From fa7cd85f77038b56536b25f43d5b58e456e59be4 Mon Sep 17 00:00:00 2001
From: "Julio C. Galindo" <54072664+stickM4N@users.noreply.github.com>
Date: Wed, 23 Jul 2025 15:43:21 +0200
Subject: [PATCH 1/3] feat: add possibility to override json schema generator
 class

---
 fastapi/openapi/utils.py | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fastapi/openapi/utils.py b/fastapi/openapi/utils.py
index 808646cc2..4c8530d34 100644
--- a/fastapi/openapi/utils.py
+++ b/fastapi/openapi/utils.py
@@ -489,6 +489,7 @@ def get_openapi(
     contact: Optional[Dict[str, Union[str, Any]]] = None,
     license_info: Optional[Dict[str, Union[str, Any]]] = None,
     separate_input_output_schemas: bool = True,
+    schema_generator: Optional[GenerateJsonSchema] = None,
 ) -> Dict[str, Any]:
     info: Dict[str, Any] = {"title": title, "version": version}
     if summary:
@@ -510,7 +511,7 @@ def get_openapi(
     operation_ids: Set[str] = set()
     all_fields = get_fields_from_routes(list(routes or []) + list(webhooks or []))
     model_name_map = get_compat_model_name_map(all_fields)
-    schema_generator = GenerateJsonSchema(ref_template=REF_TEMPLATE)
+    schema_generator = schema_generator or GenerateJsonSchema(ref_template=REF_TEMPLATE)
     field_mapping, definitions = get_definitions(
         fields=all_fields,
         schema_generator=schema_generator,

From 8cdb9637c79711e5ee393f0afe7c7f34f6376117 Mon Sep 17 00:00:00 2001
From: Yurii Motov <yurii.motov.monte@gmail.com>
Date: Fri, 1 Aug 2025 11:07:39 +0200
Subject: [PATCH 2/3] Add test

---
 tests/test_openapi_custom_schema_generator.py | 81 +++++++++++++++++++
 1 file changed, 81 insertions(+)
 create mode 100644 tests/test_openapi_custom_schema_generator.py

diff --git a/tests/test_openapi_custom_schema_generator.py b/tests/test_openapi_custom_schema_generator.py
new file mode 100644
index 000000000..62fb15399
--- /dev/null
+++ b/tests/test_openapi_custom_schema_generator.py
@@ -0,0 +1,81 @@
+import pytest
+from fastapi import FastAPI
+from fastapi._compat import PYDANTIC_V2, GenerateJsonSchema
+from fastapi.openapi.utils import get_openapi
+
+app = FastAPI()
+
+
+@app.get("/")
+def read_root():
+    pass  # pragma: no cover
+
+
+# Custom schema generator that does nothing but tracks if it was called
+class CustomJsonSchemaGenerator(GenerateJsonSchema):
+    def __init__(self):
+        super().__init__()
+        self.called = False
+
+    def generate_definitions(self, *args, **kwargs):
+        self.called = True
+        return super().generate_definitions(*args, **kwargs)
+
+
+def test_custom_schema_generator_called():
+    custom_schema_generator = CustomJsonSchemaGenerator()
+    get_openapi(
+        title=app.title,
+        version=app.version,
+        routes=app.routes,
+        schema_generator=custom_schema_generator,
+    )
+
+    if PYDANTIC_V2:
+        assert custom_schema_generator.called is True
+    else:
+        assert (  # Pydantic v1 does not use custom schema generators
+            custom_schema_generator.called is False
+        )
+
+
+@pytest.mark.parametrize("use_custom_schema_generator", [True, False])
+def test_custom_schema_generator_openapi(use_custom_schema_generator: bool):
+    custom_schema_generator = (
+        CustomJsonSchemaGenerator() if use_custom_schema_generator else None
+    )
+    openapi = get_openapi(
+        title=app.title,
+        version=app.version,
+        routes=app.routes,
+        schema_generator=custom_schema_generator,
+    )
+
+    assert openapi == OPENAPI_SCHEMA
+
+
+OPENAPI_SCHEMA = {
+    "info": {
+        "title": "FastAPI",
+        "version": "0.1.0",
+    },
+    "openapi": "3.1.0",
+    "paths": {
+        "/": {
+            "get": {
+                "operationId": "read_root__get",
+                "responses": {
+                    "200": {
+                        "content": {
+                            "application/json": {
+                                "schema": {},
+                            },
+                        },
+                        "description": "Successful Response",
+                    },
+                },
+                "summary": "Read Root",
+            },
+        },
+    },
+}

From 330e14bc10c1101813649e962bd2dcd779223e35 Mon Sep 17 00:00:00 2001
From: Yurii Motov <yurii.motov.monte@gmail.com>
Date: Fri, 1 Aug 2025 11:14:31 +0200
Subject: [PATCH 3/3] Fix test

---
 tests/test_openapi_custom_schema_generator.py | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/tests/test_openapi_custom_schema_generator.py b/tests/test_openapi_custom_schema_generator.py
index 62fb15399..b547992fc 100644
--- a/tests/test_openapi_custom_schema_generator.py
+++ b/tests/test_openapi_custom_schema_generator.py
@@ -1,6 +1,7 @@
 import pytest
 from fastapi import FastAPI
 from fastapi._compat import PYDANTIC_V2, GenerateJsonSchema
+from fastapi.openapi.constants import REF_TEMPLATE
 from fastapi.openapi.utils import get_openapi
 
 app = FastAPI()
@@ -14,7 +15,7 @@ def read_root():
 # Custom schema generator that does nothing but tracks if it was called
 class CustomJsonSchemaGenerator(GenerateJsonSchema):
     def __init__(self):
-        super().__init__()
+        super().__init__(ref_template=REF_TEMPLATE)
         self.called = False
 
     def generate_definitions(self, *args, **kwargs):