Appears when: Health check / port error
The deployment cannot reach a healthy HTTP server on the expected interface and port within its startup window. Confirm the app listens on 0.0.0.0, uses the assigned port, starts with the production command, and answers its health route quickly.
# Preserve the failing logsCapture the first startup exception and the exact configured run command.# Correct host and port bindingUse the platform-provided port and listen on 0.0.0.0.# Separate startup from heavy workMove long migrations or external warmups out of the critical health-response path.# Verify process and endpointConfirm the server remains running and the published health route responds consistently.host: 0.0.0.0port: process.env.PORTPreserve the current working revision, change one proven boundary, and verify the published path in a clean session.
Read startup logs and application configuration. The server must bind to the external interface and assigned port.
What the result tells you: A localhost-only listener is unreachable from the deployment health checker.
Use the same command configured for deployment and confirm the process stays alive.
What the result tells you: A command that exits after building leaves no web server for the health check.
Make the root or dedicated health endpoint respond without waiting on slow migrations or external services.
What the result tells you: A response beyond the health-check window looks the same as a dead server.
| Likely cause | What proves it | First safe action |
|---|---|---|
| Server bound to localhost | A localhost-only listener is unreachable from the deployment health checker. | Preserve the failing logs |
| Hard-coded or wrong port | A command that exits after building leaves no web server for the health check. | Correct host and port binding |
| Start command exits | A response beyond the health-check window looks the same as a dead server. | Separate startup from heavy work |
| Startup is blocked | A localhost-only listener is unreachable from the deployment health checker. | Verify process and endpoint |
The deployment cannot reach a healthy HTTP server on the expected interface and port within its startup window. Confirm the app listens on 0.0.0.0, uses the assigned port, starts with the production command, and answers its health route quickly.
The process is healthy inside its container but unavailable to Replit’s routing layer.
The app ignores the deployment-provided port.
The command builds files but never starts a long-running server.
Freeze generated changes, restore the last known working version, reproduce one request, collect the browser and platform logs, and change one layer at a time.
It makes the server reachable through the deployment network rather than only from inside its own runtime.
Use the port provided by the deployment environment, with a local fallback only for development.
Yes. Keep the health response fast and move slow initialization out of that request path.
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