The Agent Client Protocol Overview

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) 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: Clients (editors like Zed, JetBrains or Obsidian) manage the user environment Agents (Claude Code, Gemini CLI, OpenClaw (prev. Clawdbot)) Thinking and tool execution They communicate over stdio (agent runs as subprocess) or HTTP (remote agents) 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 Protocol Flow Complete Lifecycle Terminal Execution Flow Permission Request Flow