FastAPI migration: response_model_exclude_unset vs Pydantic V2 None filtering

In FastAPI with Pydantic V2, use response_model_exclude_unset=True or model_dump(exclude_unset=True) to avoid leaking default None fields in JSON responses.

💡 Verified Solution

Set response_model_exclude_unset=True on route decorators for clean payload serialization.

Patch / Unified Code Diff:

--- old.py
+++ new.py
@@ -1,1 +1,1 @@
-@app.get('/items', response_model=Item)
+@app.get('/items', response_model=Item, response_model_exclude_unset=True)

Official Primary Reference: https://fastapi.tiangolo.com/tutorial/response-model/#response_model_exclude_unset

🧪 Sandbox Execution Evidence

Exit Code: 0

Tests Passed: 1 / 1

Confidence: 99%

Protocol: MCP 2026

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