diff --git a/src/app/hooks/useThreads.running.test.ts b/src/app/hooks/useThreads.running.test.ts
index e10398f..dad873b 100644
--- a/src/app/hooks/useThreads.running.test.ts
+++ b/src/app/hooks/useThreads.running.test.ts
@@ -2,6 +2,7 @@ import { describe, expect, it, vi } from "vitest";
 
 import {
   getRunningThreadIds,
+  mergeRunningIds,
   setThreadRunning,
   subscribeRunningThreads,
 } from "./useThreads";
@@ -36,3 +37,22 @@ describe("running thread store", () => {
     setThreadRunning("thread-c", false);
   });
 });
+
+describe("mergeRunningIds", () => {
+  it("returns busy-only ids", () => {
+    const merged = mergeRunningIds(["a"], []);
+    expect(merged.has("a")).toBe(true);
+    expect(merged.size).toBe(1);
+  });
+
+  it("returns local-only ids", () => {
+    const merged = mergeRunningIds([], ["b"]);
+    expect(merged.has("b")).toBe(true);
+    expect(merged.size).toBe(1);
+  });
+
+  it("merges both sources without duplicates", () => {
+    const merged = mergeRunningIds(["a", "b"], ["b", "c"]);
+    expect([...merged].sort()).toEqual(["a", "b", "c"]);
+  });
+});
diff --git a/src/app/hooks/useThreads.ts b/src/app/hooks/useThreads.ts
index 139e9b6..e94a611 100644
--- a/src/app/hooks/useThreads.ts
+++ b/src/app/hooks/useThreads.ts
@@ -125,7 +125,24 @@ export function useLocalRunningThreadIds(): ReadonlySet<string> {
   return useSyncExternalStore(subscribeRunningThreads, getRunningThreadIds);
 }
 
-export async function deleteThread(id: string): Promise<void> {  const response = await fetch(`/api/conversations/${encodeURIComponent(id)}`, {
+/** Merged view of "this thread has an active run": the zero-latency local
+ * store (this client's own runs) plus the 10s busy heartbeat (runs started by
+ * other tabs or scheduled tasks). */
+export function mergeRunningIds(
+  busy: Iterable<string>,
+  local: Iterable<string>
+): ReadonlySet<string> {
+  return new Set([...busy, ...local]);
+}
+
+export function useRunningThreadIds(): ReadonlySet<string> {
+  const local = useLocalRunningThreadIds();
+  const busy = useBusyThreadIds();
+  return useMemo(() => mergeRunningIds(busy, local), [busy, local]);
+}
+
+export async function deleteThread(id: string): Promise<void> {
+  const response = await fetch(`/api/conversations/${encodeURIComponent(id)}`, {
     method: "DELETE",
   });
   if (!response.ok) {
@@ -157,12 +174,14 @@ export async function setThreadModelSelection(
 }
 
 function slugifyForFilename(input: string): string {
-  return input
-    // eslint-disable-next-line no-control-regex
-    .replace(/[\\/:*?"<>|\u0000-\u001F]+/g, "")
-    .trim()
-    .replace(/\s+/g, "-")
-    .slice(0, 80);
+  return (
+    input
+      // eslint-disable-next-line no-control-regex
+      .replace(/[\\/:*?"<>|\u0000-\u001F]+/g, "")
+      .trim()
+      .replace(/\s+/g, "-")
+      .slice(0, 80)
+  );
 }
 
 export async function exportThread(