diff --git a/apps/mobile/src/lib/projectThreadStartTurn.test.ts b/apps/mobile/src/lib/projectThreadStartTurn.test.ts
index e2a41cf9d886f..6abb3409d3680 100644
--- a/apps/mobile/src/lib/projectThreadStartTurn.test.ts
+++ b/apps/mobile/src/lib/projectThreadStartTurn.test.ts
@@ -5,6 +5,7 @@ import { serializeComposerMessageForServer } from "./composerContext";
 
 import {
   buildProjectThreadStartTurnInput,
+  deriveThreadTitleFromPrompt,
   type ProjectThreadStartTurnSpec,
 } from "./projectThreadStartTurn";
 
@@ -69,4 +70,10 @@ describe("project thread title", () => {
     expect(relaunched.message.text).toBe(wire.text);
     expect(relaunched.message.attachments).toEqual(uploadedAttachments);
   });
+
+  it("shortens a long prompt without splitting an emoji", () => {
+    expect(deriveThreadTitleFromPrompt(`${"a".repeat(68)}😀 and the rest of the prompt`)).toBe(
+      `${"a".repeat(68)}...`,
+    );
+  });
 });
diff --git a/apps/mobile/src/lib/projectThreadStartTurn.ts b/apps/mobile/src/lib/projectThreadStartTurn.ts
index 1e677d232b81f..00dbadc3aff17 100644
--- a/apps/mobile/src/lib/projectThreadStartTurn.ts
+++ b/apps/mobile/src/lib/projectThreadStartTurn.ts
@@ -87,5 +87,12 @@ export function buildProjectThreadStartTurnInput(spec: ProjectThreadStartTurnSpe
 
 export function deriveThreadTitleFromPrompt(value: string): string {
   const title = threadTitleTextFromPrompt(value) || "New thread";
-  return title.length <= 72 ? title : `${title.slice(0, 69).trimEnd()}...`;
+  if (title.length <= 72) return title;
+  let end = 69;
+  const previous = title.charCodeAt(end - 1);
+  const next = title.charCodeAt(end);
+  if (previous >= 0xd800 && previous <= 0xdbff && next >= 0xdc00 && next <= 0xdfff) {
+    end -= 1;
+  }
+  return `${title.slice(0, end).trimEnd()}...`;
 }