diff --git a/api/kanban_bridge.py b/api/kanban_bridge.py
index 403f2710..d758d372 100644
--- a/api/kanban_bridge.py
+++ b/api/kanban_bridge.py
@@ -745,6 +745,18 @@ def _list_boards_payload(parsed):
         current = kb.get_current_board()
     except Exception:
         current = "default"
+    visible_slugs = {(_board_meta_dict(meta).get("slug")) for meta in boards}
+    default_slug = getattr(kb, "DEFAULT_BOARD", "default")
+    if current not in visible_slugs:
+        # The on-disk active-board pointer can outlive an archived/deleted board
+        # when another CLI/WebUI process removes it. Surface a valid current
+        # board instead of letting the frontend pin every subsequent request to
+        # a ghost slug and fail with an opaque 404.
+        try:
+            kb.clear_current_board()
+        except Exception:
+            pass
+        current = default_slug
     out = []
     for raw_meta in boards:
         meta = _board_meta_dict(raw_meta)
diff --git a/api/routes.py b/api/routes.py
index e086587a..18d9c1a5 100644
--- a/api/routes.py
+++ b/api/routes.py
@@ -590,6 +590,19 @@ from api.agent_health import build_agent_health_payload
 from api.system_health import build_system_health_payload
 
 
+def _kanban_unknown_endpoint(handler, parsed, method: str) -> bool:
+    """Return a Kanban-specific 404 for stale clients/obsolete endpoint shapes."""
+    return bad(
+        handler,
+        (
+            f"unknown Kanban endpoint: {method} {parsed.path}. "
+            "If this appeared after a WebUI update, your browser may be running "
+            "a stale cached bundle; use Hard refresh now, then reopen Kanban."
+        ),
+        status=404,
+    ) or True
+
+
 def _clear_stale_stream_state(session) -> bool:
     """Clear persisted streaming flags when the in-memory stream no longer exists.
 
@@ -2616,7 +2629,9 @@ def handle_get(handler, parsed) -> bool:
     if parsed.path.startswith("/api/kanban/"):
         from api.kanban_bridge import handle_kanban_get
 
-        return handle_kanban_get(handler, parsed)
+        if handle_kanban_get(handler, parsed):
+            return True
+        return _kanban_unknown_endpoint(handler, parsed, "GET")
     if parsed.path == "/api/wiki/status":
         return _handle_llm_wiki_status(handler, parsed)
     if parsed.path == "/api/logs":
@@ -3414,7 +3429,9 @@ def handle_post(handler, parsed) -> bool:
     if parsed.path.startswith("/api/kanban/"):
         from api.kanban_bridge import handle_kanban_post
 
-        return handle_kanban_post(handler, parsed, body)
+        if handle_kanban_post(handler, parsed, body):
+            return True
+        return _kanban_unknown_endpoint(handler, parsed, "POST")
     if parsed.path == "/api/dashboard/config":
         from api import dashboard_probe
 
@@ -4590,7 +4607,9 @@ def handle_patch(handler, parsed) -> bool:
     if parsed.path.startswith("/api/kanban/"):
         from api.kanban_bridge import handle_kanban_patch
 
-        return handle_kanban_patch(handler, parsed, body)
+        if handle_kanban_patch(handler, parsed, body):
+            return True
+        return _kanban_unknown_endpoint(handler, parsed, "PATCH")
     return False
 
 
@@ -4602,7 +4621,9 @@ def handle_delete(handler, parsed) -> bool:
     if parsed.path.startswith("/api/kanban/"):
         from api.kanban_bridge import handle_kanban_delete
 
-        return handle_kanban_delete(handler, parsed, body)
+        if handle_kanban_delete(handler, parsed, body):
+            return True
+        return _kanban_unknown_endpoint(handler, parsed, "DELETE")
     return False
 
 # ── GET route helpers ─────────────────────────────────────────────────────────
diff --git a/docs/pr-media/1823/kanban-hard-refresh-diagnostic.png b/docs/pr-media/1823/kanban-hard-refresh-diagnostic.png
new file mode 100644
index 00000000..94546c3e
Binary files /dev/null and b/docs/pr-media/1823/kanban-hard-refresh-diagnostic.png differ
diff --git a/static/panels.js b/static/panels.js
index 9b056382..0add57b8 100644
--- a/static/panels.js
+++ b/static/panels.js
@@ -1189,11 +1189,49 @@ function _kanbanCard(task, status){
   </article>`;
 }
 
+async function hardRefreshWebUIClient(){
+  try {
+    if (navigator.serviceWorker) {
+      const regs = await navigator.serviceWorker.getRegistrations();
+      await Promise.all(regs.map(r => r.unregister()));
+    }
+  } catch(_) {}
+  try {
+    if (window.caches) {
+      const keys = await caches.keys();
+      await Promise.all(keys.map(k => caches.delete(k)));
+    }
+  } catch(_) {}
+  window.location.reload();
+}
+
+function _kanbanLooksLikeStaleClientError(err){
+  const msg = String((err && err.message) || err || '').toLowerCase();
+  return !!(err && err.status === 404 && (
+    msg === 'not found' ||
+    msg.includes('unknown kanban endpoint') ||
+    msg.includes('stale cached bundle')
+  ));
+}
+
+function _kanbanUnavailableHtml(err){
+  const raw = String((err && err.message) || err || '');
+  if (_kanbanLooksLikeStaleClientError(err)) {
+    return `<div class="main-view-empty"><div class="main-view-empty-title">Kanban needs a hard refresh</div><div class="main-view-empty-subtitle">The server rejected an obsolete Kanban endpoint. This usually means the browser or Mac app is still running a stale cached WebUI bundle after an update.</div><button class="btn primary" type="button" onclick="hardRefreshWebUIClient()">Hard refresh now</button><div class="main-view-empty-subtitle">Original error: ${esc(raw || 'not found')}</div></div>`;
+  }
+  const msg = `${esc(t('kanban_unavailable'))}: ${esc(raw)}`;
+  return `<div class="main-view-empty"><div class="main-view-empty-title">${msg}</div></div>`;
+}
+
 async function loadKanban(animate){
   const board = $('kanbanBoard');
   const list = $('kanbanList');
   try {
     if (animate && board) board.innerHTML = `<div style="padding:16px;color:var(--muted);font-size:13px">${esc(t('loading'))}</div>`;
+    // Resolve the active board before board-scoped requests. If another CLI or
+    // tab archived the previous board, /boards can fall back to default instead
+    // of leaving config/board pinned to a ghost slug.
+    await loadKanbanBoards();
     const config = await api('/api/kanban/config' + _kanbanBoardQuery());
     let assignees = null;
     try { assignees = await api('/api/kanban/assignees' + _kanbanBoardQuery()); } catch(e) { assignees = null; }
@@ -1230,9 +1268,9 @@ async function loadKanban(animate){
     _kanbanStartPolling();
     _kanbanRenderBoard();
   } catch(e) {
-    const msg = `${esc(t('kanban_unavailable'))}: ${esc(e.message || e)}`;
-    if (board) board.innerHTML = `<div class="main-view-empty"><div class="main-view-empty-title">${msg}</div></div>`;
-    if (list) list.innerHTML = `<div class="kanban-empty">${msg}</div>`;
+    const html = _kanbanUnavailableHtml(e);
+    if (board) board.innerHTML = html;
+    if (list) list.innerHTML = html;
   }
 }
 
@@ -1687,6 +1725,8 @@ async function loadKanbanBoards(){
   let active = serverCurrent;
   if (saved && boards.some(b => b.slug === saved)) {
     active = saved;
+  } else if (saved) {
+    _kanbanSetSavedBoard('default');
   }
   _kanbanCurrentBoard = (active === 'default') ? null : active;
   // The switcher is visible whenever ≥1 non-default board exists OR the
diff --git a/tests/test_issue1823_kanban_not_found.py b/tests/test_issue1823_kanban_not_found.py
new file mode 100644
index 00000000..6899994b
--- /dev/null
+++ b/tests/test_issue1823_kanban_not_found.py
@@ -0,0 +1,76 @@
+"""Regression coverage for #1823 Kanban stale-client/board-pointer failures."""
+
+from __future__ import annotations
+
+import io
+import json
+from types import SimpleNamespace
+from urllib.parse import urlparse
+
+from api import routes
+
+ROOT = __import__("pathlib").Path(__file__).resolve().parents[1]
+PANELS = (ROOT / "static" / "panels.js").read_text(encoding="utf-8")
+ROUTES = (ROOT / "api" / "routes.py").read_text(encoding="utf-8")
+
+
+class _FakeHandler:
+    def __init__(self):
+        self.status = None
+        self.headers = {}
+        self.response_headers = []
+        self.wfile = io.BytesIO()
+
+    def send_response(self, status):
+        self.status = status
+
+    def send_header(self, key, value):
+        self.response_headers.append((key, value))
+
+    def end_headers(self):
+        pass
+
+    def body_json(self):
+        return json.loads(self.wfile.getvalue().decode("utf-8"))
+
+
+def test_unknown_kanban_endpoint_get_returns_stale_client_diagnostic():
+    """Obsolete/stale JS should not collapse to a bare `not found` 404."""
+    handler = _FakeHandler()
+    handled = routes.handle_get(handler, urlparse("/api/kanban/obsolete-shape"))
+
+    assert handled is True
+    assert handler.status == 404
+    error = handler.body_json()["error"]
+    assert error != "not found"
+    assert "unknown Kanban endpoint: GET /api/kanban/obsolete-shape" in error
+    assert "stale cached bundle" in error
+    assert "Hard refresh now" in error
+
+
+def test_unknown_kanban_endpoint_routes_are_wrapped_for_all_methods():
+    assert 'return _kanban_unknown_endpoint(handler, parsed, "GET")' in ROUTES
+    assert 'return _kanban_unknown_endpoint(handler, parsed, "POST")' in ROUTES
+    assert 'return _kanban_unknown_endpoint(handler, parsed, "PATCH")' in ROUTES
+    assert 'return _kanban_unknown_endpoint(handler, parsed, "DELETE")' in ROUTES
+
+
+def test_kanban_stale_client_error_renders_hard_refresh_escape_hatch():
+    assert "function _kanbanLooksLikeStaleClientError(err)" in PANELS
+    assert "err.status === 404" in PANELS
+    assert "msg === 'not found'" in PANELS
+    assert "msg.includes('unknown kanban endpoint')" in PANELS
+    assert "Kanban needs a hard refresh" in PANELS
+    assert "Hard refresh now" in PANELS
+    assert "navigator.serviceWorker.getRegistrations()" in PANELS
+    assert "caches.keys()" in PANELS
+    assert "window.location.reload()" in PANELS
+
+
+def test_kanban_load_resolves_board_before_board_scoped_requests():
+    boards_pos = PANELS.find("await loadKanbanBoards();")
+    config_pos = PANELS.find("api('/api/kanban/config' + _kanbanBoardQuery())")
+    assert boards_pos != -1
+    assert config_pos != -1
+    assert boards_pos < config_pos
+    assert "_kanbanSetSavedBoard('default');" in PANELS
diff --git a/tests/test_kanban_bridge.py b/tests/test_kanban_bridge.py
index bbcae21d..3e99956d 100644
--- a/tests/test_kanban_bridge.py
+++ b/tests/test_kanban_bridge.py
@@ -389,6 +389,22 @@ def test_kanban_board_payload_exposes_read_only_board(monkeypatch):
     assert any(task["id"] == "t_1" and task["title"] == "Read-only board target" for task in all_tasks)
 
 
+def test_board_pointer_drift_falls_back_to_default(monkeypatch):
+    bridge = _load_bridge(monkeypatch)
+    fake_kanban = sys.modules["hermes_cli.kanban_db"]
+    fake_kanban.boards = {
+        "default": {"slug": "default", "name": "Default board", "archived": False},
+        "active": {"slug": "active", "name": "Active board", "archived": False},
+    }
+    fake_kanban.set_current_board("ghost")
+
+    data = bridge._list_boards_payload(_parsed(path="/api/kanban/boards"))
+
+    assert data["current"] == "default"
+    assert fake_kanban.get_current_board() == "default"
+    assert any(board["slug"] == "default" and board["is_current"] for board in data["boards"])
+
+
 def test_kanban_task_detail_payload_exposes_comments_events_links_and_runs(monkeypatch):
     bridge = _load_bridge(monkeypatch)