diff --git a/extensions/qa-matrix/src/runners/contract/scenario-runtime-shared.test.ts b/extensions/qa-matrix/src/runners/contract/scenario-runtime-shared.test.ts
index 20d6b3594d9f..37c55b3c7f44 100644
--- a/extensions/qa-matrix/src/runners/contract/scenario-runtime-shared.test.ts
+++ b/extensions/qa-matrix/src/runners/contract/scenario-runtime-shared.test.ts
@@ -1,6 +1,9 @@
 // Qa Matrix tests cover scenario runtime shared plugin behavior.
 import { afterEach, describe, expect, it, vi } from "vitest";
-import { resolveMatrixQaNoReplyWindowMs } from "./scenario-runtime-shared.js";
+import {
+  buildMatrixReplyArtifact,
+  resolveMatrixQaNoReplyWindowMs,
+} from "./scenario-runtime-shared.js";
 
 describe("matrix scenario runtime shared", () => {
   afterEach(() => {
@@ -19,4 +22,18 @@ describe("matrix scenario runtime shared", () => {
       expect(resolveMatrixQaNoReplyWindowMs(30_000)).toBe(8_000);
     }
   });
+
+  it("keeps reply previews UTF-16 safe without changing empty-body artifacts", () => {
+    const event = {
+      kind: "message" as const,
+      roomId: "!room:matrix-qa.test",
+      eventId: "$event",
+      sender: "@sut:matrix-qa.test",
+      type: "m.room.message",
+    };
+    const prefix = "a".repeat(199);
+
+    expect(buildMatrixReplyArtifact({ ...event, body: `${prefix}😀tail` }).bodyPreview).toBe(prefix);
+    expect(buildMatrixReplyArtifact({ ...event, body: " " }).bodyPreview).toBe("");
+  });
 });
diff --git a/extensions/qa-matrix/src/runners/contract/scenario-runtime-shared.ts b/extensions/qa-matrix/src/runners/contract/scenario-runtime-shared.ts
index 4f0acad5b30f..f924d6afe1bc 100644
--- a/extensions/qa-matrix/src/runners/contract/scenario-runtime-shared.ts
+++ b/extensions/qa-matrix/src/runners/contract/scenario-runtime-shared.ts
@@ -1,5 +1,6 @@
 // Qa Matrix plugin module implements scenario runtime shared behavior.
 import { randomUUID } from "node:crypto";
+import { truncateUtf16Safe } from "openclaw/plugin-sdk/text-utility-runtime";
 import { createMatrixQaClient, type MatrixQaRoomObserver } from "../../substrate/client.js";
 import type { MatrixQaObservedEvent } from "../../substrate/events.js";
 import type { MatrixQaFaultProxyObserver } from "../../substrate/fault-proxy.js";
@@ -189,7 +190,7 @@ export function buildMatrixReplyArtifact(
 ): MatrixQaReplyArtifact {
   const replyBody = event.body?.trim();
   return {
-    bodyPreview: replyBody?.slice(0, 200),
+    bodyPreview: replyBody === undefined ? undefined : truncateUtf16Safe(replyBody, 200),
     eventId: event.eventId,
     mentions: event.mentions,
     relatesTo: event.relatesTo,