extensions/acpx/src/runtime-internals/mcp-proxy.mjs

#!/usr/bin/env node

import { spawn } from "node:child_process";
import { createInterface } from "node:readline";

function splitCommandLine(value) {
  const parts = [];
  let current = "";
  let quote = null;
  let escaping = false;

  for (const ch of value) {
    if (escaping) {
      current += ch;
      escaping = false;
      continue;
    }
    if (ch === "\\" && quote !== "'") {
      escaping = true;
      continue;
    }
    if (quote) {
      if (ch === quote) {
        quote = null;
      } else {
        current += ch;
      }
      continue;
    }
    if (ch === "'" || ch === '"') {
      quote = ch;
      continue;
    }
    if (/\s/.test(ch)) {
      if (current.length > 0) {
        parts.push(current);
        current = "";
      }
      continue;
    }
    current += ch;
  }

  if (escaping) {
    current += "\\";
  }
  if (quote) {
    throw new Error("Invalid agent command: unterminated quote");
  }
  if (current.length > 0) {
    parts.push(current);
  }
  if (parts.length === 0) {
    throw new Error("Invalid agent command: empty command");
  }
  return {
    command: parts[0],
    args: parts.slice(1),
  };
}

function decodePayload(argv) {
  const payloadIndex = argv.indexOf("--payload");
  if (payloadIndex < 0) {
    throw new Error("Missing --payload");
  }
  const encoded = argv[payloadIndex + 1];
  if (!encoded) {
    throw new Error("Missing MCP proxy payload value");
  }
  const parsed = JSON.parse(Buffer.from(encoded, "base64url").toString("utf8"));
  if (!parsed || typeof parsed !== "object" || Array.isArray(parsed)) {
    throw new Error("Invalid MCP proxy payload");
  }
  if (typeof parsed.targetCommand !== "string" || parsed.targetCommand.trim() === "") {
    throw new Error("MCP proxy payload missing targetCommand");
  }
  const mcpServers = Array.isArray(parsed.mcpServers) ? parsed.mcpServers : [];
  return {
    targetCommand: parsed.targetCommand,
    mcpServers,
  };
}

function shouldInject(method) {
  return method === "session/new" || method === "session/load" || method === "session/fork";
}

function rewriteLine(line, mcpServers) {
  if (!line.trim()) {
    return line;
  }
  try {
    const parsed = JSON.parse(line);
    if (
      !parsed ||
      typeof parsed !== "object" ||
      Array.isArray(parsed) ||
      !shouldInject(parsed.method) ||
      !parsed.params ||
      typeof parsed.params !== "object" ||
      Array.isArray(parsed.params)
    ) {
      return line;
    }
    const next = {
      ...parsed,
      params: {
        ...parsed.params,
        mcpServers,
      },
    };
    return JSON.stringify(next);
  } catch {
    return line;
  }
}

const { targetCommand, mcpServers } = decodePayload(process.argv.slice(2));
const target = splitCommandLine(targetCommand);
const child = spawn(target.command, target.args, {
  stdio: ["pipe", "pipe", "inherit"],
  env: process.env,
});

if (!child.stdin || !child.stdout) {
  throw new Error("Failed to create MCP proxy stdio pipes");
}

const input = createInterface({ input: process.stdin });
input.on("line", (line) => {
  child.stdin.write(`${rewriteLine(line, mcpServers)}\n`);
});
input.on("close", () => {
  child.stdin.end();
});

child.stdout.pipe(process.stdout);

child.on("error", (error) => {
  process.stderr.write(`${error instanceof Error ? error.message : String(error)}\n`);
  process.exit(1);
});

child.on("close", (code, signal) => {
  if (signal) {
    process.kill(process.pid, signal);
    return;
  }
  process.exit(code ?? 0);
});