---
title: The Agent Client Protocol Overview
link: https://www.philschmid.de/acp-overview
source: philschmid-de
published: 2026-02-01T00:00:00Z
updated: 2026-02-01T00:00:00Z
first_seen: 2026-09-27T19:29:15.293681927Z
summary: The Agent Client Protocol (ACP) is an open standard abstracts the events and outputs of AI agents and provides a common interface for editors to interact with them. Similar to MCP but for agent to client (UI) communication.
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---

With the explosion of AI coding agents—Claude Code, Gemini CLI, OpenCode, Goose, Codex CLI, we've entered an era where every major AI lab ships their own terminal-based coding assistant. But there is one problem. Each agent has its own events or outputs.

The [Agent Client Protocol (ACP)](https://agentclientprotocol.com/get-started/introduction) is an an open standard abstracts the events and outputs of AI agents and provides a common interface for editors to interact with them. Similar to MCP but for agent to client (UI) communication.

ACP is a JSON-RPC 2.0 protocol with a simple premise:

1. **Clients** (editors like Zed, JetBrains or Obsidian) manage the user environment
2. **Agents** (Claude Code, Gemini CLI, OpenClaw (prev. Clawdbot)) Thinking and tool execution
3. They communicate over **stdio** (agent runs as subprocess) or **HTTP** (remote agents)

![acp](https://www.philschmid.de/static/blog/acp-overview/acp.png)

## Protocol Events

| Method                       | Direction      | Purpose                                   |
| ---------------------------- | -------------- | ----------------------------------------- |
| `initialize`                 | Client → Agent | Negotiate protocol version & capabilities |
| `session/new`                | Client → Agent | Create new conversation session           |
| `session/load`               | Client → Agent | Resume existing session                   |
| `session/prompt`             | Client → Agent | Send user message                         |
| `session/set_mode`           | Client → Agent | Switch agent operating mode               |
| `fs/read_text_file`          | Agent → Client | Read file (includes unsaved edits)        |
| `fs/write_text_file`         | Agent → Client | Write/create file                         |
| `terminal/create`            | Agent → Client | Start shell command                       |
| `terminal/output`            | Agent → Client | Get command output                        |
| `terminal/wait_for_exit`     | Agent → Client | Wait for command completion               |
| `terminal/kill`              | Agent → Client | Terminate running command                 |
| `terminal/release`           | Agent → Client | Clean up terminal                         |
| `session/request_permission` | Agent → Client | Request user approval for action          |

### Notifications (One-Way)

| Notification     | Direction      | Purpose                               |
| ---------------- | -------------- | ------------------------------------- |
| `session/update` | Agent → Client | Stream progress, messages, tool calls |
| `session/cancel` | Client → Agent | Abort current operation               |

### Session Update Types

| Update Type             | Description            | Content                    |
| ----------------------- | ---------------------- | -------------------------- |
| `plan`                  | Agent's execution plan | List of steps with status  |
| `agent_message_chunk`   | Streamed text response | Text content block         |
| `user_message_chunk`    | Echo of user input     | Text content block         |
| `thought_message_chunk` | Agent's reasoning      | Text content block         |
| `tool_call`             | New tool invocation    | ID, title, kind, status    |
| `tool_call_update`      | Tool progress/result   | Status change, content     |
| `available_commands`    | Slash commands         | List of available commands |
| `mode_change`           | Mode switch            | New mode info              |

### Tool Call Status Flow

![tool](https://www.philschmid.de/static/blog/acp-overview/tool.png)

## Protocol Flow

### Complete Lifecycle

![protocol](https://www.philschmid.de/static/blog/acp-overview/full.png)

### Terminal Execution Flow

![terminal](https://www.philschmid.de/static/blog/acp-overview/terminal.png)

### Permission Request Flow

![permission](https://www.philschmid.de/static/blog/acp-overview/permission.png)