MCP

px0 already holds two things a coding agent wants: a brain that can answer questions from what you have read, and a set of workflows that can be run. A CLI can be shelled out to, but MCP means the agent can discover what is there and what each thing takes, rather than having the shape of px0 explained to it in a prompt.

px0 mcp serve
px0 mcp serve --allow-runs

px0 mcp serve speaks MCP on stdin and stdout, one JSON-RPC message per line, until stdin closes. No MCP SDK is involved - the surface needed is initialize, tools/list, and tools/call over stdio.

`--allow-runs`

Let a client run workflows.

Input: flag, no value. Default off.
Off by default because a workflow can post, send, and file things. Without it, workflow_run is not even listed, and calling it returns an error explaining how to enable it.

What it exposes

Tool	What it does	Needs `--allow-runs`
`brain_ask`	Answer a question from the brain, citing the files used	No
`brain_search`	Return matching passages, without a model call	No
`workflows_list`	Every workflow, with its schedule and whether it is disabled	No
`guidelines_list`	Every guideline file	No
`guideline_read`	One guideline verbatim, to follow it	No
`workflow_run`	Run a workflow, optionally as a dry run	Yes

brain_ask and brain_search take an optional kind (blog, paper, doc, video, stub) and k. Nothing under the brain’s private folder is ever returned, the same guarantee that holds for px0 brain search and px0 brain ask on the command line.

Registering it with a client

Claude Code, and most MCP clients, take a command and its arguments:

{
  "mcpServers": {
    "px0": {
      "command": "px0",
      "args": ["mcp", "serve"],
      "env": { "PX0_HOME": "/Users/you/.px0" }
    }
  }
}

Set PX0_HOME when the client’s environment differs from your shell’s, since that is what selects the store.

Key	Effect
`retrieval.backend`	Which retrieval backend `brain_ask` and `brain_search` use
`retrieval.k_default`	Passages retrieved when a call names no `k`
`model.harness_cmd`	The backend `brain_ask` asks to write the answer

Exit codes

Code	When
`0`	stdin closed and the server stopped
`1`	No store, or an unknown subcommand

Get started

Workflows

Tools and connections

Brain

Guidelines

Runs

Reference

`--allow-runs`

What it exposes

Registering it with a client

Exit codes

​--allow-runs

​What it exposes

​Registering it with a client

​Related configuration

​Exit codes

`--allow-runs`

What it exposes

Registering it with a client

Related configuration

Exit codes