fix(mcp): accept reserved _meta field in tools/call params (#2670)

* fix(mcp): accept reserved _meta field in tools/call params

The memory MCP server rejected any tools/call whose params contained a key
other than name/arguments, returning -32602 "Unknown or missing memory tool."

MCP clients (e.g. Claude Code) attach the spec-reserved `_meta` field
(such as progressToken) to request params, so every tool call from a
compliant client failed and the entire memory MCP surface was unreachable —
even though initialize/tools-list and the `ecc memory` CLI kept working.

Per the MCP base protocol, `_meta` is reserved for request metadata and
must be accepted. Add it to the params key allowlist.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* test(mcp): validate _meta shape and cover tools/call param allowlist

Address CodeRabbit review on #2670:
- Validate params._meta when present: accept metadata objects, reject null,
  arrays, and scalar values (reuses isRecord). Keeps _meta optional and
  preserves existing name/arguments/unexpected-key rejection.
- Add regression tests: accept _meta with progressToken, reject malformed
  _meta values, and continue rejecting unrelated top-level params.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

scripts/memory-mcp.mjs

@@ -437,7 +437,10 @@ function createMemoryMcpService(options = {}) {
           !isRecord(params)
           || typeof name !== 'string'
           || !TOOL_BY_NAME.has(name)
-          || Object.keys(params).some(key => !['name', 'arguments'].includes(key))
+          // `_meta` is reserved by MCP for request metadata (e.g. progressToken); accept it,
+          // but when present it must be a metadata object — reject null, arrays, and scalars.
+          || (Object.prototype.hasOwnProperty.call(params, '_meta') && !isRecord(params._meta))
+          || Object.keys(params).some(key => !['name', 'arguments', '_meta'].includes(key))
         ) {
           return jsonRpcError(message.id, -32602, 'Unknown or missing memory tool.');
         }

tests/scripts/memory-mcp.test.js

@@ -136,6 +136,7 @@ async function withClient(fn, options = {}) {
       'tools/call',
       { name, arguments: toolArguments }
     ),
+    callToolRaw: params => request('tools/call', params),
   };
 
   try {
@@ -180,6 +181,40 @@ async function main() {
     });
   });
 
+  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.
+      const withMeta = await client.callToolRaw({
+        name: 'memory_doctor',
+        arguments: {},
+        _meta: { progressToken: 'progress-123' },
+      });
+      assert.ok(Array.isArray(withMeta.content));
+
+      // Baseline: no `_meta` still works.
+      const withoutMeta = await client.callToolRaw({
+        name: 'memory_doctor',
+        arguments: {},
+      });
+      assert.ok(Array.isArray(withoutMeta.content));
+
+      // A malformed `_meta` (null, array, or scalar) must be rejected.
+      for (const badMeta of [null, ['not', 'an', 'object'], 'string', 42, true]) {
+        await assert.rejects(
+          client.callToolRaw({ name: 'memory_doctor', arguments: {}, _meta: badMeta }),
+          /-32602/,
+          `expected _meta=${JSON.stringify(badMeta)} to be rejected`
+        );
+      }
+
+      // Unrelated top-level params must still be rejected.
+      await assert.rejects(
+        client.callToolRaw({ name: 'memory_doctor', arguments: {}, unexpected: true }),
+        /-32602/
+      );
+    });
+  });
+
   await test('starts when the npm bin invokes the server through a symlink', async () => {
     const binRoot = fs.mkdtempSync(path.join(os.tmpdir(), 'ecc-memory-bin-'));
     const binPath = path.join(binRoot, 'ecc-memory-mcp');