When Your Frontend Hits an API That Doesn't Exist — Debugging 405 Method Not Allowed

Got a bug report: "Renaming a project returns 405 Method Not Allowed" on a self-built dashboard. The cause was simple, but it's a classic gotcha when maintaining SPAs long-term, so I'm documenting it. Symptoms On the project list page, inline-editing a project name and saving triggers POST /api/simple-tasks/rename 405 (METHOD NOT ALLOWED) in the browser console. Flask's server log showed the same 405. Root Cause: Frontend-Backend Route Mismatch Flask had /api/update-project-name registered in a different context , but the frontend JavaScript was calling /api/simple-tasks/rename . In other words: The frontend (SPA) was written to use a new API path The backend (Flask) never got the corresponding route added Flask returns 405 because it doesn't know the path. You might expect 404, but when another route with the same prefix exists, Flask's routing can interpret it as "path matches but method doesn't" — hence 405 instead of 404. The Fix Added the corresponding endpoint near the existing s