diff --git a/extensions/qa-matrix/src/runners/contract/scenario-catalog.ts b/extensions/qa-matrix/src/runners/contract/scenario-catalog.ts
index 8e864032a29..ba33718867c 100644
--- a/extensions/qa-matrix/src/runners/contract/scenario-catalog.ts
+++ b/extensions/qa-matrix/src/runners/contract/scenario-catalog.ts
@@ -44,6 +44,7 @@ export type MatrixQaScenarioId =
   | "matrix-room-membership-loss"
   | "matrix-homeserver-restart-resume"
   | "matrix-mention-gating"
+  | "matrix-mxid-prefixed-command-block"
   | "matrix-mention-metadata-spoof-block"
   | "matrix-observer-allowlist-override"
   | "matrix-allowlist-block"
@@ -449,6 +450,14 @@ export const MATRIX_QA_SCENARIOS: MatrixQaScenarioDefinition[] = [
     timeoutMs: 8_000,
     title: "Matrix room message without mention does not trigger",
   },
+  {
+    id: "matrix-mxid-prefixed-command-block",
+    timeoutMs: 8_000,
+    title: "Matrix MXID-prefixed control commands stay gated",
+    configOverrides: {
+      groupPolicy: "open",
+    },
+  },
   {
     id: "matrix-mention-metadata-spoof-block",
     timeoutMs: 8_000,
diff --git a/extensions/qa-matrix/src/runners/contract/scenario-runtime.ts b/extensions/qa-matrix/src/runners/contract/scenario-runtime.ts
index 91c869828dc..dc9147b9d13 100644
--- a/extensions/qa-matrix/src/runners/contract/scenario-runtime.ts
+++ b/extensions/qa-matrix/src/runners/contract/scenario-runtime.ts
@@ -247,6 +247,18 @@ export async function runMatrixQaScenario(
         token,
       });
     }
+    case "matrix-mxid-prefixed-command-block": {
+      const token = buildMatrixQaToken("MATRIX_QA_MXID_COMMAND");
+      return await runNoReplyScenario({
+        accessToken: context.observerAccessToken,
+        actorId: "observer",
+        actorUserId: context.observerUserId,
+        body: `${context.sutUserId} /new`,
+        mentionUserIds: [context.sutUserId],
+        context,
+        token,
+      });
+    }
     case "matrix-mention-metadata-spoof-block": {
       const token = buildMatrixQaToken("MATRIX_QA_METADATA_SPOOF");
       return await runNoReplyScenario({
diff --git a/extensions/qa-matrix/src/runners/contract/scenarios.test.ts b/extensions/qa-matrix/src/runners/contract/scenarios.test.ts
index 21ff8a7c89c..81686ef4c8d 100644
--- a/extensions/qa-matrix/src/runners/contract/scenarios.test.ts
+++ b/extensions/qa-matrix/src/runners/contract/scenarios.test.ts
@@ -106,6 +106,7 @@ describe("matrix live qa scenarios", () => {
       "matrix-room-membership-loss",
       "matrix-homeserver-restart-resume",
       "matrix-mention-gating",
+      "matrix-mxid-prefixed-command-block",
       "matrix-mention-metadata-spoof-block",
       "matrix-observer-allowlist-override",
       "matrix-allowlist-block",
@@ -516,6 +517,51 @@ describe("matrix live qa scenarios", () => {
     });
   });
 
+  it("blocks MXID-prefixed Matrix control commands from non-allowlisted observers", async () => {
+    const primeRoom = vi.fn().mockResolvedValue("observer-sync-start");
+    const sendTextMessage = vi.fn().mockResolvedValue("$observer-command-trigger");
+    const waitForOptionalRoomEvent = vi.fn().mockImplementation(async (params) => {
+      expect(params.since).toBe("observer-sync-start");
+      return {
+        matched: false,
+        since: "observer-sync-next",
+      };
+    });
+
+    createMatrixQaClient.mockReturnValue({
+      primeRoom,
+      sendTextMessage,
+      waitForOptionalRoomEvent,
+    });
+
+    const scenario = MATRIX_QA_SCENARIOS.find(
+      (entry) => entry.id === "matrix-mxid-prefixed-command-block",
+    );
+    expect(scenario).toBeDefined();
+
+    await expect(runMatrixQaScenario(scenario!, matrixQaScenarioContext())).resolves.toMatchObject({
+      artifacts: {
+        actorUserId: "@observer:matrix-qa.test",
+        driverEventId: "$observer-command-trigger",
+      },
+    });
+
+    expect(createMatrixQaClient).toHaveBeenCalledWith({
+      accessToken: "observer-token",
+      baseUrl: "http://127.0.0.1:28008/",
+    });
+    expect(sendTextMessage).toHaveBeenCalledWith({
+      body: "@sut:matrix-qa.test /new",
+      mentionUserIds: ["@sut:matrix-qa.test"],
+      roomId: "!main:matrix-qa.test",
+    });
+    expect(waitForOptionalRoomEvent).toHaveBeenCalledWith(
+      expect.objectContaining({
+        roomId: "!main:matrix-qa.test",
+      }),
+    );
+  });
+
   it("queues a Matrix trigger during restart before proving incremental sync continues", async () => {
     const callOrder: string[] = [];
     const primeRoom = vi.fn().mockResolvedValue("driver-sync-start");