diff --git a/backend/app/auth.py b/backend/app/auth.py
index 68b235f..c654dca 100644
--- a/backend/app/auth.py
+++ b/backend/app/auth.py
@@ -419,15 +419,22 @@ def _complete_link(
 
 @router.post("/logout")
 async def logout(request: Request):
-    """Sign out the active account. If other accounts have authenticated in this browser,
-    switch to the most recent one instead of fully logging out."""
-    active = request.session.get("user_id")
+    """Sign the requesting tab's active account out of this browser (removing it from the wallet).
+    Per-tab aware: the account is taken from the X-Siftlode-Account header when present, so logging
+    out in one tab doesn't disturb another tab running a different account. If the account being
+    removed was the session default, promote the most recent remaining account (or clear the
+    session when none remain)."""
+    active, is_default = resolved_user_id(request)
     remaining = [a for a in (request.session.get("account_ids") or []) if a != active]
-    if remaining:
-        request.session["account_ids"] = remaining
-        request.session["user_id"] = remaining[-1]
-        return JSONResponse({"ok": True, "switched": True})
-    request.session.clear()
+    request.session["account_ids"] = remaining
+    if is_default:
+        if remaining:
+            request.session["user_id"] = remaining[-1]
+            return JSONResponse({"ok": True, "switched": True})
+        request.session.clear()
+        return JSONResponse({"ok": True, "switched": False})
+    # A non-default (per-tab) account signed out: it's gone from the wallet and the session default
+    # is untouched. The tab drops its own override and falls back to the default on reload.
     return JSONResponse({"ok": True, "switched": False})
 
 
@@ -659,20 +666,52 @@ def password_reset_confirm(payload: dict, db: Session = Depends(get_db)) -> dict
     return {"ok": True}
 
 
+# A tab can act as a different signed-in account than the session default by sending this header
+# (per-tab identity: two tabs in one browser, two accounts). Lowercased — Starlette header lookup
+# is case-insensitive, but we compare explicitly below.
+ACTIVE_ACCOUNT_HEADER = "x-siftlode-account"
+
+
+def resolved_user_id(request: Request) -> tuple[int | None, bool]:
+    """Which account this request acts as, and whether that's the session's default account.
+
+    The signed cookie holds the *wallet* (`account_ids`, every account signed into this browser)
+    plus a default `user_id`. A request may override the default with the X-Siftlode-Account
+    header, but ONLY for an account already in the wallet — so a tab can't impersonate an account
+    that never authenticated here. Everything else (WebSocket, plain navigations) keeps using the
+    cookie default.
+    """
+    default_id = request.session.get("user_id")
+    wallet = request.session.get("account_ids") or []
+    hdr = request.headers.get(ACTIVE_ACCOUNT_HEADER)
+    if hdr:
+        try:
+            hid = int(hdr)
+        except ValueError:
+            hid = None
+        if hid is not None and hid in wallet:
+            return hid, hid == default_id
+    return default_id, True
+
+
 def current_user(request: Request, db: Session = Depends(get_db)) -> User:
-    user_id = request.session.get("user_id")
+    user_id, is_default = resolved_user_id(request)
     if not user_id:
         raise HTTPException(status_code=401, detail="Not authenticated")
     user = db.get(User, user_id)
     if user is None or not user.is_active or user.is_suspended:
         # Suspended/deactivated mid-session → drop the session so the block takes effect at once.
-        request.session.clear()
+        # But only nuke the whole session when the SESSION DEFAULT identity is gone; a stale
+        # per-tab header account just 401s that one tab (other tabs' default may still be valid).
+        if is_default:
+            request.session.clear()
         raise HTTPException(status_code=401, detail="Not authenticated")
-    # Always keep the active account in the switchable list — covers sessions created before
+    # Always keep the session default in the switchable list — covers sessions created before
     # multi-session existed (their account_ids was never seeded), so the switcher isn't empty.
+    default_id = request.session.get("user_id")
     accounts = request.session.get("account_ids") or []
-    if user_id not in accounts:
-        accounts.append(user_id)
+    if default_id and default_id not in accounts:
+        accounts.append(default_id)
         request.session["account_ids"] = accounts[-MAX_SESSION_ACCOUNTS:]
     return user
 
diff --git a/backend/app/main.py b/backend/app/main.py
index 944e294..7115870 100644
--- a/backend/app/main.py
+++ b/backend/app/main.py
@@ -131,6 +131,13 @@ app.include_router(setup_routes.router)
 
 # The built SPA (populated by the Docker frontend build stage).
 STATIC_DIR = Path(__file__).parent / "static_spa"
+# index.html is unhashed and references the content-hashed /assets bundles, so it MUST NOT be
+# heuristically cached — otherwise, after a deploy, a browser keeps serving the old index.html
+# (pointing at the previous bundle) and runs stale code until a hard refresh. `no-cache` lets it
+# stay cached but forces revalidation (cheap 304 via the FileResponse ETag) on every load. The
+# hashed bundles under /assets can cache forever (a new build changes their filename).
+INDEX_HTML = STATIC_DIR / "index.html"
+INDEX_HEADERS = {"Cache-Control": "no-cache"}
 app.mount(
     "/assets",
     StaticFiles(directory=STATIC_DIR / "assets", check_dir=False),
@@ -140,7 +147,7 @@ app.mount(
 
 @app.get("/")
 async def index() -> FileResponse:
-    return FileResponse(STATIC_DIR / "index.html")
+    return FileResponse(INDEX_HTML, headers=INDEX_HEADERS)
 
 
 @app.get("/{full_path:path}")
@@ -155,4 +162,4 @@ async def spa_fallback(full_path: str) -> FileResponse:
         candidate = (STATIC_DIR / full_path).resolve()
         if candidate.is_file() and STATIC_DIR.resolve() in candidate.parents:
             return FileResponse(candidate)
-    return FileResponse(STATIC_DIR / "index.html")
+    return FileResponse(INDEX_HTML, headers=INDEX_HEADERS)
diff --git a/backend/app/routes/messages.py b/backend/app/routes/messages.py
index 9718e1d..53a68e8 100644
--- a/backend/app/routes/messages.py
+++ b/backend/app/routes/messages.py
@@ -363,7 +363,19 @@ async def messages_ws(ws: WebSocket) -> None:
     """Live push channel: authenticated by the session cookie, registers the connection so
     sent messages reach this user's open tabs instantly. We don't consume client→server frames
     (sending goes through POST /api/messages); the receive loop only detects disconnect."""
-    uid = ws.session.get("user_id") if "session" in ws.scope else None
+    # A browser WebSocket can't send custom headers, so the per-tab account (see
+    # X-Siftlode-Account on HTTP) rides in the ?account= query param instead — honoured only for
+    # an account already in this browser's wallet. Falls back to the session default.
+    sess = ws.session if "session" in ws.scope else {}
+    uid = sess.get("user_id")
+    q = ws.query_params.get("account")
+    if q:
+        try:
+            qid = int(q)
+        except ValueError:
+            qid = None
+        if qid is not None and qid in (sess.get("account_ids") or []):
+            uid = qid
     if not uid:
         await ws.close(code=1008)
         return
diff --git a/frontend/src/components/NavSidebar.tsx b/frontend/src/components/NavSidebar.tsx
index 0518be8..578a7ec 100644
--- a/frontend/src/components/NavSidebar.tsx
+++ b/frontend/src/components/NavSidebar.tsx
@@ -20,7 +20,7 @@ import {
   Tv,
   UserPlus,
 } from "lucide-react";
-import { api, type Me } from "../lib/api";
+import { api, clearActiveAccount, setActiveAccount, type Me } from "../lib/api";
 import { useLiveQuery } from "../lib/useLiveQuery";
 import { getUnreadCount, subscribe } from "../lib/notifications";
 import type { Page } from "../lib/urlState";
@@ -94,7 +94,14 @@ export default function NavSidebar({
   }, [acctOpen]);
 
   async function logout() {
-    await fetch("/auth/logout", { method: "POST", credentials: "include" });
+    // Signs THIS tab's active account out of the browser wallet (server is per-tab aware); then
+    // drops this tab's override and reloads onto whatever account remains the default.
+    try {
+      await api.logout();
+    } catch {
+      /* clear locally and reload regardless */
+    }
+    clearActiveAccount();
     location.reload();
   }
 
@@ -106,21 +113,20 @@ export default function NavSidebar({
   });
   const otherAccounts = (accountsQuery.data ?? []).filter((a) => !a.active);
 
-  async function switchTo(id: number) {
-    try {
-      await api.switchAccount(id);
-      // Drop the previous account's in-module sub-view/overlay before the reload (which would
-      // otherwise preserve history.state across the swap). Otherwise the new account lands on a
-      // stale sub-view — e.g. an open chat thread whose partnerId belongs to the OLD identity
-      // (often the new account's own id → an empty self-thread). Start at the module root instead.
-      const st = { ...(window.history.state || {}) };
-      delete st._sub;
-      delete st._ov;
-      window.history.replaceState(st, "");
-      location.reload(); // cleanest way to reload all per-user state for the new account
-    } catch {
-      /* a revoked/removed account — leave the popover open */
-    }
+  function switchTo(id: number) {
+    // Per-tab switch: point THIS tab at the account and reload; other tabs keep their own
+    // identity. No server round-trip — req() sends the account header (validated against the
+    // browser wallet server-side), so the cookie's default account is left untouched.
+    setActiveAccount(id);
+    // Drop the previous account's in-module sub-view/overlay before the reload (which would
+    // otherwise preserve history.state across the swap). Otherwise the new account lands on a
+    // stale sub-view — e.g. an open chat thread whose partnerId belongs to the OLD identity
+    // (often the new account's own id → an empty self-thread). Start at the module root instead.
+    const st = { ...(window.history.state || {}) };
+    delete st._sub;
+    delete st._ov;
+    window.history.replaceState(st, "");
+    location.reload(); // cleanest way to reload all per-user state for the new account
   }
 
   // Durable, server-backed unread count for the inbox badge. Polled live (pauses when the
diff --git a/frontend/src/lib/api.ts b/frontend/src/lib/api.ts
index cc2a873..9dee231 100644
--- a/frontend/src/lib/api.ts
+++ b/frontend/src/lib/api.ts
@@ -266,6 +266,38 @@ const setupHeaders = (token: string) => ({
   "X-Setup-Token": token,
 });
 
+// --- Per-tab active account --------------------------------------------------------------
+// The signed session cookie is the browser's account "wallet"; which account a given TAB acts
+// as is chosen here and sent per-request, so two tabs can run two accounts at once. Stored in
+// sessionStorage (per-tab, survives F5, not shared across tabs). null = use the session default.
+const ACTIVE_ACCOUNT_HEADER = "X-Siftlode-Account";
+const ACTIVE_ACCOUNT_KEY = "siftlode.activeAccount";
+
+export function getActiveAccount(): number | null {
+  try {
+    const raw = sessionStorage.getItem(ACTIVE_ACCOUNT_KEY);
+    if (!raw) return null;
+    const n = Number(raw);
+    return Number.isFinite(n) ? n : null;
+  } catch {
+    return null;
+  }
+}
+export function setActiveAccount(id: number): void {
+  try {
+    sessionStorage.setItem(ACTIVE_ACCOUNT_KEY, String(id));
+  } catch {
+    /* ignore */
+  }
+}
+export function clearActiveAccount(): void {
+  try {
+    sessionStorage.removeItem(ACTIVE_ACCOUNT_KEY);
+  } catch {
+    /* ignore */
+  }
+}
+
 async function req(url: string, opts: RequestInit = {}, cfg: ReqConfig = {}): Promise<any> {
   const method = opts.method ?? "GET";
   const canRetry = cfg.idempotent ?? method === "GET";
@@ -274,9 +306,15 @@ async function req(url: string, opts: RequestInit = {}, cfg: ReqConfig = {}): Pr
   for (;;) {
     let r: Response;
     try {
+      const active = getActiveAccount();
       r = await fetch(url, {
         credentials: "include",
-        headers: { "Content-Type": "application/json" },
+        headers: {
+          "Content-Type": "application/json",
+          // Per-tab identity override (only setup calls pass their own headers, and those are
+          // pre-auth, so this is safe to put in the defaults that `...opts` may replace).
+          ...(active != null ? { [ACTIVE_ACCOUNT_HEADER]: String(active) } : {}),
+        },
         ...opts,
       });
     } catch (e) {
@@ -600,6 +638,10 @@ export const api = {
     req("/api/me/account", { method: "DELETE" }),
   switchAccount: (userId: number): Promise<{ ok: boolean; user_id: number }> =>
     req("/api/me/switch", { method: "POST", body: JSON.stringify({ user_id: userId }) }),
+  // Signs the current tab's active account out of the browser wallet (per-tab aware via the
+  // X-Siftlode-Account header that req() attaches).
+  logout: (): Promise<{ ok: boolean; switched: boolean }> =>
+    req("/auth/logout", { method: "POST" }),
   version: (): Promise<VersionInfo> => req("/api/version"),
   tags: (): Promise<Tag[]> => req("/api/tags"),
   status: (): Promise<SyncStatus> => req("/api/sync/status"),
diff --git a/frontend/src/lib/messagesSocket.ts b/frontend/src/lib/messagesSocket.ts
index cafe909..73acd5e 100644
--- a/frontend/src/lib/messagesSocket.ts
+++ b/frontend/src/lib/messagesSocket.ts
@@ -2,7 +2,7 @@
 // to all of a user's open tabs when a message is stored; subscribers react (e.g. refetch the
 // thread + conversations). Auto-reconnects with backoff; a low-frequency poll elsewhere is the
 // safety net if the socket is down.
-import type { Message, MessageUser } from "./api";
+import { getActiveAccount, type Message, type MessageUser } from "./api";
 
 // A pushed message plus both parties, so the dock can open/flash the right window.
 export interface IncomingMessage {
@@ -20,7 +20,11 @@ let reconnectTimer: ReturnType<typeof setTimeout> | null = null;
 
 function url(): string {
   const proto = location.protocol === "https:" ? "wss" : "ws";
-  return `${proto}://${location.host}/api/messages/ws`;
+  // A WebSocket can't carry the X-Siftlode-Account header, so the per-tab account rides in the
+  // query string (validated against the browser wallet server-side).
+  const account = getActiveAccount();
+  const qs = account != null ? `?account=${account}` : "";
+  return `${proto}://${location.host}/api/messages/ws${qs}`;
 }
 
 function open() {