MCP Protocol Overview

Model Context Protocol (MCP) Overview

The Model Context Protocol (MCP) is a standardized protocol for communication between AI language models and external systems. MCP Client Tester provides comprehensive support for testing and debugging all aspects of this protocol.

Protocol Architecture

MCP follows a client-server architecture where:

• MCP Client: Typically an AI application (like Claude Desktop, custom chatbots, or AI assistants)
• MCP Server: Provides tools, resources, and other capabilities to the client
• Transport Layer: Handles communication between client and server

graph LR
    A[AI Client] <-->|MCP Protocol| B[MCP Server]
    B <--> C[External Systems]
    B <--> D[Databases]
    B <--> E[APIs]
    B <--> F[File Systems]

Core Protocol Features

1. Tools

Tools are functions that clients can invoke on the server. They represent actions the AI can take.

Tool Call Request

{
  "jsonrpc": "2.0",
  "id": "1",
  "method": "tools/call",
  "params": {
    "name": "search_database",
    "arguments": {
      "query": "user preferences",
      "limit": 10
    }
  }
}

Tool Call Response

{
  "jsonrpc": "2.0",
  "id": "1",
  "result": {
    "content": [
      {
        "type": "text",
        "text": "Found 3 matching records..."
      }
    ]
  }
}

2. Resources

Resources provide read-only data that clients can access. They represent information the AI can read.

Resource Read Request

{
  "jsonrpc": "2.0",
  "id": "2",
  "method": "resources/read",
  "params": {
    "uri": "file:///path/to/document.txt"
  }
}

3. Prompts

Prompts are reusable templates that can be invoked by clients with parameters.

Prompt Get Request

{
  "jsonrpc": "2.0",
  "id": "3",
  "method": "prompts/get",
  "params": {
    "name": "code_review",
    "arguments": {
      "language": "python",
      "file_path": "main.py"
    }
  }
}

4. Sampling

Servers can request the client to generate content using the LLM.

Sampling Request

{
  "jsonrpc": "2.0",
  "method": "sampling/create",
  "params": {
    "messages": [
      {
        "role": "user",
        "content": "Analyze this code for potential issues"
      }
    ],
    "max_tokens": 1000
  }
}

5. Progress & Cancellation

Long-running operations support progress reporting and cancellation.

Progress Notification

{
  "jsonrpc": "2.0",
  "method": "progress",
  "params": {
    "progressToken": "operation_123",
    "progress": 45,
    "total": 100
  }
}

Transport Protocols

MCP supports multiple transport mechanisms, each with different characteristics:

STDIO

Advantages

• Simple process-based communication
• Automatic lifecycle management
• No network configuration needed
• Ideal for local development

Limitations

• Local-only communication
• No concurrent connections
• Process overhead

HTTP

Advantages

• Standard HTTP infrastructure
• Works through firewalls/proxies
• Request/response semantics
• Good for REST-style interactions

Limitations

• No server-initiated messages
• Polling required for notifications
• Higher latency for real-time updates

Server-Sent Events

Advantages

• Real-time server-to-client updates
• Built on standard HTTP
• Automatic reconnection
• Efficient for notifications

Limitations

• Unidirectional (server to client)
• Requires separate request channel
• Browser connection limits

HTTP Streaming

Advantages

• Bidirectional communication
• Low latency
• Efficient for real-time apps
• Single connection

Limitations

• Complex connection management
• Proxy/firewall challenges
• Requires streaming support

Message Types

MCP uses JSON-RPC 2.0 as its base messaging protocol with three message types:

Requests

Messages that expect a response from the recipient.

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

Responses

Replies to request messages, containing either results or errors.

Success Response

{
  "jsonrpc": "2.0",
  "id": "unique-request-id",
  "result": {
    // Method-specific result data
  }
}

Error Response

{
  "jsonrpc": "2.0",
  "id": "unique-request-id",
  "error": {
    "code": -32601,
    "message": "Method not found",
    "data": {
      // Optional error details
    }
  }
}

Notifications

One-way messages that don’t expect a response.

{
  "jsonrpc": "2.0",
  "method": "notification_method",
  "params": {
    // Notification-specific parameters
  }
}

Protocol Lifecycle

Understanding the typical flow of MCP communication:

sequenceDiagram
    participant C as Client
    participant S as Server

    C->>S: Initialize connection
    S-->>C: Initialization response

    C->>S: tools/list
    S-->>C: Available tools

    C->>S: resources/list
    S-->>C: Available resources

    loop Normal Operation
        C->>S: tools/call
        S-->>C: Tool result

        S->>C: progress (notification)

        C->>S: resources/read
        S-->>C: Resource content
    end

    C->>S: Disconnect

Error Handling

MCP defines standard error codes for common scenarios:

Code	Name	Description
-32700	Parse error	Invalid JSON
-32600	Invalid Request	Invalid JSON-RPC
-32601	Method not found	Unknown method
-32602	Invalid params	Invalid parameters
-32603	Internal error	Server error
-32000	Server error	Server-specific error

Tip

MCP Client Tester automatically detects and logs all error conditions, helping you identify and debug protocol issues quickly.

Testing with MCP Client Tester

MCP Client Tester provides comprehensive testing capabilities for all protocol features:

Protocol Validation

• Message Format: Validates JSON-RPC 2.0 compliance
• Method Signatures: Checks parameter and response formats
• Error Handling: Tests error conditions and recovery
• Transport Compliance: Validates transport-specific behavior

Client Capability Detection

• Feature Support: Detects supported MCP features
• Protocol Version: Identifies MCP version support
• Transport Preferences: Determines optimal transport methods
• Error Recovery: Tests client error handling

Performance Analysis

• Message Timing: Measures request/response latency
• Throughput: Tests message processing rates
• Resource Usage: Monitors memory and connection usage
• Concurrent Connections: Tests multi-client scenarios

Best Practices

When implementing or testing MCP:

1. Handle All Message Types: Ensure proper support for requests, responses, and notifications
2. Implement Error Handling: Provide meaningful error messages and proper error codes
3. Support Progress Reporting: For long-running operations, implement progress notifications
4. Validate Parameters: Always validate input parameters and provide clear error messages
5. Test Multiple Transports: Different clients may prefer different transport methods
6. Monitor Performance: Track message timing and resource usage
7. Document Capabilities: Clearly document supported tools, resources, and features

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Ready to dive deeper? Continue with Transport Protocols to understand the different communication methods, or explore Message Types for detailed protocol specifications.