diff --git a/apps/web/src/components/ThreadNotificationCoordinator.badge.browser.test.tsx b/apps/web/src/components/ThreadNotificationCoordinator.badge.browser.test.tsx
index 695378034f4b..3f8d6d1db57f 100644
--- a/apps/web/src/components/ThreadNotificationCoordinator.badge.browser.test.tsx
+++ b/apps/web/src/components/ThreadNotificationCoordinator.badge.browser.test.tsx
@@ -145,10 +145,14 @@ function toV2ThreadShell(input: typeof thread) {
   };
 }
 
-function shell(overrides: Partial<typeof thread> = {}) {
+function shell(overrides: Partial<typeof thread> = {}, ...others: Partial<typeof thread>[]) {
   return {
     status: "live",
-    snapshot: Option.some({ threads: [toV2ThreadShell({ ...thread, ...overrides })] }),
+    snapshot: Option.some({
+      threads: [overrides, ...others].map((override) =>
+        toV2ThreadShell({ ...thread, ...override }),
+      ),
+    }),
   };
 }
 function complete(environment = "one", completedAt = "2026-09-13T08:00:00Z") {
@@ -197,16 +201,43 @@ afterEach(async () => {
   vi.unstubAllGlobals();
 });
 
-it("counts notifying threads across environments, replaces repeat alerts, and clears on focus", async () => {
+it("uses bounded tags across threads and environments, replaces repeat alerts, and clears on focus", async () => {
+  const environmentId = "fd6fff30-f90b-4ee7-9812-4cf497791b90";
+  const otherEnvironmentId = "25b62992-b996-4ee3-8d82-d988840e45c1";
+  const threadId = "a8395a98-a186-4fe7-9d4c-fb8ba63b7588";
+  const otherThreadId = "257598ad-ae4f-4422-a073-e03f4b4f5f0d";
+  const completed = (id: string, completedAt = "2026-09-13T08:00:00Z") => ({
+    id,
+    latestTurn: { turnId: "turn", state: "completed", completedAt },
+  });
+  state.environmentIds = [environmentId, otherEnvironmentId];
+  state.shells.set(environmentId, shell({ id: threadId }, { id: otherThreadId }));
+  state.shells.set(otherEnvironmentId, shell({ id: threadId }));
   await render();
-  complete();
+  state.shells.set(environmentId, shell(completed(threadId), { id: otherThreadId }));
   await render();
   expect(state.badge).toHaveBeenLastCalledWith(1);
-  complete("one", "2026-09-13T08:01:00Z");
-  complete("two");
+  state.shells.set(environmentId, shell(completed(threadId), completed(otherThreadId)));
+  state.shells.set(otherEnvironmentId, shell(completed(threadId)));
   await render();
-  expect(state.badge).toHaveBeenLastCalledWith(2);
-  expect(TestNotification.sent[0]!.close).toHaveBeenCalledOnce();
+  expect(state.badge).toHaveBeenLastCalledWith(3);
+  state.shells.set(
+    environmentId,
+    shell(completed(threadId, "2026-09-13T08:01:00Z"), completed(otherThreadId)),
+  );
+  await render();
+  expect(state.badge).toHaveBeenLastCalledWith(3);
+  expect(TestNotification.sent).toHaveLength(4);
+  const [first, otherThread, otherEnvironment, replacement] = TestNotification.sent;
+  for (const notification of TestNotification.sent) {
+    expect(notification.tag).toMatch(/^[0-9a-f]{16}$/);
+  }
+  expect(new Set([first!.tag, otherThread!.tag, otherEnvironment!.tag]).size).toBe(3);
+  expect(replacement!.tag).toBe(first!.tag);
+  expect(first!.close).toHaveBeenCalledOnce();
+  expect(otherThread!.close).not.toHaveBeenCalled();
+  expect(otherEnvironment!.close).not.toHaveBeenCalled();
+  expect(replacement!.close).not.toHaveBeenCalled();
   focused = true;
   window.dispatchEvent(new Event("focus"));
   expect(state.badge).toHaveBeenLastCalledWith(0);
@@ -214,7 +245,7 @@ it("counts notifying threads across environments, replaces repeat alerts, and cl
     TestNotification.sent.every((notification) => notification.close.mock.calls.length > 0),
   ).toBe(true);
   focused = false;
-  complete("two", "2026-09-13T08:02:00Z");
+  state.shells.set(otherEnvironmentId, shell(completed(threadId, "2026-09-13T08:02:00Z")));
   await render();
   expect(state.badge).toHaveBeenLastCalledWith(1);
 });
diff --git a/apps/web/src/components/ThreadNotificationCoordinator.browser.test.tsx b/apps/web/src/components/ThreadNotificationCoordinator.browser.test.tsx
index 93de18b6b3e3..d106c67baa91 100644
--- a/apps/web/src/components/ThreadNotificationCoordinator.browser.test.tsx
+++ b/apps/web/src/components/ThreadNotificationCoordinator.browser.test.tsx
@@ -282,7 +282,7 @@ describe("thread notifications", () => {
     expect(state.notification).toHaveBeenCalledTimes(1);
     expect(state.notification).toHaveBeenCalledWith(title, {
       body: "Fix the login form",
-      tag: "env-1:thread-1",
+      tag: expect.any(String),
       silent: true,
     });
   });
@@ -355,7 +355,7 @@ describe("thread notifications", () => {
     expect(state.add).not.toHaveBeenCalled();
     expect(state.notification).toHaveBeenCalledWith("Thread completed", {
       body: "Fix the login form",
-      tag: "env-1:thread-1",
+      tag: expect.any(String),
       silent: true,
     });
   });
diff --git a/apps/web/src/components/ThreadNotificationCoordinator.tsx b/apps/web/src/components/ThreadNotificationCoordinator.tsx
index fab97abeeef0..bb7fdcc29a8b 100644
--- a/apps/web/src/components/ThreadNotificationCoordinator.tsx
+++ b/apps/web/src/components/ThreadNotificationCoordinator.tsx
@@ -23,6 +23,7 @@ import {
   hasNotificationSound,
   playNotificationSound,
   setNotificationBadge,
+  threadNotificationTag,
   unlockNotificationAudio,
 } from "../threadNotifications";
 import { resolveSidebarThreadStatus } from "./Sidebar.logic";
@@ -218,7 +219,7 @@ function EnvironmentNotifications({
       try {
         const notification = new Notification(title, {
           body: thread.title,
-          tag: `${environmentId}:${thread.id}`,
+          tag: threadNotificationTag(environmentId, thread.id),
           silent: true,
         });
         onNotification(environmentId, notification);
diff --git a/apps/web/src/threadNotifications.ts b/apps/web/src/threadNotifications.ts
index 3453a2931e94..0766f977d252 100644
--- a/apps/web/src/threadNotifications.ts
+++ b/apps/web/src/threadNotifications.ts
@@ -1,3 +1,4 @@
+import type { EnvironmentId, ThreadId } from "@t3tools/contracts";
 import type { ClientSettings } from "@t3tools/contracts/settings";
 
 import completionUrl from "./assets/notification-completion.mp3";
@@ -19,6 +20,21 @@ export function hasDesktopNotifications(mode: NotificationMode) {
   return mode === "notifications" || mode === "notifications-and-sound";
 }
 
+const TAG_HASH_OFFSET_BASIS = 0xcbf29ce484222325n;
+const TAG_HASH_PRIME = 0x100000001b3n;
+const TAG_HASH_MASK = 0xffffffffffffffffn;
+
+/** Windows drops toast notifications with the 73-character environment/thread tag. */
+export function threadNotificationTag(environmentId: EnvironmentId, threadId: ThreadId): string {
+  const key = `${environmentId}:${threadId}`;
+  let hash = TAG_HASH_OFFSET_BASIS;
+  for (let index = 0; index < key.length; index += 1) {
+    hash ^= BigInt(key.charCodeAt(index));
+    hash = (hash * TAG_HASH_PRIME) & TAG_HASH_MASK;
+  }
+  return hash.toString(16).padStart(16, "0");
+}
+
 let originalFavicon: HTMLLinkElement | undefined;
 let badgeFavicon: HTMLLinkElement | undefined;