Protocol Revision: 2025-03-26
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

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Lifecycle Phases

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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": "2025-03-26",
    "capabilities": {
      "roots": {
        "listChanged": true
      },
      "sampling": {}
    },
    "clientInfo": {
      "name": "ExampleClient",
      "version": "1.0.0"
    }
  }
}
The initialize request MUST NOT be part of a JSON-RPC batch, as other requests and notifications are not possible until initialization has completed. This also permits backwards compatibility with prior protocol versions that do not explicitly support JSON-RPC batches. The server MUST respond with its own capabilities and information: 
{
  "jsonrpc": "2.0",
  "id": 1,
  "result": {
    "protocolVersion": "2025-03-26",
    "capabilities": {
      "logging": {},
      "prompts": {
        "listChanged": true
      },
      "resources": {
        "subscribe": true,
        "listChanged": true
      },
      "tools": {
        "listChanged": true
      }
    },
    "serverInfo": {
      "name": "ExampleServer",
      "version": "1.0.0"
    },
    "instructions": "Optional instructions for the client"
  }
}
After successful initialization, the client MUST send an initialized notification to indicate it is ready to begin normal operations: 
{
  "jsonrpc": "2.0",
  "method": "notifications/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.

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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.

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Capability Negotiation

Client and server capabilities establish which optional protocol features will be available during the session. Key capabilities include:
CategoryCapabilityDescription
ClientrootsAbility to provide filesystem roots
ClientsamplingSupport for LLM sampling requests
ClientexperimentalDescribes support for non-standard experimental features
ServerpromptsOffers prompt templates
ServerresourcesProvides readable resources
ServertoolsExposes callable tools
ServerloggingEmits structured log messages
ServercompletionsSupports argument autocompletion
ServerexperimentalDescribes support for non-standard experimental features
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)

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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

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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:

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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.

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HTTP

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

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Timeouts

Implementations SHOULD establish timeouts for all sent requests, to prevent hung connections and resource exhaustion. When the request has not received a success or error response within the timeout period, the sender SHOULD issue a cancellation notification for that request and stop waiting for a response. SDKs and other middleware SHOULD allow these timeouts to be configured on a per-request basis. Implementations MAY choose to reset the timeout clock when receiving a progress notification corresponding to the request, as this implies that work is actually happening. However, implementations SHOULD always enforce a maximum timeout, regardless of progress notifications, to limit the impact of a misbehaving client or server.

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Error Handling

Implementations SHOULD be prepared to handle these error cases:
  • Protocol version mismatch
  • Failure to negotiate required capabilities
  • Request timeouts
Example initialization error: 
{
  "jsonrpc": "2.0",
  "id": 1,
  "error": {
    "code": -32602,
    "message": "Unsupported protocol version",
    "data": {
      "supported": ["2024-11-05"],
      "requested": "1.0.0"
    }
  }
}