diff --git a/apps/web/src/hooks/useMarkLiveCompletedThreadsUnread.test.ts b/apps/web/src/hooks/useMarkLiveCompletedThreadsUnread.test.ts
new file mode 100644
index 000000000000..cab82605a207
--- /dev/null
+++ b/apps/web/src/hooks/useMarkLiveCompletedThreadsUnread.test.ts
@@ -0,0 +1,223 @@
+import {
+  EnvironmentId,
+  ProjectId,
+  ProviderInstanceId,
+  ThreadId,
+  TurnId,
+  type OrchestrationLatestTurnState,
+  type OrchestrationThreadShell,
+} from "@t3tools/contracts";
+import { describe, expect, it } from "vite-plus/test";
+
+import {
+  transitionCompletedThreadUnreadState,
+  type CompletedThreadUnreadEnvironment,
+  type CompletedThreadUnreadState,
+} from "./useMarkLiveCompletedThreadsUnread";
+
+const ENVIRONMENT_ID = EnvironmentId.make("environment-1");
+const COMPLETED_AT = "2026-06-17T18:30:00.000Z";
+
+function makeThread(input: {
+  readonly id: string;
+  readonly state?: OrchestrationLatestTurnState;
+  readonly completedAt?: string | null;
+  readonly updatedAt?: string;
+  readonly archivedAt?: string | null;
+}): OrchestrationThreadShell {
+  const threadId = ThreadId.make(input.id);
+  const state = input.state ?? "completed";
+  return {
+    id: threadId,
+    projectId: ProjectId.make("project-1"),
+    title: input.id,
+    modelSelection: {
+      instanceId: ProviderInstanceId.make("codex"),
+      model: "gpt-5.6-sol",
+    },
+    runtimeMode: "full-access",
+    interactionMode: "default",
+    branch: "main",
+    worktreePath: null,
+    latestTurn: {
+      turnId: TurnId.make(`turn-${input.id}`),
+      state,
+      requestedAt: "2026-06-17T18:00:00.000Z",
+      startedAt: "2026-06-17T18:00:01.000Z",
+      completedAt: input.completedAt === undefined ? COMPLETED_AT : input.completedAt,
+      assistantMessageId: null,
+    },
+    createdAt: "2026-06-17T18:00:00.000Z",
+    updatedAt: input.updatedAt ?? COMPLETED_AT,
+    archivedAt: input.archivedAt ?? null,
+    settledOverride: null,
+    settledAt: null,
+    session: null,
+    latestUserMessageAt: "2026-06-17T18:00:00.000Z",
+    hasPendingApprovals: false,
+    hasPendingUserInput: false,
+    hasActionableProposedPlan: false,
+  };
+}
+
+function environment(
+  threads: ReadonlyArray<OrchestrationThreadShell>,
+  isLive = true,
+): CompletedThreadUnreadEnvironment {
+  return { environmentId: ENVIRONMENT_ID, isLive, threads };
+}
+
+function bootstrap(threads: ReadonlyArray<OrchestrationThreadShell> = []) {
+  return transitionCompletedThreadUnreadState(new Map(), [environment(threads)]);
+}
+
+describe("transitionCompletedThreadUnreadState", () => {
+  it("keeps completed threads in the initial historical snapshot read", () => {
+    const historical = makeThread({ id: "historical" });
+
+    const transition = bootstrap([historical]);
+
+    expect(transition.actions).toEqual([]);
+    expect(transition.state.get(ENVIRONMENT_ID)?.get(historical.id)).toBe(COMPLETED_AT);
+  });
+
+  it("marks a first-seen live completed thread unread when updatedAt equals completedAt", () => {
+    const initial = bootstrap();
+    const completed = makeThread({ id: "live-equal" });
+
+    const transition = transitionCompletedThreadUnreadState(initial.state, [
+      environment([completed]),
+    ]);
+
+    expect(transition.actions).toEqual([
+      { environmentId: ENVIRONMENT_ID, threadId: completed.id, completedAt: COMPLETED_AT },
+    ]);
+  });
+
+  it("marks a first-seen live completed thread unread when updatedAt is after completedAt", () => {
+    const initial = bootstrap();
+    const completed = makeThread({
+      id: "live-later",
+      updatedAt: "2026-06-17T18:31:00.000Z",
+    });
+
+    const transition = transitionCompletedThreadUnreadState(initial.state, [
+      environment([completed]),
+    ]);
+
+    expect(transition.actions).toEqual([
+      { environmentId: ENVIRONMENT_ID, threadId: completed.id, completedAt: COMPLETED_AT },
+    ]);
+  });
+
+  it("marks a completed thread introduced by a post-bootstrap snapshot unread", () => {
+    const historical = makeThread({ id: "historical" });
+    const initial = bootstrap([historical]);
+    const added = makeThread({ id: "snapshot-addition" });
+
+    const transition = transitionCompletedThreadUnreadState(initial.state, [
+      environment([historical, added]),
+    ]);
+
+    expect(transition.actions).toEqual([
+      { environmentId: ENVIRONMENT_ID, threadId: added.id, completedAt: COMPLETED_AT },
+    ]);
+  });
+
+  it("marks a previously running background thread when it completes", () => {
+    const running = makeThread({ id: "background", state: "running", completedAt: null });
+    const initial = bootstrap([running]);
+    const completed = makeThread({ id: "background" });
+
+    const transition = transitionCompletedThreadUnreadState(initial.state, [
+      environment([completed]),
+    ]);
+
+    expect(transition.actions).toEqual([
+      { environmentId: ENVIRONMENT_ID, threadId: completed.id, completedAt: COMPLETED_AT },
+    ]);
+  });
+
+  it("preserves observations across reconnect snapshots without repeating unread actions", () => {
+    const completed = makeThread({ id: "completed" });
+    const initial = bootstrap([completed]);
+
+    const synchronizing = transitionCompletedThreadUnreadState(initial.state, [
+      environment([], false),
+    ]);
+    const reconnected = transitionCompletedThreadUnreadState(synchronizing.state, [
+      environment([completed]),
+    ]);
+
+    expect(synchronizing.actions).toEqual([]);
+    expect(synchronizing.state).toEqual(initial.state);
+    expect(reconnected.actions).toEqual([]);
+  });
+
+  it("does not repeat an unread action after reconnecting", () => {
+    const initial = bootstrap();
+    const completed = makeThread({ id: "completed" });
+    const firstSeen = transitionCompletedThreadUnreadState(initial.state, [
+      environment([completed]),
+    ]);
+    const synchronizing = transitionCompletedThreadUnreadState(firstSeen.state, [
+      environment([completed], false),
+    ]);
+    const reconnected = transitionCompletedThreadUnreadState(synchronizing.state, [
+      environment([completed]),
+    ]);
+
+    expect(firstSeen.actions).toHaveLength(1);
+    expect(reconnected.actions).toEqual([]);
+  });
+
+  it("waits for a completion-consistent updatedAt before marking unread", () => {
+    const initial = bootstrap();
+    const stale = makeThread({
+      id: "stale",
+      updatedAt: "2026-06-17T18:29:59.000Z",
+    });
+    const staleTransition = transitionCompletedThreadUnreadState(initial.state, [
+      environment([stale]),
+    ]);
+    const consistent = makeThread({ id: "stale" });
+    const consistentTransition = transitionCompletedThreadUnreadState(staleTransition.state, [
+      environment([consistent]),
+    ]);
+
+    expect(staleTransition.actions).toEqual([]);
+    expect(consistentTransition.actions).toEqual([
+      { environmentId: ENVIRONMENT_ID, threadId: consistent.id, completedAt: COMPLETED_AT },
+    ]);
+  });
+
+  it("does not mark interrupted or error turns as completed", () => {
+    const initial = bootstrap();
+    const interrupted = makeThread({ id: "interrupted", state: "interrupted" });
+    const failed = makeThread({ id: "failed", state: "error" });
+
+    const transition = transitionCompletedThreadUnreadState(initial.state, [
+      environment([interrupted, failed]),
+    ]);
+
+    expect(transition.actions).toEqual([]);
+    expect(transition.state.get(ENVIRONMENT_ID)).toEqual(
+      new Map([
+        [interrupted.id, null],
+        [failed.id, null],
+      ]),
+    );
+  });
+
+  it("treats a removed and re-added environment as a fresh historical bootstrap", () => {
+    const historical = makeThread({ id: "historical" });
+    const initial = bootstrap([historical]);
+    const removed = transitionCompletedThreadUnreadState(initial.state, []);
+    const readded = transitionCompletedThreadUnreadState(removed.state, [
+      environment([historical]),
+    ]);
+
+    expect(removed.state).toEqual(new Map() satisfies CompletedThreadUnreadState);
+    expect(readded.actions).toEqual([]);
+  });
+});
diff --git a/apps/web/src/hooks/useMarkLiveCompletedThreadsUnread.ts b/apps/web/src/hooks/useMarkLiveCompletedThreadsUnread.ts
new file mode 100644
index 000000000000..cf9f73514165
--- /dev/null
+++ b/apps/web/src/hooks/useMarkLiveCompletedThreadsUnread.ts
@@ -0,0 +1,126 @@
+import { useAtomValue } from "@effect/atom-react";
+import { scopedThreadKey, scopeThreadRef } from "@t3tools/client-runtime/environment";
+import type { EnvironmentId, OrchestrationThreadShell, 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 { environmentShell } from "../state/shell";
+import { useUiStateStore } from "../uiStateStore";
+
+export interface CompletedThreadUnreadEnvironment {
+  readonly environmentId: EnvironmentId;
+  readonly isLive: boolean;
+  readonly threads: ReadonlyArray<OrchestrationThreadShell>;
+}
+
+export type CompletedThreadUnreadState = ReadonlyMap<
+  EnvironmentId,
+  ReadonlyMap<ThreadId, string | null>
+>;
+
+export interface CompletedThreadUnreadAction {
+  readonly environmentId: EnvironmentId;
+  readonly threadId: ThreadId;
+  readonly completedAt: string;
+}
+
+export interface CompletedThreadUnreadTransition {
+  readonly state: CompletedThreadUnreadState;
+  readonly actions: ReadonlyArray<CompletedThreadUnreadAction>;
+}
+
+function eligibleCompletedAt(thread: OrchestrationThreadShell): string | null {
+  if (thread.latestTurn?.state !== "completed" || !thread.latestTurn.completedAt) {
+    return null;
+  }
+
+  const completedAt = Date.parse(thread.latestTurn.completedAt);
+  const updatedAt = Date.parse(thread.updatedAt);
+  if (!Number.isFinite(completedAt) || !Number.isFinite(updatedAt) || updatedAt < completedAt) {
+    return null;
+  }
+
+  return thread.latestTurn.completedAt;
+}
+
+export function transitionCompletedThreadUnreadState(
+  previousState: CompletedThreadUnreadState,
+  environments: ReadonlyArray<CompletedThreadUnreadEnvironment>,
+): CompletedThreadUnreadTransition {
+  const nextState = new Map<EnvironmentId, ReadonlyMap<ThreadId, string | null>>();
+  const actions: CompletedThreadUnreadAction[] = [];
+
+  for (const environment of environments) {
+    const previousCompletions = previousState.get(environment.environmentId);
+    if (!environment.isLive) {
+      // Keep the last live observations while a known environment reconnects,
+      // but do not treat bootstrap or catch-up snapshots as new completions.
+      if (previousCompletions !== undefined) {
+        nextState.set(environment.environmentId, previousCompletions);
+      }
+      continue;
+    }
+
+    const currentCompletions = new Map<ThreadId, string | null>();
+    for (const thread of environment.threads) {
+      const completedAt = eligibleCompletedAt(thread);
+      currentCompletions.set(thread.id, completedAt);
+
+      if (
+        // An absent previous map means this is the environment's initial live
+        // snapshot, whose historical completions establish the read baseline.
+        previousCompletions !== undefined &&
+        completedAt !== null &&
+        thread.archivedAt === null &&
+        (!previousCompletions.has(thread.id) || previousCompletions.get(thread.id) !== completedAt)
+      ) {
+        actions.push({
+          environmentId: environment.environmentId,
+          threadId: thread.id,
+          completedAt,
+        });
+      }
+    }
+    nextState.set(environment.environmentId, currentCompletions);
+  }
+
+  return { state: nextState, actions };
+}
+
+const completedThreadUnreadEnvironmentsAtom = Atom.make(
+  (get): ReadonlyArray<CompletedThreadUnreadEnvironment> => {
+    const environments: CompletedThreadUnreadEnvironment[] = [];
+    for (const environmentId of get(environmentCatalog.catalogValueAtom).entries.keys()) {
+      const shellState = get(environmentShell.stateValueAtom(environmentId));
+      environments.push({
+        environmentId,
+        isLive: shellState.status === "live",
+        threads: Option.match(shellState.snapshot, {
+          onNone: () => [],
+          onSome: (snapshot) => snapshot.threads,
+        }),
+      });
+    }
+    return environments;
+  },
+).pipe(Atom.withLabel("completed-thread-unread:environments"));
+
+export function useMarkLiveCompletedThreadsUnread(): void {
+  const environments = useAtomValue(completedThreadUnreadEnvironmentsAtom);
+  const stateRef = useRef<CompletedThreadUnreadState>(new Map());
+
+  useEffect(() => {
+    const transition = transitionCompletedThreadUnreadState(stateRef.current, environments);
+    stateRef.current = transition.state;
+
+    const uiState = useUiStateStore.getState();
+    for (const action of transition.actions) {
+      uiState.markThreadUnread(
+        scopedThreadKey(scopeThreadRef(action.environmentId, action.threadId)),
+        action.completedAt,
+      );
+    }
+  }, [environments]);
+}
diff --git a/apps/web/src/routes/__root.tsx b/apps/web/src/routes/__root.tsx
index bc8f8507b814..058afed611e2 100644
--- a/apps/web/src/routes/__root.tsx
+++ b/apps/web/src/routes/__root.tsx
@@ -32,6 +32,7 @@ import { resolveAndPersistPreferredEditor } from "../editorPreferences";
 import { applyAppearanceFontVariables, isFontFamilyAvailable } from "~/appearanceFonts";
 import { loadHostFontFamily } from "../hostFonts";
 import { useClientSettings } from "../hooks/useSettings";
+import { useMarkLiveCompletedThreadsUnread } from "../hooks/useMarkLiveCompletedThreadsUnread";
 import {
   deriveLogicalProjectKeyFromSettings,
   derivePhysicalProjectKeyFromPath,
@@ -140,6 +141,7 @@ function RootRouteView() {
         <ConfirmDialogHost />
         <SlowRpcRequestToastCoordinator />
         <HostedStaticEnvironmentBootstrap />
+        <CompletedThreadUnreadTracker />
         {primaryEnvironmentAuthenticated ? <EventRouter /> : null}
         {primaryEnvironmentAuthenticated ? <ProviderUpdateLaunchNotification /> : null}
         {appShell}
@@ -151,6 +153,11 @@ function RootRouteView() {
   );
 }
 
+function CompletedThreadUnreadTracker() {
+  useMarkLiveCompletedThreadsUnread();
+  return null;
+}
+
 function GlassAppearanceSync() {
   const glassOpacity = useClientSettings((settings) => settings.glassOpacity);