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Clear out junk files and repair common Windows errorsFree Scan →Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →Claude Code can use Fable as a second-model advisor while Opus 5.5 remains the model doing the work. Claude decides whether to consult Fable—for example, before committing to an approach, after a recurring error, or before calling a task complete—and then applies its guidance. The feature is experimental, requires the Anthropic API, and does not guarantee a consultation on any particular turn.
What Fable does when Opus 5.5 is the main model
Anthropic describes the advisor as a server-side tool for consulting another model at selected moments. Opus 5.5 remains responsible for the session’s ordinary turns and work; Fable provides a second perspective when Claude Code chooses to ask. The advisor receives the full conversation, including tool calls and their results, and returns guidance for the main model to consider before continuing. See Anthropic’s Claude Code advisor documentation for the feature’s mechanics.
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This is not the same as switching the session’s main model to Fable. Nor is it a deterministic review gate: an advisor consultation is optional and model-driven.
How to set Fable as an advisor
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In Claude Code, run
/advisor fableto choose Fable as the advisor.What’s actually slowing this PC down?
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If Claude Code asks for consent, run
/model fable, accept the usage-credit prompt, then select Fable as the advisor. -
To save the selection in settings, set
"advisorModel": "fable"in a settings file. -
To use Fable for a single launch, start Claude Code with
claude --advisor fable.
Anthropic’s current documentation says Fable 5.1 requires Claude Code v2.1.257 or later. Check the official compatibility information and your organization’s model allowlist if the advisor is unavailable. The command also requires access to Fable. On some plans, advisor use consumes usage credits and requires one-time consent.
When Claude Code consults Fable
Claude decides when to call the advisor. Anthropic says consultations tend to happen before committing to an approach, when an error keeps recurring, and before declaring a task complete. The timing is not controlled by a fixed rule or user-configured trigger. You can ask Claude to consult the advisor in your prompt, but that does not guarantee a call; there is also no setting to force a call or cap the number of calls.
That makes Fable useful as an optional second opinion, not as a required approval step. If a task needs predictable human review or a guaranteed check, do not treat advisor use as a substitute for that process.
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Requirements and availability
The advisor requires the Anthropic API. Anthropic’s documentation says it is not available through Amazon Bedrock, Claude Platform on AWS, Google Cloud’s Agent Platform, or Microsoft Foundry. The feature is experimental, so its behavior, pricing, and availability may change.
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Feature-flag fetching is required for activation. Settings such as
DISABLE_TELEMETRYcan prevent activation because they disable feature-flag fetching.Free tools Windows power users keep installed
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CLAUDE_CODE_DISABLE_ADVISOR_TOOL=1disables the advisor feature entirely. -
Organization model allowlists can affect whether Fable is selectable.
How advisor use affects cost
Each advisor call adds the advisor model’s token rates to the main model’s usage. Fable processes the full transcript again on every call; its read is not cached. Consequently, total cost depends on how often Claude consults Fable and how long the conversation has become. Anthropic says toggling the advisor leaves the main model’s prompt cache intact, but that does not cache the advisor’s transcript read.
Anthropic separately says Opus 5.5 costs 40% less to run than Opus 5 on typical workloads at default settings, according to its September 22, 2026 Opus 5.5 announcement. That is a comparison between the two main models, not evidence that adding Fable as an advisor saves money. Anthropic’s advisor documentation explains token billing but does not report measured cost or quality results for the Opus 5.5–Fable pairing.
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| Approach | Who handles routine turns? | How the additional judgment is used | Key consideration |
|---|---|---|---|
| Opus 5.5 with Fable advisor | Opus 5.5 | Claude may consult Fable at selected decision points. | Calls are optional, and each one adds usage for a fresh full-transcript read. |
| Switch the main model to Fable | Fable | Fable is the main model rather than an occasional advisor. | This changes which model performs the session’s routine work. |
opusplan or subagents |
Depends on the selected setup | These are distinct Claude Code approaches, not the same advisor mechanism. | Choose based on the workflow you need; the advisor is specifically an optional second-model consultation. |
For a direct comparison of configurations and their supported behavior, consult Anthropic’s advisor documentation. The official materials describe how the advisor works, but do not establish that this exact pairing improves output quality or reduces total cost.
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