When a first-party MCP server (one using your claude.ai login) rejects the connection, Claude now emits a structured error with code FIRST_PARTY_AUTH_REJECTED and a more actionable message:
- HTTP 403: explains the token may be missing a required scope, and suggests running
/login - HTTP 401: tells you the token may have expired, and suggests
/login
The new code enables better diagnostics and retry logic — if a 401 occurs after sending an access token, Claude automatically refreshes the token and retries the request once.
Evidence
First-party auth error (search for "FIRST_PARTY_AUTH_REJECTED" and "missing a scope this server needs")
Strings lifted out of the shipped bundle, so the claim above can be checked against them.