Lifecycle

The Model Context Protocol (MCP) defines a rigorous lifecycle for client-server connections that ensures proper capability negotiation and state management.

1. Initialization: Capability negotiation and protocol version agreement
2. Operation: Normal protocol communication
3. Shutdown: Graceful termination of the connection

sequenceDiagram
    participant Client
    participant Server

    Note over Client,Server: Initialization Phase
    activate Client
    Client->>+Server: initialize request
    Server-->>Client: initialize response
    Client--)Server: initialized notification

    Note over Client,Server: Operation Phase
    rect rgb(200, 220, 250)
        note over Client,Server: Normal protocol operations
    end

    Note over Client,Server: Shutdown
    Client--)-Server: Disconnect
    deactivate Server
    Note over Client,Server: Connection closed

Lifecycle Phases

Initialization

The initialization phase MUST be the first interaction between client and server. During this phase, the client and server:

• Establish protocol version compatibility
• Exchange and negotiate capabilities
• Share implementation details

The client MUST initiate this phase by sending an initialize request containing:

• Protocol version supported
• Client capabilities
• Client implementation information

{
  "jsonrpc": "2.0",
  "id": 1,
  "method": "initialize",
  "params": {
    "protocolVersion": "2024-11-05",
    "capabilities": {
      "roots": {
        "listChanged": true
      },
      "sampling": {}
    },
    "clientInfo": {
      "name": "ExampleClient",
      "version": "1.0.0"
    }
  }
}

The server MUST respond with its own capabilities and information:

{
  "jsonrpc": "2.0",
  "id": 1,
  "result": {
    "protocolVersion": "2024-11-05",
    "capabilities": {
      "logging": {},
      "prompts": {
        "listChanged": true
      },
      "resources": {
        "subscribe": true,
        "listChanged": true
      },
      "tools": {
        "listChanged": true
      }
    },
    "serverInfo": {
      "name": "ExampleServer",
      "version": "1.0.0"
    }
  }
}

After successful initialization, the client MUST send an initialized notification to indicate it is ready to begin normal operations:

{
  "jsonrpc": "2.0",
  "method": "initialized"
}

• The client SHOULD NOT send requests other than pings before the server has responded to the initialize request.
• The server SHOULD NOT send requests other than pings and logging before receiving the initialized notification.

Version Negotiation

In the initialize request, the client MUST send a protocol version it supports. This SHOULD be the latest version supported by the client.

If the server supports the requested protocol version, it MUST respond with the same version. Otherwise, the server MUST respond with another protocol version it supports. This SHOULD be the latest version supported by the server.

If the client does not support the version in the server’s response, it SHOULD disconnect.

Capability Negotiation

Client and server capabilities establish which optional protocol features will be available during the session.

Key capabilities include:

Capability objects can describe sub-capabilities like:

• listChanged: Support for list change notifications (for prompts, resources, and tools)
• subscribe: Support for subscribing to individual items’ changes (resources only)

Operation

During the operation phase, the client and server exchange messages according to the negotiated capabilities.

Both parties SHOULD:

• Respect the negotiated protocol version
• Only use capabilities that were successfully negotiated

Shutdown

During the shutdown phase, one side (usually the client) cleanly terminates the protocol connection. No specific shutdown messages are defined—instead, the underlying transport mechanism should be used to signal connection termination:

stdio

For the stdio transport, the client SHOULD initiate shutdown by:

1. First, closing the input stream to the child process (the server)
2. Waiting for the server to exit, or sending SIGTERM if the server does not exit within a reasonable time
3. Sending SIGKILL if the server does not exit within a reasonable time after SIGTERM

The server MAY initiate shutdown by closing its output stream to the client and exiting.

HTTP

For HTTP transports, shutdown is indicated by closing the associated HTTP connection(s).

Error Handling

Implementations SHOULD be prepared to handle these error cases:

• Protocol version mismatch
• Failure to negotiate required capabilities
• Initialize request timeout
• Shutdown timeout

Implementations SHOULD implement appropriate timeouts for all requests, to prevent hung connections and resource exhaustion.

Example initialization error:

{
  "jsonrpc": "2.0",
  "id": 1,
  "error": {
    "code": -32602,
    "message": "Unsupported protocol version",
    "data": {
      "supported": ["2024-11-05"],
      "requested": "1.0.0"
    }
  }
}