diff --git a/frontend/src/app/workspace/agents/[agent_name]/chats/[thread_id]/page.tsx b/frontend/src/app/workspace/agents/[agent_name]/chats/[thread_id]/page.tsx
index 49134d7aa4..c08f8161d9 100644
--- a/frontend/src/app/workspace/agents/[agent_name]/chats/[thread_id]/page.tsx
+++ b/frontend/src/app/workspace/agents/[agent_name]/chats/[thread_id]/page.tsx
@@ -67,6 +67,7 @@ import { isHiddenFromUIMessage } from "@/core/messages/utils";
 import { modelTaskInput } from "@/core/models/control";
 import { useModels } from "@/core/models/hooks";
 import { useModelControl } from "@/core/models/use-model-control";
+import { notificationBody } from "@/core/notification/body";
 import { useNotification } from "@/core/notification/hooks";
 import { useThreadScheduledTaskEvents } from "@/core/scheduled-tasks/events";
 import { useLocalSettings, useThreadSettings } from "@/core/settings";
@@ -221,18 +222,12 @@ export default function AgentChatPage() {
       // it is open: it has been read.
       markThreadRead();
       if (document.hidden || !document.hasFocus()) {
-        let body = "Conversation finished";
         const lastMessage = state.messages[state.messages.length - 1];
-        if (lastMessage) {
-          const textContent = textOfMessage(lastMessage);
-          if (textContent) {
-            body =
-              textContent.length > 200
-                ? textContent.substring(0, 200) + "..."
-                : textContent;
-          }
-        }
-        showNotification(state.title, { body });
+        showNotification(state.title, {
+          body: notificationBody(
+            lastMessage ? textOfMessage(lastMessage) : null,
+          ),
+        });
       }
     },
   });
diff --git a/frontend/src/components/workspace/chats/chat-page.tsx b/frontend/src/components/workspace/chats/chat-page.tsx
index bfed5e25db..c7bbaa4d75 100644
--- a/frontend/src/components/workspace/chats/chat-page.tsx
+++ b/frontend/src/components/workspace/chats/chat-page.tsx
@@ -62,6 +62,7 @@ import { isHiddenFromUIMessage } from "@/core/messages/utils";
 import { modelTaskInput } from "@/core/models/control";
 import { useModels } from "@/core/models/hooks";
 import { useModelControl } from "@/core/models/use-model-control";
+import { notificationBody } from "@/core/notification/body";
 import { useNotification } from "@/core/notification/hooks";
 import { useProject } from "@/core/projects";
 import { useThreadScheduledTaskEvents } from "@/core/scheduled-tasks/events";
@@ -231,18 +232,12 @@ export default function ChatPage() {
       // it is open: it has been read.
       markThreadRead();
       if (document.hidden || !document.hasFocus()) {
-        let body = "Conversation finished";
         const lastMessage = state.messages.at(-1);
-        if (lastMessage) {
-          const textContent = textOfMessage(lastMessage);
-          if (textContent) {
-            body =
-              textContent.length > 200
-                ? textContent.substring(0, 200) + "..."
-                : textContent;
-          }
-        }
-        showNotification(state.title, { body });
+        showNotification(state.title, {
+          body: notificationBody(
+            lastMessage ? textOfMessage(lastMessage) : null,
+          ),
+        });
       }
     },
   });
diff --git a/frontend/src/core/notification/body.ts b/frontend/src/core/notification/body.ts
new file mode 100644
index 0000000000..ecf0ec472f
--- /dev/null
+++ b/frontend/src/core/notification/body.ts
@@ -0,0 +1,27 @@
+/** Longest notification body, counted in code points. */
+export const MAX_NOTIFICATION_BODY_CODE_POINTS = 200;
+
+/** Ellipsis appended to a shortened notification body. */
+const ELLIPSIS = "...";
+
+/**
+ * Body for a "finished in the background" notification.
+ *
+ * The text is bounded by code points rather than UTF-16 code units: a
+ * `String.prototype.length` / `substring` cut can land between the two halves
+ * of an astral character (an emoji) and leave a lone surrogate, which the
+ * notification renders as U+FFFD. Summaries that fit the budget are returned
+ * whole even when they contain astral characters.
+ */
+export function notificationBody(
+  text: string | null | undefined,
+  fallback = "Conversation finished",
+): string {
+  if (!text) {
+    return fallback;
+  }
+  const characters = Array.from(text);
+  return characters.length > MAX_NOTIFICATION_BODY_CODE_POINTS
+    ? `${characters.slice(0, MAX_NOTIFICATION_BODY_CODE_POINTS).join("")}${ELLIPSIS}`
+    : text;
+}
diff --git a/frontend/tests/unit/core/notification/body.test.ts b/frontend/tests/unit/core/notification/body.test.ts
new file mode 100644
index 0000000000..405d647470
--- /dev/null
+++ b/frontend/tests/unit/core/notification/body.test.ts
@@ -0,0 +1,36 @@
+import { expect, test } from "@rstest/core";
+
+import {
+  MAX_NOTIFICATION_BODY_CODE_POINTS,
+  notificationBody,
+} from "@/core/notification/body";
+
+const LONE_SURROGATE =
+  /[\uD800-\uDBFF](?![\uDC00-\uDFFF])|(?<![\uD800-\uDBFF])[\uDC00-\uDFFF]/;
+
+test("quotes the last message when it fits", () => {
+  expect(notificationBody("Done in 3 steps.")).toBe("Done in 3 steps.");
+});
+
+test("falls back when there is no text to quote", () => {
+  expect(notificationBody(null)).toBe("Conversation finished");
+  expect(notificationBody(undefined)).toBe("Conversation finished");
+  expect(notificationBody("")).toBe("Conversation finished");
+  expect(notificationBody("", "Custom fallback")).toBe("Custom fallback");
+});
+
+test("a body that fits the code-point budget keeps its trailing emoji", () => {
+  // 200 code points but 201 UTF-16 units: UTF-16 counting would cut this.
+  const text = `${"a".repeat(MAX_NOTIFICATION_BODY_CODE_POINTS - 1)}😀`;
+  expect(notificationBody(text)).toBe(text);
+});
+
+test("a truncated body never ends in half an emoji", () => {
+  // 201 code points; the old UTF-16 cut landed inside the surrogate pair.
+  const text = `${"a".repeat(MAX_NOTIFICATION_BODY_CODE_POINTS - 1)}😀b`;
+  const body = notificationBody(text);
+  expect(body).toBe(
+    `${"a".repeat(MAX_NOTIFICATION_BODY_CODE_POINTS - 1)}😀...`,
+  );
+  expect(body).not.toMatch(LONE_SURROGATE);
+});