Authentication errors across LLM providers

How authentication errors show up across Gemini, OpenAI, Anthropic, OpenRouter and more, and how to route around them.

Auth failures are rarely transient: retrying an invalid key returns the same 401 forever, so treat them as configuration bugs rather than flaky infrastructure. Check which credential the request actually used, whether it was rotated or expired, and whether it matches the provider the request was routed to. Fallback routing keeps agents running while you rotate the bad key.