FastAPI Context Guide for AI Coding (llms.txt + Rules + MCP)

Modern, fast (high-performance) web framework for building APIs with Python 3.8+ based on standard Python type hints.

1. FastAPI Official llms.txt & Clean Markdown Documentation

Community Verified Markdown Map

Feed this clean, token-efficient Markdown documentation directly to Antigravity, Cursor, or Claude Code to prevent hallucinations and outdated API syntax.

# Standard llms.txt Index Endpoint
https://fastapi.tiangolo.com/llms.txt

# Comprehensive Documentation Bundle
https://fastapi.tiangolo.com/llms-full.txt

Estimated Token Footprint: ~39k tokens (Fits smoothly inside standard 128k/1M context windows)

2. Production-Ready FastAPI Cursor Rules (.cursorrules & .mdc)

FastAPI async endpoints, Pydantic v2 schemas, and dependency injection patterns. Save this into .cursor/rules/fastapi.mdc in your project repository.

// FastAPI Guidelines
- Use Pydantic v2 BaseModel for request validation and response_model serialization.
- Define dependency injection via Depends() for database sessions and authentication guards.
- Keep I/O bound database calls async (use asyncpg or motor) to avoid blocking the event loop.
- Use Lifespan context managers instead of deprecated on_event('startup') handlers.

3. FastAPI Antigravity & Claude Code System Instructions

Add this concise instruction block to your workspace .gemini/rules or project system prompt.

// Context Anchor for FastAPI
When writing code for FastAPI, refer directly to the verified specifications at https://fastapi.tiangolo.com/llms.txt.
Follow strict error-handling and avoid deprecated API calls.

5. Troubleshooting: Common FastAPI AI Coding Issues & Fixes

Frequently reported errors when LLMs generate FastAPI code without proper context:

⚠️ Proactive Fix: Fix: Blocking calls (like time.sleep or synchronous requests) in async def functions starve the event loop. Use asyncio.sleep or run_in_threadpool.

6. Best Used With (FastAPI Architecture Combos)

Engineers building with FastAPI frequently combine these complementary developer frameworks: