Fable Mode MCP server

Fable Mode

<!-- mcp-name: io.github.REX-codebase/fable-mode -->

Agents that think before they write.

&nbsp; &nbsp; &nbsp; &nbsp;

Fable Mode is an open-source control plane for AI coding agents.

It makes an agent deliberate, produce evidence, and survive adversarial review before it earns permission to write to your workspace. The gates are mechanical, not prompt advice: no timer, no proof, no write access.

Demo

KERR // ORRERY

One self-contained HTML file. Raw WebGL, zero libraries, zero external assets, and zero build step.

https://github.com/user-attachments/assets/8287bbfe-e3ee-4dcf-ba0f-f9ff22ae79bd

7 renders rejected before final · 2 bugs caught · 10/10 red-team probes passed

Fable Mode overview

https://github.com/user-attachments/assets/27f4f8a2-b1bb-4398-a08c-bc9fd93d69d7

Quick start

A session starts locked. Confidence does not unlock it.

1. Install the MCP server using one of the options below.
2. Add the optional Agent Skill if you want the full workflow.
3. Ask your agent to use Fable Mode for a concrete coding task and choose a time budget.

A new session starts with execution locked:

{
  "action": "create_session",
  "session_name": "demo-refactor",
  "objective": "Refactor the parser without changing public behavior",
  "time_budget_minutes": 2
}

The agent then records evidence and an invariant. An early unlockexecution request is rejected until the authority timer and proof prerequisites pass. Use getstatus at any point to see the active phase, remaining time, evidence counts, and lock state.

The same gates guard every phase: evidence receipts for claims, a five-vector red-team swarm for code, and a sealed record of what was verified.

One package, two agent environments

Fable ships as one PyPI package. The same package contains the runtime, stdio MCP server, and complete Agent Skill. Setup is explicit so installing an MCP server never silently activates workspace instructions.

Run setup from the project the agent will work in:

uvx --from fable-engine==1.3.9 fable-mode setup --yes

This resolves the pinned package in an isolated uv environment and copies the bundled skill to .agents/skills/fable-mode. Use --dry-run to preview or --target for another skill directory. For a persistent install, use:

python -m pip install fable-engine
fable-mode setup --yes

Then choose only the invocation that matches the agent environment.

Native MCP client

Run fable-engine as the stdio server. For example:

// Claude Code: claude mcp add fable-engine -- uvx --from fable-engine==1.3.9 fable-engine
// Cursor or another JSON-configured client:
{
  "mcpServers": {
    "fable-engine": {
      "command": "uvx",
      "args": ["--from", "fable-engine==1.3.9", "fable-engine"]
    }
  }
}

Shortened. The full README is on GitHub.