Fastapi expert

FastAPI Expert Skill Profile

Specialized backend implementation guide for production-grade async Python APIs using FastAPI 0.115+, Pydantic V2, SQLAlchemy 2.0 async, and Python 3.12+.

Critical Requirements (MUST DO)

• Type hints throughout — FastAPI and Pydantic depend on them
• Pydantic V2 syntax exclusively (model_config, field_validator, ConfigDict)
• Annotated[T, Depends()] type alias pattern for all dependencies
• lifespan context manager — never @app.on_event (deprecated, has a double-fire bug)
• async def for all I/O routes; def for CPU-bound routes (FastAPI runs those in a threadpool)
• PyJWT for JWT — never python-jose (CVE-2024-33663, abandoned since 2022)
• uv for dependency management, ruff for linting
• RFC 9457 Problem Details (application/problem+json) for all error responses — see references/error-responses.md
• Cursor-based pagination by default for all collection endpoints — see references/pagination.md
• URI prefix versioning from day one (/v1/) — never ship unversioned public endpoints
• Idempotency-Key header support for non-idempotent POST/PATCH operations
• Retry-After on all 429 and 503 responses

Anti-Pattern Quick Reference

❌ Avoid	✅ Use Instead
@app.on_event("startup/shutdown")	lifespan context manager
python-jose / jose import	PyJWT (import jwt)
ORJSONResponse / UJSONResponse	Default Pydantic/Rust serializer (FastAPI 0.130+)
Column() in SQLAlchemy models	mapped_column() with Mapped[]
orm_mode = True / class Config	model_config = ConfigDict(from_attributes=True)
.dict() / .json()	.model_dump() / .model_dump_json()
@pytest.mark.asyncio on every test	asyncio_mode = "auto" in pyproject.toml
TestClient in async tests	httpx.AsyncClient with ASGITransport
allow_origins=["*"] + allow_credentials=True	Explicit origin list
time.sleep() in async def	await asyncio.sleep() or use def route
pip / poetry	uv
Optional[X]	X | None (Python 3.10+)
200 OK with error in body	Proper 4xx/5xx + RFC 9457 application/problem+json
{"error": {"message": ...}} custom format	RFC 9457 Problem Details
?page=N offset pagination by default	Cursor pagination (?cursor=...)
Verb URIs (/getUsers, /createUser)	Noun resource URIs (/users)
Unversioned public endpoints	/v1/ URI prefix on all routes

Reference Files

File	Contents
references/pydantic-v2.md	Schemas, validators, ConfigDict, computed_field, settings
references/endpoints-routing.md	Router setup, CRUD, query parameter models (0.115+), dependencies
references/authentication.md	PyJWT, OAuth2, RBAC, httpOnly cookies, refresh tokens
references/async-sqlalchemy.md	Engine, AsyncAttrs, DatabaseSessionManager, CRUD, lifespan
references/testing-async.md	anyio auto mode, AsyncClient, NullPool, rollback isolation
references/security.md	CORS, rate limiting, security headers, middleware stack
references/project-structure.md	Domain-based layout, app factory, module conventions
references/error-responses.md	RFC 9457 Problem Details, FastAPI exception handlers, status code guide
references/pagination.md	Cursor pagination, async SQLAlchemy keyset queries, response envelope
references/versioning.md	URI versioning, APIRouter prefix, breaking changes, Sunset headers