type-required.schema/default/TopLevel.ts
+++ b/head/schema-typescript/test/inputs/schema/object-type-required.schema/default/TopLevel.ts
@@ -11,7 +11,7 @@ export interface TopLevel {
     empty: object;
     foo:   string;
     bar?:  string;
-    [property: string]: unknown | object | string | string;
+    [property: string]: unknown;
 }
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-typescript/test/inputs/schema/optional-constraints.schema/default/TopLevel.ts b/head/schema-typescript/test/inputs/schema/optional-constraints.schema/default/TopLevel.ts
index 1d5b5b5..8cf1679 100644
--- a/base/schema-typescript/test/inputs/schema/optional-constraints.schema/default/TopLevel.ts
+++ b/head/schema-typescript/test/inputs/schema/optional-constraints.schema/default/TopLevel.ts
@@ -13,7 +13,7 @@ export interface TopLevel {
     optPattern?: string;
     optString?:  string;
     reqZeroMin:  number;
-    [property: string]: unknown | number | number | string | string | number;
+    [property: string]: unknown;
 }
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-typescript/test/inputs/schema/pattern.schema/default/TopLevel.ts b/head/schema-typescript/test/inputs/schema/pattern.schema/default/TopLevel.ts
index 4e30028..665a7fc 100644
--- a/base/schema-typescript/test/inputs/schema/pattern.schema/default/TopLevel.ts
+++ b/head/schema-typescript/test/inputs/schema/pattern.schema/default/TopLevel.ts
@@ -11,7 +11,7 @@ export interface TopLevel {
     pattern1: string;
     pattern2: string;
     union:    string;
-    [property: string]: unknown | string | string | string;
+    [property: string]: unknown;
 }
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-typescript/test/inputs/schema/postman-collection.schema/default/TopLevel.ts b/head/schema-typescript/test/inputs/schema/postman-collection.schema/default/TopLevel.ts
index 2deba04..34dfb4e 100644
--- a/base/schema-typescript/test/inputs/schema/postman-collection.schema/default/TopLevel.ts
+++ b/head/schema-typescript/test/inputs/schema/postman-collection.schema/default/TopLevel.ts
@@ -14,12 +14,12 @@ export interface TopLevel {
     item?:     TopLevel[];
     name?:     string;
     response?: Response[];
-    [property: string]: unknown | TopLevel[] | string | Response[];
+    [property: string]: unknown;
 }
 
 export interface Response {
     body?: string;
-    [property: string]: unknown | string;
+    [property: string]: unknown;
 }
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-typescript/test/inputs/schema/prefix-items.schema/default/TopLevel.ts b/head/schema-typescript/test/inputs/schema/prefix-items.schema/default/TopLevel.ts
index a39d9af..ee2e5bf 100644
--- a/base/schema-typescript/test/inputs/schema/prefix-items.schema/default/TopLevel.ts
+++ b/head/schema-typescript/test/inputs/schema/prefix-items.schema/default/TopLevel.ts
@@ -10,7 +10,7 @@
 export interface TopLevel {
     open:  Open[];
     tuple: Open[];
-    [property: string]: unknown | Open[] | Open[];
+    [property: string]: unknown;
 }
 
 export type Open = boolean | number;
diff --git a/base/schema-typescript/test/inputs/schema/recursive-union-flattening.schema/default/TopLevel.ts b/head/schema-typescript/test/inputs/schema/recursive-union-flattening.schema/default/TopLevel.ts
index 1d4abd2..3fc7608 100644
--- a/base/schema-typescript/test/inputs/schema/recursive-union-flattening.schema/default/TopLevel.ts
+++ b/head/schema-typescript/test/inputs/schema/recursive-union-flattening.schema/default/TopLevel.ts
@@ -13,12 +13,12 @@ export type TopLevelElement = unknown[] | boolean | number | null | TopLevelObje
 
 export interface TopLevelObject {
     x?: X;
-    [property: string]: unknown | X;
+    [property: string]: unknown;
 }
 
 export interface XObject {
     x?: X;
-    [property: string]: unknown | X;
+    [property: string]: unknown;
 }
 
 export type XElement = unknown[] | boolean | number | null | XObject | string;
diff --git a/base/schema-typescript/test/inputs/schema/ref-id-files.schema/default/TopLevel.ts b/head/schema-typescript/test/inputs/schema/ref-id-files.schema/default/TopLevel.ts
index 7a1dd40..4b340a1 100644
--- a/base/schema-typescript/test/inputs/schema/ref-id-files.schema/default/TopLevel.ts
+++ b/head/schema-typescript/test/inputs/schema/ref-id-files.schema/default/TopLevel.ts
@@ -9,7 +9,7 @@
 
 export interface TopLevel {
     foo: number;
-    [property: string]: unknown | number;
+    [property: string]: unknown;
 }
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-typescript/test/inputs/schema/ref-remote.schema/default/TopLevel.ts b/head/schema-typescript/test/inputs/schema/ref-remote.schema/default/TopLevel.ts
index 9ca2844..a3e9b2b 100644
--- a/base/schema-typescript/test/inputs/schema/ref-remote.schema/default/TopLevel.ts
+++ b/head/schema-typescript/test/inputs/schema/ref-remote.schema/default/TopLevel.ts
@@ -12,7 +12,7 @@
  */
 export interface TopLevel {
     next?: TopLevel;
-    [property: string]: unknown | TopLevel;
+    [property: string]: unknown;
 }
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-typescript/test/inputs/schema/renaming-bug.schema/default/TopLevel.ts b/head/schema-typescript/test/inputs/schema/renaming-bug.schema/default/TopLevel.ts
index a588b63..f2841a1 100644
--- a/base/schema-typescript/test/inputs/schema/renaming-bug.schema/default/TopLevel.ts
+++ b/head/schema-typescript/test/inputs/schema/renaming-bug.schema/default/TopLevel.ts
@@ -11,60 +11,60 @@ export interface TopLevel {
     version?:  string;
     fruits?:   Fruit[];
     vehicles?: Vehicle[];
-    [property: string]: unknown | string | Fruit[] | Vehicle[];
+    [property: string]: unknown;
 }
 
 export interface Fruit {
     apple?:   boolean;
     berries?: Berry[];
     orange?:  boolean;
-    [property: string]: unknown | boolean | Berry[] | boolean;
+    [property: string]: unknown;
 }
 
 export interface Berry {
     color?:  Color;
     name?:   string;
     shapes?: Shape[];
-    [property: string]: unknown | Color | string | Shape[];
+    [property: string]: unknown;
 }
 
 export interface Color {
     rgb?: number;
-    [property: string]: unknown | number;
+    [property: string]: unknown;
 }
 
 export interface Shape {
     geometry?: Geometry;
     history?:  History;
-    [property: string]: unknown | Geometry | History;
+    [property: string]: unknown;
 }
 
 export interface Geometry {
     rectShape?:     RectShape;
     circularShape?: CircularShape[];
-    [property: string]: unknown | RectShape | CircularShape[];
+    [property: string]: unknown;
 }
 
 export interface CircularShape {
     shapeName?: string;
-    [property: string]: unknown | string;
+    [property: string]: unknown;
 }
 
 export interface RectShape {
     parts?: Part[];
-    [property: string]: unknown | Part[];
+    [property: string]: unknown;
 }
 
 export interface Part {
     depth?:  string;
     length?: string;
     width?:  string;
-    [property: string]: unknown | string | string | string;
+    [property: string]: unknown;
 }
 
 export interface History {
     class?: string;
-    [property: string]: unknown | string;
+    [property: string]: unknown;
 }
 
 export interface Vehicle {
@@ -74,25 +74,25 @@ export interface Vehicle {
     subModule?: boolean;
     type?:      VehicleType;
     year?:      string;
-    [property: string]: unknown | string | string | Speed | boolean | VehicleType | string;
+    [property: string]: unknown;
 }
 
 export interface Speed {
     velocity?: Limit;
-    [property: string]: unknown | Limit;
+    [property: string]: unknown;
 }
 
 export interface Limit {
     maximum?: number;
     minimum?: number;
-    [property: string]: unknown | number | number;
+    [property: string]: unknown;
 }
 
 export interface VehicleType {
     name?:   Name;
     width?:  Axis;
     length?: Axis;
-    [property: string]: unknown | Name | Axis | Axis;
+    [property: string]: unknown;
 }
 
 export type Axis = "X" | "Y" | "Z" | "YZ" | "ZX" | "XY";
diff --git a/base/schema-typescript/test/inputs/schema/required-draft3.schema/default/TopLevel.ts b/head/schema-typescript/test/inputs/schema/required-draft3.schema/default/TopLevel.ts
index 808c46a..5d752aa 100644
--- a/base/schema-typescript/test/inputs/schema/required-draft3.schema/default/TopLevel.ts
+++ b/head/schema-typescript/test/inputs/schema/required-draft3.schema/default/TopLevel.ts
@@ -9,7 +9,7 @@
 
 export interface TopLevel {
     longitude: number;
-    [property: string]: unknown | number;
+    [property: string]: unknown;
 }
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-typescript/test/inputs/schema/required-non-properties.schema/default/TopLevel.ts b/head/schema-typescript/test/inputs/schema/required-non-properties.schema/default/TopLevel.ts
index f291bbe..b5a3f4a 100644
--- a/base/schema-typescript/test/inputs/schema/required-non-properties.schema/default/TopLevel.ts
+++ b/head/schema-typescript/test/inputs/schema/required-non-properties.schema/default/TopLevel.ts
@@ -10,7 +10,7 @@
 export interface TopLevel {
     foo: number;
     bar: unknown;
-    [property: string]: unknown | number | unknown;
+    [property: string]: unknown;
 }
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-typescript/test/inputs/schema/required.schema/default/TopLevel.ts b/head/schema-typescript/test/inputs/schema/required.schema/default/TopLevel.ts
index 808c46a..5d752aa 100644
--- a/base/schema-typescript/test/inputs/schema/required.schema/default/TopLevel.ts
+++ b/head/schema-typescript/test/inputs/schema/required.schema/default/TopLevel.ts
@@ -9,7 +9,7 @@
 
 export interface TopLevel {
     longitude: number;
-    [property: string]: unknown | number;
+    [property: string]: unknown;
 }
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-typescript/test/inputs/schema/rust-cycle-breaker-union.schema/default/TopLevel.ts b/head/schema-typescript/test/inputs/schema/rust-cycle-breaker-union.schema/default/TopLevel.ts
index fd13718..b76a76b 100644
--- a/base/schema-typescript/test/inputs/schema/rust-cycle-breaker-union.schema/default/TopLevel.ts
+++ b/head/schema-typescript/test/inputs/schema/rust-cycle-breaker-union.schema/default/TopLevel.ts
@@ -10,13 +10,13 @@
 export interface TopLevel {
     next?: Next;
     value: string;
-    [property: string]: unknown | Next | string;
+    [property: string]: unknown;
 }
 
 export interface Node {
     next?: Next;
     value: string;
-    [property: string]: unknown | Next | string;
+    [property: string]: unknown;
 }
 
 export type Next = null | Node | string;
diff --git a/base/schema-typescript/test/inputs/schema/schema-constraints.schema/default/TopLevel.ts b/head/schema-typescript/test/inputs/schema/schema-constraints.schema/default/TopLevel.ts
index 88efcfe..80dbe26 100644
--- a/base/schema-typescript/test/inputs/schema/schema-constraints.schema/default/TopLevel.ts
+++ b/head/schema-typescript/test/inputs/schema/schema-constraints.schema/default/TopLevel.ts
@@ -10,7 +10,7 @@
 export interface TopLevel {
     minMaxLength: string;
     percent:      number;
-    [property: string]: unknown | string | number;
+    [property: string]: unknown;
 }
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-typescript/test/inputs/schema/simple-ref.schema/default/TopLevel.ts b/head/schema-typescript/test/inputs/schema/simple-ref.schema/default/TopLevel.ts
index 9ca2844..a3e9b2b 100644
--- a/base/schema-typescript/test/inputs/schema/simple-ref.schema/default/TopLevel.ts
+++ b/head/schema-typescript/test/inputs/schema/simple-ref.schema/default/TopLevel.ts
@@ -12,7 +12,7 @@
  */
 export interface TopLevel {
     next?: TopLevel;
-    [property: string]: unknown | TopLevel;
+    [property: string]: unknown;
 }
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-typescript/test/inputs/schema/strict-optional.schema/default/TopLevel.ts b/head/schema-typescript/test/inputs/schema/strict-optional.schema/default/TopLevel.ts
index 9e409d2..208356c 100644
--- a/base/schema-typescript/test/inputs/schema/strict-optional.schema/default/TopLevel.ts
+++ b/head/schema-typescript/test/inputs/schema/strict-optional.schema/default/TopLevel.ts
@@ -9,7 +9,7 @@
 
 export interface TopLevel {
     foo: number | null;
-    [property: string]: unknown | number | null;
+    [property: string]: unknown;
 }
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-typescript/test/inputs/schema/top-level-array.schema/default/TopLevel.ts b/head/schema-typescript/test/inputs/schema/top-level-array.schema/default/TopLevel.ts
index 3a48dba..bfa6424 100644
--- a/base/schema-typescript/test/inputs/schema/top-level-array.schema/default/TopLevel.ts
+++ b/head/schema-typescript/test/inputs/schema/top-level-array.schema/default/TopLevel.ts
@@ -12,7 +12,7 @@ export type TopLevel = TextClassificationOutputElement[];
 export interface TextClassificationOutputElement {
     label: string;
     score: number;
-    [property: string]: unknown | string | number;
+    [property: string]: unknown;
 }
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-typescript/test/inputs/schema/tuple.schema/default/TopLevel.ts b/head/schema-typescript/test/inputs/schema/tuple.schema/default/TopLevel.ts
index 7d12391..d90f7f5 100644
--- a/base/schema-typescript/test/inputs/schema/tuple.schema/default/TopLevel.ts
+++ b/head/schema-typescript/test/inputs/schema/tuple.schema/default/TopLevel.ts
@@ -9,7 +9,7 @@
 
 export interface TopLevel {
     tuple: Tuple[];
-    [property: string]: unknown | Tuple[];
+    [property: string]: unknown;
 }
 
 export type Tuple = boolean | number;
diff --git a/base/schema-typescript/test/inputs/schema/unevaluated-properties.schema/default/TopLevel.ts b/head/schema-typescript/test/inputs/schema/unevaluated-properties.schema/default/TopLevel.ts
index 38465bc..90b3f3e 100644
--- a/base/schema-typescript/test/inputs/schema/unevaluated-properties.schema/default/TopLevel.ts
+++ b/head/schema-typescript/test/inputs/schema/unevaluated-properties.schema/default/TopLevel.ts
@@ -9,14 +9,14 @@
 
 export interface TopLevel {
     config?: Config;
-    [property: string]: unknown | Config;
+    [property: string]: unknown;
 }
 
 export interface Config {
     closed?:   Closed;
     name?:     string;
     settings?: { [key: string]: Item[] };
-    [property: string]: unknown | Closed | string | { [key: string]: Item[] };
+    [property: string]: unknown;
 }
 
 export type Closed = unknown[] | boolean | object | number | number | null | string;
@@ -24,7 +24,7 @@ export type Closed = unknown[] | boolean | object | number | number | null | str
 export interface Item {
     key:   string;
     value: string;
-    [property: string]: unknown | string | string;
+    [property: string]: unknown;
 }
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-typescript/test/inputs/schema/union-int-double.schema/default/TopLevel.ts b/head/schema-typescript/test/inputs/schema/union-int-double.schema/default/TopLevel.ts
index 14c5bc6..bcd40b1 100644
--- a/base/schema-typescript/test/inputs/schema/union-int-double.schema/default/TopLevel.ts
+++ b/head/schema-typescript/test/inputs/schema/union-int-double.schema/default/TopLevel.ts
@@ -9,7 +9,7 @@
 
 export interface TopLevel {
     value: Value;
-    [property: string]: unknown | Value;
+    [property: string]: unknown;
 }
 
 export type Value = number | string;
diff --git a/base/schema-typescript/test/inputs/schema/union.schema/default/TopLevel.ts b/head/schema-typescript/test/inputs/schema/union.schema/default/TopLevel.ts
index a44e0f6..1ba82fe 100644
--- a/base/schema-typescript/test/inputs/schema/union.schema/default/TopLevel.ts
+++ b/head/schema-typescript/test/inputs/schema/union.schema/default/TopLevel.ts
@@ -13,7 +13,7 @@ export interface TopLevelElement {
     one?:   number;
     two:    boolean;
     three?: number;
-    [property: string]: unknown | number | boolean | number;
+    [property: string]: unknown;
 }
 
 // Converts JSON strings to/from your types
diff --git a/base/schema-typescript/test/inputs/schema/uuid.schema/default/TopLevel.ts b/head/schema-typescript/test/inputs/schema/uuid.schema/default/TopLevel.ts
index 7bb8abb..052065c 100644
--- a/base/schema-typescript/test/inputs/schema/uuid.schema/default/TopLevel.ts
+++ b/head/schema-typescript/test/inputs/schema/uuid.schema/default/TopLevel.ts
@@ -14,7 +14,7 @@ export interface TopLevel {
     one:           string;
     optional?:     string;
     unionWithEnum: UnionWithEnumUnion;
-    [property: string]: unknown | (null | string)[] | string[] | null | string | string | string | UnionWithEnumUnion;
+    [property: string]: unknown;
 }
 
 export type UnionWithEnumUnion = UnionWithEnumEnum | string;
diff --git a/base/schema-typescript/test/inputs/schema/vega-lite.schema/default/TopLevel.ts b/head/schema-typescript/test/inputs/schema/vega-lite.schema/default/TopLevel.ts
index 37e19d3..0ada8e0 100644
--- a/base/schema-typescript/test/inputs/schema/vega-lite.schema/default/TopLevel.ts
+++ b/head/schema-typescript/test/inputs/schema/vega-lite.schema/default/TopLevel.ts
@@ -1706,7 +1706,7 @@ export interface RangeConfig {
      * Default range palette for the `shape` channel.
      */
     symbol?: string[];
-    [property: string]: RangeConfigValue | Category | Category | Category | Category | Category | string[];
+    [property: string]: RangeConfigValue | Category | string[] | undefined;
 }
 
 /**
diff --git a/head/schema-typescript-effect-schema/test/inputs/schema/mixed-additional-properties.schema/default/TopLevel.ts b/head/schema-typescript-effect-schema/test/inputs/schema/mixed-additional-properties.schema/default/TopLevel.ts
new file mode 100644
index 0000000..aa875ea
--- /dev/null
+++ b/head/schema-typescript-effect-schema/test/inputs/schema/mixed-additional-properties.schema/default/TopLevel.ts
@@ -0,0 +1,4 @@
+import * as S from "effect/Schema";
+
+export const TopLevel = S.Record({ key: S.String, value: S.Union(S.Number, S.String)});
+export type TopLevel = S.Schema.Type<typeof TopLevel>;
diff --git a/head/schema-typescript-zod/test/inputs/schema/mixed-additional-properties.schema/default/TopLevel.ts b/head/schema-typescript-zod/test/inputs/schema/mixed-additional-properties.schema/default/TopLevel.ts
new file mode 100644
index 0000000..576f9be
--- /dev/null
+++ b/head/schema-typescript-zod/test/inputs/schema/mixed-additional-properties.schema/default/TopLevel.ts
@@ -0,0 +1,4 @@
+import * as z from "zod";
+
+export const TopLevelSchema = z.record(z.string(), z.union([z.number(), z.string()]));
+export type TopLevel = z.infer<typeof TopLevelSchema>;
