Protocol Specification¶

The formal specification for the Model Context Protocol, defining message formats, behavior requirements, and implementation guidelines.

Specification Overview¶

The MCP specification is organized into modular components:

📋 Protocol Overview

Core protocol architecture, design principles, and module organization.

📨 Message Types

Complete reference for all request, response, and notification formats.

🔒 Security Model

Authentication, authorization, and transport security requirements.

📊 Version History

Protocol evolution, version compatibility, and migration guides.

✅ Conformance Tests

Test suite to verify protocol compliance and implementation correctness.

Quick Reference¶

Message Format¶

All MCP messages follow JSON-RPC 2.0:

{
  "jsonrpc": "2.0",
  "id": "unique-id",
  "method": "namespace/operation",
  "params": {
    // Method-specific parameters
  }
}

Core Namespaces¶

Namespace	Purpose	Example Methods
`initialize`	Session setup	`initialize`
`tools`	Tool operations	`tools/list`, `tools/call`
`resources`	Resource access	`resources/list`, `resources/read`
`prompts`	Prompt templates	`prompts/list`, `prompts/get`
`completion`	AI completions	`completion/complete`

Transport Requirements¶

MCP supports multiple transports:

stdio: Local process communication
HTTP/HTTPS: Network API access
WebSocket: Real-time bidirectional

Implementation Guidelines¶

1. Version Negotiation¶

Always start with capability negotiation:

{
  "method": "initialize",
  "params": {
    "protocolVersion": "1.0",
    "capabilities": {
      "tools": true,
      "resources": true
    }
  }
}

2. Error Handling¶

Use standard JSON-RPC error codes:

{
  "error": {
    "code": -32602,
    "message": "Invalid params",
    "data": {
      "param": "toolName",
      "issue": "Tool not found"
    }
  }
}

3. Schema Validation¶

All parameters must validate against JSON Schema:

{
  "inputSchema": {
    "type": "object",
    "properties": {
      "query": {
        "type": "string",
        "minLength": 1
      }
    },
    "required": ["query"]
  }
}

Compliance Levels¶

Basic Compliance¶

✅ JSON-RPC 2.0 message format
✅ Initialize handshake
✅ At least one of: tools, resources, or prompts
✅ Standard error codes

Full Compliance¶

✅ All basic requirements
✅ Complete capability negotiation
✅ Schema validation
✅ Batch request support
✅ Proper lifecycle management

Extended Features¶

✅ Streaming responses
✅ Resource subscriptions
✅ Dynamic capability updates
✅ Custom extensions

Design Principles¶

The MCP specification follows these core principles:

Simplicity - Easy to implement correctly
Extensibility - Add features without breaking compatibility
Security - Secure by default
Performance - Efficient for real-world use
Interoperability - Work across platforms and languages

Reading the Specification¶

For implementers:

Start with the Protocol Overview
Understand Message Types
Review the Security Model
Test with Conformance Tests

Specification Status

The MCP specification is actively maintained. Check Version History for the latest updates and planned changes.

Contributing to the Spec¶

Found an issue or have a suggestion? The specification is developed openly:

Review process on GitHub
Request for Comments (RFC) system
Regular community reviews
Backward compatibility commitment