✨ Serialize JSON with Pydantic when there's a response model

fastapi/_compat/v2.py

@@ -199,6 +199,32 @@ class ModelField:
             exclude_none=exclude_none,
         )
 
+    def serialize_json(
+        self,
+        value: Any,
+        *,
+        include: IncEx | None = None,
+        exclude: IncEx | None = None,
+        by_alias: bool = True,
+        exclude_unset: bool = False,
+        exclude_defaults: bool = False,
+        exclude_none: bool = False,
+    ) -> bytes:
+        # What calls this code passes a value that already called
+        # self._type_adapter.validate_python(value)
+        # This uses Pydantic's dump_json() which serializes directly to JSON
+        # bytes in one pass (via Rust), avoiding the intermediate Python dict
+        # step of dump_python(mode="json") + json.dumps().
+        return self._type_adapter.dump_json(
+            value,
+            include=include,
+            exclude=exclude,
+            by_alias=by_alias,
+            exclude_unset=exclude_unset,
+            exclude_defaults=exclude_defaults,
+            exclude_none=exclude_none,
+        )
+
     def __hash__(self) -> int:
         # Each ModelField is unique for our purposes, to allow making a dict from
         # ModelField to its JSON Schema.

fastapi/routing.py

@@ -271,6 +271,7 @@ async def serialize_response(
     exclude_none: bool = False,
     is_coroutine: bool = True,
     endpoint_ctx: EndpointContext | None = None,
+    dump_json: bool = False,
 ) -> Any:
     if field:
         if is_coroutine:
@@ -286,8 +287,8 @@ async def serialize_response(
                 body=response_content,
                 endpoint_ctx=ctx,
             )
-
+        serializer = field.serialize_json if dump_json else field.serialize
-        return field.serialize(
+        return serializer(
             value,
             include=include,
             exclude=exclude,
@@ -443,6 +444,14 @@ def get_request_handler(
                     response_args["status_code"] = current_status_code
                 if solved_result.response.status_code:
                     response_args["status_code"] = solved_result.response.status_code
+                # Use the fast path (dump_json) when no custom response
+                # class was set and a response field with a TypeAdapter
+                # exists. Serializes directly to JSON bytes via Pydantic's
+                # Rust core, skipping the intermediate Python dict +
+                # json.dumps() step.
+                use_dump_json = response_field is not None and isinstance(
+                    response_class, DefaultPlaceholder
+                )
                 content = await serialize_response(
                     field=response_field,
                     response_content=raw_response,
@@ -454,7 +463,15 @@ def get_request_handler(
                     exclude_none=response_model_exclude_none,
                     is_coroutine=is_coroutine,
                     endpoint_ctx=endpoint_ctx,
+                    dump_json=use_dump_json,
                 )
+                if use_dump_json:
+                    response = Response(
+                        content=content,
+                        media_type="application/json",
+                        **response_args,
+                    )
+                else:
                     response = actual_response_class(content, **response_args)
                 if not is_body_allowed_for_status_code(response.status_code):
                     response.body = b""