Compare the deployed inputs
Read the build and runtime logs first, then check the exact command, dependency lockfile, runtime version, and required environment variables. A value present in a local shell or ignored configuration file may not exist in the hosting environment.
Check case-sensitive paths, files excluded from the deployment, and code that expects writable local disk. Hosted filesystems may be read-only, ephemeral, or shared differently; persistent user data should use an intended storage service.
Reproduce and isolate
Use a clean build and the same production start command locally when possible. Separate build-time variables from runtime secrets, and verify that each is configured in the correct environment or deployment target.
Change one condition at a time and compare logs for the failing request. Avoid printing secret values while debugging; record whether a value is present and validate its shape without exposing credentials.