diff --git a/frontend/src/core/scheduled-tasks/format.ts b/frontend/src/core/scheduled-tasks/format.ts
index b1f95e9b22..4ff2e5f22c 100644
--- a/frontend/src/core/scheduled-tasks/format.ts
+++ b/frontend/src/core/scheduled-tasks/format.ts
@@ -270,14 +270,22 @@ export function displayTimeZone(
  * A run summary short enough to quote mid-sentence: its first sentence, no
  * trailing sentence punctuation (the template adds its own), at most `max`
  * characters with an ellipsis. The full text stays in the linked run.
+ *
+ * The budget counts code points, so an emoji at the cut is never split into a
+ * lone surrogate (`conversation-outline.ts` and `tool-detail-preview.ts`
+ * follow the same rule for their own limits).
  */
 export function briefReason(summary: string, max = 120): string {
   const text = summary.replace(/\s+/g, " ").trim();
   const sentenceEnd = /[。！？]|[.!?](?=\s|$)/.exec(text);
   const first = sentenceEnd ? text.slice(0, sentenceEnd.index + 1) : text;
   const bare = first.replace(/[\s。．.！!？?；;，,、:：]+$/u, "");
-  if (bare.length <= max) {
+  const characters = Array.from(bare);
+  if (characters.length <= max) {
     return bare;
   }
-  return `${bare.slice(0, max - 1).trimEnd()}…`;
+  return `${characters
+    .slice(0, Math.max(0, max - 1))
+    .join("")
+    .trimEnd()}…`;
 }
diff --git a/frontend/tests/unit/core/scheduled-tasks/format.test.ts b/frontend/tests/unit/core/scheduled-tasks/format.test.ts
index 806f182df7..4c305391a9 100644
--- a/frontend/tests/unit/core/scheduled-tasks/format.test.ts
+++ b/frontend/tests/unit/core/scheduled-tasks/format.test.ts
@@ -199,4 +199,17 @@ describe("briefReason", () => {
     expect(reason).toHaveLength(120);
     expect(reason.endsWith("…")).toBe(true);
   });
+
+  it("counts code points so a cut never splits an emoji", () => {
+    // 120 code points but 121 UTF-16 units: the whole summary fits.
+    const fits = `${"a".repeat(118)}😀b`;
+    expect(briefReason(fits)).toBe(fits);
+
+    // Cut lands before the extra text, after the whole emoji: no lone surrogate.
+    const overflow = `${"a".repeat(118)}😀bc`;
+    expect(briefReason(overflow)).toBe(`${"a".repeat(118)}😀…`);
+    expect(briefReason(overflow)).not.toMatch(
+      /[\uD800-\uDBFF](?![\uDC00-\uDFFF])/,
+    );
+  });
 });