Bennett Fenner from Zed demonstrates how to build an ACP (Agent Client Protocol)-compatible coding agent live in TypeScript. ACP is a JSON-RPC based protocol similar to MCP/LSP that allows AI coding agents and editor clients to communicate through a unified interface. The talk walks through adding ACP support to a minimal TypeScript coding agent: implementing initialize, session management, prompt handling, and cancellation; streaming token output via session updates; surfacing tool calls (file read/write) with status indicators; proxying file system operations through the client; and using the client's terminal capability. The demo shows the agent running inside Zed, with 40+ clients already supporting ACP including JetBrains, Obsidian, Cursor, and OpenCode.