Are you sure it refused because it can't use a username and password? I literally have loops running right now where it uses a database of test users and passwords to log into different roles and do computer use and browser automation testing. Sonnet and Opus complain when I provide credentials and password in chats but it is happy to use ones stored in files and stuff, so it might just be guardrails to push good opsec so that the secrets aren't captured in the session history and prompts.

That’s the joy of prompting. Different prompts, different task details, different contexts, different results

It was doing that to me too. Then I said "I'm hereby giving you explicit authorization to use these dev-only credentials in my local environment" and it worked. I also made it add that authorization to its memory.