test(FUX-039): add cross-instance toast dedupe regression test

Co-authored-by: Fusion <noreply@runfusion.ai>

packages/dashboard/app/components/__tests__/RuntimeFallbackBadge.test.tsx

@@ -3,6 +3,7 @@ import { describe, it, expect, vi, beforeEach, afterEach } from "vitest";
 import { render, screen, act } from "@testing-library/react";
 import { RuntimeFallbackBadge } from "../RuntimeFallbackBadge";
 import { ToastProvider, useToast } from "../../hooks/useToast";
+import { __resetRuntimeFallbackToastDedupeStoreForTests } from "../../hooks/useRuntimeFallbackStatus";
 import type { TaskRuntimeFallbackResponse } from "../../api/legacy";
 
 const legacyMocks = vi.hoisted(() => ({
@@ -88,6 +89,10 @@ describe("RuntimeFallbackBadge", () => {
   beforeEach(() => {
     vi.useFakeTimers();
     legacyMocks.fetchTaskRuntimeFallback.mockReset();
+    // The toast dedupe store is module-level/shared by design (that is the
+    // fix under test) — reset it between test cases so one test's "already
+    // toasted" state does not leak into the next.
+    __resetRuntimeFallbackToastDedupeStoreForTests();
   });
 
   afterEach(() => {
@@ -238,12 +243,46 @@ describe("RuntimeFallbackBadge", () => {
     expect(legacyMocks.fetchTaskRuntimeFallback).toHaveBeenCalled();
     expect(screen.getByTestId("runtime-fallback-badge")).toBeInTheDocument();
   });
+
+  it("fires exactly one toast when the same task/event is observed by two simultaneously-mounted badge instances (e.g. ActiveAgentsPanel + AgentsView rendering the same task concurrently)", async () => {
+    legacyMocks.fetchTaskRuntimeFallback.mockResolvedValue(fallbackWithHint);
+
+    render(
+      <ToastProvider>
+        <RuntimeFallbackBadge taskId="FN-100" isInViewport={true} projectId="proj-1" />
+        <RuntimeFallbackBadge taskId="FN-100" isInViewport={true} projectId="proj-1" />
+        <ToastPeek />
+      </ToastProvider>,
+    );
+
+    await act(async () => {
+      await vi.advanceTimersByTimeAsync(0);
+    });
+
+    // Both instances polled and both observed the same new eventId on their
+    // first poll, but the shared module-level dedupe store means only one of
+    // them should have won the "claim" and fired a toast.
+    expect(screen.getAllByTestId("toast-entry")).toHaveLength(1);
+    expect(screen.getAllByTestId("toast-entry")[0].textContent).toContain("hermes");
+
+    // Both badges still render independently (dedupe only affects the toast,
+    // not the per-instance badge display).
+    expect(screen.getAllByTestId("runtime-fallback-badge")).toHaveLength(2);
+
+    // Further polls with the same event on both instances must not add a
+    // second toast either.
+    await act(async () => {
+      await vi.advanceTimersByTimeAsync(30_000);
+    });
+    expect(screen.getAllByTestId("toast-entry")).toHaveLength(1);
+  });
 });
 
 describe("RuntimeFallbackBadge — mobile breakpoint", () => {
   beforeEach(() => {
     vi.useFakeTimers();
     legacyMocks.fetchTaskRuntimeFallback.mockReset();
+    __resetRuntimeFallbackToastDedupeStoreForTests();
   });
 
   afterEach(() => {

packages/dashboard/app/hooks/useRuntimeFallbackStatus.ts

@@ -14,11 +14,65 @@
  * This hook only polls while `enabled` is true (callers should pass
  * `isInViewport` so off-screen cards do not generate background traffic).
  */
-import { useEffect, useRef, useState } from "react";
+import { useEffect, useState } from "react";
 import { fetchTaskRuntimeFallback, type TaskRuntimeFallbackResponse } from "../api/legacy";
 
 const POLL_INTERVAL_MS = 30_000;
 
+// Toast dedupe must be shared across ALL hook instances in the process, not
+// scoped per-instance: the same task/event can be observed simultaneously by
+// multiple mounted badges (e.g. ActiveAgentsPanel + AgentsView board/list +
+// TaskCard all rendering the same in-progress task at once), each running
+// its own useRuntimeFallbackStatus() call. A per-instance ref only dedupes
+// within one component instance's own poll history, so the same eventId
+// would independently look "newly observed" to every instance and fire one
+// toast each. Module-level state is shared across every call site because
+// there is exactly one copy of this module per process/bundle.
+//
+// Keyed by `${taskId}:${eventId}` (not eventId alone) so ids are unambiguous
+// even if two different tasks' audit logs ever produced colliding event ids.
+// Bounded via a simple FIFO eviction (insertion order === Map iteration
+// order) so a long-lived dashboard session touching many tasks over many
+// hours cannot grow this unboundedly; runtime-fallback events are rare
+// (at most one per agent session), so a few hundred entries comfortably
+// covers realistic session lengths without needing TTL bookkeeping.
+const MAX_TOASTED_EVENTS = 500;
+const toastedEventKeys = new Map<string, true>();
+
+function toastKey(taskId: string, eventId: string): string {
+  return `${taskId}:${eventId}`;
+}
+
+/**
+ * Returns true and records the key the first time it is seen; returns false
+ * on every subsequent call for the same key, regardless of which hook
+ * instance/component asks. This is the single shared gate all simultaneously
+ * mounted badge instances for the same task funnel through.
+ */
+function claimToastOnce(taskId: string, eventId: string): boolean {
+  const key = toastKey(taskId, eventId);
+  if (toastedEventKeys.has(key)) {
+    return false;
+  }
+  toastedEventKeys.set(key, true);
+  if (toastedEventKeys.size > MAX_TOASTED_EVENTS) {
+    const oldestKey = toastedEventKeys.keys().next().value;
+    if (oldestKey !== undefined) {
+      toastedEventKeys.delete(oldestKey);
+    }
+  }
+  return true;
+}
+
+/**
+ * Test-only escape hatch: clears the shared module-level dedupe store between
+ * test cases so one test's "already toasted" state cannot leak into the
+ * next. Not used by production code paths.
+ */
+export function __resetRuntimeFallbackToastDedupeStoreForTests(): void {
+  toastedEventKeys.clear();
+}
+
 export interface RuntimeFallbackStatus {
   /** True only when the latest resolution has wasConfigured=false and a non-empty runtimeHint. */
   showBadge: boolean;
@@ -55,10 +109,6 @@ export function useRuntimeFallbackStatus(
   projectId?: string,
 ): RuntimeFallbackStatus {
   const [status, setStatus] = useState<RuntimeFallbackStatus>(IDLE_STATUS);
-  // Dedupe key for toasts: last audit event ID we already toasted for. Persists across
-  // polls/re-renders for the lifetime of the component so the toast fires exactly once
-  // per newly-observed fallback session, not on every poll.
-  const lastToastedEventIdRef = useRef<string | null>(null);
 
   useEffect(() => {
     if (!enabled || !taskId) {
@@ -83,10 +133,10 @@ export function useRuntimeFallbackStatus(
         return;
       }
 
-      const isNewlyObserved = data.eventId !== null && data.eventId !== lastToastedEventIdRef.current;
-      if (isNewlyObserved && data.eventId) {
-        lastToastedEventIdRef.current = data.eventId;
-      }
+      // Dedupe against the shared module-level store (not a per-instance ref)
+      // so a fallback event toasts exactly once across every simultaneously
+      // mounted badge instance for this task, not once per instance.
+      const isNewlyObserved = data.eventId !== null && taskId !== undefined && claimToastOnce(taskId, data.eventId);
 
       setStatus({
         showBadge: true,