From ef9471546dc624f693d34fb405601eb5dabe4b3d Mon Sep 17 00:00:00 2001
From: jaylfc <jaylfc25@gmail.com>
Date: Mon, 26 Oct 2026 01:09:58 +0000
Subject: [PATCH] Session mode availability must require both units

Docs-Reviewed: behavior change limited to device gate logic, no API contract change requiring doc updates

```
FAILED tests/test_session_mode.py::test_state_unavailable_on_desk_without_fallback - assert True is False
FAILED tests/test_session_mode.py::test_switch_refused_on_desk_without_fallback - AttributeError: 'dict' object has no attribute 'status_code'
2 failed, 4 passed in 0.29s
```

All tests pass after fix.
---
 .../tsk-mtf4dv-session-mode-needs-fallback.md |  1 +
 tests/test_session_mode.py                    | 48 +++++++++++++++++++
 tinyagentos/routes/system.py                  |  6 ++-
 3 files changed, 53 insertions(+), 2 deletions(-)
 create mode 100644 changelog.d/tsk-mtf4dv-session-mode-needs-fallback.md

diff --git a/changelog.d/tsk-mtf4dv-session-mode-needs-fallback.md b/changelog.d/tsk-mtf4dv-session-mode-needs-fallback.md
new file mode 100644
index 000000000..0a18b0c69
--- /dev/null
+++ b/changelog.d/tsk-mtf4dv-session-mode-needs-fallback.md
@@ -0,0 +1 @@
+### Fixed\n- Session mode availability now requires both taos-kiosk.service and plasma-mobile.service; desks without the fallback unit are not offered a switch.
diff --git a/tests/test_session_mode.py b/tests/test_session_mode.py
index c748bd721..a741222bc 100644
--- a/tests/test_session_mode.py
+++ b/tests/test_session_mode.py
@@ -92,3 +92,51 @@ async def json(self):
     resp = await system_routes.set_session_mode(_Req())
     assert resp.status_code == 400
     assert started == []
+
+
+@pytest.mark.asyncio
+async def test_state_unavailable_on_desk_without_fallback(monkeypatch):
+    async def fake(*args):
+        if args[0] == "cat":
+            unit = args[-1]
+            if unit == "taos-kiosk.service":
+                return 0, "# unit"
+            if unit == "plasma-mobile.service":
+                return 1, "No files found"
+            return 0, "# unit"
+        return 3, "inactive"
+
+    monkeypatch.setattr(system_routes, "_systemctl", fake)
+    state = await system_routes._session_mode_state()
+    assert state["available"] is False
+
+
+@pytest.mark.asyncio
+async def test_switch_refused_on_desk_without_fallback(monkeypatch):
+    started: list[str] = []
+
+    async def fake(*args):
+        if args[0] == "start":
+            started.append(args[-1])
+            return 0, ""
+        if args[0] == "cat":
+            unit = args[-1]
+            if unit == "taos-kiosk.service":
+                return 0, "# unit"
+            if unit == "plasma-mobile.service":
+                return 1, "No files found"
+            return 1, "No files found"
+        return 3, "inactive"
+
+    monkeypatch.setattr(system_routes, "_systemctl", fake)
+
+    class _Req:
+        async def body(self):
+            return b'{"mode":"kiosk"}'
+
+        async def json(self):
+            return {"mode": "kiosk"}
+
+    resp = await system_routes.set_session_mode(_Req())
+    assert resp.status_code == 404
+    assert started == []
diff --git a/tinyagentos/routes/system.py b/tinyagentos/routes/system.py
index 2947bb279..36ad015f1 100644
--- a/tinyagentos/routes/system.py
+++ b/tinyagentos/routes/system.py
@@ -358,7 +358,9 @@ async def _session_mode_state() -> dict:
     # and rc is not. An unknown unit reports "inactive" too, so availability is
     # probed separately with `cat`, which fails only when the unit is absent.
     cat_rc, _ = await _systemctl("cat", _SESSION_UNITS["kiosk"])
-    available = cat_rc == 0
+    # availability needs both units because a desk has the kiosk unit and no fallback
+    plasma_cat_rc, _ = await _systemctl("cat", _SESSION_UNITS["plasma"])
+    available = cat_rc == 0 and plasma_cat_rc == 0
     _, plasma_state = await _systemctl("is-active", _SESSION_UNITS["plasma"])
     if kiosk_state == "active":
         current = "kiosk"
@@ -397,7 +399,7 @@ async def set_session_mode(request: Request):
     state = await _session_mode_state()
     if not state["available"]:
         return JSONResponse(
-            {"error": "this device has no taOS kiosk session installed"}, status_code=404,
+            {"error": "this device has no second session to switch to"}, status_code=404,
         )
     # Starting the target stops the other: the units Conflict. Doing it as two
     # jobs (stop-then-start) would leave a window with NO session on screen.