🚨 Fable 5 completely unusable: 400 error on every request when advisorModel is set to Sonnet

Summary

Fable 5 is completely broken out of the box. Switching to claude-fable-5 via /model immediately causes a 400 on every single request. The error is not triggered by calling the advisor — it fires the moment any prompt is sent, because the tool schema itself (sent in every API request) is rejected.

Users who set "advisorModel": "sonnet" (explicitly or as a common recommendation in docs/guides) are fully blocked from using Fable 5 with zero indication of why.

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Error

API Error: 400 tools.10.model: 'claude-sonnet-4-6' cannot be used as an advisor
when the request model is 'claude-fable-5'.

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Root Cause

The advisor tool definition is included in every API request with a hardcoded model field (e.g. "model": "claude-sonnet-4-6"). When the main request model is claude-fable-5, the API rejects the entire request — not the tool call, but the request itself.

This means:

• The error happens before any response
• Denying the advisor tool via permissions.deny does not fix it (tool def still sent)
• There is no in-app error message explaining what's wrong or how to fix it

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Steps to Reproduce

1. Add to ~/.claude/settings.json:

{
  "advisorModel": "sonnet"
}

1. Run /model → select Fable 5
2. Type any prompt and submit

Result: API Error: 400 tools.10.model: 'claude-sonnet-4-6' cannot be used as an advisor when the request model is 'claude-fable-5'.

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Impact

| Scenario | Result |
|---|---|
| /model → Fable 5, advisorModel: "sonnet" | 💥 Every request fails |
| /model → Fable 5, advisorModel: "fable" | ✅ Works |
| /model → Sonnet 4.6, advisorModel: "sonnet" | ✅ Works |
| No advisorModel set, /model → Fable 5 | ❓ Untested — may also fail |

Anyone who followed setup guides or has advisorModel: "sonnet" in their config cannot use Fable 5 at all without manual intervention.

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Expected Behavior

Claude Code should auto-match advisorModel to the active session model when no explicit override is given — or at minimum, validate at model-switch time and warn the user, instead of silently failing on every request.

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Workaround

Manually edit ~/.claude/settings.json:

{
  "advisorModel": "fable"
}

⚠️ This breaks the advisor when switching back to Sonnet. There is currently no way to make both work simultaneously without manually toggling the setting each time.

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Environment

• Platform: macOS 25.5.0 (Darwin)
• Claude Code CLI (latest)
• Main model: claude-fable-5
• Advisor model: claude-sonnet-4-6 (via alias "sonnet")