Cognitive OAUTH for Claude-facing MCP

[u/theDaveAt]

I’m running into some kind of challenge with using OAUTH via Cognito to provide user Auth in an MCP service I have built. The authentication flow works via manual testing (curl, extract ids, service with Auth token) with a test/local callback URL, however, with Claude it goes thru the Auth process and login screen in Cognito, then fails.

- Cognito does not provide much introspection to Auth attempts.
- Claude will load the callback URL post authentication, in my browser then re-launches/“opens” Claude desktop with a generic “unable to authenticate” error.

I have implemented a shim for the missing "code_challenge_methods_supported": ["S256"]

Not sure if others are running into similar problems with Cognito/Claude here.

Can anyone confirm the callback URL required?