diff --git a/ui/src/pages/overview/event-log.test.ts b/ui/src/pages/overview/event-log.test.ts
new file mode 100644
index 00000000000..ff55941c650
--- /dev/null
+++ b/ui/src/pages/overview/event-log.test.ts
@@ -0,0 +1,39 @@
+/* @vitest-environment jsdom */
+
+import { render } from "lit";
+import { describe, expect, it } from "vitest";
+import { formatEventPayload } from "../../lib/presenter.ts";
+import { renderOverviewEventLog } from "./event-log.ts";
+
+function createBoundaryPayload() {
+  const emptyPayload = formatEventPayload({ message: "" });
+  const valueStart = emptyPayload.indexOf('""') + 1;
+  return { message: `${"a".repeat(119 - valueStart)}🙂tail` };
+}
+
+describe("renderOverviewEventLog", () => {
+  it("keeps payload previews UTF-16 safe at the truncation boundary", () => {
+    const payload = createBoundaryPayload();
+    const formatted = formatEventPayload(payload);
+    const oldPreview = formatted.slice(0, 120);
+    expect(oldPreview.charCodeAt(oldPreview.length - 1)).toBe(0xd83d);
+
+    const container = document.createElement("div");
+    render(
+      renderOverviewEventLog({
+        events: [
+          {
+            ts: 1,
+            event: "agent.message",
+            payload,
+          },
+        ],
+      }),
+      container,
+    );
+
+    const preview = container.querySelector(".ov-event-log-payload")?.textContent ?? "";
+    expect(preview).toBe(formatted.slice(0, 119));
+    expect(preview.charCodeAt(preview.length - 1)).not.toBe(0xd83d);
+  });
+});
diff --git a/ui/src/pages/overview/event-log.ts b/ui/src/pages/overview/event-log.ts
index 94d04d4e303..9363609d578 100644
--- a/ui/src/pages/overview/event-log.ts
+++ b/ui/src/pages/overview/event-log.ts
@@ -3,7 +3,7 @@ import { html, nothing } from "lit";
 import type { EventLogEntry } from "../../api/event-log.ts";
 import { icons } from "../../components/icons.ts";
 import { t } from "../../i18n/index.ts";
-import { formatTimeMs } from "../../lib/format.ts";
+import { formatTimeMs, truncateText } from "../../lib/format.ts";
 import { formatEventPayload } from "../../lib/presenter.ts";
 
 type OverviewEventLogProps = {
@@ -32,7 +32,7 @@ export function renderOverviewEventLog(props: OverviewEventLogProps) {
               <span class="ov-event-log-name">${entry.event}</span>
               ${entry.payload
                 ? html`<span class="ov-event-log-payload muted"
-                    >${formatEventPayload(entry.payload).slice(0, 120)}</span
+                    >${truncateText(formatEventPayload(entry.payload), 120).text}</span
                   >`
                 : nothing}
             </div>