Fastapi Code Review

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name: fastapi-code-review description: Reviews FastAPI code for routing patterns, dependency injection, validation, and async handlers. Use when reviewing FastAPI apps, checking APIRouter setup, Depends() usage, or response models.

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FastAPI Code Review

Quick Reference

| Issue Type | Reference | |------------|-----------| | APIRouter setup, response_model, status codes | references/routes.md | | Depends(), yield deps, cleanup, shared deps | references/dependencies.md | | Pydantic models, HTTPException, 422 handling | references/validation.md | | Async handlers, blocking I/O, background tasks | references/async.md |

Review Checklist

[ ] APIRouter with proper prefix and tags

[ ] All routes specify response_model for type safety

[ ] Correct HTTP methods (GET, POST, PUT, DELETE, PATCH)

[ ] Proper status codes (200, 201, 204, 404, etc.)

[ ] Dependencies use Depends() not manual calls

[ ] Yield dependencies have proper cleanup

[ ] Request/Response models use Pydantic

[ ] HTTPException with status code and detail

[ ] All route handlers are async def

[ ] No blocking I/O (requests, time.sleep, open())

[ ] Background tasks for non-blocking operations

[ ] No bare except in route handlers

Valid Patterns (Do NOT Flag)

These are idiomatic FastAPI patterns that may appear problematic but are correct:

Pydantic validates request body automatically - No manual validation needed when using typed Pydantic models as parameters

Dependency injection for database sessions - Sessions come from Depends(), not passed as function arguments

HTTPException for all HTTP errors - FastAPI handles conversion to proper HTTP responses

Async def endpoint without await - May be using sync dependencies or simple operations; FastAPI handles this

Type annotation on Depends() - This is documentation/IDE support, not a type assertion

Query/Path/Body defaults - FastAPI processes these at runtime, not traditional Python defaults

Returning dict from endpoint - Pydantic converts automatically if response_model is set

Context-Sensitive Rules

Only flag issues when the context warrants it:

Flag missing validation ONLY IF the field isn't already in a Pydantic model with validators

Flag missing auth ONLY IF the endpoint isn't using Depends() with an auth dependency

Flag missing error handling ONLY IF HTTPException isn't raised appropriately for error cases

Flag sync in async ONLY IF the operation is actually blocking (file I/O, network calls, CPU-bound), not just non-async

Gates (FastAPI-specific)

Run once per FastAPI-related finding, after you can anchor file:line for the handler (see review-verification-protocol) and before the finding text ships. If a step’s pass condition is not met, do not assert the finding as written—gather evidence, withdraw, downgrade severity, or rephrase as a question.

Gate 1 — Route decorator and response surface

| Step | Action | Pass condition | |------|--------|---------------------| | 1a | Open the handler’s route decorator in the repo (not from memory). | file:line for @router.* / @app.* (or the site that registers this handler). | | 1b | Record HTTP method, response_model=, and status_code= on that decorator (or note they are absent). | Snippet from that line or explicit absent with the same file:line. |

Gate 2 — Blocking or “should be async”

| Step | Action | Pass condition | |------|--------|---------------------| | 2a | Read the full handler body. | file:line range covering the body. | | 2b | If claiming blocking I/O: name each blocking call (e.g. requests., open(, time.sleep, sync DB/ORM). | Each call has file:line, or withdraw the finding if none after the read. |

Gate 3 — Depends, validation, auth

| Step | Action | Pass condition | |------|--------|---------------------| | 3a | List parameters: Depends / Annotated[..., Depends], Pydantic models, Body/Query/Path, Request/Response. | Names + mechanism tied to file:line on the signature. | | 3b | If claiming missing auth: search the handler file (and its APIRouter module if separate) for Depends, Security, HTTPBearer, or project auth dependencies. | Citation to an existing hook, or search result: paths searched + N matches (zero is allowed). | | 3c | If claiming missing validation: confirm the argument is not already a Pydantic model or constrained Query/Path/Body. | Type/source with file:line, or withdraw if validation already applies. |

FastAPI Framework Behaviors

FastAPI + Pydantic handle many concerns automatically:

Request validation via Pydantic models

Response serialization via response_model

Dependency injection for cross-cutting concerns

Exception handling via exception handlers

Before flagging "missing" functionality, verify FastAPI isn't handling it.

When to Load References

Reviewing route definitions → routes.md

Reviewing dependency injection → dependencies.md

Reviewing Pydantic models/validation → validation.md

Reviewing async route handlers → async.md

Review Questions

1. Do all routes have explicit response models and status codes? 2. Are dependencies injected via Depends() with proper cleanup? 3. Do all Pydantic models validate inputs correctly? 4. Are all route handlers async and non-blocking?

Before Submitting Findings

1. For each FastAPI-related finding, complete Gates (FastAPI-specific) above. 2. Load and follow review-verification-protocol (Pre-Report checklist and Verification by Issue Type) before reporting any issue.