A cached-but-unusable OAuth token (expired/revoked, or a refresh the IdP
rejects) makes the MCP SDK fall through to the authorization-code flow even
though build_oauth_auth's guard only checks token-file existence. In a
non-interactive context (systemd gateway, cron, background MCP discovery)
_redirect_handler then printed an auth URL / launched a browser flow no
operator can complete, and _wait_for_callback bound a localhost listener and
blocked for the full 300s timeout — gating gateway adapter startup and, on
retry, colliding on the callback port (OSError: [Errno 98] Address already
in use).
Re-check interactivity at the redirect/callback boundary and raise an
actionable OAuthNonInteractiveError before printing a URL, opening a browser,
or binding a listener. The guard holds regardless of whether a token file
exists (the point the token-file guard cannot cover), and only triggers on
the authorization-code path, so valid/refreshable tokens keep working
non-interactively. Both build_oauth_auth and MCPOAuthManager reuse these
handlers, so the sibling construction path is covered too.
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