Chapter 14: The Model Context Protocol (MCP)

TL;DR

• MCP is the structured protocol for backend communication in Worka.
• Packs expose tools through MCP methods like tool.call.
• In Worka v2, MCP runs in‑process inside the pack’s WASM runtime.
• Example Request from Host:

  {
    "jsonrpc": "2.0",
    "method": "tool.call",
    "params": {
      "name": "your_tool_name",
      "arguments": { "param1": "value1" }
    }
  }

• Example Response from Server:

  {
    "jsonrpc": "2.0",
    "result": { "some_data": "your_response" }
  }

• Your server can also send elicit requests back to the host to get user input or to securely call other tools.

---

Worka uses the Model Context Protocol (MCP) as the contract between packs and the host. Packs expose tools; the host invokes them with structured input.

Understanding the basics of MCP is helpful, but remember that in practice, you will use a library like the rmcp crate for Rust that handles the low-level protocol details for you.

The Core Flow: tool.call

The primary job of an MCP server is to expose Tools and respond to requests from the Host to execute them. This is done via the tool.call method.

When a pack’s UI or an agent wants to run a tool, the host issues a tool.call request and passes structured arguments.

Request: POST /

{
  "jsonrpc": "2.0",
  "method": "tool.call",
  "params": {
    "name": "greet",
    "arguments": {
      "name": "Alice"
    }
  }
}

Your server must parse this request, find the function corresponding to the tool name (greet in this case), execute it with the provided arguments, and then send back a JSON response.

Successful Response:

{
  "jsonrpc": "2.0",
  "result": {
    "greeting": "Hello, Alice!"
  }
}

Error Response:

{
  "jsonrpc": "2.0",
  "error": {
    "code": -32602,
    "message": "Invalid params: Missing 'name' argument"
  }
}

Advanced Communication: elicit

Communication is not just one-way. A powerful feature of MCP is that your server can pause its work and send a request back to the Host. This is called an elicitation.

This is an advanced topic covered later, but there are two main uses:

1. To get input from the user: Your tool can ask the Host to render a form and wait for the user to fill it out.
2. To call another tool: Your tool can securely ask the Host to call a tool on another pack, including the virtual packs like worka/db.

This elicit mechanism is the foundation for building complex, interactive, and interconnected workflows on the Worka platform.