AI Gateway - Standardize provider credential error responses in AI Gateway

AI Gateway's REST API now returns consistent responses when an AI provider rejects credentials. The change applies to POST /ai/run ↗︎.

Scenario Previous AI Gateway response New AI Gateway response ElevenLabs Provider-specific UserCredentialsError with HTTP 403 HTTP 401 with error code 2009 Google Vertex HTTP 500 for rejected credentials, with upstream retries HTTP 401 with error code 2009; the request fails without retrying the provider All other providers HTTP 402 or another provider-specific status for rejected credentials HTTP 401 with error code 2009 When using Unified Billing, the provider rejects the credentials Provider-specific authentication error HTTP 503

Update applications that handle AI Gateway REST API errors to treat HTTP 401 as an invalid or rejected provider credential.

For details about providing provider credentials, refer to Bring your own provider keys.