Transports in the Model Context Protocol (MCP) provide the foundation for communication between clients and servers. A transport handles the underlying mechanics of how messages are sent and received.

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Message Format

MCP uses JSON-RPC 2.0 as its wire format. The transport layer is responsible for converting MCP protocol messages into JSON-RPC format for transmission and converting received JSON-RPC messages back into MCP protocol messages. There are three types of JSON-RPC messages used:

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Requests

{
  jsonrpc: "2.0",
  id: number | string,
  method: string,
  params?: object
}

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Responses

{
  jsonrpc: "2.0",
  id: number | string,
  result?: object,
  error?: {
    code: number,
    message: string,
    data?: unknown
  }
}

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Notifications

{
  jsonrpc: "2.0",
  method: string,
  params?: object
}

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Built-in Transport Types

MCP includes two standard transport implementations:

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Standard Input/Output (stdio)

The stdio transport enables communication through standard input and output streams. This is particularly useful for local integrations and command-line tools. Use stdio when:
  • Building command-line tools
  • Implementing local integrations
  • Needing simple process communication
  • Working with shell scripts
const server = new Server({
  name: "example-server",
  version: "1.0.0"
}, {
  capabilities: {}
});

const transport = new StdioServerTransport();
await server.connect(transport);

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Server-Sent Events (SSE)

SSE transport enables server-to-client streaming with HTTP POST requests for client-to-server communication. Use SSE when:
  • Only server-to-client streaming is needed
  • Working with restricted networks
  • Implementing simple updates
import express from "express";

const app = express();

const server = new Server({
  name: "example-server",
  version: "1.0.0"
}, {
  capabilities: {}
});

let transport: SSEServerTransport | null = null;

app.get("/sse", (req, res) => {
  transport = new SSEServerTransport("/messages", res);
  server.connect(transport);
});

app.post("/messages", (req, res) => {
  if (transport) {
    transport.handlePostMessage(req, res);
  }
});

app.listen(3000);

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Custom Transports

MCP makes it easy to implement custom transports for specific needs. Any transport implementation just needs to conform to the Transport interface: You can implement custom transports for:
  • Custom network protocols
  • Specialized communication channels
  • Integration with existing systems
  • Performance optimization
interface Transport {
  // Start processing messages
  start(): Promise<void>;

  // Send a JSON-RPC message
  send(message: JSONRPCMessage): Promise<void>;

  // Close the connection
  close(): Promise<void>;

  // Callbacks
  onclose?: () => void;
  onerror?: (error: Error) => void;
  onmessage?: (message: JSONRPCMessage) => void;
}

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Error Handling

Transport implementations should handle various error scenarios:
  1. Connection errors
  2. Message parsing errors
  3. Protocol errors
  4. Network timeouts
  5. Resource cleanup
Example error handling: 
class ExampleTransport implements Transport {
  async start() {
    try {
      // Connection logic
    } catch (error) {
      this.onerror?.(new Error(`Failed to connect: ${error}`));
      throw error;
    }
  }

  async send(message: JSONRPCMessage) {
    try {
      // Sending logic
    } catch (error) {
      this.onerror?.(new Error(`Failed to send message: ${error}`));
      throw error;
    }
  }
}

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Best Practices

When implementing or using MCP transport:
  1. Handle connection lifecycle properly
  2. Implement proper error handling
  3. Clean up resources on connection close
  4. Use appropriate timeouts
  5. Validate messages before sending
  6. Log transport events for debugging
  7. Implement reconnection logic when appropriate
  8. Handle backpressure in message queues
  9. Monitor connection health
  10. Implement proper security measures

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Security Considerations

When implementing transport:

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Authentication and Authorization

  • Implement proper authentication mechanisms
  • Validate client credentials
  • Use secure token handling
  • Implement authorization checks

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Data Security

  • Use TLS for network transport
  • Encrypt sensitive data
  • Validate message integrity
  • Implement message size limits
  • Sanitize input data

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Network Security

  • Implement rate limiting
  • Use appropriate timeouts
  • Handle denial of service scenarios
  • Monitor for unusual patterns
  • Implement proper firewall rules

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Debugging Transport

Tips for debugging transport issues:
  1. Enable debug logging
  2. Monitor message flow
  3. Check connection states
  4. Validate message formats
  5. Test error scenarios
  6. Use network analysis tools
  7. Implement health checks
  8. Monitor resource usage
  9. Test edge cases
  10. Use proper error tracking