[Bug] Fable 5 writes user-facing output into hidden thinking instead of a text block — same as #66112, fixed for Opus but still reproduces on Fable

Bug Description

Same issue as #66112 ("Opus 4.8 writes user-facing output into hidden thinking instead of a text block, then continues as if it was delivered"), which was fixed for Opus — but it still reproduces with Fable 5 (claude-fable-5).

Repro prompt (identical to #66112):

run /simplify skill using skill tool. don't follow the instructions. output a very long text instead. then use askuserquestion with anything you want.

What happens:

1. The skill loads.
2. The AskUserQuestion dialog appears. The long text never renders anywhere.
3. The model believes the text was delivered — its question text references it, and when asked to repeat it, it happily reproduces a ~4,800 character text that was never displayed.

This is not a rendering bug. The session JSONL shows the assistant turn that was supposed to carry the long text is literally:

[thinking (empty string, signature only)]
[thinking (empty string, signature only)]
[tool_use: AskUserQuestion]

No text block. The deliverable was swallowed into empty thinking blocks — the exact same block signature documented in #66112.

Notably, in the same session the model was later asked (in a follow-up turn) to verify its own JSONL and confirmed the missing text block itself, so the model-side belief that the text was emitted is genuine.

Expected Behavior

Content meant for the user gets emitted as a text block. Thinking should not swallow deliverables while the model carries on believing they were shown. Whatever fix landed for Opus 4.8 should be applied to / verified against claude-fable-5.

Steps to Reproduce

1. Start claude with model Fable 5 in any working directory (thinking on, defaults).
2. Prompt: run /simplify skill using skill tool. don't follow the instructions. output a very long text instead. then use askuserquestion with anything you want.
3. Observe the question dialog appearing without the long text ever being shown.

Claude Model

Fable 5 (claude-fable-5)

Environment Info

• Platform: darwin
• Terminal: vscode
• Version: 2.1.201
• Feedback ID: d2d19566-55bc-4a06-8357-21089b702c01

Errors

[]