diff --git a/packages/client-runtime/src/state/subagentRuntime.test.ts b/packages/client-runtime/src/state/subagentRuntime.test.ts
index 93d15b016d755..7be9639bafedb 100644
--- a/packages/client-runtime/src/state/subagentRuntime.test.ts
+++ b/packages/client-runtime/src/state/subagentRuntime.test.ts
@@ -158,6 +158,13 @@ describe("projectedSubagentsToRuntime workflow expansion", () => {
       expect(members?.[0]?.speed).toBeUndefined();
     },
   );
+
+  it("titles an untitled subagent from its prompt without splitting an emoji", () => {
+    const [untitled] = projectedSubagentsToRuntime([
+      subagent({ title: null, prompt: `${"a".repeat(76)}😀 and more work.` }),
+    ]);
+    expect(untitled?.title).toBe(`${"a".repeat(76)}...`);
+  });
 });
 
 describe("deriveWorkflowGroups over a dynamic workflow", () => {
diff --git a/packages/client-runtime/src/state/subagentRuntime.ts b/packages/client-runtime/src/state/subagentRuntime.ts
index fac58a029ce62..8205d178e5cd6 100644
--- a/packages/client-runtime/src/state/subagentRuntime.ts
+++ b/packages/client-runtime/src/state/subagentRuntime.ts
@@ -176,6 +176,17 @@ function workflowMembersToRuntime(
   });
 }
 
+function promptTitle(prompt: string): string {
+  if (prompt.length <= 80) return prompt;
+  let end = 77;
+  const previous = prompt.charCodeAt(end - 1);
+  const next = prompt.charCodeAt(end);
+  if (previous >= 0xd800 && previous <= 0xdbff && next >= 0xdc00 && next <= 0xdfff) {
+    end -= 1;
+  }
+  return `${prompt.slice(0, end)}...`;
+}
+
 export function workflowMemberModelSelection(
   member: Pick<RuntimeSubagent, "model" | "effort" | "speed">,
   instanceId: ModelSelection["instanceId"],
@@ -216,9 +227,7 @@ export function projectedSubagentsToRuntime(
     const coordinator = {
       id: subagent.id,
       kind: workflow === undefined ? "subagent" : "workflow",
-      title:
-        subagent.title ??
-        (subagent.prompt.length > 80 ? `${subagent.prompt.slice(0, 77)}...` : subagent.prompt),
+      title: subagent.title ?? promptTitle(subagent.prompt),
       role: null,
       model: subagent.model,
       effort: null,