FastAPI is the modern, high-performance Python web framework for building APIs that combines Python type hints with automatic OpenAPI documentation generation and async/await support — the dominant framework for deploying ML models, building LLM application backends, and creating AI microservices due to its exceptional developer experience, performance parity with Node.js, and native integration with the Python ML ecosystem.

What Is FastAPI?

• Definition: A Python web framework built on Starlette (ASGI web toolkit) and Pydantic (data validation) that uses Python type hints to define request/response schemas — automatically generating OpenAPI documentation, validating incoming requests, and serializing responses without additional boilerplate.
• Performance: FastAPI achieves performance comparable to Node.js and Go for async workloads by running on ASGI (Asynchronous Server Gateway Interface) with Uvicorn — benchmarks consistently place it among the fastest Python frameworks, limited only by Python's GIL for CPU-bound work.
• Type-Driven: The same Python type annotations that define your editor's autocomplete also define the API's validation rules, OpenAPI schema, and error messages — a single definition drives everything.
• Auto-Docs: FastAPI automatically generates interactive Swagger UI at /docs and ReDoc at /redoc from your endpoint function signatures — zero additional documentation effort for standard endpoints.
• Ecosystem: Developed by Sebastián Ramírez (tiangolo) in 2018 — now the most popular Python API framework on GitHub for new projects, having displaced Flask as the ML model serving standard.

Why FastAPI Matters for AI/ML

• ML Model Serving: Deploy any PyTorch/TensorFlow/Sklearn model as an HTTP API in ~20 lines of FastAPI code — model loads on startup, predict endpoint accepts structured JSON, returns predictions with automatic validation.
• LLM Application Backends: FastAPI powers the backends of AI applications — chat history management, streaming token responses via SSE, tool call handling, and user session management all supported natively.
• Async LLM Calls: Native async/await enables efficient concurrent LLM API calls — one FastAPI worker handles hundreds of concurrent OpenAI API requests without blocking, unlike sync Flask.
• Pydantic Integration: Request validation using Pydantic models catches malformed inputs before they reach model inference code — FastAPI returns structured 422 error responses with field-level validation messages automatically.
• Background Tasks: FastAPI supports background tasks for async processing — trigger model inference asynchronously and return a job ID, poll for completion, enabling long-running AI pipeline execution without blocking.

Core FastAPI Patterns

Basic ML Model Serving: from fastapi import FastAPI from pydantic import BaseModel import torch

app = FastAPI() model = torch.load("model.pt").eval()

class PredictRequest(BaseModel): text: str max_length: int = 100

class PredictResponse(BaseModel): prediction: str confidence: float

@app.post("/predict", response_model=PredictResponse) async def predict(request: PredictRequest) -> PredictResponse: with torch.no_grad(): output = model.generate(request.text, max_length=request.max_length) return PredictResponse(prediction=output.text, confidence=output.score)

LLM Streaming (SSE): from fastapi.responses import StreamingResponse from openai import AsyncOpenAI

openai = AsyncOpenAI()

@app.post("/chat/stream") async def chat_stream(request: ChatRequest): async def generate(): async with openai.chat.completions.stream( model="gpt-4o", messages=request.messages ) as stream: async for text in stream.text_stream: yield f"data: {json.dumps({"token": text})}

" yield "data: [DONE]

"

return StreamingResponse(generate(), media_type="text/event-stream")

Dependency Injection (auth, DB connections): from fastapi import Depends, HTTPException, status from fastapi.security import HTTPBearer

security = HTTPBearer()

def verify_api_key(credentials: HTTPAuthorizationCredentials = Depends(security)): if credentials.credentials not in valid_api_keys: raise HTTPException(status_code=401, detail="Invalid API key") return credentials.credentials

@app.post("/embed", dependencies=[Depends(verify_api_key)]) async def embed(request: EmbedRequest): return {"embeddings": embed_model.encode(request.texts).tolist()}

FastAPI vs Flask vs Django

FastAPI is the Python API framework that makes building production ML serving infrastructure fast, correct, and well-documented by default — by leveraging Python type hints for simultaneous validation, serialization, and documentation generation, FastAPI eliminates the boilerplate that previously made Python API development slow and error-prone.