Middleware & Error Handling

Add CORS, request IDs, and consistent exception handlers.

Recipe

Quick-reference recipe card - copy-paste ready.

from fastapi import FastAPI, Request
from fastapi.middleware.cors import CORSMiddleware
 
app = FastAPI()
app.add_middleware(CORSMiddleware, allow_origins=["https://app.example"], allow_methods=["*"], allow_headers=["*"])
 
@app.middleware("http")
async def add_request_id(request: Request, call_next):
    response = await call_next(request)
    response.headers["X-Request-Id"] = request.headers.get("X-Request-Id", "generated")
    return response

When to reach for this:

Browser clients need CORS
Uniform JSON errors
Cross-cutting request metadata

Working Example

from fastapi import FastAPI, HTTPException, Request
from fastapi.responses import JSONResponse
 
app = FastAPI()
 
class AppError(Exception):
    def __init__(self, code: str, status: int = 400):
        self.code = code
        self.status = status
 
@app.exception_handler(AppError)
async def app_error_handler(_: Request, exc: AppError):
    return JSONResponse(status_code=exc.status, content={"error": exc.code})
 
@app.get("/boom")
def boom():
    raise AppError("invalid_state", 422)

What this demonstrates:

CORS middleware
HTTP middleware for headers
Registered exception handlers

Deep Dive

How It Works

Middleware wraps requests outermost-first for add_middleware order.
Exception handlers convert domain errors to stable JSON.
Log server-side details; return safe messages to clients.

Gotchas

Boundary validation skipped - Invalid data reaches persistence layers.. Fix: Validate with Pydantic or framework forms at the edge..
Leaking stack traces - Clients see internal errors.. Fix: Map exceptions to stable HTTP responses..
Blocking async event loops - Workers stall under concurrent load.. Fix: Use async drivers or threadpool wrappers..
Secrets in source control - Credentials leak via git history.. Fix: Load secrets from env or a vault at runtime..
Missing observability - Incidents are hard to debug.. Fix: Add structured logs, metrics, and request IDs..

Alternatives

Alternative	Use When	Don't Use When
Alternate framework in this cookbook	Team standard or existing monolith	Greenfield API with different constraints
Managed BaaS	CRUD-only MVP	Custom auth, workflows, or compliance needs
gRPC	Internal high-performance RPC	Public HTTP clients and browser access

FAQs

When should I adopt FastAPI middleware?

Use it when the patterns and trade-offs on this page match your API or data boundary.

What is the top production mistake with FastAPI middleware?

Skipping validation, timeouts, or explicit error contracts at the HTTP edge.

How do I test FastAPI middleware?

Use the framework test client, override dependencies, and assert status plus JSON shape.

Does FastAPI middleware work with Python 3.14?

Yes - examples target Python 3.14 with pinned framework versions from the stack footer.

How does FastAPI middleware relate to Pydantic 2?

Validate and serialize at boundaries; keep services working with typed domain objects.

Sync or async?

Prefer async routes when I/O dominates; keep CPU work small or offload to workers.

Where should business logic live?

Thin handlers; services own rules; repositories own queries.

How do I document APIs?

Publish OpenAPI or schema docs that match response models in code.

How do I handle versioning?

Explicit URL or header versioning with deprecation windows - avoid silent breaks.

What should I read next?

Follow the Related links for the next layer of depth in this section.

How do I stay secure?

Authenticate callers, authorize per resource, rate-limit, and never log secrets.

Performance first step?

Measure DB and upstream latency before swapping frameworks.

FastAPI Basics - Core routes and models
Dependency Injection - Per-request providers
Testing FastAPI - Test client patterns
Deploying FastAPI - Production serving

Stack versions: This page was written for Python 3.14.0 (stable 3.14, maintenance 3.13), FastAPI 0.115+, Django 5.2, Flask 3.1, Pydantic 2, PyTorch 2.6+, pandas 2.2+, Polars 1.x, ruff 0.9+, and uv 0.6+.

Recipe

Quick-reference recipe card - copy-paste ready.

from fastapi import FastAPI, Request
from fastapi.middleware.cors import CORSMiddleware
 
app = FastAPI()
app.add_middleware(CORSMiddleware, allow_origins=["https://app.example"], allow_methods=["*"], allow_headers=["*"])
 
@app.middleware("http")
async def add_request_id(request: Request, call_next):
    response = await call_next(request)
    response.headers["X-Request-Id"] = request.headers.get("X-Request-Id", "generated")
    return response

When to reach for this:

Browser clients need CORS
Uniform JSON errors
Cross-cutting request metadata

Working Example

from fastapi import FastAPI, HTTPException, Request
from fastapi.responses import JSONResponse
 
app = FastAPI()
 
class AppError(Exception):
    def __init__(self, code: str, status: int = 400):
        self.code = code
        self.status = status
 
@app.exception_handler(AppError)
async def app_error_handler(_: Request, exc: AppError):
    return JSONResponse(status_code=exc.status, content={"error": exc.code})
 
@app.get("/boom")
def boom():
    raise AppError("invalid_state", 422)

What this demonstrates:

CORS middleware
HTTP middleware for headers
Registered exception handlers

Deep Dive

How It Works

Middleware wraps requests outermost-first for add_middleware order.
Exception handlers convert domain errors to stable JSON.
Log server-side details; return safe messages to clients.

Gotchas

Boundary validation skipped - Invalid data reaches persistence layers.. Fix: Validate with Pydantic or framework forms at the edge..
Leaking stack traces - Clients see internal errors.. Fix: Map exceptions to stable HTTP responses..
Blocking async event loops - Workers stall under concurrent load.. Fix: Use async drivers or threadpool wrappers..
Secrets in source control - Credentials leak via git history.. Fix: Load secrets from env or a vault at runtime..
Missing observability - Incidents are hard to debug.. Fix: Add structured logs, metrics, and request IDs..

Alternatives

Alternative	Use When	Don't Use When
Alternate framework in this cookbook	Team standard or existing monolith	Greenfield API with different constraints
Managed BaaS	CRUD-only MVP	Custom auth, workflows, or compliance needs
gRPC	Internal high-performance RPC	Public HTTP clients and browser access

FAQs

When should I adopt FastAPI middleware?

Use it when the patterns and trade-offs on this page match your API or data boundary.

What is the top production mistake with FastAPI middleware?

Skipping validation, timeouts, or explicit error contracts at the HTTP edge.

How do I test FastAPI middleware?

Use the framework test client, override dependencies, and assert status plus JSON shape.

Does FastAPI middleware work with Python 3.14?

Yes - examples target Python 3.14 with pinned framework versions from the stack footer.

How does FastAPI middleware relate to Pydantic 2?

Validate and serialize at boundaries; keep services working with typed domain objects.

Sync or async?

Prefer async routes when I/O dominates; keep CPU work small or offload to workers.

Where should business logic live?

Thin handlers; services own rules; repositories own queries.

How do I document APIs?

Publish OpenAPI or schema docs that match response models in code.

How do I handle versioning?

Explicit URL or header versioning with deprecation windows - avoid silent breaks.

What should I read next?

Follow the Related links for the next layer of depth in this section.

How do I stay secure?

Authenticate callers, authorize per resource, rate-limit, and never log secrets.

Performance first step?

Measure DB and upstream latency before swapping frameworks.

FastAPI Basics - Core routes and models
Dependency Injection - Per-request providers
Testing FastAPI - Test client patterns
Deploying FastAPI - Production serving

Stack versions: This page was written for Python 3.14.0 (stable 3.14, maintenance 3.13), FastAPI 0.115+, Django 5.2, Flask 3.1, Pydantic 2, PyTorch 2.6+, pandas 2.2+, Polars 1.x, ruff 0.9+, and uv 0.6+.

Middleware & Error Handling

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Middleware & Error Handling