Fable MCP server

mcp-server-fable

An MCP (Model Context Protocol) server for Anthropic Claude Fable 5 — Anthropic's most capable, most expensive model ($10 / $50 per 1M input / output tokens, ~2× Opus). Built in Rust, it exposes Fable as specialized MCP tools (plan, critique, ask) so any MCP client can use them.

Fable is not a day-to-day chat model here. This server is built for the one thing that justifies the price: using Fable to plan and critique, then handing the result to a cheaper model (Sonnet / Haiku / Opus) to execute. It works in Claude Code, Claude Desktop, Cursor, custom agents, and any other environment that supports MCP servers over stdio.

Communicates via stdio using JSON-RPC 2.0. Structurally it mirrors mcp-server-claude-chat, but is deliberately adapted to Fable's API surface and purpose-built for the plan-then-execute pattern.

Why mcp-server-fable (vs. Claude Code's built-in advisor)?

Claude Code has a powerful native advisor feature (/advisor fable or the advisor tool). It lets a fast executor model (typically Sonnet or Haiku) dynamically ask Fable for guidance on hard decisions inside a single session.

This MCP server takes a complementary approach that is useful in more situations:

• Explicit tools with handoff-optimized prompts — plan and critique use fixed, carefully written system prompts designed so the output can be passed verbatim to a cheaper model. Plans are numbered, unambiguous, include exact paths/signatures, edge cases, acceptance criteria, and out-of-scope notes. Critiques are deliberately coverage-first (report everything; filter downstream).
• Works everywhere MCP works — Not limited to Claude Code. Use it in Claude Desktop, Cursor, Windsurf, custom agent frameworks, VS Code MCP extensions, scripts, or any future tool that supports the Model Context Protocol.
• You control the orchestration — Call Fable for planning or review exactly when you (or your multi-agent system) decide, instead of relying on the model to escalate.
• Clean composability — Treat Fable as a reusable specialist service alongside your other MCP servers. Perfect for modern "expensive model for judgment, cheap model for execution" workflows.
• Correct Fable-specific behavior — Proper effort levels, refusal handling (no silent fallback), long timeouts, and accurate per-call cost reporting at Fable rates.

Many teams are converging on the same pattern the community discovered: use Fable narrowly for architecture, planning, and review, then execute with cheaper models. This server gives you first-class, portable tools for the "Fable parts" of that pattern.

See the "Technical details" section below for the specific Fable API differences that also required a dedicated implementation.

Subscription vs. API credits

This server uses the Anthropic API with an API key (API credits) — the only supported, terms-compliant way to drive Claude from a third-party tool. A Claude Pro/Max subscription is not usable here. Point base_url at an Anthropic-compatible gateway if you run one.

Technical details: Fable API differences

Fable's Messages API differs from the Opus-era shape (this is why a dedicated server was needed rather than reusing a general Claude chat wrapper):

Shortened. The full README is on GitHub.