What
Claude Code now runs a background check that validates the current OAuth bearer token (the credential sent with requests to prove who is logged in) and measures how long that check takes. It only runs this for first-party sessions (i.e. logged in directly with Claude, not through another integration), logs a warning if the check fails, and records the timing and outcome. A related helper classifies how the session is authenticated into categories such as claude_ai_oauth, oauth_token_env, auth_token_env, api_key_helper, or wif.
Why
This gives Anthropic visibility into authentication health and latency without affecting normal usage, helping diagnose login or token problems by authentication method.