ruby-sdk/docs/_server/logging.md

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default	Logging	17

Logging

The MCP Ruby SDK supports structured logging through the notify_log_message method, following the MCP Logging specification.

The notifications/message notification is used for structured logging between client and server.

{: .warning }

MCP Logging (logging/setLevel and notifications/message) is deprecated as of protocol version 2026-07-28 (SEP-2577), while remaining fully supported under 2025-11-25. Use stderr or OpenTelemetry for new servers.

{: .note }

On the modern lifecycle, where logging/setLevel does not exist, the level comes per request from the io.modelcontextprotocol/logLevel _meta member: it authorizes notifications/message for that request only, delivered on the request's own response stream. A request without the member (or with an unrecognized level) receives no log messages.

Log Levels

The SDK supports 8 log levels with increasing severity:

• debug - Detailed debugging information
• info - General informational messages
• notice - Normal but significant events
• warning - Warning conditions
• error - Error conditions
• critical - Critical conditions
• alert - Action must be taken immediately
• emergency - System is unusable

How Logging Works

1. Client Configuration: The client sends a logging/setLevel request to configure the minimum log level
2. Server Filtering: The server only sends log messages at the configured level or higher severity
3. Notification Delivery: Log messages are sent as notifications/message to the client

For example, if the client sets the level to "error" (severity 4), the server will send messages with levels: error, critical, alert, and emergency.

For more details, see the MCP Logging specification.

Usage Example:

The client first configures the level with a logging/setLevel request:

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

The server then emits messages at each severity; only those at or above the configured level are delivered:

server = MCP::Server.new(name: "my_server")
transport = MCP::Server::Transports::StdioTransport.new(server)

server.notify_log_message(
  data: { message: "Application started successfully" },
  level: "info"
)

server.notify_log_message(
  data: { message: "Configuration file not found, using defaults" },
  level: "warning"
)

server.notify_log_message(
  data: {
    error: "Database connection failed",
    details: { host: "localhost", port: 5432 }
  },
  level: "error",
  logger: "DatabaseLogger" # Optional logger name
)

Key Features:

• Server has capability logging to send log messages
• Messages are only sent if a transport is configured
• Messages are filtered based on the client's configured log level
• If the log level hasn't been set by the client, no messages will be sent

Transport Support

• stdio: Notifications are sent as JSON-RPC 2.0 messages to stdout
• Streamable HTTP: Notifications are sent as JSON-RPC 2.0 messages over HTTP with streaming (chunked transfer or SSE)