GitHub issue #87640 in anthropics/claude-code, opened 2026-08-18 by sagimedina1, records a report that the Fable 5 `[reasoning_extraction]` safeguard rejected a message whose user text was the single word `Hi`; the reporter states the classifier guards against prompts attempting to extract chain-of-thought, that nothing in the conversation did that, and that the same session context works normally on Opus 5, pointing at Fable-only safeguards rather than at the conversation.

“Fable 5 returns `API Error: Fable 5's safeguards flagged this message` with `Details: [reasoning_extraction]` on a message whose user text is the single word `Hi`.”

“The `[reasoning_extraction]` classifier guards against prompts attempting to extract the model's chain of thought. Nothing in the conversation does that — the first and only user turn was a greeting.”

“The same session context works normally on Opus 5 — only Fable 5 rejects it, which points at the additional Fable-only safeguards rather than at the conversation.”