fix(msteams): truncate reflection prompt on UTF-16 boundary

Leon-SK668 · Leon-SK668 · commit d4da0d3afaa9 · 2026-06-25T08:13:05.000+08:00
buildReflectionPrompt truncated the thumbed-down response with
String.slice(0, 500) on a UTF-16 code-unit index, so an astral
character straddling the 500-char cap was cut into a lone surrogate in
the reflection prompt built for the LLM.

Use truncateUtf16Safe so truncation never splits a surrogate pair,
keeping the existing 500-char budget and '...' suffix.

Adds tests asserting the prompt stays UTF-16 well formed when truncating
and that a boundary emoji is preserved when it fits.
diff --git a/extensions/msteams/src/feedback-reflection-prompt.ts b/extensions/msteams/src/feedback-reflection-prompt.ts
@@ -1,5 +1,6 @@
 // Msteams plugin module implements feedback reflection prompt behavior.
 import { normalizeOptionalLowercaseString } from "openclaw/plugin-sdk/string-coerce-runtime";
+import { truncateUtf16Safe } from "openclaw/plugin-sdk/text-utility-runtime";
 
 /** Max chars of the thumbed-down response to include in the reflection prompt. */
 const MAX_RESPONSE_CHARS = 500;
@@ -19,7 +20,7 @@ export function buildReflectionPrompt(params: {
   if (params.thumbedDownResponse) {
     const truncated =
       params.thumbedDownResponse.length > MAX_RESPONSE_CHARS
-        ? `${params.thumbedDownResponse.slice(0, MAX_RESPONSE_CHARS)}...`
+        ? `${truncateUtf16Safe(params.thumbedDownResponse, MAX_RESPONSE_CHARS)}...`
         : params.thumbedDownResponse;
     parts.push(`\nYour response was:\n> ${truncated}`);
   }
diff --git a/extensions/msteams/src/feedback-reflection.test.ts b/extensions/msteams/src/feedback-reflection.test.ts
@@ -19,6 +19,11 @@ import { msteamsRuntimeStub } from "./test-support/runtime.js";
 
 const previousStateDir = process.env.OPENCLAW_STATE_DIR;
 
+// Matches an unpaired UTF-16 surrogate (lone high or lone low), without relying
+// on the ES2024 String.prototype.isWellFormed() runtime API.
+const UNPAIRED_SURROGATE_RE =
+  /[\uD800-\uDBFF](?![\uDC00-\uDFFF])|(?<![\uD800-\uDBFF])[\uDC00-\uDFFF]/;
+
 describe("buildFeedbackEvent", () => {
   it("builds a well-formed custom event", () => {
     const event = buildFeedbackEvent({
@@ -73,6 +78,26 @@ describe("buildReflectionPrompt", () => {
     expect(prompt.length).toBeLessThan(longResponse.length + 500);
   });
 
+  it("does not split UTF-16 surrogate pairs when truncating a thumbed-down response", () => {
+    const thumbedDownResponse = `${"a".repeat(499)}🦞${"b".repeat(20)}`;
+
+    const prompt = buildReflectionPrompt({ thumbedDownResponse });
+
+    expect(prompt).not.toMatch(UNPAIRED_SURROGATE_RE);
+    expect(prompt).toContain(`${"a".repeat(499)}...`);
+    expect(prompt).not.toContain("\ud83e");
+    expect(prompt).not.toContain("\udd9e");
+  });
+
+  it("keeps a boundary emoji when it fully fits before the truncation cap", () => {
+    const thumbedDownResponse = `${"a".repeat(498)}🦞${"b".repeat(20)}`;
+
+    const prompt = buildReflectionPrompt({ thumbedDownResponse });
+
+    expect(prompt).not.toMatch(UNPAIRED_SURROGATE_RE);
+    expect(prompt).toContain(`${"a".repeat(498)}🦞...`);
+  });
+
   it("includes user comment when provided", () => {
     const prompt = buildReflectionPrompt({
       thumbedDownResponse: "Some response",
