GenMCP.TelemetryLogger (gen_mcp v2.0.0)

A :telemetry handler that logs the events emitted by the :gen_mcp application.

Attaching it gives you ready-made Logger output for the lifecycle and transport events the library emits, each at a fixed log level. It is the quickest way to see what the server is doing without writing your own :telemetry handler.

Attaching the logger

Attach the handler once when your application boots, from your Application.start/2 callback:

defmodule MyApp.Application do
  use Application

  @impl true
  def start(_type, _args) do
    :ok = GenMCP.TelemetryLogger.attach()

    children = [
      # your supervision tree
    ]

    Supervisor.start_link(children, strategy: :one_for_one, name: MyApp.Supervisor)
  end
end

GenMCP.attach_default_logger/1 is a thin wrapper over attach/1, so calling either one has the same effect. Pass filters to narrow what is logged, for example to keep only the more severe events:

:ok = GenMCP.TelemetryLogger.attach(min_log_level: :error)

See attach/1 for the full list of filters.

Events

Here are the emitted events for the library, and the corresponding log level used for each one.

[:gen_mcp, :server, :init] with a log level of :debug
[:gen_mcp, :server, :start_error] with a log level of :error
[:gen_mcp, :transport, :request_rejected] with a log level of :debug
[:gen_mcp, :transport, :server_crashed] with a log level of :error
[:gen_mcp, :transport, :version_rejected] with a log level of :debug

Summary

Functions

attach(filters \\ [])

Attaches the telemetry handler that logs :gen_mcp events.

Functions

attach(filters \\ [])

Attaches the telemetry handler that logs :gen_mcp events.

Call this once at startup (see the module doc for placement in Application.start/2). It subscribes a single :telemetry handler, named after this module, to the events listed in the module doc, and returns :ok on success.

By default every event is logged at its mapped level. Pass filters to subscribe to a subset:

:min_log_level - keep only events whose mapped level is at least this severe. For example min_log_level: :error drops the :debug events and keeps the :error ones.
:prefixes - a list of event-name prefixes; keep only events whose name starts with one of them. For example prefixes: [[:gen_mcp, :transport]] keeps only the transport events.

Examples

Attach every event at its default level:

:ok = GenMCP.TelemetryLogger.attach()

Attach only transport events logged at :error or above:

:ok =
  GenMCP.TelemetryLogger.attach(
    min_log_level: :error,
    prefixes: [[:gen_mcp, :transport]]
  )