When the MCP OAuth callback server's preferred port (default 3000) was unavailable, `OAuthCallbackFlow.#startCallbackServer` silently bound a random port and forwarded the mismatched `redirect_uri` to the authorization server. Providers that validate redirect URIs against a registered callback (e.g. Atlassian) returned an opaque HTTP 500, leaving the local flow waiting for a callback that never arrived until the 5-minute timeout fired.
Added `OAuthCallbackFlowOptions.allowPortFallback` (default `true`, preserving every existing AI-provider flow) and threaded `allowPortFallback: false` through `MCPOAuthFlow`'s `resolveCallbackOptions`. With fallback disabled, login now throws a `ConfigurationError` that names the busy port and the remediation (free the port, or set `oauth.callbackPort`/`oauth.redirectUri` in `mcp.json`) before opening the browser. The existing `oauth.redirectUri`-strict path is reworded along the same lines so callers see one consistent message family.
Fixes#3887
roboomp
d2c767507b