[Bug]: Fallback model has no reasoning selector and always starts with Low effort #27449

Before submitting

• I searched existing issues and did not find a duplicate of this fallback-settings problem.
• I included enough detail to reproduce or investigate the problem.

Area

apps/web

Steps to reproduce

1. Open Settings → New threads.
2. Set the default Model to Opus 5.5, with reasoning set to Medium.
3. Set Fallback model to 6.1 Sol.
4. Try to choose a reasoning effort for the fallback model.
5. Start a new thread when the default model's provider is out of usage and T3 uses the fallback.

Expected behavior

The fallback model should expose its own supported reasoning-effort selector, just like the default model. The selected effort should be saved and used when fallback starts a new thread.

Actual behavior

The default Model row shows both the model and a Medium reasoning control. The Fallback model row only shows 6.1 Sol, with no reasoning control.

When fallback is used, it always starts with Low reasoning in my experience. There is no way to configure a different fallback reasoning level in these settings.

Impact

Minor bug or occasional failure

Every fallback thread starts at an unwanted reasoning level, requiring manual adjustment.

Version or commit

Exact build not recorded.

Environment

T3 Code on macOS. Default model: Opus 5.5 / Medium. Fallback model: 6.1 Sol.

Screenshots, recordings, or supporting files

The screenshot demonstrates the missing selector; the Low behavior is my reported observation.

Related issue

#26707 concerns a missing reasoning selector for built-in Sol in normal threads. This report concerns the separate Fallback model setting and the reasoning used when fallback activates; the root cause has not been established.