[BUG] Session falls back from Fable 5 to Opus 4.8 mid-task with no way to opt out

What's Wrong?

I set the session model to Claude Fable 5 (claude-fable-5) and started a large refactor of my personal application. Partway through the task, the session switched to Opus 4.8 (claude-opus-4-8) automatically. No prompt, no option to decline, no option to pause and wait for the Fable 5 limit to reset.

To be clear, this is not a problem with my application or my setup. It is the fallback behavior itself.

Why this matters in my case: I have already taken this codebase as far as Opus 4.8 can take it across many prior sessions. The entire reason I am refactoring now is to get Fable 5's perspective on the architecture. When the session drops to Opus 4.8 mid-stream, I am spending usage on a model that has nothing new to offer this project, and the refactor ends up with mixed output where I cannot tell which model produced which changes. That is worse than the session simply stopping.

Steps to Reproduce

1. Start a session with the model set to claude-fable-5
2. Run a long refactoring task until the Fable 5 usage threshold is reached
3. The session continues mid-task on claude-opus-4-8 without asking

Expected Behavior

Any one of these would resolve it:

1. Prompt at the switch point with real choices: continue on the fallback model, pause until the limit resets, or stop
2. A setting to disable automatic fallback entirely, so the session pauses instead of downgrading
3. At minimum, an unmissable marker in the session showing exactly where the switch happened and which output came from which model

Actual Behavior

The session downgrades to Opus 4.8 mid-task and keeps working as if nothing changed. The user has no control over whether or when this happens.

Impact

• Usage spent on a model I did not choose and have already exhausted for this codebase
• Mixed model output inside a single refactor, so the results cannot be attributed or trusted as Fable 5 work
• A session planned entirely around Fable 5 loses its purpose partway through

Environment Info

• Platform: darwin
• Terminal: vscode
• Version: 2.1.198
• Feedback ID: 9f4b485f-f4af-484a-bc86-eb15db7c6371

Errors

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