Claude Code can use Fable as a second-model advisor while Opus 5.5 remains the model doing the work. Claude—not a fixed rule or user-configured checkpoint—decides whether to consult Fable, typically before committing to an approach, after a recurring error, or before marking a task complete. The feature is experimental, requires the Anthropic API, and may add usage-credit charges.
What Fable does as an advisor
Anthropic describes the advisor as a server-side tool: Claude Code’s main model can send the conversation to a second model, receive its guidance, and apply that guidance before continuing. With this pairing, Opus 5.5 stays in charge of the session and performs the coding work; Fable supplies an additional perspective when Claude chooses to ask for one. The advisor receives the full conversation, including tool calls and their results, not just the latest user message. Anthropic’s advisor documentation explains the feature and its supported pairings.
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When Claude Code consults Fable
Claude controls the timing. Anthropic says, “Claude decides when to call the advisor.” It may consult before settling on an approach, when the same error keeps recurring, or before declaring a task complete. Those are examples, not guaranteed triggers: invoking the advisor is model-driven rather than a deterministic review step.
You can ask Claude to consult the advisor in your prompt, but there is no setting that forces a call or caps the number of calls. If you need a predictable review every time, treat the advisor as optional assistance rather than a substitute for an explicit human or scripted review.
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Set Fable as the advisor
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In a Claude Code session, run
/advisor fableto select Fable as the advisor. -
If prompted to give consent, run
/model fable, accept the usage-credit consent prompt, then select Fable as the advisor. -
To save the choice in settings, set
"advisorModel": "fable"in a Claude Code settings file.Special offer. See more information about Outbyte and uninstall instructions. Please review EULA and Privacy policy. -
To select it for a single launch, start Claude Code with
claude --advisor fable.
The current compatibility documentation says Fable 5.1 requires Claude Code v2.1.257 or later. Check Anthropic’s compatibility information and your organization’s model allowlist if the command or setting does not activate.
Requirements, availability, and controls
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The advisor is experimental and requires the Anthropic API. Anthropic says it is not available through Amazon Bedrock, Claude Platform on AWS, Google Cloud’s Agent Platform, or Microsoft Foundry. Behavior, pricing, and availability can change.
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Claude Code relies on feature-flag fetching for advisor activation. Anthropic notes that settings such as
DISABLE_TELEMETRY, which disable feature-flag fetching, can prevent activation. The environment variableCLAUDE_CODE_DISABLE_ADVISOR_TOOL=1disables the advisor tool entirely.Crashes, No Sound, or Screen Glitches?
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On some plans, Fable advisor use draws on usage credits and requires one-time consent before it can be selected. Follow the consent prompt if Claude Code presents one.
How advisor use affects cost
Each advisor call adds the advisor model’s token rates to the main model’s usage. The advisor processes the full transcript anew on every call; its transcript read is not cached. So the additional cost depends on how often Claude consults Fable and how much conversation it must read, rather than being a fixed per-session amount. The main model’s prompt cache remains intact when the advisor is toggled.
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Anthropic separately says Opus 5.5 costs 40% less to run than Opus 5 on typical workloads at default settings. That is a comparison between those main models—not evidence that adding Fable to Opus 5.5 reduces cost. Anthropic’s Opus 5.5 announcement provides the model-release context; the advisor documentation describes billing mechanics but does not publish a measured cost or quality result for this exact pairing.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Advisor, model switching, or another review approach?
The choice depends on whether you want Opus 5.5 to remain the routine worker while another model weighs in selectively, or want to change how the session is run. Anthropic’s documentation discusses the advisor alongside model switching, opusplan, and subagents; these are different workflows rather than interchangeable guarantees.
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Use the advisor when you want optional second-model input at moments Claude judges useful, while Opus 5.5 remains the main model.
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Switch models when you want a different model to handle the session’s main work, not merely advise Opus 5.5.
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Choose a predictable review process when a task requires a review at a specific point every time. Advisor timing is controlled by Claude and cannot be guaranteed through a setting.
There is no published combined-pairing trial result establishing that Opus 5.5 plus Fable produces better output or costs less than Opus 5.5 alone. The defensible expectation is narrower: this setup makes another model available for selected consultations, with added usage that varies by call frequency and transcript length.
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