Long-Running Operations & Non-Blocking Asynchronous Tasks

Standard MCP tools are strictly synchronous: the host issues a tools/call request and blocks waiting for the response. When an agent triggers operations requiring minutes or hours (such as building container images, fine-tuning a model, or executing extensive database migrations), synchronous execution risks socket disconnections, proxy timeouts, and frozen user interfaces.

import { McpServer } from "@modelcontextprotocol/sdk/server/mcp.js";
import { z } from "zod";

const server = new McpServer({ name: "cloud-builder", version: "1.0.0" });

server.tool(
  "run_container_build",
  { repoUrl: z.string().url(), tag: z.string() },
  async ({ repoUrl, tag }, extra) => {
    const taskId = "build_" + Date.now();
    
    // Spawn background worker process
    spawnBuildWorker(taskId, repoUrl, tag, async (current, total, msg) => {
      // Stream real-time progress back to the host client
      if (extra._meta?.progressToken) {
        await extra.sendProgressNotification({
          progressToken: extra._meta.progressToken,
          progress: current,
          total: total
        });
      }
    });

    return {
      content: [{ type: "text", text: "Build started asynchronously. Task ID: " + taskId }]
    };
  }
);