uvicorn
Lightning-fast ASGI server for Python — production-grade async web server using uvloop and httptools. uvicorn features: HTTP/1.1 and HTTP/2, WebSocket support, worker processes (--workers N), hot reload (--reload for dev), SSL/TLS (--ssl-keyfile/--ssl-certfile), Unix socket support, graceful shutdown, access logging, --proxy-headers for reverse proxy, --root-path for sub-path deployment, --limit-concurrency for backpressure, programmatic startup via uvicorn.run() and Config, Gunicorn integration via UvicornWorker, and lifespan protocol support.
Score Breakdown
⚙ Agent Friendliness
🔒 Security
ASGI server. Use behind nginx/Caddy for SSL termination, rate limiting, and static files. --proxy-headers only when behind trusted proxy. Limit with --limit-concurrency for DoS protection. Do not expose uvicorn directly to internet without reverse proxy. Worker crashes = service degradation, not total failure with --workers.
⚡ Reliability
Best When
Serving FastAPI, Starlette, or other ASGI applications in production — uvicorn is the standard ASGI server with the best performance for Python async web applications.
Avoid When
WSGI apps (use gunicorn), Windows with multiple workers, or when Gunicorn's process management features are needed without uvicorn workers.
Use Cases
- • Agent FastAPI server — uvicorn main:app --host 0.0.0.0 --port 8080 --workers 4 — production; agent deploys FastAPI app with 4 worker processes; each worker handles requests independently; --workers requires fork() (not available on Windows)
- • Agent development server — uvicorn main:app --reload --host 127.0.0.1 --port 8000 — development; agent runs with auto-reload on file changes; --reload watches for .py changes; do NOT use --reload in production
- • Agent programmatic startup — import uvicorn; if __name__ == '__main__': uvicorn.run('main:app', host='0.0.0.0', port=8080, workers=4, log_level='info') — Python; agent starts uvicorn from Python code; useful for embedding in scripts or testing
- • Agent Gunicorn+Uvicorn — gunicorn main:app -w 4 -k uvicorn.workers.UvicornWorker -b 0.0.0.0:8080 — production; agent uses Gunicorn's process manager with uvicorn workers; best of both: Gunicorn process management + uvicorn async performance; recommended for production Kubernetes
- • Agent SSL termination — uvicorn main:app --ssl-keyfile key.pem --ssl-certfile cert.pem --host 0.0.0.0 --port 443 — SSL; agent runs with TLS directly (less common — usually offload to nginx/Caddy); --ssl-keyfile and --ssl-certfile for PEM files
Not For
- • WSGI applications — uvicorn is ASGI only; Flask/Django WSGI apps need gunicorn; or use Django ASGI mode
- • Standalone web server — uvicorn should be behind nginx/Caddy in production for static files, SSL, rate limiting
- • Windows multiprocessing — --workers N uses fork() which doesn't work on Windows; single process on Windows
Interface
Authentication
No auth — ASGI server. Application handles auth.
Pricing
uvicorn is BSD 3-Clause licensed. Free for all use.
Agent Metadata
Known Gotchas
- ⚠ pip install uvicorn[standard] for performance — base pip install uvicorn skips uvloop and httptools; these C extensions provide 2-3x performance improvement; agent code: requirements.txt: uvicorn[standard]; or install separately: pip install uvloop httptools; check: uvicorn --version shows if uvloop/httptools active
- ⚠ --workers not available on Windows — uvicorn --workers 4 uses os.fork() which Windows doesn't support; single worker only on Windows; agent Docker images: use Linux base images for multi-worker; Windows dev: single worker acceptable; production: always Linux with --workers
- ⚠ String import 'module:app' vs object — uvicorn.run('main:app') (string) for --reload; uvicorn.run(app) (object) for programmatic without reload; --reload requires string import to re-import module; agent code: use string form 'myapp.main:app' for production; import object for embedded testing
- ⚠ --proxy-headers required behind reverse proxy — when behind nginx/Caddy, X-Forwarded-For and X-Forwarded-Proto headers contain real client IP and protocol; without --proxy-headers: request.client.host is proxy IP; agent code: add --proxy-headers in Kubernetes/Docker Compose deployments behind ingress; --forwarded-allow-ips='*' for trusted proxies
- ⚠ lifespan protocol for startup/shutdown — @asynccontextmanager; async def lifespan(app): await startup(); yield; await shutdown(); app = FastAPI(lifespan=lifespan) — startup/shutdown hooks; uvicorn implements lifespan protocol; agent code: use lifespan for database pool creation on startup and cleanup on shutdown; replaces deprecated on_event handlers
- ⚠ Log format and level configuration — uvicorn default: access log to stdout, error log to stderr; --log-level info/warning/error/critical; --no-access-log to disable access log; --log-config path/to/log_config.json for custom format; agent code: parse uvicorn logs as JSON: uvicorn --log-config log_config.json with json-format formatter
Alternatives
Full Evaluation Report
Detailed scoring breakdown, competitive positioning, security analysis, and improvement recommendations for uvicorn.
Scores are editorial opinions as of 2026-03-06.