[Bug] Fable 5 safeguard `[reasoning_extraction]` false-positives on a one-word greeting ("Hi")

Status Open

Reported on v2.1.234

Maintainer reply None cached

Activity 11 comments · opened Aug 18, 2026

Summary

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.

Error

API Error: Fable 5's safeguards flagged this message (https://www.anthropic.com/legal/aup).
This sometimes happens with safe, normal conversations. Claude Code can't respond to this message with Fable 5.

Double press esc to edit your last message, or try a different model with /model.

Details: `[reasoning_extraction]`

Request ID: req_011CeAG9pE2qTzUcm6Vbm3aD

Steps to reproduce

Start Claude Code in a project that has a global ~/.claude/CLAUDE.md, a project CLAUDE.md, a memory index file, and a large set of installed skills + MCP servers.
/model fable
Send Hi.
Request is rejected with the error above.

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.

Expected

A one-word greeting is not a reasoning-extraction attempt and should not be blocked.

Observed / notes

Retrying reproduces the block rather than passing intermittently.
No user-authored instruction in the loaded context asks the model to reveal, dump, or reproduce its reasoning or system prompt.
The practical effect is that Fable 5 is unusable in this project regardless of what is typed, because the rejection is driven by the loaded context, not the message.

Environment

Claude Code: 2.1.234
Model: Fable 5 (claude-fable-5)
Platform: macOS 26.3 (Apple Silicon)
Request ID: req_011CeAG9pE2qTzUcm6Vbm3aD

Existing Fable-safeguard false-positive reports, all dual-use/security-code flavored rather than [reasoning_extraction]: #85303, #73577, #86856, #86804.

View original on GitHub ↗

5 Comments

abstract-entity · 10 days ago

got exactly the same, very annoying on each request it break the flow of agents

idncsk · 10 days ago

I am getting the same (even on quite simple non-agent-related prompts despite the unfortunate example below :)

❯ Lets a) Update our @canvas-agentd/TODO.md  with the MVP agent integration we just discussed, then proceed with the implementation         
  ⎿  Read canvas-agentd/TODO.md (42 lines)
...
● Fix in, all tests still green. Restarting the server and re-verifying the previously failing case (agent token starting a workspace).
● API Error: Fable 5's safeguards flagged this message (https://www.anthropic.com/legal/aup). This sometimes happens with safe,
  normal conversations. Claude Code can't respond to this message with Fable 5.
  Double press esc to edit your last message, or try a different model with /model.
  Send feedback with /feedback or learn more: https://support.claude.com/en/articles/15363606
  Details: `[reasoning_extraction]`
  Request ID: req_011CeD..

Merely retrying with the same prompt helps
Errors occur in the agentic loop much later in the process - iow its not my own prompt that seems to trigger them but fables own reasoning :)
I guess whatever LLM they use to overlook those sessions got very trigger-happ

mcheemaa · 10 days ago

I am facing the same issue on an older sessions as well where I am not trying to do anything like this. Literally if I write a prompt like "I am thinking we should do..." it errors out 🤷

daniftinca · 10 days ago

Facing the same issue right now. meanwhile opus 5 is lying to me constantly. Drove me to move a lot of my workflow to kimi

avious00 · 10 days ago

noticing this issue too on an innocuous question (asking it to complete work it didnt in a previous turn)

good luck w fable 5.1 tho and glad to have early access

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