Tutorial 10

Logging

Demonstrates the MCP server logging API for sending **structured log messages** to the client.

Demonstrates the MCP server logging API for sending structured log messages to the client.

How It Works

Servers send log messages to clients via SendLogMessage. An optional SetLogHandler controls which log levels are allowed — returning an error suppresses that log.

s := finemcp.NewServer("logging", "1.0.0")

// Suppress debug-level logs
s.SetLogHandler(func(ctx context.Context, level finemcp.LogLevel) error {
    if level == finemcp.LogLevelDebug {
        return fmt.Errorf("debug suppressed")
    }
    return nil
})

Log Levels

Constant	Description
`finemcp.LogLevelDebug`	Detailed diagnostic info
`finemcp.LogLevelInfo`	General operational messages
`finemcp.LogLevelWarning`	Potential issues
`finemcp.LogLevelError`	Error conditions

Example

The do-work tool emits logs at all four levels:

tool, _ := finemcp.NewTool("do-work",
    func(ctx context.Context, input []byte) ([]byte, error) {
        s.SendLogMessage(ctx, finemcp.LogLevelDebug, "worker",
            map[string]any{"msg": "starting work", "step": 0})
        s.SendLogMessage(ctx, finemcp.LogLevelInfo, "worker",
            map[string]any{"msg": "processing", "step": 1})
        s.SendLogMessage(ctx, finemcp.LogLevelWarning, "worker",
            map[string]any{"msg": "slow query detected", "ms": 450})
        s.SendLogMessage(ctx, finemcp.LogLevelError, "worker",
            map[string]any{"msg": "retrying failed operation", "attempt": 2})
        return []byte("Work done. Check logs."), nil
    },
    finemcp.WithDescription("Perform work and emit log messages"),
)

SendLogMessage API

s.SendLogMessage(ctx context.Context, level LogLevel, logger string, data any) error

Parameter	Description
`level`	Log severity level
`logger`	Logger name (e.g., `"worker"`, `"db"`)
`data`	Structured data (maps, strings, etc.) sent as JSON

Log messages are delivered as notifications/message:

{
  "jsonrpc": "2.0",
  "method": "notifications/message",
  "params": { "level": "warning", "logger": "worker", "data": { "msg": "slow query", "ms": 450 } }
}

Testing with curl

go run ./10-logging

# Initialize
curl -X POST http://localhost:8080 -H "Content-Type: application/json" -d '{
  "jsonrpc": "2.0", "id": 1, "method": "initialize",
  "params": { "protocolVersion": "2025-03-26", "clientInfo": { "name": "curl", "version": "1.0.0" }, "capabilities": {} }
}'

# Trigger logging
curl -X POST http://localhost:8080 -H "Content-Type: application/json" -d '{
  "jsonrpc": "2.0", "id": 3, "method": "tools/call",
  "params": { "name": "do-work" }
}'