diff --git a/packages/provider-core/src/server/subagentProjection.test.ts b/packages/provider-core/src/server/subagentProjection.test.ts
new file mode 100644
index 0000000000000..767764db0cae5
--- /dev/null
+++ b/packages/provider-core/src/server/subagentProjection.test.ts
@@ -0,0 +1,13 @@
+import { describe, expect, it } from "vite-plus/test";
+
+import { subagentThreadTitle } from "./subagentProjection.ts";
+
+describe("subagentThreadTitle", () => {
+  it("clips a long prompt without splitting a surrogate pair", () => {
+    const title = (prompt: string) =>
+      subagentThreadTitle({ parentTitle: "Parent", prompt, ordinal: 1 });
+
+    expect(title(`${"a".repeat(68)}😀 and more work.`)).toBe(`${"a".repeat(68)}...`);
+    expect(title(`${"a".repeat(67)}😀 and more work.`)).toBe(`${"a".repeat(67)}😀...`);
+  });
+});
diff --git a/packages/provider-core/src/server/subagentProjection.ts b/packages/provider-core/src/server/subagentProjection.ts
index 3775818892502..dbd929192e04d 100644
--- a/packages/provider-core/src/server/subagentProjection.ts
+++ b/packages/provider-core/src/server/subagentProjection.ts
@@ -39,8 +39,14 @@ export function subagentThreadTitle(input: {
   if (detail === undefined) {
     return `${input.parentTitle} subagent ${input.ordinal}`;
   }
-  const clipped = detail.length > 72 ? `${detail.slice(0, 69)}...` : detail;
-  return clipped;
+  if (detail.length <= 72) return detail;
+  let end = 69;
+  const previous = detail.charCodeAt(end - 1);
+  const next = detail.charCodeAt(end);
+  if (previous >= 0xd800 && previous <= 0xdbff && next >= 0xdc00 && next <= 0xdfff) {
+    end -= 1;
+  }
+  return `${detail.slice(0, end)}...`;
 }
 
 export function makeSubagentChildThread(input: {