Claude Code advisor: Opus 5.5 main model, Fable 5.2 advisor

Anthropic's Claude Code now supports an advisor tool that lets a main model do the work while consulting a stronger model only at key decision points, such as planning, recurring errors and final checks. With Opus 5.5 as the main model and Fable 5.2 as the advisor, the advisor receives the full conversation including tool calls and returns guidance, but never edits files itself; Claude Code and the API both enforce that the advisor ranks at or above the main model. The advisor can be set per session with /advisor, persisted via the advisorModel setting, or passed with the --advisor flag, and toggling it mid-session does not break the main model's prompt cache.

The Claude Code advisor lets one model do the work while a stronger model is consulted only at the moments that decide the outcome. With Opus 5.5 as the main model and Fable 5.2 as the advisor, you get Fable's judgement on the plan, on recurring errors and on the final check, without running Fable on every turn. The advisor is a server tool https://platform.claude.com/docs/en/agents-and-tools/tool-use/advisor-tool that runs on Anthropic's side. At a key moment the main model calls it; the advisor model receives the full conversation, every tool call and result included, and returns guidance. The main model applies that guidance and keeps working. The advisor never edits files itself, and when the main model's own evidence contradicts the advice a recommended step fails, a file says otherwise , it surfaces the conflict instead of following blindly Claude Code docs https://code.claude.com/docs/en/advisor . There are three ways, from the docs: 1. Inside a session: set it and save it as your default /advisor fable 2. In your settings file ~/.claude/settings.json : a persistent default { "advisorModel": "fable" } 3. For one session only, without changing the saved setting claude --advisor fable fable , opus and sonnet are aliases for Claude Code's current default in each family; a full model id such as claude-opus-5-5 also works. To check what is active, run /advisor with no argument. On our Mac Claude Code 2.1.322 it printed this before and after adding the setting: bash $ claude -p "/advisor" Advisor: off Usage: /advisor <fable|opus|sonnet|off after adding "advisorModel": "fable" to ~/.claude/settings.json $ claude -p "/advisor" Advisor: Fable 5.2 Usage: /advisor <fable|opus|sonnet|off On some plans, Fable as the advisor first needs a one-time consent: run /model fable and continue on Fable once, then pick Fable as the advisor. Until then Claude Code points you to /model fable and does not save Fable as the advisor. The rule: the advisor must rank at or above the session's main model. Claude Code checks the pairing before each request, and the API checks it again. | Main model | Accepted advisor | Note | |---|---|---| | Opus 5 or Opus 5.5 | Fable, Opus 5 or later | Opus does the work, Fable advises | | Sonnet 5.5 | Fable, Opus 5 or later, Sonnet 5.5 | Fable advice without running Fable throughout | | Fable 5.2 | Fable 5.2 only | Claude Code does not attach an Opus or Sonnet advisor to Fable | | Haiku 4.5 | Fable, Opus, Sonnet, Haiku 5.5 | Haiku 4.5 can call an advisor but cannot be one | Subagents inherit the advisor and run the same check against their own model. If you pick an advisor below your main model, Claude Code saves it but does not attach it. Claude decides, not a fixed rule. In practice it tends to consult: You can ask for one directly "consult the advisor before you continue" . There is no setting that caps or forces calls; say in your instructions if you want more or fewer. During a call the transcript shows an Advising line; press Ctrl+O to read the guidance. Turning the advisor on or off mid-session does not break the main model's prompt cache. Use the advisor for long tasks where most turns are routine but the plan decides the outcome: large refactors, a bug that keeps returning, work you want checked before it is called done. If you already run Codex CLI next to Claude Code https://systemdesignsimple.com/claude-code-vs-codex-cli/ for a second opinion, the advisor gives you one inside the Claude session. For short tasks, or work where every turn needs the strongest model, switch the main model with /model instead. To hand a whole subtask to another model, use a subagent with its own model. DISABLE TELEMETRY set . /advisor off turns it off; CLAUDE CODE DISABLE ADVISOR TOOL=1 disables the tool entirely. It is the coordinator pattern turned around: instead of a strong model directing many workers, one capable worker asks a stronger reviewer only at the decision points. Most turns of a coding task are routine read a file, run a test, make a small edit , and those do not need the strongest model. The few that set direction do. Paying for judgement only where it changes the result is the same trade-off as a second AI that reviews the first https://systemdesignsimple.com/one-ai-writes-one-ai-reviews/ , applied inside one session. For what Fable 5.2 itself can do, see the Claude Fable 5.2 review https://systemdesignsimple.com/claude-fable-5-2-mythos-5-2/ ; for the rest of the setup that makes long sessions reliable, see how to use Claude like a professional https://systemdesignsimple.com/claude-like-a-pro/ .