fix(memory-mcp): accept the reserved _meta param on ping

tools/list and tools/call on main already admit `_meta` — MCP reserves it
for request metadata and a client may attach it to any request. ping still
refused every parameter, so a client that sends `_meta` on everything
(Codex does) got -32602 on its keepalive.

Rebased onto main and narrowed: when this branch was first written the same
gap existed on tools/list, which has since been fixed upstream. Only the
ping handler is left, so only the ping handler is touched.

Refs #2810

scripts/memory-mcp.mjs

@@ -426,8 +426,18 @@ function createMemoryMcpService(options = {}) {
         return jsonRpcError(message.id, -32002, 'Server is not initialized.');
       }
       if (message.method === 'ping') {
-        if (message.params && Object.keys(message.params).length > 0) {
-          return jsonRpcError(message.id, -32602, 'ping does not accept parameters.');
+        const params = message.params ?? {};
+        // `_meta` is reserved by MCP for request metadata (e.g. progressToken) and
+        // may ride on any request, which is why `tools/list` and `tools/call` below
+        // both admit it. `ping` rejected every parameter, so a client that attaches
+        // `_meta` to everything — Codex does — got -32602 on its keepalive. Present
+        // means it must be a metadata object; nothing else is accepted. (#2810)
+        if (
+          !isRecord(params)
+          || (Object.prototype.hasOwnProperty.call(params, '_meta') && !isRecord(params._meta))
+          || Object.keys(params).some(key => key !== '_meta')
+        ) {
+          return jsonRpcError(message.id, -32602, 'ping accepts no parameters other than _meta.');
         }
         return jsonRpcResult(message.id, {});
       }

tests/scripts/memory-mcp.test.js

@@ -260,6 +260,7 @@ async function withClient(fn, options = {}) {
         { name, arguments: toolArguments }
       ),
       callToolRaw: params => request('tools/call', params),
+      ping: params => request('ping', params),
     };
     phase = 'callback';
     await Promise.race([Promise.resolve().then(() => fn(client, fixture)), transportFailure]);
@@ -393,6 +394,25 @@ async function main() {
     });
   });
 
+  await test('accepts the reserved _meta param on ping and rejects malformed values (#2810)', async () => {
+    await withClient(async client => {
+      assert.deepStrictEqual(await client.ping({ _meta: { progressToken: 'progress-1' } }), {});
+      assert.deepStrictEqual(await client.ping(), {});
+      assert.deepStrictEqual(await client.ping({}), {});
+
+      for (const badMeta of [null, ['not', 'an', 'object'], 'string', 42, true]) {
+        await assert.rejects(
+          client.ping({ _meta: badMeta }),
+          /-32602/,
+          `expected ping _meta=${JSON.stringify(badMeta)} to be rejected`
+        );
+      }
+
+      await assert.rejects(client.ping({ unexpected: true }), /-32602/);
+      await assert.rejects(client.ping({ _meta: {}, unexpected: true }), /-32602/);
+    });
+  });
+
   await test('accepts the reserved _meta param on tools/call and rejects malformed values', async () => {
     await withClient(async client => {
       // A valid `_meta` object (e.g. progressToken) must not block the tool call.