Fixing 401 Unauthorized Errors in LLM APIs: A Practical Checklist

A 401 Unauthorized response from an LLM API means the server did not accept the credentials presented, which can stem from several different causes rather than just an incorrect key. The guide separates 401 from 403 and 429 errors, which point to permissions and rate limits respectively. Common culprits include missing or misnamed environment variables, malformed authorization headers, and stray whitespace copied along with the key. It recommends sending a single raw request with curl to determine whether the key itself is valid, and checking for key rotation or a mismatch between the key and the base URL. The author also advises against retrying 401 errors, since they are not transient and retries can trigger lockouts, so the error should be handled explicitly and fail fast.