FastAPI

This skill provides comprehensive guidance for building APIs with FastAPI.

Quick Navigation

When to Use

• Creating REST APIs with Python
• Adding endpoints with automatic validation
• Implementing OAuth2/JWT authentication
• Working with Pydantic models
• Adding dependency injection
• Configuring CORS, middleware
• Uploading files, handling forms
• Testing API endpoints

Installation

Requires Python 3.10+. Install: pip install "fastapi[standard]" (full with uvicorn) or pip install fastapi (minimal). Add python-multipart for forms/files.

Release Highlights (0.139.2 -> 0.141.1)

• Frontend dev experience (0.141.0): checkdir on app.frontend() now defaults to "auto", which skips the frontend-directory existence check (with a warning) when FASTAPIENV=development and enforces it otherwise. fastapi dev sets FASTAPI_ENV=development automatically unless already set, so running the dev server before the frontend is built no longer raises an error.
• Frontend fixes (0.141.1): background tasks and response headers produced by dependencies used on frontend routes (dependency support was added in 0.139.0) are now applied correctly instead of being silently dropped. FASTAPI_ENV is documented in the FastAPI CLI guide.
• Dependency-injection memory refactor (0.140.0 – 0.140.7): internal dependency trees are no longer repeatedly flattened for OpenAPI generation, request-parameter resolution, and body-field handling, reducing memory use in large applications; the internal lru_cache limit for dependency resolution was also raised to fit bigger apps. This is an internal optimization with no required code changes.
• Streaming fixes (0.140.8 – 0.140.13): includerouter() no longer loses the declared stream item type from a nested router's yield-based endpoints; responsemodel* params (e.g. responsemodelexclude) are now honored on plain (non-generator) endpoints that return an Iterable[Model], not only on yield-based ones; statuscode set on SSE/JSONL streaming endpoints is now honored instead of always returning 200; a dedicated API reference page for fastapi.sse was added.
• jsonableencoder fix (0.140.9): excludedefaults now propagates into dict keys and values, not only into top-level model fields.

Release Highlights (0.137.0 -> 0.139.2)

• Frontend serving (0.138.0): app.frontend("/", directory="dist") serves a built static frontend directly from the FastAPI app. 0.139.0 adds dependencies support in app.frontend(), enabling automatic cookie authentication for frontends, and 0.139.1 fixes fallback for dotted paths like /users/john.doe.
• Router internals (0.137.0): include_router() now preserves APIRouter/APIRoute instances instead of cloning routes, so routes added after inclusion are reflected automatically and memory use drops. Custom APIRouter/APIRoute subclasses gain .matches()/.handle() (alpha) for advanced routing.
• Thread safety (0.139.2): route building is refactored to be thread-safe, mainly relevant for tests running across parallel threads.

Release Highlights (0.133.0 → 0.136.1)

• 0.134.0: streaming JSON Lines and streaming binary data support using yield.
• 0.135.0: first-class Server-Sent Events (SSE) support (EventSourceResponse).
• 0.135.1: fix around TaskGroup usage in request async exit stack (stability fix).
• 0.136.1: FastAPI updates its Pydantic v2 code to avoid deprecations and bumps Starlette to 1.0.0.

Patch Notes (0.136.2 → 0.136.3)

• SSE responses now validate event fields more strictly, so malformed ServerSentEvent payloads fail earlier instead of quietly streaming invalid frames.
• Header parameters no longer accept underscore-named incoming headers when convertunderscores=True (the default). If a client truly sends underscore headers, declare Header(convertunderscores=False) and verify that your proxy chain allows them.

Quick Start

from fastapi import FastAPI

app = FastAPI()

@app.get("/")
def read_root():
    return {"Hello": "World"}

@app.get("/items/{item_id}")
def read_item(item_id: int, q: str | None = None):
    return {"item_id": item_id, "q": q}

Run: fastapi dev main.py

Core Patterns

Type-Safe Parameters

from typing import Annotated
from fastapi import Path, Query

@app.get("/items/{item_id}")
def read_item(
    item_id: Annotated[int, Path(ge=1)],
    q: Annotated[str | None, Query(max_length=50)] = None
):
    return {"item_id": item_id, "q": q}

Request Body with Validation

from pydantic import BaseModel, Field

class Item(BaseModel):
    name: str = Field(min_length=1, max_length=100)
    price: float = Field(gt=0)

@app.post("/items/", response_model=Item)
def create_item(item: Item):
    return item

Dependencies

from fastapi import Depends

async def get_db():
    db = SessionLocal()
    try:
        yield db
    finally:
        db.close()

@app.get("/users/")
def list_users(db: Annotated[Session, Depends(get_db)]):
    return db.query(User).all()

Authentication

from fastapi.security import OAuth2PasswordBearer

oauth2_scheme = OAuth2PasswordBearer(tokenUrl="token")

async def get_current_user(token: Annotated[str, Depends(oauth2_scheme)]):
    return decode_token(token)

@app.get("/users/me")
def read_me(user: Annotated[User, Depends(get_current_user)]):
    return user

API Documentation

• Swagger UI: /docs
• ReDoc: /redoc
• OpenAPI: /openapi.json

Best Practices

• Use Annotated[Type, ...] for parameters
• Define Pydantic models for request/response
• Use response_model for output filtering
• Add status_code for proper HTTP codes
• Use tags for API organization
• Add dependencies at router/app level for auth

Prohibitions

• ❌ Return raw database models (use response models)
• ❌ Store passwords in plain text (use bcrypt/passlib)
• ❌ Mix Body with Form/File in same endpoint
• ❌ Use sync blocking I/O in async endpoints
• ❌ Skip HTTPException for error handling

Links

• Documentation
• Releases
• GitHub
• PyPI