[BUG] Session does not use selected model (it uses another model), model display/select inconsistencies (VS Code extension)

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

Version 2.1.259

Selecting a model works sometimes, sometimes it uses another model (= not the one I selected).
I often switch between Opus 4.8 and Fable 5.1.
(I set "ANTHROPIC_DEFAULT_OPUS_MODEL": "claude-opus-4-8[1m]" to force Opus 4.8)

Either it shows Fable, but uses Opus or it shows Opus and uses Fable (I double checked it in the session JSON, so I'm sure it isn't just a display bug).
Sometimes the model tag suddenly switches after the first agent message.
It gets worse when I use multiple tabs.

And even the selection itself is mostly wrong, see this screenshot of a running session (the info contradicts):

<img width="689" height="423" alt="Image" src="https://github.com/user-attachments/assets/6ecbc846-d58e-41a0-aafb-51286f479604" />

The tag shows Opus 4.8, but the selection shows that Fable 5.1 is currently active.

When I restart VS Code, even an existing session may switch to another model when I continue.
The same may happen with the effort level.
This didn't happen in the version I had installed before the update (-> 2.1.220).

It's super annoying.
The only solution: I have to switch to another model and effort level and then to my desired model/effort, then I always get the correct one.

What Should Happen?

It should use the selected model and not a random one.

Error Messages/Logs

Steps to Reproduce

See above.

Claude Model

Not sure / Multiple models

Is this a regression?

Yes, this worked in a previous version

Last Working Version

2.1.220 (at least for me)

Claude Code Version

2.1.259

Platform

Anthropic API

Operating System

Ubuntu/Debian Linux

Terminal/Shell

Non-interactive/CI environment

Additional Information

_No response_