diff --git a/src/tools/mongodb/update/updateMany.ts b/src/tools/mongodb/update/updateMany.ts
index 599115327..98a852911 100644
--- a/src/tools/mongodb/update/updateMany.ts
+++ b/src/tools/mongodb/update/updateMany.ts
@@ -47,6 +47,11 @@ export class UpdateManyTool extends MongoDBToolBase {
         const provider = await this.ensureConnected();
 
         this.assertMqlIsAllowed(filter);
+        // The update document is also MQL and, in its aggregation-pipeline form, can carry
+        // server-side JS operators ($function/$where/$accumulator) and write stages ($out/$merge).
+        // It must go through the same guard as the filter so `disableServerSideJs` / readOnly /
+        // disabled-tools restrictions can't be bypassed via the update argument.
+        this.assertMqlIsAllowed(update);
 
         // Check if update operation uses an index if enabled
         if (this.config.indexCheck) {
diff --git a/tests/integration/tools/mongodb/update/updateMany.test.ts b/tests/integration/tools/mongodb/update/updateMany.test.ts
index 9a32f8878..3ccaef1f1 100644
--- a/tests/integration/tools/mongodb/update/updateMany.test.ts
+++ b/tests/integration/tools/mongodb/update/updateMany.test.ts
@@ -295,4 +295,33 @@ describeWithMongoDB("updateMany tool with server-side JavaScript operators", (in
             }
         });
     }
+
+    // The `update` document is MQL too, so it must go through the same guard as the `filter`
+    // (the regression this protects against). Unlike a filter, a classic update document does not
+    // execute the operator server-side, so the enabled case only asserts the guard lets it through.
+    for (const jsDisabled of [true, false]) {
+        it(`${jsDisabled ? "rejects" : "allows"} an update document carrying $function when disableServerSideJs is ${jsDisabled}`, async () => {
+            integration.mcpServer().userConfig.disableServerSideJs = jsDisabled;
+            await integration.connectMcpClient();
+            const response = await integration.mcpClient().callTool({
+                name: "update-many",
+                arguments: {
+                    database: integration.randomDbName(),
+                    collection: "people",
+                    filter: {},
+                    update: {
+                        $set: {
+                            flagged: { $function: { body: "function () { return true; }", args: [], lang: "js" } },
+                        },
+                    },
+                },
+            });
+            const content = getResponseContent(response.content);
+            if (jsDisabled) {
+                expect(content).toContain(`The "$function" operator is not allowed.`);
+            } else {
+                expect(content).not.toContain("server-side JavaScript operators");
+            }
+        });
+    }
 });