[BUG] Xcode 26.6 Plugin Defaults to Fable, even when not available

### Preflight Checklist

- [x] I have searched [existing issues](https://github.com/anthropics/claude-code/issues?q=is%3Aissue%20state%3Aopen%20label%3Abug) 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?

I had been using the Claude Agent plugin in Xcode 26.3. After updating to 26.6, I had to re-install the plugin and re-authenticate. Initial use worked, but after 2-3 prompts, I got the following error: "API Error: Fable 5 requires usage credits. Update Claude Code to the latest version to learn more". In Xcode > Settings > Intelligence > Claude Agent the Model was set to Default, which it had always been. Much thrashing ensued, finally fixed by changing the Model to Sonnet and restarting Xcode. The issue then is that "Default" now apparently resolves to Fable, even for accounts (like mine) which do not have usage credits enabled, and are therefore unable to use Fable.

### What Should Happen?

The Default setting should resolve to a model the user has access to, preferably the model they had been using before the Xcode update.

### Error Messages/Logs

```shell

```

### Steps to Reproduce

1. Run Xcode 26.3 with Claude Agent's Model set to Default.
2. Update to Xcode 26.6

### Claude Model

Sonnet (default)

### Is this a regression?

Yes, this worked in a previous version

### Last Working Version

_No response_

### Claude Code Version

2.1.216

### Platform

Anthropic API

### Operating System

macOS

### Terminal/Shell

Terminal.app (macOS)

### Additional Information

_No response_

Start at the Claude Agent Model setting in Xcode and trace how Default is selected after the Xcode 26.6 update; the payload names no repository files or tests. Reproduce with Xcode 26.3 and 26.6 using Claude Code 2.1.216 and an account without Fable usage credits, then verify that Default selects an accessible model.

Written by the indexing model from the issue text.