diff --git a/fix-python-fastapi-additionalproperties-true-parent.md b/fix-python-fastapi-additionalproperties-true-parent.md
new file mode 100644
index 000000000000..01c9d3a80f08
--- /dev/null
+++ b/fix-python-fastapi-additionalproperties-true-parent.md
@@ -0,0 +1,97 @@
+# Fix Plan: python-fastapi additionalProperties:true generates wrong parent class
+
+## Issue
+GitHub issue #20153: `python-fastapi` generator produces invalid imports and wrong base class for
+models that have `additionalProperties: true` (a boolean) at the schema level.
+
+**Broken output:**
+```python
+from openapi_server.models.object import object
+class HTTPRequest(object):
+    ...
+```
+
+**Expected output:**
+```python
+from pydantic import BaseModel
+class HTTPRequest(BaseModel):
+    ...
+```
+
+## Root Cause
+
+`ModelUtils.isMapSchema()` returns `true` for any schema with `additionalProperties: true` because
+the boolean value counts as "map-like" per the OAS spec. This triggers
+`updateModelForObject()` → `addAdditionPropertiesToCodeGenModel()` → `addParentContainer()` →
+`addParentFromContainer()`, which sets `model.parent = toInstantiationType(schema)`.
+
+For `additionalProperties: true`, `ModelUtils.getAdditionalProperties()` returns `new Schema()`
+(an empty schema), and `getSchemaType(new Schema())` returns `"object"`.
+
+So `model.parent` becomes `"object"` — Python's built-in type. Then in
+`AbstractPythonCodegen.postProcessModels()`:
+
+```java
+if (!StringUtils.isEmpty(model.parent)) {
+    modelImports.add(model.parent);  // → "from openapi_server.models.object import object"
+}
+```
+
+And the mustache template:
+```mustache
+class {{classname}}({{#parent}}{{{.}}}{{/parent}}{{^parent}}BaseModel{{/parent}}):
+```
+
+generates `class HTTPRequest(object):` instead of `class HTTPRequest(BaseModel):`.
+
+`PythonClientCodegen` already overrides `addAdditionPropertiesToCodeGenModel()` to avoid this,
+but `PythonFastAPIServerCodegen` and `AbstractPythonConnexionServerCodegen` do not — they fall
+through to `DefaultCodegen` which calls `addParentContainer()`.
+
+## Fix
+
+Move the correct `addAdditionPropertiesToCodeGenModel()` override from `PythonClientCodegen` up
+to `AbstractPythonCodegen`. This ensures ALL Python generators (FastAPI, Connexion, client) share
+the same correct behaviour without duplicating the fix.
+
+```java
+// AbstractPythonCodegen.java
+@Override
+protected void addAdditionPropertiesToCodeGenModel(CodegenModel codegenModel, Schema schema) {
+    final Schema additionalProperties = ModelUtils.getAdditionalProperties(schema);
+    if (additionalProperties != null) {
+        codegenModel.additionalPropertiesType = getSchemaType(additionalProperties);
+    }
+    // Do NOT call addParentContainer() — Python uses the `additional_properties` field
+    // pattern in templates rather than class inheritance to represent additionalProperties.
+    // Calling the default would set model.parent = "object" which generates an invalid
+    // "from openapi_server.models.object import object" import.
+}
+```
+
+Remove the duplicate override from `PythonClientCodegen`.
+
+## Test
+
+Add a regression test in `PythonFastAPIServerCodegenTest` using a new fixture spec
+`src/test/resources/bugs/issue_20153.yaml` that includes a schema with
+`additionalProperties: true` and named properties (modelling the TAMS `http-request.json`).
+
+The test verifies:
+- `from openapi_server.models.object import object` is NOT present in the generated model
+- `class HttpRequest(BaseModel):` IS present (correct base class)
+- `additional_properties: Dict[str, Any] = {}` IS present (the correct pattern)
+
+## Files Changed
+
+1. `modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/AbstractPythonCodegen.java`
+   — add `addAdditionPropertiesToCodeGenModel()` override
+
+2. `modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/PythonClientCodegen.java`
+   — remove now-duplicate `addAdditionPropertiesToCodeGenModel()` override
+
+3. `modules/openapi-generator/src/test/resources/bugs/issue_20153.yaml`
+   — new minimal OAS 3.1 fixture
+
+4. `modules/openapi-generator/src/test/java/org/openapitools/codegen/python/PythonFastAPIServerCodegenTest.java`
+   — new regression test `testAdditionalPropertiesTrueUsesBaseModel`
diff --git a/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/AbstractPythonCodegen.java b/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/AbstractPythonCodegen.java
index ee16c3805081..294486c781c3 100644
--- a/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/AbstractPythonCodegen.java
+++ b/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/AbstractPythonCodegen.java
@@ -1529,6 +1529,25 @@ public boolean isDataTypeString(String dataType) {
         return "str".equals(dataType);
     }
 
+    /**
+     * Python generators represent additionalProperties via a dedicated `additional_properties`
+     * field in the generated model class rather than through class inheritance. Setting
+     * model.parent to "object" (the return value of toInstantiationType when additionalProperties
+     * is the boolean true) causes the template to emit an invalid import:
+     *   from openapi_server.models.object import object
+     * and an incorrect base class declaration:
+     *   class Foo(object):
+     * Override here to only set additionalPropertiesType (used for typing) without calling
+     * addParentContainer(), keeping model.parent null so the template falls back to BaseModel.
+     */
+    @Override
+    protected void addAdditionPropertiesToCodeGenModel(CodegenModel codegenModel, Schema schema) {
+        final Schema additionalProperties = ModelUtils.getAdditionalProperties(schema);
+        if (additionalProperties != null) {
+            codegenModel.additionalPropertiesType = getSchemaType(additionalProperties);
+        }
+    }
+
     /* The definition for a Python type.
      *
      * This encapsulate all the type definition: the actual type, and potentially:
diff --git a/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/PythonClientCodegen.java b/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/PythonClientCodegen.java
index fdf19feff107..99fbdd6044a3 100644
--- a/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/PythonClientCodegen.java
+++ b/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/PythonClientCodegen.java
@@ -17,7 +17,6 @@
 
 package org.openapitools.codegen.languages;
 
-import io.swagger.v3.oas.models.media.Schema;
 import io.swagger.v3.oas.models.security.SecurityScheme;
 import lombok.Setter;
 import org.apache.commons.lang3.Strings;
@@ -25,7 +24,6 @@
 import org.openapitools.codegen.meta.GeneratorMetadata;
 import org.openapitools.codegen.meta.Stability;
 import org.openapitools.codegen.meta.features.*;
-import org.openapitools.codegen.utils.ModelUtils;
 import org.openapitools.codegen.utils.ProcessUtils;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
@@ -457,15 +455,6 @@ public String generatorLanguageVersion() {
         return "3.10+";
     }
 
-    @Override
-    protected void addAdditionPropertiesToCodeGenModel(CodegenModel codegenModel, Schema schema) {
-        final Schema additionalProperties = ModelUtils.getAdditionalProperties(schema);
-
-        if (additionalProperties != null) {
-            codegenModel.additionalPropertiesType = getSchemaType(additionalProperties);
-        }
-    }
-
     @Override
     public String escapeReservedWord(String name) {
         if (this.reservedWordsMappings().containsKey(name)) {
diff --git a/modules/openapi-generator/src/test/java/org/openapitools/codegen/python/PythonFastAPIServerCodegenTest.java b/modules/openapi-generator/src/test/java/org/openapitools/codegen/python/PythonFastAPIServerCodegenTest.java
index ff381bacecbd..bdeadadf0a9b 100644
--- a/modules/openapi-generator/src/test/java/org/openapitools/codegen/python/PythonFastAPIServerCodegenTest.java
+++ b/modules/openapi-generator/src/test/java/org/openapitools/codegen/python/PythonFastAPIServerCodegenTest.java
@@ -106,6 +106,21 @@ public void testToPythonExamplePrefersExampleOverExamples() {
         Assert.assertEquals(codegen.exposeToPythonExample(cp), "\"doggie\"");
     }
 
+    @Test(description = "schema with additionalProperties:true inherits BaseModel, not object (issue #20153)")
+    public void testAdditionalPropertiesTrueUsesBaseModel() throws IOException {
+        final DefaultCodegen codegen = new PythonFastAPIServerCodegen();
+        final String outputPath = generateFiles(codegen, "src/test/resources/bugs/issue_20153.yaml");
+        final Path model = Paths.get(outputPath + "src/openapi_server/models/http_request.py");
+
+        assertFileExists(model);
+        // Must use BaseModel as the base class, not Python's built-in object type
+        assertFileContains(model, "class HttpRequest(BaseModel):");
+        // Must carry the additional_properties field for the additionalProperties:true schema
+        assertFileContains(model, "additional_properties: Dict[str, Any] = {}");
+        // Must NOT generate a bad import that tries to import object as a module
+        assertFileNotContains(model, "from openapi_server.models.object import object");
+    }
+
     @Test(description = "binary multipart form fields are typed as FastAPI UploadFile")
     public void testBinaryMultipartFieldUsesUploadFile() throws IOException {
         final DefaultCodegen codegen = new PythonFastAPIServerCodegen();
diff --git a/modules/openapi-generator/src/test/resources/bugs/issue_20153.yaml b/modules/openapi-generator/src/test/resources/bugs/issue_20153.yaml
new file mode 100644
index 000000000000..90ec3d9d0e04
--- /dev/null
+++ b/modules/openapi-generator/src/test/resources/bugs/issue_20153.yaml
@@ -0,0 +1,45 @@
+openapi: "3.1.0"
+info:
+  title: Issue 20153 - additionalProperties true generates wrong parent class
+  version: "1.0"
+paths:
+  /requests:
+    post:
+      operationId: createRequest
+      requestBody:
+        required: true
+        content:
+          application/json:
+            schema:
+              $ref: '#/components/schemas/HttpRequest'
+      responses:
+        "200":
+          description: OK
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/HttpRequest'
+components:
+  schemas:
+    HttpRequest:
+      title: HTTP Request
+      description: Gives information on a particular http request a client should perform
+      type: object
+      required:
+        - url
+      properties:
+        url:
+          description: The URL to make the request to
+          type: string
+        body:
+          description: The body of the request
+          type: string
+        content-type:
+          description: The content type to use
+          type: string
+        headers:
+          description: Additional headers to include
+          type: object
+          additionalProperties:
+            type: string
+      additionalProperties: true