diff --git a/apps/web/src/hooks/useMarkFirstSeenCompletedThreadsUnread.test.ts b/apps/web/src/hooks/useMarkFirstSeenCompletedThreadsUnread.test.ts
new file mode 100644
index 00000000000..34e78f56c84
--- /dev/null
+++ b/apps/web/src/hooks/useMarkFirstSeenCompletedThreadsUnread.test.ts
@@ -0,0 +1,72 @@
+import { scopedThreadKey, scopeThreadRef } from "@t3tools/client-runtime/environment";
+import { EnvironmentId, ThreadId } from "@t3tools/contracts";
+import { describe, expect, it } from "vite-plus/test";
+
+import { resolveFirstSeenCompletedThreads } from "./useMarkFirstSeenCompletedThreadsUnread";
+
+const localEnvironmentId = EnvironmentId.make("environment-local");
+const remoteEnvironmentId = EnvironmentId.make("environment-remote");
+
+function thread(
+  id: string,
+  state: "completed" | "running" = "completed",
+  environmentId = localEnvironmentId,
+) {
+  return {
+    environmentId,
+    id: ThreadId.make(id),
+    latestTurn: {
+      state,
+      completedAt: "2026-06-18T09:00:00.000Z",
+    },
+  } as const;
+}
+
+describe("resolveFirstSeenCompletedThreads", () => {
+  it("seeds initial snapshot history without marking it unread", () => {
+    const result = resolveFirstSeenCompletedThreads({
+      threads: [thread("historical")],
+      environmentSnapshotIds: [localEnvironmentId],
+      previouslySeenThreadKeysByEnvironment: new Map(),
+    });
+
+    expect(result.newlyUnreadThreads).toEqual([]);
+    expect(result.nextSeenThreadKeysByEnvironment.get(localEnvironmentId)).toEqual(
+      new Set([scopedThreadKey(scopeThreadRef(localEnvironmentId, ThreadId.make("historical")))]),
+    );
+  });
+
+  it("marks a completed thread that first appears after bootstrap unread", () => {
+    const historicalKey = scopedThreadKey(
+      scopeThreadRef(localEnvironmentId, ThreadId.make("historical")),
+    );
+    const completedKey = scopedThreadKey(
+      scopeThreadRef(localEnvironmentId, ThreadId.make("completed")),
+    );
+    const result = resolveFirstSeenCompletedThreads({
+      threads: [thread("historical"), thread("completed")],
+      environmentSnapshotIds: [localEnvironmentId],
+      previouslySeenThreadKeysByEnvironment: new Map([
+        [localEnvironmentId, new Set([historicalKey])],
+      ]),
+    });
+
+    expect(result.newlyUnreadThreads).toEqual([
+      {
+        threadKey: completedKey,
+        completedAt: "2026-06-18T09:00:00.000Z",
+      },
+    ]);
+  });
+
+  it("does not mark a new unfinished thread or a thread outside a snapshot environment", () => {
+    const result = resolveFirstSeenCompletedThreads({
+      threads: [thread("running", "running"), thread("remote", "completed", remoteEnvironmentId)],
+      environmentSnapshotIds: [localEnvironmentId],
+      previouslySeenThreadKeysByEnvironment: new Map([[localEnvironmentId, new Set()]]),
+    });
+
+    expect(result.newlyUnreadThreads).toEqual([]);
+    expect(result.nextSeenThreadKeysByEnvironment.has(remoteEnvironmentId)).toBe(false);
+  });
+});
diff --git a/apps/web/src/hooks/useMarkFirstSeenCompletedThreadsUnread.ts b/apps/web/src/hooks/useMarkFirstSeenCompletedThreadsUnread.ts
new file mode 100644
index 00000000000..241c998ae53
--- /dev/null
+++ b/apps/web/src/hooks/useMarkFirstSeenCompletedThreadsUnread.ts
@@ -0,0 +1,97 @@
+import { useAtomValue } from "@effect/atom-react";
+import { scopedThreadKey, scopeThreadRef } from "@t3tools/client-runtime/environment";
+import type { EnvironmentId, ThreadId } from "@t3tools/contracts";
+import * as Option from "effect/Option";
+import { Atom } from "effect/unstable/reactivity";
+import { useEffect, useRef } from "react";
+
+import { environmentCatalog } from "../connection/catalog";
+import { useThreadShells } from "../state/entities";
+import { environmentShell } from "../state/shell";
+import { useUiStateStore } from "../uiStateStore";
+
+const environmentSnapshotIdsAtom = Atom.make((get): ReadonlyArray<EnvironmentId> => {
+  const environmentIds: EnvironmentId[] = [];
+  for (const environmentId of get(environmentCatalog.catalogValueAtom).entries.keys()) {
+    if (Option.isSome(get(environmentShell.stateValueAtom(environmentId)).snapshot)) {
+      environmentIds.push(environmentId);
+    }
+  }
+  return environmentIds;
+}).pipe(Atom.withLabel("completed-thread-unread:snapshot-environments"));
+
+interface FirstSeenThreadInput {
+  readonly environmentId: EnvironmentId;
+  readonly id: ThreadId;
+  readonly latestTurn: {
+    readonly state: string;
+    readonly completedAt: string | null;
+  } | null;
+}
+
+export function resolveFirstSeenCompletedThreads(input: {
+  readonly threads: ReadonlyArray<FirstSeenThreadInput>;
+  readonly environmentSnapshotIds: ReadonlyArray<EnvironmentId>;
+  readonly previouslySeenThreadKeysByEnvironment: ReadonlyMap<EnvironmentId, ReadonlySet<string>>;
+}): {
+  readonly nextSeenThreadKeysByEnvironment: Map<EnvironmentId, Set<string>>;
+  readonly newlyUnreadThreads: ReadonlyArray<{
+    readonly threadKey: string;
+    readonly completedAt: string | null;
+  }>;
+} {
+  const snapshotEnvironmentIds = new Set(input.environmentSnapshotIds);
+  const nextSeenThreadKeysByEnvironment = new Map<EnvironmentId, Set<string>>();
+  const newlyUnreadThreads: Array<{
+    readonly threadKey: string;
+    readonly completedAt: string | null;
+  }> = [];
+  for (const environmentId of snapshotEnvironmentIds) {
+    nextSeenThreadKeysByEnvironment.set(environmentId, new Set());
+  }
+
+  for (const thread of input.threads) {
+    if (!snapshotEnvironmentIds.has(thread.environmentId)) {
+      continue;
+    }
+
+    const threadKey = scopedThreadKey(scopeThreadRef(thread.environmentId, thread.id));
+    nextSeenThreadKeysByEnvironment.get(thread.environmentId)?.add(threadKey);
+
+    const previousThreadKeys = input.previouslySeenThreadKeysByEnvironment.get(
+      thread.environmentId,
+    );
+    if (
+      previousThreadKeys !== undefined &&
+      !previousThreadKeys.has(threadKey) &&
+      thread.latestTurn?.state === "completed"
+    ) {
+      newlyUnreadThreads.push({
+        threadKey,
+        completedAt: thread.latestTurn.completedAt,
+      });
+    }
+  }
+
+  return { nextSeenThreadKeysByEnvironment, newlyUnreadThreads };
+}
+
+export function useMarkFirstSeenCompletedThreadsUnread(): void {
+  const threads = useThreadShells();
+  const environmentSnapshotIds = useAtomValue(environmentSnapshotIdsAtom);
+  const seenThreadKeysByEnvironmentRef = useRef<Map<EnvironmentId, Set<string>>>(new Map());
+
+  useEffect(() => {
+    const { nextSeenThreadKeysByEnvironment, newlyUnreadThreads } =
+      resolveFirstSeenCompletedThreads({
+        threads,
+        environmentSnapshotIds,
+        previouslySeenThreadKeysByEnvironment: seenThreadKeysByEnvironmentRef.current,
+      });
+    for (const thread of newlyUnreadThreads) {
+      useUiStateStore.getState().markThreadUnread(thread.threadKey, thread.completedAt);
+    }
+
+    seenThreadKeysByEnvironmentRef.current = nextSeenThreadKeysByEnvironment;
+  }, [environmentSnapshotIds, threads]);
+}
diff --git a/apps/web/src/routes/__root.tsx b/apps/web/src/routes/__root.tsx
index 346991d114d..cb7530c0345 100644
--- a/apps/web/src/routes/__root.tsx
+++ b/apps/web/src/routes/__root.tsx
@@ -27,6 +27,7 @@ import {
   toastManager,
 } from "../components/ui/toast";
 import { resolveAndPersistPreferredEditor } from "../editorPreferences";
+import { useMarkFirstSeenCompletedThreadsUnread } from "../hooks/useMarkFirstSeenCompletedThreadsUnread";
 import { useClientSettings } from "../hooks/useSettings";
 import {
   deriveLogicalProjectKeyFromSettings,
@@ -277,6 +278,8 @@ function AuthenticatedTracingBootstrap() {
 }
 
 function EventRouter() {
+  useMarkFirstSeenCompletedThreadsUnread();
+
   const navigate = useNavigate();
   const pathname = useLocation({ select: (loc) => loc.pathname });
   const projectGroupingSettings = useClientSettings(selectProjectGroupingSettings);