MCP — Model Context Protocol

What is MCP?

MCP (Model Context Protocol) is how AI assistants like Claude and ChatGPT call external tools. Your server exposes "tools" — functions with names, descriptions, and input schemas — and the AI model decides when to call them during a conversation.

How it works

All communication uses JSON-RPC 2.0 over HTTP POST to a single endpoint (e.g., /mcp):

Client sends initialize to establish the session

Server responds with its name, version, and capabilities

Client sends tools/list to discover available tools

Server returns a list of tools with descriptions and input schemas

Client sends tools/call with a tool name and arguments

Server executes the tool and returns the result

Implementation example

app.post('/mcp', (req, res) => {
  const { method, params, id } = req.body;

  if (method === 'initialize') {
    return res.json({ jsonrpc: '2.0', id, result: {
      protocolVersion: '2024-11-05',
      capabilities: { tools: {} },
      serverInfo: { name: 'my-service', version: '1.0' }
    }});
  }

  if (method === 'tools/list') {
    return res.json({ jsonrpc: '2.0', id, result: {
      tools: [{
        name: 'search',
        description: 'Search for items',
        inputSchema: {
          type: 'object',
          properties: { query: { type: 'string' } },
          required: ['query']
        }
      }]
    }});
  }

  if (method === 'tools/call') {
    const { name, arguments: args } = params;
    return res.json({ jsonrpc: '2.0', id, result: {
      content: [{ type: 'text', text: 'Search results...' }]
    }});
  }

  res.json({ jsonrpc: '2.0', id, error: { code: -32601, message: 'Method not found' }});
});

Key concepts

Tools: Functions your server exposes (e.g., "search", "create_invoice")
inputSchema: JSON Schema describing what arguments each tool accepts
Streamable HTTP: The standard transport — all messages via POST to one endpoint
SSE: Server-Sent Events — optional, used for server-to-client streaming via GET

Spec maturity

Formal specification. MCP is maintained by Anthropic with a versioned spec. Current protocol version: 2024-11-05. Supported natively by Claude, ChatGPT, and many AI tools.

Learn more