cursor-agent-api-proxy/src/acp-client.mjs

/**
 * Minimal ACP (Agent Client Protocol) JSON-RPC client over stdio.
 * Newline-delimited JSON-RPC 2.0 per ACP transport spec.
 */

export class AcpClient {
  /**
   * @param {object} opts
   * @param {import("node:stream").Writable} opts.stdin
   * @param {import("node:stream").Readable} opts.stdout
   * @param {(msg: string) => void} [opts.log]
   */
  constructor({ stdin, stdout, log, requestTimeoutMs }) {
    this.stdin = stdin;
    this.stdout = stdout;
    this.log = log || (() => {});
    this.requestTimeoutMs =
      requestTimeoutMs ?? parseInt(process.env.ACP_REQUEST_TIMEOUT_MS || "300000", 10);
    this._buf = "";
    this._nextId = 1;
    /** @type {Map<number, {resolve: Function, reject: Function}>} */
    this._pending = new Map();
    /** @type {(update: object) => void} */
    this.onSessionUpdate = null;
    /**
     * Handle incoming JSON-RPC requests from the agent (permissions, cursor extensions).
     * @type {(method: string, params: object) => Promise<object>}
     */
    this.onIncomingRequest = null;
    this._closed = false;
    this.agentCapabilities = null;

    stdout.setEncoding("utf8");
    stdout.on("data", (chunk) => this._onData(chunk));
  }

  _send(obj) {
    if (this._closed || this.stdin.destroyed) return;
    this.stdin.write(`${JSON.stringify(obj)}\n`);
  }

  _onData(chunk) {
    this._buf += chunk;
    let nl;
    while ((nl = this._buf.indexOf("\n")) >= 0) {
      const line = this._buf.slice(0, nl).trim();
      this._buf = this._buf.slice(nl + 1);
      if (!line) continue;
      let msg;
      try {
        msg = JSON.parse(line);
      } catch (e) {
        this.log(`[acp] parse error: ${e.message}`);
        continue;
      }
      this._dispatch(msg).catch((e) => this.log(`[acp] dispatch error: ${e.message}`));
    }
  }

  async _dispatch(msg) {
    if (msg.method === "session/update") {
      this.onSessionUpdate?.(msg.params || {});
      return;
    }

    if (msg.method && msg.id != null && !Object.prototype.hasOwnProperty.call(msg, "result") && !msg.error) {
      const handler = this.onIncomingRequest;
      if (!handler) {
        this._send({
          jsonrpc: "2.0",
          id: msg.id,
          error: { code: -32603, message: "no incoming request handler" },
        });
        return;
      }
      try {
        const result = await handler(msg.method, msg.params || {});
        this._send({ jsonrpc: "2.0", id: msg.id, result: result ?? {} });
      } catch (e) {
        this._send({
          jsonrpc: "2.0",
          id: msg.id,
          error: { code: -32000, message: e.message || "handler failed" },
        });
      }
      return;
    }

    if (msg.id != null && (msg.result !== undefined || msg.error !== undefined)) {
      const p = this._pending.get(msg.id);
      if (!p) return;
      this._pending.delete(msg.id);
      if (msg.error) {
        const err = new Error(msg.error.message || "ACP error");
        err.code = msg.error.code;
        err.data = msg.error.data;
        p.reject(err);
      } else {
        p.resolve(msg.result);
      }
    }
  }

  /**
   * @param {string} method
   * @param {object} [params]
   * @returns {Promise<object>}
   */
  request(method, params = {}) {
    const id = this._nextId++;
    return new Promise((resolve, reject) => {
      const timer = setTimeout(() => {
        if (!this._pending.has(id)) return;
        this._pending.delete(id);
        reject(new Error(`ACP request timeout after ${this.requestTimeoutMs}ms: ${method}`));
      }, this.requestTimeoutMs);

      this._pending.set(id, {
        resolve: (value) => {
          clearTimeout(timer);
          resolve(value);
        },
        reject: (err) => {
          clearTimeout(timer);
          reject(err);
        },
      });
      this._send({ jsonrpc: "2.0", id, method, params });
    });
  }

  async initialize() {
    const result = await this.request("initialize", {
      protocolVersion: 1,
      clientCapabilities: {
        fs: { readTextFile: false, writeTextFile: false },
        terminal: false,
      },
      clientInfo: { name: "cursor-agent-api-proxy", version: "1.6.0" },
    });
    this.agentCapabilities = result.agentCapabilities || null;
    if (!result.agentCapabilities?.promptCapabilities?.image) {
      throw new Error("cursor-agent ACP does not advertise image prompt capability");
    }
    return result;
  }

  async authenticateIfNeeded() {
    try {
      return await this.request("authenticate", { methodId: "cursor_login" });
    } catch (e) {
      this.log(`[acp] authenticate skipped or failed: ${e.message}`);
      return null;
    }
  }

  close() {
    this._closed = true;
    for (const [, p] of this._pending) {
      p.reject(new Error("ACP client closed"));
    }
    this._pending.clear();
  }
}