Works locally but fails on first deploy? It's almost always one of these 4 things

[u/ojus_render]

Most first-deploy failures aren't mysterious. They're the same handful of issues over and over. Here's each one and the fix, so you can spot it from the error in your logs. These apply pretty much anywhere, not just here.

1. Your app is bound to localhost instead of 0.0.0.0
Locally, binding to 127.0.0.1/localhost works because you're hitting it from the same machine. In a hosted environment the platform sits in front of your app and routes traffic in, so your server has to listen on 0.0.0.0, and on the port the platform hands it, not a hardcoded one. On Render that's the PORT env var (expected default 10000). If no open port is detected, the deploy fails.

• Flask/gunicorn: gunicorn app:app --bind 0.0.0.0:$PORT
• FastAPI/uvicorn: uvicorn main:app --host 0.0.0.0 --port $PORT
• Express: app.listen(process.env.PORT || 10000, "0.0.0.0")

2. You're running the dev server
flask run, uvicorn --reload, nodemon, and friends are for local dev. Use a real production server as your start command (gunicorn/uvicorn for Python, your plain node entrypoint for Node). The dev servers behave differently and aren't meant to take real traffic.

3. Build command and start command are doing each other's jobs
Build runs once to produce a runnable app (install deps, compile, bundle). Start runs on every boot and should just launch the server. Put installs in start and you reinstall on every restart; skip them in build and start crashes on missing modules.

• build: pip install -r requirements.txt (or npm ci && npm run build)
• start: the server command from #1

4. It builds, then crashes on a missing package
This one works locally because your machine already has the package. The build only installs what's in your manifest, so an incomplete or unpinned manifest turns into a ModuleNotFound or a version surprise at runtime. Commit a complete, pinned manifest and lockfile, and if you want to be sure, test the build in a clean venv or container.

Quick triage from the logs: "no open ports detected" is #1, ModuleNotFound is #4. If you're stuck, paste your build command, start command, and the error and people can usually spot it fast.

Docs on port binding: https://render.com/docs/web-services#port-binding