What is the message format of MCP? What are the common message types?

MCP's message format is based on the JSON-RPC 2.0 protocol, with extensions to support interactions between AI models and external systems. Here's a detailed explanation of the message format:

Basic Message Structure

All MCP messages follow the basic JSON-RPC 2.0 format:

json
{
  "jsonrpc": "2.0",
  "id": "unique-request-id",
  "method": "method-name",
  "params": { ... }
}

1. Request Message

Used for clients to send tool invocation requests to the server:

json
{
  "jsonrpc": "2.0",
  "id": "req-123",
  "method": "tools/call",
  "params": {
    "name": "calculate",
    "arguments": {
      "expression": "2 + 2"
    }
  }
}

2. Response Message

Server returns execution results:

json
{
  "jsonrpc": "2.0",
  "id": "req-123",
  "result": {
    "content": [
      {
        "type": "text",
        "text": "Result: 4"
      }
    ]
  }
}

3. Error Response

Returned when a request fails:

json
{
  "jsonrpc": "2.0",
  "id": "req-123",
  "error": {
    "code": -32602,
    "message": "Invalid params",
    "data": {
      "details": "Parameter 'expression' cannot be empty"
    }
  }
}

4. Notification Message

Server-initiated messages (no response required):

json
{
  "jsonrpc": "2.0",
  "method": "notifications/progress",
  "params": {
    "progress": 0.5,
    "message": "Processing..."
  }
}

Common Method Types

tools/list - Get list of available tools

json
{
  "jsonrpc": "2.0",
  "id": "req-001",
  "method": "tools/list"
}

resources/list - Get list of available resources

json
{
  "jsonrpc": "2.0",
  "id": "req-002",
  "method": "resources/list"
}

resources/read - Read resource content

json
{
  "jsonrpc": "2.0",
  "id": "req-003",
  "method": "resources/read",
  "params": {
    "uri": "file:///data/config.json"
  }
}

prompts/list - Get list of prompts

json
{
  "jsonrpc": "2.0",
  "id": "req-004",
  "method": "prompts/list"
}

Error Codes

MCP defines standard error codes:

Code	Name	Description
-32700	Parse error	JSON parse error
-32600	Invalid Request	Invalid request
-32601	Method not found	Method does not exist
-32602	Invalid params	Invalid parameters
-32603	Internal error	Internal error
-32000	Server error	Server error

Content Types

MCP supports multiple content types:

json
{
  "type": "text",
  "text": "Plain text content"
}

json
{
  "type": "image",
  "data": "base64-encoded-image-data",
  "mimeType": "image/png"
}

json
{
  "type": "resource",
  "uri": "file:///data/report.pdf",
  "mimeType": "application/pdf"
}

Message Streaming

For long-running operations, streaming responses are supported:

json
{
  "jsonrpc": "2.0",
  "id": "req-005",
  "method": "tools/call",
  "params": {
    "name": "generate_report",
    "arguments": {
      "stream": true
    }
  }
}

Best Practices:

1. Unique ID: Each request must have a unique ID
2. Type Validation: Strictly validate parameter types and formats
3. Error Handling: Provide detailed error information and data
4. Timeout Handling: Implement request timeout mechanisms
5. Logging: Log all messages for debugging and auditing

Understanding MCP's message format is crucial for implementing compatible servers and clients.