App-scoped Vercel MCP connector fails client-credentials token exchange

I set up an app-scoped connector for the Software Factory guide:

Client Credentials: enabled
User Authorization: disabled
Refresh Tokens: enabled
Scopes: empty
supportedSubjectTypes: app

The stored dynamic client registers with tokenEndpointAuthMethod "none" (public client). Token exchange for client_credentials then fails, so the MCP connection cannot authenticate as the app principal.

Expected: an app-scoped connector with Client Credentials enabled should obtain a token and call the allowlisted MCP tools without a user sign-in.

Actual: authorization does not complete for principalType "app".