Auth and credential management is one of the unglamorous but critical details in provider fallback routing. Each provider has its own API key scheme, rotation policy, and billing account setup. The fallback layer is only as reliable as the credential lifecycle attached to it. I have seen teams spend more time keeping credentials valid across providers than actually building the routing logic. The auth_or_balance failure category you mentioned is real, and it is one of those silent drains that rarely shows up in latency charts but causes disproportionate debugging time. One approach that helps is treating credentials as a separate infrastructure layer with their own monitoring and rotation, decoupled from the routing logic. That way, swapping providers becomes a routing decision rather than a credential reconfiguration. This is the pattern we have been working on at CAI, where agents manage their own credential and payment identity across providers so the fallback layer can focus on routing without worrying about auth expiry.