---
title: "What is MSP, the Muse Code Session Protocol?"
url: https://helicon.sh/glossary/msp
description: "MSP is the JSON-RPC protocol that applications use to drive a Muse Code session over stdio, instead of parsing terminal output. What it carries and why it matters."
updated: 2026-09-19
site: Helicon
---

# What is MSP, the Muse Code Session Protocol?

The Muse Code Session Protocol, MSP, is the JSON-RPC interface a Muse Code host exposes over stdio when you run `muse serve`. It carries sessions, turns, tool calls, approvals and usage as typed events, so a client can render structure rather than parsing terminal text. Meta ships an MIT licensed SDK for it.

## What travels over it

- Sessions, keyed to a working directory.
- Turns, with the model and reasoning effort used.
- Tool calls, with the command and its output.
- Approval requests, with the mode that produced them.
- Usage, which is where cost and plan figures come from.

## Why protocol beats scraping

A wrapper that parses TUI output breaks whenever the TUI changes and can never see anything the TUI does not print. A protocol client receives typed events, so approvals are objects you can queue, diffs are structured, and usage is a number rather than a guess.

## Who can use it

Anyone. The SDK is MIT licensed, which makes client applications the intended path rather than a tolerated hack. That is the basis for saying Helicon is built the supported way even though it is unofficial.

## Frequently asked questions

### Is MSP the same as the Agent Client Protocol?

No. ACP is a general protocol several editors use for any agent. MSP is Muse Code's own interface, so it carries Muse Code specific things like its exact approval modes and usage reporting.

### Do I need to know about MSP to use a GUI?

No. It matters only because a protocol native client behaves better than one scraping a terminal.

### Is the SDK open source?

Yes, MIT licensed.

## Related

- [muse serve](https://helicon.sh/glossary/muse-serve): muse serve starts a Muse Code protocol host that applications can drive over stdio. What it is for, and how a GUI uses one host per workspace.
- [the Agent Client Protocol](https://helicon.sh/glossary/acp): ACP is a general protocol that lets editors talk to any conforming coding agent. How it differs from a Muse Code specific protocol, and what gets lost in the bridge.
- [Zed with ACP](https://helicon.sh/compare/zed-acp): Running Muse Code inside Zed through an Agent Client Protocol bridge, compared with a standalone Muse Code desktop app. Editor native versus session native.

---

Helicon is a free, MIT licensed, unofficial community client for Meta's Muse Code CLI. Not made, sponsored or endorsed by Meta. Source: https://helicon.sh/. Machine readable index: https://helicon.sh/llms.txt