Fable Mode — Claude Code MCP server: what it does, how to install
Enforces deliberation timers and structured proof receipts before letting AI agents edit code in your project.
claude mcp add fable-mode -- uvx fable-engineWhat Fable Mode does
Agents tend to jump straight into editing files without thorough analysis. Fable Mode addresses this with mechanical write gates exposed via MCP tools. When a session starts, write access stays locked. An agent must declare an objective and spend a dedicated time budget on analysis. Calling unlock_execution early fails immediately until the internal timer and proof steps conclude.
The workflow follows four stages: deliberate under a countdown, record proof receipts with invariants, survive an adversarial red-team check across five vectors, and finally gain write rights. Running the server requires Python 3.10 or newer. The underlying fable-engine package has zero external runtime dependencies and stores session records on disk, configurable via FABLE_DATA_DIR.
Without valid proof and an expired timer, the engine strictly rejects write permissions. Teams can also enable an optional adjudicator that validates proof receipts through an external review request before code changes begin.
What it helps with
- Prevent premature file edits during task deliberation
- Store verifiable proof receipts and invariants prior to modifications
- Run automated five-vector adversarial checks against proposed logic
- Inspect deliberation timers and current gate state with
get_status
Example prompts for the agent
- Start a 3-minute Fable Mode session to refactor our parser without breaking existing behavior.
- Check session progress using
get_statusto see how much deliberation time remains. - Record invariant proofs and trigger the red-team checks before requesting file access.
How to install Fable Mode
Platform CLI
claude mcp add fable-mode -- uvx fable-engineuvx runs the command named after the package; if the README names another one, use uvx --from fable-engine <command>
Platform CLI
codex mcp add fable-mode -- uvx fable-engineuvx runs the command named after the package; if the README names another one, use uvx --from fable-engine <command>
Config file
{
"mcpServers": {
"fable-mode": {
"command": "uvx",
"args": [
"fable-engine"
]
}
}
}uvx runs the command named after the package; if the README names another one, use uvx --from fable-engine <command>
Config file
{
"servers": {
"fable-mode": {
"type": "stdio",
"command": "uvx",
"args": [
"fable-engine"
]
}
}
}uvx runs the command named after the package; if the README names another one, use uvx --from fable-engine <command>
Platform CLI
gemini mcp add fable-mode uvx fable-engineuvx runs the command named after the package; if the README names another one, use uvx --from fable-engine <command>
Step by step, with commands for each platform: How to add an MCP server to Claude Code, Codex, Cursor, VS Code, Gemini CLI and more
Questions about Fable Mode
How do I install Fable Mode in Claude Code?
Run claude mcp add fable-mode -- uvx fable-engine in your terminal. You need Python 3.10 or newer installed on your machine.
What does Fable Mode do?
It enforces strict write barriers, preventing agents from altering workspace files until a deliberation timer expires, evidence receipts are recorded, and adversarial tests pass.
Does Fable Mode require third-party Python dependencies?
The underlying fable-engine package has zero external runtime dependencies. It saves session records locally, with the destination controlled by FABLE_DATA_DIR.
