diff --git a/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/DartDioClientCodegen.java b/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/DartDioClientCodegen.java
index 077f79904a69..8daf4c9cc23e 100644
--- a/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/DartDioClientCodegen.java
+++ b/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/DartDioClientCodegen.java
@@ -660,10 +660,134 @@ public void postProcessModelProperty(CodegenModel model, CodegenProperty propert
                             items.getAdditionalProperties().dataType
                     ));
                 }
+
+                // Recursively register builder factories for every nested
+                // container reachable from this property. Without this step,
+                // shapes like Map<String, List<X>> (e.g. an OpenAPI object
+                // with `additionalProperties: { type: array, items: ...}`)
+                // never get a `BuiltList<X>` factory registered, and
+                // built_value throws "No builder factory for BuiltList<X>"
+                // at deserialization time.
+                registerNestedBuilderFactories(property);
             }
         }
     }
 
+    /**
+     * Walk the CodegenProperty tree and register a built_value
+     * BuilderFactory for every container node, including the top one.
+     * Handles arbitrary nesting like {@code Map<String, List<X>>},
+     * {@code List<Map<String, X>>}, {@code List<List<X>>}, etc.
+     */
+    private void registerNestedBuilderFactories(CodegenProperty prop) {
+        if (prop == null || !prop.isContainer || prop.items == null) {
+            return;
+        }
+        // Recurse first so deeper containers are registered too.
+        registerNestedBuilderFactories(prop.items);
+
+        if (prop.items.isContainer) {
+            // Truly nested container (e.g. Map<String, List<X>>):
+            // must use composite form because the simple constructor
+            // cannot express the nested FullType.
+            BuilderFactoryExpr expr = renderBuilderFactory(prop);
+            if (expr != null) {
+                addBuiltValueSerializer(BuiltValueSerializer.composite(
+                        expr.fullTypeArgs,
+                        expr.builderInstantiation));
+            }
+        } else {
+            // Leaf container (e.g. List<X>, Map<String, X>): use the
+            // same simple constructor the rest of the codegen uses so
+            // the Set deduplicates correctly.
+            addBuiltValueSerializer(new BuiltValueSerializer(
+                    prop.isArray,
+                    prop.getUniqueItems(),
+                    prop.isMap,
+                    prop.items.isNullable,
+                    prop.items.dataType));
+        }
+    }
+
+    /**
+     * Render the FullType argument list and the matching Builder
+     * instantiation for a container CodegenProperty.
+     *
+     * @return null if {@code prop} is not a container we can render.
+     */
+    private BuilderFactoryExpr renderBuilderFactory(CodegenProperty prop) {
+        if (prop == null || !prop.isContainer || prop.items == null) {
+            return null;
+        }
+        String innerFullType = renderInnerFullType(prop.items);
+        String innerDart = renderDartType(prop.items);
+
+        if (prop.isArray) {
+            String collection = prop.getUniqueItems() ? "BuiltSet" : "BuiltList";
+            String builder = prop.getUniqueItems() ? "SetBuilder" : "ListBuilder";
+            return new BuilderFactoryExpr(
+                    collection + ", [FullType(" + innerFullType + ")]",
+                    builder + "<" + innerDart + ">");
+        }
+        if (prop.isMap) {
+            return new BuilderFactoryExpr(
+                    "BuiltMap, [FullType(String), FullType(" + innerFullType + ")]",
+                    "MapBuilder<String, " + innerDart + ">");
+        }
+        return null;
+    }
+
+    /**
+     * What goes inside {@code FullType(...)} for this property:
+     * a leaf type name like {@code "Foo"}, or a nested expression like
+     * {@code "BuiltList, [FullType(Foo)]"}.
+     */
+    private String renderInnerFullType(CodegenProperty prop) {
+        if (prop == null) return "dynamic";
+        if (!prop.isContainer || prop.items == null) {
+            return prop.dataType;
+        }
+        String inner = renderInnerFullType(prop.items);
+        if (prop.isArray) {
+            String collection = prop.getUniqueItems() ? "BuiltSet" : "BuiltList";
+            return collection + ", [FullType(" + inner + ")]";
+        }
+        if (prop.isMap) {
+            return "BuiltMap, [FullType(String), FullType(" + inner + ")]";
+        }
+        return prop.dataType;
+    }
+
+    /**
+     * Render the Dart type literal (e.g. {@code BuiltMap<String, BuiltList<Foo>>})
+     * used inside the {@code () => XBuilder<...>()} lambda.
+     */
+    private String renderDartType(CodegenProperty prop) {
+        if (prop == null) return "dynamic";
+        if (!prop.isContainer || prop.items == null) {
+            return prop.dataType;
+        }
+        String inner = renderDartType(prop.items);
+        if (prop.isArray) {
+            String collection = prop.getUniqueItems() ? "BuiltSet" : "BuiltList";
+            return collection + "<" + inner + ">";
+        }
+        if (prop.isMap) {
+            return "BuiltMap<String, " + inner + ">";
+        }
+        return prop.dataType;
+    }
+
+    private static final class BuilderFactoryExpr {
+        final String fullTypeArgs;
+        final String builderInstantiation;
+
+        BuilderFactoryExpr(String fullTypeArgs, String builderInstantiation) {
+            this.fullTypeArgs = fullTypeArgs;
+            this.builderInstantiation = builderInstantiation;
+        }
+    }
+
     @Override
     public OperationsMap postProcessOperationsWithModels(OperationsMap objs, List<ModelMap> allModels) {
         super.postProcessOperationsWithModels(objs, allModels);
@@ -817,12 +941,45 @@ static class BuiltValueSerializer {
 
         @Getter final String dataType;
 
+        // When non-null, the serializer is rendered verbatim from these
+        // pre-computed strings instead of being dispatched through the
+        // isArray/isMap branches in serializers.mustache. Used for
+        // arbitrarily nested container types where dataType alone can't
+        // express the FullType expression (e.g. Map<String, List<X>>).
+        @Getter final String fullTypeArgs;
+
+        @Getter final String builderInstantiation;
+
         private BuiltValueSerializer(boolean isArray, boolean uniqueItems, boolean isMap, boolean isNullable, String dataType) {
             this.isArray = isArray;
             this.uniqueItems = uniqueItems;
             this.isMap = isMap;
             this.isNullable = isNullable;
             this.dataType = dataType;
+            this.fullTypeArgs = null;
+            this.builderInstantiation = null;
+        }
+
+        private BuiltValueSerializer(String fullTypeArgs, String builderInstantiation) {
+            this.isArray = false;
+            this.uniqueItems = false;
+            this.isMap = false;
+            this.isNullable = false;
+            this.dataType = "";
+            this.fullTypeArgs = fullTypeArgs;
+            this.builderInstantiation = builderInstantiation;
+        }
+
+        /**
+         * Build a serializer for a nested-container BuilderFactory whose
+         * type signature can't be expressed by the simple
+         * (isArray, isMap, dataType) form. {@code fullTypeArgs} is the
+         * argument list for {@code FullType(...)} (without the wrapping
+         * call) and {@code builderInstantiation} is the inside of
+         * {@code () => ...()}.
+         */
+        public static BuiltValueSerializer composite(String fullTypeArgs, String builderInstantiation) {
+            return new BuiltValueSerializer(fullTypeArgs, builderInstantiation);
         }
 
         public boolean isArray() {
@@ -842,11 +999,18 @@ public boolean equals(Object o) {
             if (this == o) return true;
             if (o == null || getClass() != o.getClass()) return false;
             BuiltValueSerializer that = (BuiltValueSerializer) o;
+            if (fullTypeArgs != null || that.fullTypeArgs != null) {
+                return Objects.equals(fullTypeArgs, that.fullTypeArgs)
+                        && Objects.equals(builderInstantiation, that.builderInstantiation);
+            }
             return isArray == that.isArray && uniqueItems == that.uniqueItems && isMap == that.isMap && isNullable == that.isNullable && dataType.equals(that.dataType);
         }
 
         @Override
         public int hashCode() {
+            if (fullTypeArgs != null) {
+                return Objects.hash(fullTypeArgs, builderInstantiation);
+            }
             return Objects.hash(isArray, uniqueItems, isMap, isNullable, dataType);
         }
     }
diff --git a/modules/openapi-generator/src/main/resources/dart/libraries/dio/serialization/built_value/serializers.mustache b/modules/openapi-generator/src/main/resources/dart/libraries/dio/serialization/built_value/serializers.mustache
index 0ae5dc8af49e..67ff3ea18cda 100644
--- a/modules/openapi-generator/src/main/resources/dart/libraries/dio/serialization/built_value/serializers.mustache
+++ b/modules/openapi-generator/src/main/resources/dart/libraries/dio/serialization/built_value/serializers.mustache
@@ -23,6 +23,11 @@ part 'serializers.g.dart';
 ])
 Serializers serializers = (_$serializers.toBuilder(){{#builtValueSerializers}}
       ..addBuilderFactory(
+{{#fullTypeArgs}}
+        const FullType({{{fullTypeArgs}}}),
+        () => {{{builderInstantiation}}}(),
+{{/fullTypeArgs}}
+{{^fullTypeArgs}}
 {{#isArray}}
         const FullType(Built{{#uniqueItems}}Set{{/uniqueItems}}{{^uniqueItems}}List{{/uniqueItems}}, [FullType{{#isNullable}}.nullable{{/isNullable}}({{dataType}})]),
         () => {{#uniqueItems}}Set{{/uniqueItems}}{{^uniqueItems}}List{{/uniqueItems}}Builder<{{dataType}}>(),
@@ -31,6 +36,7 @@ Serializers serializers = (_$serializers.toBuilder(){{#builtValueSerializers}}
         const FullType(BuiltMap, [FullType(String), FullType{{#isNullable}}.nullable{{/isNullable}}({{dataType}})]),
         () => MapBuilder<String, {{dataType}}{{#isNullable}}?{{/isNullable}}>(),
 {{/isMap}}
+{{/fullTypeArgs}}
       ){{/builtValueSerializers}}
       {{#models}}{{#model}}{{#vendorExtensions.x-is-parent}}..add({{classname}}.serializer)
       {{/vendorExtensions.x-is-parent}}{{/model}}{{/models}}..add(const OneOfSerializer())
diff --git a/modules/openapi-generator/src/test/java/org/openapitools/codegen/dart/dio/DartDioClientCodegenTest.java b/modules/openapi-generator/src/test/java/org/openapitools/codegen/dart/dio/DartDioClientCodegenTest.java
index e19857605731..fe9a045cebbd 100644
--- a/modules/openapi-generator/src/test/java/org/openapitools/codegen/dart/dio/DartDioClientCodegenTest.java
+++ b/modules/openapi-generator/src/test/java/org/openapitools/codegen/dart/dio/DartDioClientCodegenTest.java
@@ -142,6 +142,50 @@ public void testImportMappingsInSerializersAndBarrelFile() throws IOException {
                 "package:my_api/src/model/custom_address.dart");
     }
 
+    /**
+     * Regression test for missing BuilderFactory entries on container
+     * types reachable only via {@code additionalProperties}.
+     *
+     * Before the fix, a property like
+     * {@code Map<String, List<Widget>>} (an object schema with
+     * {@code additionalProperties: { type: array, items: ... }}) ended
+     * up in the generated Dart class but no
+     * {@code addBuilderFactory(BuiltList<Widget>, ...)} call was
+     * emitted in {@code serializers.dart}. built_value then failed at
+     * runtime with
+     * {@code Bad state: No builder factory for BuiltList<Widget>}.
+     *
+     * The fix walks every model property's container tree and registers
+     * a factory for each nested layer.
+     */
+    @Test
+    public void testNestedAdditionalPropertiesGetBuilderFactories() throws IOException {
+        File output = Files.createTempDirectory("test").toFile();
+        output.deleteOnExit();
+
+        final CodegenConfigurator configurator = new CodegenConfigurator()
+                .setGeneratorName("dart-dio")
+                .setInputSpec("src/test/resources/3_0/dart-dio/built_value_additional_properties_factory.yaml")
+                .setOutputDir(output.getAbsolutePath().replace("\\", "/"));
+
+        ClientOptInput opts = configurator.toClientOptInput();
+        Generator generator = new DefaultGenerator().opts(opts);
+        List<File> files = generator.generate();
+        files.forEach(File::deleteOnExit);
+
+        Path serializers = output.toPath().resolve("lib/src/serializers.dart");
+
+        // Inner container: List<Widget>.
+        TestUtils.assertFileContains(serializers,
+                "const FullType(BuiltList, [FullType(Widget)]),",
+                "() => ListBuilder<Widget>(),");
+
+        // Outer container: Map<String, List<Widget>>.
+        TestUtils.assertFileContains(serializers,
+                "const FullType(BuiltMap, [FullType(String), FullType(BuiltList, [FullType(Widget)])]),",
+                "() => MapBuilder<String, BuiltList<Widget>>(),");
+    }
+
     @Test
     public void verifyDartDioGeneratorRuns() throws IOException {
         File output = Files.createTempDirectory("test").toFile();
diff --git a/modules/openapi-generator/src/test/resources/3_0/dart-dio/built_value_additional_properties_factory.yaml b/modules/openapi-generator/src/test/resources/3_0/dart-dio/built_value_additional_properties_factory.yaml
new file mode 100644
index 000000000000..9ada6354eefd
--- /dev/null
+++ b/modules/openapi-generator/src/test/resources/3_0/dart-dio/built_value_additional_properties_factory.yaml
@@ -0,0 +1,51 @@
+openapi: "3.0.0"
+info:
+  title: Built-value additionalProperties BuilderFactory test
+  version: "1.0.0"
+paths:
+  /catalog/{id}:
+    get:
+      operationId: getCatalog
+      parameters:
+        - name: id
+          in: path
+          required: true
+          schema: { type: integer }
+      responses:
+        "200":
+          description: OK
+          content:
+            application/json:
+              schema:
+                $ref: "#/components/schemas/Catalog"
+components:
+  schemas:
+    Widget:
+      type: object
+      required:
+        - id
+        - name
+      properties:
+        id:
+          type: integer
+        name:
+          type: string
+    # `additionalProperties: { type: array, items: ... }` is the canonical
+    # "map of array of $ref" shape. Without the BuilderFactory walk,
+    # deserialization throws
+    #   Bad state: No builder factory for BuiltList<Widget>
+    WidgetsByCategory:
+      type: object
+      additionalProperties:
+        type: array
+        items:
+          $ref: "#/components/schemas/Widget"
+    Catalog:
+      type: object
+      required:
+        - id
+      properties:
+        id:
+          type: integer
+        widgetsByCategory:
+          $ref: "#/components/schemas/WidgetsByCategory"
diff --git a/samples/openapi3/client/petstore/dart-dio/petstore_client_lib_fake/lib/src/serializers.dart b/samples/openapi3/client/petstore/dart-dio/petstore_client_lib_fake/lib/src/serializers.dart
index f55ca699ac78..4a1884a3eca2 100644
--- a/samples/openapi3/client/petstore/dart-dio/petstore_client_lib_fake/lib/src/serializers.dart
+++ b/samples/openapi3/client/petstore/dart-dio/petstore_client_lib_fake/lib/src/serializers.dart
@@ -136,6 +136,42 @@ Serializers serializers = (_$serializers.toBuilder()
         const FullType(BuiltMap, [FullType(String), FullType(String)]),
         () => MapBuilder<String, String>(),
       )
+      ..addBuilderFactory(
+        const FullType(BuiltList, [FullType(BuiltList, [FullType(int)])]),
+        () => ListBuilder<BuiltList<int>>(),
+      )
+      ..addBuilderFactory(
+        const FullType(BuiltMap, [FullType(String), FullType(num)]),
+        () => MapBuilder<String, num>(),
+      )
+      ..addBuilderFactory(
+        const FullType(BuiltMap, [FullType(String), FullType(Animal)]),
+        () => MapBuilder<String, Animal>(),
+      )
+      ..addBuilderFactory(
+        const FullType(BuiltList, [FullType(ReadOnlyFirst)]),
+        () => ListBuilder<ReadOnlyFirst>(),
+      )
+      ..addBuilderFactory(
+        const FullType(BuiltList, [FullType(BuiltList, [FullType(ReadOnlyFirst)])]),
+        () => ListBuilder<BuiltList<ReadOnlyFirst>>(),
+      )
+      ..addBuilderFactory(
+        const FullType(BuiltMap, [FullType(String), FullType(BuiltMap, [FullType(String), FullType(String)])]),
+        () => MapBuilder<String, BuiltMap<String, String>>(),
+      )
+      ..addBuilderFactory(
+        const FullType(BuiltMap, [FullType(String), FullType(int)]),
+        () => MapBuilder<String, int>(),
+      )
+      ..addBuilderFactory(
+        const FullType(BuiltList, [FullType(BuiltList, [FullType(num)])]),
+        () => ListBuilder<BuiltList<num>>(),
+      )
+      ..addBuilderFactory(
+        const FullType(BuiltList, [FullType(num)]),
+        () => ListBuilder<num>(),
+      )
       ..addBuilderFactory(
         const FullType(BuiltList, [FullType(User)]),
         () => ListBuilder<User>(),
@@ -144,6 +180,10 @@ Serializers serializers = (_$serializers.toBuilder()
         const FullType(BuiltSet, [FullType(String)]),
         () => SetBuilder<String>(),
       )
+      ..addBuilderFactory(
+        const FullType(BuiltMap, [FullType(String), FullType(BuiltList, [FullType(int)])]),
+        () => MapBuilder<String, BuiltList<int>>(),
+      )
       ..addBuilderFactory(
         const FullType(BuiltSet, [FullType(Pet)]),
         () => SetBuilder<Pet>(),
@@ -153,21 +193,45 @@ Serializers serializers = (_$serializers.toBuilder()
         () => ListBuilder<Pet>(),
       )
       ..addBuilderFactory(
-        const FullType(BuiltMap, [FullType(String), FullType.nullable(JsonObject)]),
-        () => MapBuilder<String, JsonObject?>(),
-      )
-      ..addBuilderFactory(
-        const FullType(BuiltMap, [FullType(String), FullType(int)]),
-        () => MapBuilder<String, int>(),
+        const FullType(BuiltList, [FullType(JsonObject)]),
+        () => ListBuilder<JsonObject>(),
       )
       ..addBuilderFactory(
         const FullType(BuiltList, [FullType(ModelEnumClass)]),
         () => ListBuilder<ModelEnumClass>(),
       )
+      ..addBuilderFactory(
+        const FullType(BuiltList, [FullType.nullable(JsonObject)]),
+        () => ListBuilder<JsonObject>(),
+      )
+      ..addBuilderFactory(
+        const FullType(BuiltList, [FullType(Tag)]),
+        () => ListBuilder<Tag>(),
+      )
+      ..addBuilderFactory(
+        const FullType(BuiltList, [FullType(ModelFile)]),
+        () => ListBuilder<ModelFile>(),
+      )
+      ..addBuilderFactory(
+        const FullType(BuiltList, [FullType(int)]),
+        () => ListBuilder<int>(),
+      )
+      ..addBuilderFactory(
+        const FullType(BuiltMap, [FullType(String), FullType.nullable(JsonObject)]),
+        () => MapBuilder<String, JsonObject?>(),
+      )
+      ..addBuilderFactory(
+        const FullType(BuiltMap, [FullType(String), FullType(bool)]),
+        () => MapBuilder<String, bool>(),
+      )
       ..addBuilderFactory(
         const FullType(BuiltList, [FullType(String)]),
         () => ListBuilder<String>(),
       )
+      ..addBuilderFactory(
+        const FullType(BuiltMap, [FullType(String), FullType(JsonObject)]),
+        () => MapBuilder<String, JsonObject>(),
+      )
       ..add(Animal.serializer)
       ..add(ParentWithNullable.serializer)
       ..add(const OneOfSerializer())