[Bug] Claude Fable 5 automatically switches to different model mid-session without notification

Bug Description
Subject: Claude Fable 5 is being switched to another model during routine software
development

【What happens】
While using Fable 5 (claude-fable-5) in Claude Code CLI, the model is
automatically switched mid-task during ordinary software development work,
interrupting the session.

【What I was doing】
Refactoring work on an internal SaaS product. Specifically, a mechanical
rename of URL segments in a Next.js app
(/home/[account]/xxxxxxxxx/ → /home/[account]/lyyyyyyy) following a TDD flow:

• git mv on directories
• String replacement of URL literals in config files
• Regression checks against the existing test suite

All of this is mechanical substitution with no design decisions involved.
There is no security-sensitive content, no dual-use capability, and no
offensive tooling anywhere in the task. It is routine maintenance of a
standard business application.

【Impact】

1. The in-progress TDD cycle is cut in half.
2. Accumulated session context (investigation results, plan, implementation

approach) does not carry over usefully, causing rework.

1. Multi-stage workflows designed around Fable 5 (subagent delegation,

phase management) get handed to a model with different behavior and
quality characteristics mid-flight, which breaks output consistency.

【On the trigger】
No reason for the switch is surfaced to the user, so I have not been able to
identify what triggered it. Given the nature of the work, I suspect a
safety-related classifier fired incorrectly, but this is speculation and
I cannot confirm it.

【Requests】

1. Improve classifier precision so routine software development does not

trigger an automatic switch.

1. Surface the reason (or at least the category) when a switch occurs.

Without a reason, users can neither work around it nor report it precisely.

1. Provide a setting to disable automatic switching for development use.

Model consistency directly affects output quality across long
implementation sessions.

【Environment】

• Claude Code CLI
• Model: claude-fable-5
• OS: macOS (Darwin 24.6.0)
• Project: TypeScript / Next.js / Supabase monorepo

Environment Info

• Platform: darwin
• Terminal: ghostty
• Version: 2.1.220
• Feedback ID: 99b9751a-b66c-4393-b34a-e9845ccb7018

Errors

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