What is Agent Client Protocol (ACP) and how does OpenACP implement it?

[0xMag1ci4n]

I keep seeing "ACP" mentioned in the docs. What exactly is the Agent Client Protocol and how does OpenACP use it under the hood?

[jasondbranding]

Agent Client Protocol (ACP) is an open standard for communication between AI coding agents and client applications. Developed at agentclientprotocol.org.

How OpenACP implements ACP:

1. Agent subprocess: OpenACP spawns the agent as a subprocess and communicates via the ACP SDK (@agentclientprotocol/sdk)
2. Event system: Receives events from the agent: text, thought, tool_call, tool_update, plan, usage, commands_update, image_content, audio_content, session_end, error, system_message
3. Permission requests: Agent sends a permission request → OpenACP displays approve/deny buttons on the messaging app → sends result back to agent
4. Agent Registry: OpenACP uses the ACP Registry (agentclientprotocol.com) to discover and install agents. The registry supports: npx, uvx, binary, and custom distributions

How it works:

User message → OpenACP → AgentInstance (ACP subprocess) → Agent executes
Agent events ← AgentInstance ← OpenACP ← User receives results

Each AgentInstance implements the full ACP Client interface, converts ACP events into OpenACP AgentEvent types, and handles permission requests.
Related: What is OpenACP · Agents