Logging

The Model Context Protocol (MCP) provides a standardized way for servers to send structured log messages to clients. Clients can control logging verbosity by setting minimum log levels, with servers sending notifications containing severity levels, optional logger names, and arbitrary JSON-serializable data.

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User Interaction Model

Implementations are free to expose logging through any interface pattern that suits their needs—the protocol itself does not mandate any specific user interaction model.

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Capabilities

Servers that emit log message notifications MUST declare the logging capability:

{
  "capabilities": {
    "logging": {}
  }
}

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Log Levels

The protocol follows the standard syslog severity levels specified in RFC 5424:

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Protocol Messages

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Setting Log Level

To configure the minimum log level, clients MAY send a logging/setLevel request:

Request:

{
  "jsonrpc": "2.0",
  "id": 1,
  "method": "logging/setLevel",
  "params": {
    "level": "info"
  }
}

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Log Message Notifications

Servers send log messages using notifications/message notifications:

{
  "jsonrpc": "2.0",
  "method": "notifications/message",
  "params": {
    "level": "error",
    "logger": "database",
    "data": {
      "error": "Connection failed",
      "details": {
        "host": "localhost",
        "port": 5432
      }
    }
  }
}

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Message Flow

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Error Handling

Servers SHOULD return standard JSON-RPC errors for common failure cases:

• Invalid log level: -32602 (Invalid params)
• Configuration errors: -32603 (Internal error)

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Implementation Considerations

1. Servers SHOULD:

   • Rate limit log messages
   • Include relevant context in data field
   • Use consistent logger names
   • Remove sensitive information

2. Clients MAY:

   • Present log messages in the UI
   • Implement log filtering/search
   • Display severity visually
   • Persist log messages

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Security

1. Log messages MUST NOT contain:

   • Credentials or secrets
   • Personal identifying information
   • Internal system details that could aid attacks

2. Implementations SHOULD:

   • Rate limit messages
   • Validate all data fields
   • Control log access
   • Monitor for sensitive content