✅ Add test for Pydantic serialization

tests/test_dump_json_fast_path.py

@@ -0,0 +1,51 @@
+from unittest.mock import patch
+
+from fastapi import FastAPI
+from fastapi.responses import JSONResponse
+from fastapi.testclient import TestClient
+from pydantic import BaseModel
+
+
+class Item(BaseModel):
+    name: str
+    price: float
+
+
+app = FastAPI()
+
+
+@app.get("/default")
+def get_default() -> Item:
+    return Item(name="widget", price=9.99)
+
+
+@app.get("/explicit", response_class=JSONResponse)
+def get_explicit() -> Item:
+    return Item(name="widget", price=9.99)
+
+
+client = TestClient(app)
+
+
+def test_default_response_class_skips_json_dumps():
+    """When no response_class is set, the fast path serializes directly to
+    JSON bytes via Pydantic's dump_json and never calls json.dumps."""
+    with patch(
+        "starlette.responses.json.dumps", wraps=__import__("json").dumps
+    ) as mock_dumps:
+        response = client.get("/default")
+    assert response.status_code == 200
+    assert response.json() == {"name": "widget", "price": 9.99}
+    mock_dumps.assert_not_called()
+
+
+def test_explicit_response_class_uses_json_dumps():
+    """When response_class is explicitly set to JSONResponse, the normal path
+    is used and json.dumps is called via JSONResponse.render()."""
+    with patch(
+        "starlette.responses.json.dumps", wraps=__import__("json").dumps
+    ) as mock_dumps:
+        response = client.get("/explicit")
+    assert response.status_code == 200
+    assert response.json() == {"name": "widget", "price": 9.99}
+    mock_dumps.assert_called_once()