Advisor tool: Fable 5 (claude-fable-5) consistently returns error_code "unavailable"; Opus 4.8 succeeds under identical config

Description

The advisor tool fails 100% of the time (2/2 attempts, two independent sessions) when advisorModel is set to fable (claude-fable-5), while the exact same executor (Sonnet 5) succeeds immediately when advisorModel is set to opus (claude-opus-4-8).

Environment

• Claude Code version: 2.1.205 (macOS)
• Executor model: Sonnet 5 (claude-sonnet-5)
• Advisor model under test: Fable 5 (claude-fable-5)

Steps to reproduce

1. Set "advisorModel": "fable" in settings (or pass --advisor fable) with executor --model sonnet.
2. Trigger an advisor call from within a session (interactively via the advisor tool, or headless: claude -p "<prompt instructing the model to call advisor>" --advisor fable --model sonnet).
3. Observe the tool result.

Expected behavior

Per the Model compatibility table, Claude Sonnet 5 → Claude Fable 5 is a documented, supported executor/advisor pair. A successful advisor_tool_result (variant advisor_redacted_result for Fable, per docs) is expected.

Actual behavior

Every attempt returns:

{"type": "advisor_tool_result_error", "error_code": "unavailable"}

Reproduced twice in independent sessions:

• requestId req_011CcqsqAHekKpGEFnCpymPy, 2026-07-09T04:47:29.042Z
• requestId req_011CcqtrbPQdvShwenumrYy4, 2026-07-09T05:00:37.074Z

No advisor_message iteration ever appears in usage.iterations for these calls — the advisor sub-inference appears to fail before any Fable-side tokens are consumed. This also means Fable advisor usage never shows up in account usage/billing despite advisorModel being correctly configured, which is what led me to investigate in the first place.

Control test (same executor, different advisor model)

Identical setup, only --advisor opus instead of --advisor fable:

claude -p "<explicit instruction to call advisor>" --advisor opus --model sonnet

succeeded immediately — requestId req_011CcquLsfyRnp8vEJGHTnam, 2026-07-09T05:07:00.116Z, advisorModel: "claude-opus-4-8", returned a proper advisor_result with real text content.

Why this matters

This makes the documented "Sonnet executor + Fable advisor" cost-optimization pattern (as described in Anthropic's own "the advisor strategy" post) non-functional in practice — the advisor call is attempted, correctly reaches the server, and is rejected every time, silently degrading to "no advice" while the executor continues unassisted. Since error_code: "unavailable" is documented as a generic catch-all, it isn't possible to self-diagnose further from the client side.

Ask

Could someone confirm whether Fable 5 (or Mythos 5) is currently fully enabled as an advisor model for standard accounts, or if this is a known rollout/availability gap?