VibePager · How it works

The Agent Client Protocol, and why it matters on a phone

Before ACP, every editor that wanted to host a coding agent wrote a bespoke integration per agent — and anything outside an editor had to scrape a terminal. ACP is why that stopped.

Updated September 2026

What it is

The Agent Client Protocol is an open standard created by Zed Industries and released in August 2025. It defines how a client (an editor, or in our case a phone) talks to an agent (Claude Code, OpenCode, Gemini CLI…) over JSON-RPC 2.0 on standard input and output.

The comparison people reach for is MCP, and it is the right one but pointed the other way. MCP gives an agent access to tools. ACP gives a client access to an agent. They compose: an ACP client drives an agent that uses MCP tools.

Why it changes what a phone can do

Without a protocol, remote-controlling a coding agent means one of two bad options: write a custom integration for every agent, or read its terminal output and guess. Guessing is how you get a remote control that works until the agent changes a log line.

With ACP the agent tells you, structurally, what it is doing: this is a message, this is a tool call, this is a permission request with these options. That is exactly what a phone screen needs — it is the difference between showing you a wall of terminal text and showing you a card with the file, the diff and two buttons.

• Permission requests arrive as data, with their options — so they become buttons, not text you have to parse.
• Tool calls are identified, so "editing a file" and "running a command" can look different and be approved differently.
• Model and mode options come from the agent, so the picker shows what your install actually has instead of a hardcoded list.
• Sessions can be resumed, which is what lets you continue on the phone a conversation you started at the desk.

Who speaks it

The registry has grown past two dozen agents. Names you are likely to recognise include Claude Code, Codex CLI, GitHub Copilot CLI, Cursor, Gemini CLI, OpenCode, Kimi CLI, Kiro CLI, Qwen Code, Goose, Cline, Factory Droid, OpenHands, Mistral Vibe and Junie by JetBrains. On the client side: Zed natively, JetBrains IDEs, and community plugins for Neovim and Emacs.

How VibePager uses it

One ACP adapter covers OpenCode, Gemini CLI, Kimi and Kilo Code — the same handshake, unchanged, for all four. Claude Code and Codex have their own adapters because they predate the protocol and speak their own dialects. The phone does not know or care which is which: by the time an event reaches your screen it has the same shape either way.

Questions

What is the Agent Client Protocol?

An open standard from Zed Industries, released in August 2025, for how a client talks to a coding agent over JSON-RPC 2.0 on stdin and stdout. It is the agent-side counterpart to MCP: MCP gives agents tools, ACP gives clients agents.

ACP vs MCP — do I need both?

They are not alternatives. An ACP client drives an agent; that agent may use MCP servers for its tools. You can use either without the other.

If my agent speaks ACP, does it work with VibePager?

Not automatically. The supported list is Claude Code, Codex, OpenCode, Kilo Code, Gemini CLI and Kimi, plus Roo Code and Cline when installed. ACP is what makes adding another one cheap.

Is ACP only for editors?

No — that is the point. Anything that can speak JSON-RPC over a pipe can be a client, which is why a phone can be one.