diff --git a/packages/client-runtime/src/state/subagentDisplay.test.ts b/packages/client-runtime/src/state/subagentDisplay.test.ts
index 5bdd434388ec6..9b1536a7aacb1 100644
--- a/packages/client-runtime/src/state/subagentDisplay.test.ts
+++ b/packages/client-runtime/src/state/subagentDisplay.test.ts
@@ -36,6 +36,6 @@ it("normalizes Unicode whitespace and ignores a large hidden suffix", () => {
     subagentDetailPreview({ status: "running", progress: "\uFEFF\u2029", result: " Done " }),
   ).toBe("Done");
   expect(subagentDetailPreview({ status: "failed", result: "x".repeat(279) + "😀" })).toBe(
-    "x".repeat(279) + "\uD83D…",
+    "x".repeat(279) + "…",
   );
 });
diff --git a/packages/client-runtime/src/state/subagentDisplay.ts b/packages/client-runtime/src/state/subagentDisplay.ts
index 124c45e264f79..5f1a65fb8a22e 100644
--- a/packages/client-runtime/src/state/subagentDisplay.ts
+++ b/packages/client-runtime/src/state/subagentDisplay.ts
@@ -132,7 +132,15 @@ export function subagentDetailPreview(input: {
       if (compact.length > 0) compact += " ";
     }
     compact += detail[index];
-    if (compact.length > 280) return `${compact.slice(0, 280).trimEnd()}…`;
+    if (compact.length > 280) {
+      let end = 280;
+      const previous = compact.charCodeAt(end - 1);
+      const next = compact.charCodeAt(end);
+      if (previous >= 0xd800 && previous <= 0xdbff && next >= 0xdc00 && next <= 0xdfff) {
+        end -= 1;
+      }
+      return `${compact.slice(0, end).trimEnd()}…`;
+    }
   }
   return compact || null;
 }