feat: log local MCP requests to stderr by method

The stdio server writes each request's method, and the tool or resource it
names, to stderr, which clients such as Claude Desktop copy into their logs.
Arguments and results are never logged.

server/mcp/stdio.js

@@ -14,13 +14,29 @@ import { parseErrorResponse } from '../../src/tools/mcp/index.js';
 import { createLocalMcpServer } from './server.js';
 import { DEFAULT_API_BASE } from './services.js';
 
+/**
+ * A one-line description of a request for the diagnostic log: its method,
+ * and the tool or resource it names. Arguments and data are never logged.
+ */
+export function describeRequest(message) {
+  const method = typeof message?.method === 'string' ? message.method : null;
+  if (!method) return null;
+  const subject =
+    method === 'tools/call'
+      ? message.params?.name
+      : method === 'resources/read'
+        ? message.params?.uri
+        : null;
+  return typeof subject === 'string' ? `${method} ${subject}` : method;
+}
+
 /** Serve newline-delimited JSON-RPC until `input` ends. */
-export async function serveStdio(server, { input, output }) {
+export async function serveStdio(server, { input, output, log = () => {} }) {
   const lines = createInterface({ input, crlfDelay: Infinity });
   const pending = new Set();
   for await (const line of lines) {
     if (!line.trim()) continue;
-    const work = respond(server, line).then((response) => {
+    const work = respond(server, line, log).then((response) => {
       if (response) output.write(`${JSON.stringify(response)}\n`);
     });
     pending.add(work);
@@ -29,13 +45,15 @@ export async function serveStdio(server, { input, output }) {
   await Promise.all(pending);
 }
 
-async function respond(server, line) {
+async function respond(server, line, log) {
   let message;
   try {
     message = JSON.parse(line);
   } catch {
     return parseErrorResponse();
   }
+  const described = describeRequest(message);
+  if (described) log(described);
   return server.handle(message);
 }
 
@@ -59,6 +77,7 @@ if (process.argv[1] && fileURLToPath(import.meta.url) === process.argv[1]) {
     await serveStdio(createLocalMcpServer(options), {
       input: process.stdin,
       output: process.stdout,
+      log: (line) => console.error(`<- ${line}`),
     });
   } catch (error) {
     console.error(error.message);