Agent Client Protocol (ACP)

ACP is an open protocol that standardizes communication between code editors and AI coding agents — the same role LSP plays for language tooling.

The Problem It Solves

Before ACP, every editor (VS Code, JetBrains, Zed) had to build a custom integration for every agent it wanted to support. ACP breaks this N×M problem into N+M: editors implement ACP once and gain access to all compliant agents; agents implement ACP once and work across all compliant editors.

How It Works

ACP runs over JSON-RPC 2.0. Two deployment modes:

• Local: agent runs as a subprocess, communication over stdin/stdout
• Remote: cloud-hosted agent communicates via HTTP or WebSocket

Sessions start with a capability negotiation handshake (initialize), then proceed through prompt turns:

1. Editor sends session/prompt with user content
2. Agent streams session/update notifications back: plans, thinking blocks, tool calls, message chunks
3. Agent requests tool permissions from the editor when needed
4. Agent responds with a stop reason: end_turn, max_tokens, refusal, or cancelled

ACP in the harness-kit Comparator

ACP-compatible harnesses (like Claude Code) are marked with an ACP badge in the comparator's Setup and Results phases. Once full ACP transport is implemented, the execution layer will exchange structured JSON-RPC events instead of parsing raw terminal output — producing richer, typed activity data for comparison.

Relationship to MCP

ACP and MCP compose rather than compete:

• MCP provides the tools an agent can call
• ACP defines how the editor and agent communicate during a session

ACP passes MCP server configurations to the agent at session startup; the agent connects to those MCP servers directly. ACP can also proxy MCP requests back through the editor.

harness-kit and ACP

harness-kit operates at the configuration layer — it defines which plugins, skills, MCP servers, and instructions an agent loads. ACP operates at the runtime layer — how the editor and agent exchange messages. These are complementary:

• harness-kit configures what an agent has access to
• ACP standardizes how the editor talks to that agent during work

Resources

• ACP Documentation
• GitHub: agentclientprotocol/agent-client-protocol
• ACP Agent Registry
• Official SDKs: Python · TypeScript · Rust · Kotlin · Java