[BUG] Fable 5 advisor silently disabled by its own safety classifier — no Opus fallback, generic "unavailable", sticky-off for the session

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

Environment

• Claude Code v2.1.172
• VS Code integrated terminal
• Subscription (Max) account, direct Anthropic API auth
• Main model: Opus 4.8
• advisorModel: claude-fable-5

Per the Claude Code advisor accepted-pairs table, an Opus 4.6+ main accepts a Fable advisor, so this is a documented-valid pairing.

What happens

When the transcript contains content that trips Fable 5's safety classifier (cyber / bio / frontier_llm / reasoning_extraction), the advisor call fails with:

"The advisor tool is unavailable. Do not try to use it again."

The executor (Opus 4.8) then continues without any advice, and because of the "do not try again" instruction the advisor stays effectively disabled for the rest of the session.

Repro

1. Main model Opus 4.8, set advisorModel to claude-fable-5.
2. Put classifier-triggering material in context (e.g. an offensive-security/CTF or biology-adjacent file, or a CLAUDE.md/skill that instructs the model to "explain/show your reasoning").
3. Force an advisor call: "consult the advisor before continuing."
4. Observe the advisor call return "unavailable" and the executor proceed with no advice; subsequent advisor calls stay disabled.

Expected

Consistent with the main-model path, a Fable advisor refusal should fall back to an Opus advisor (Fable→Opus 4.8), so the executor still receives a second opinion — instead of silently losing the advisor.

Actual / root cause

The advisor is a server-side model call made inside tool execution. The Refusals-and-fallback docs note that the fallbacks configuration does not propagate into model calls made inside tool execution, and that declines after server tools run return the refusal rather than advancing to a fallback. So the Fable advisor's classifier decline never reaches the Fable→Opus fallback machinery; it surfaces to the executor as a generic advisor error (apparently advisor_tool_result_error with error_code "unavailable"), with no category information and no Opus substitution.

Impact

• The advisor is disabled precisely in the domains where a stronger reviewer is most valuable (security audits, biology-adjacent code).
• reasoning_extraction can trip on benign coding contexts whenever prompts/skills ask the model to explain its reasoning, so this is not limited to security/bio work.
• Net effect: Fable cannot be relied on as an advisor for any Opus/Sonnet/Haiku main, despite being an accepted pairing.

What Should Happen?

Requests

1. Apply classifier fallback to the advisor sub-inference (Fable→Opus 4.8 advisor), or auto-select a policy-compatible advisor model when Fable declines.
2. Replace the generic "advisor tool is unavailable" with a distinct message that names the refusal category (cyber/bio/frontier_llm/reasoning_extraction), so it isn't confused with config/auth failures.
3. Treat a classifier decline per-request, not sticky per-session — don't disable the advisor for the rest of the session after a single fire.
4. Tune the classifiers to fire on genuinely harmful intent rather than topic adjacency, and/or loosen the threshold to cut false positives. As currently shipped they are conservative enough to block benign defensive-security work, beneficial life-sciences work, and ordinary ML work, plus reasoning_extraction on routine "explain your reasoning" prompts. This is a the most serious problem of the Fable 5. The false-positive rate is the root driver of every problem above; reducing it would resolve most of them even before the fallback/UX fixes land.

Error Messages/Logs

Steps to Reproduce

1. Main model Opus 4.8, set advisorModel to claude-fable-5.
2. Put classifier-triggering material in context (e.g. an offensive-security/CTF or biology-adjacent file, or a CLAUDE.md/skill that instructs the model to "explain/show your reasoning").
3. Force an advisor call: "consult the advisor before continuing."
4. Observe the advisor call return "unavailable" and the executor proceed with no advice; subsequent advisor calls stay disabled.

Claude Model

Other

Is this a regression?

No, this never worked

Last Working Version

_No response_

Claude Code Version

2.1.172

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

VS Code integrated terminal

Additional Information

_No response_