[Feature Request] Add model performance monitoring and transparent API behavior documentation

Summary

When a newer model is released, older models (in this case Sonnet 5) appear to degrade sharply. I spent 4–5 hours stuck on a simple bug with Sonnet 5. Switching to Fable resolved it immediately.

If this is intentional routing or quality throttling, it is opaque and feels like a way to burn extra tokens. Please document expected model behavior and add a way to monitor quality regressions.

I have used Sonnet for more than six months across multiple accounts.

What happened

1. I was debugging a simple issue in Claude Code using Sonnet 5.
2. The model looped, repeated itself, and failed to make progress for 4–5 hours.
3. I switched the model to Fable.
4. The same task completed successfully.

Why this matters

• Large token spend with no useful output
• No signal that the model is underperforming
• No public documentation of how older models change after a new release
• Hard to trust Claude Code for production work if quality can drop without notice

Requested outcome

• Transparent documentation of model versions, routing, and any known quality changes after a new release
• In-product monitoring or a warning when a session is looping / not making progress
• A way to pin a known-good model version so behavior does not silently change
• Confirmation that older models are not intentionally degraded

Environment

• Platform: darwin
• Terminal: iTerm.app
• Version: 2.1.274
• Feedback ID: b0777965-efe8-403e-9a8b-e7b6e2c22a85

Errors

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