Logging

logging/setLevel, legacy only.

Note

logging/setLevel and the Logging feature as a whole were deprecated in 2026-07-28. Modern clients set the level per request via _meta["io.modelcontextprotocol/logLevel"] — see request-scoped streaming.

Legacy clients set a server-wide log level once, and the server pushes notifications/message at or above that level over the SSE stream.

defineMcpHandler({
  name: "my-server",
  version: "1.0.0",
  onLogLevelSet(level, event) {
    console.log(`Client requested log level: ${level}`);
  },
  tools: [/* ... */],
});

The level is validated against the eight MCP levels before your callback runs:

debug · info · notice · warning · error · critical · alert · emergency

Declaring onLogLevelSet is what advertises logging: {} in the initialize capabilities. The callback is a notification hook, not bookkeeping: h3-mcp already records the level per session, so nothing you write has to.

Important

Log messages are sent to the client. Treat them as output for a third party: no credentials, no tokens, no internal hostnames or stack traces. Everything that applies to error messages applies here.

To actually send a log message, use event.context.mcp.log() — the same call on both eras. It gates on the level in force (here, the one this session set) and needs no configuration:

event.context.mcp?.log?.("warning", { retries: 3 }, "importer");

On legacy the message travels on the request's own SSE response, so the client must send Accept: text/event-stream with the call. Anything logged below the requested level, or by a request whose client asked for no logs at all, is dropped.