Here’s a quick checklist to debug:

• Check proxy_pass configuration: Ensure that proxy_pass points to http://ip:port/ without including the subpath (e.g., /book/). If you include the subpath, FastAPI won't match routes properly and will return 404.

• Verify trailing slash handling: FastAPI distinguishes between /book and /book/. Missing or inconsistent slashes can cause 404 errors. Make sure your URLs are consistent, or add a rewrite rule in Nginx to enforce a trailing slash.

• Check CORS settings: If accessing from browsers, CORS misconfiguration can prevent requests from reaching the backend, appearing like 404s. Make sure you add proper CORS middleware to your FastAPI applications.

• Ensure backend service availability: Confirm that your FastAPI apps are running, listening on 0.0.0.0 (not 127.0.0.1), and reachable at the specified IP and port. You can test direct access using curl or a browser to verify backend health.

If the issue persists, can you show your current Nginx config? It will help pinpoint the problem faster.