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Loom

MCPServer

The MCPServer system provides a standardized way to connect to tool servers that implement the Model Context Protocol (MCP). It supports both SSE (Server-Sent Events) and stdio (standard input/output) transports.

Creating a Server Connection

There are two built-in classes for connection:

  • MCPServerSSE: Connects to a remote MCP server using Server-Sent Events.
  • MCPServerStdio: Launches and communicates with a local MCP server using standard input/output.
import { MCPServerSSE, MCPServerStdio } from "loom-agents/mcp";


// Using SSE
const mcpSSE = new MCPServerSSE(new URL("http://localhost:3000/mcp"));


// Using stdio
const mcpStdio = new MCPServerStdio("node", ["./tool-server.js"]);

Listing Available Tools

Each server may expose a set of tools that can be queried:

const tools = await mcpSSE.getTools();
console.log(tools);
// {
//   tools: [
//     {
//       name: "calculate-bmi",
//       inputSchema: {
//         type: "object",
//         properties: [Object],
//         required: [Array],
//         additionalProperties: false,
//         $schema: "http://json-schema.org/draft-07/schema#",
//       },
//     },
//     {
//       name: "fetch-weather",
//       inputSchema: {
//         type: "object",
//         properties: [Object],
//         required: [Array],
//         additionalProperties: false,
//         $schema: "http://json-schema.org/draft-07/schema#",
//       },
//     },
//   ];
// }

This will automatically connect the client if it hasn’t already.

Calling a Tool

Once connected, tools can be called directly by name:

const result = await mcpSSE.callTool({
  name: "calculate-bmi",
  arguments: {
    "weightKg":70,
    "heightM":1.75
  }
});


console.log(result);
// {
//   content: [
//     {
//       type: "text",
//       text: "22.857142857142858",
//     },
//   ];
// }


// Note, Loom parses MCP results for errors and sends the content / output off `.content`
// [
//  {
//    type: "text",
//    text: "22.857142857142858",
//  },
//]

Extending MCPServer

If you want to implement a custom transport or extend tool behaviors, you can subclass MCPServerBase.

class CustomMCPServer extends MCPServerBase {
  constructor(myTransport: CustomTransport) {
    super(myTransport);
  }


  // Custom behavior
}

Internals

The MCPServerBase manages client connection state and handles listing and calling tools. It lazily initializes the client only when needed:

await this.client.connect(this.transport);

It caches tool metadata after the first call to getTools() to avoid redundant lookups.

  • Agent: Interacts with tools through an MCPServer
  • Tool Definition: Learn how tools are defined and exposed
  • Trace: Structured tracking of tool calls and agent interactions
  • Loom Configuration: Customize global AI behavior and context handling