diff --git a/changelog.d/tsk-l6jtud-handset-probe-fallback-unit.md b/changelog.d/tsk-l6jtud-handset-probe-fallback-unit.md
new file mode 100644
index 000000000..1e677642f
--- /dev/null
+++ b/changelog.d/tsk-l6jtud-handset-probe-fallback-unit.md
@@ -0,0 +1,4 @@
+### Fixed: a desk machine set up with the kiosk session is no longer treated as a phone
+- `hardware._detect_device_class()` now returns "mobile" only when BOTH `taos-kiosk.service` AND `plasma-mobile.service` units exist (via `systemctl cat`).
+- `routes.system._session_mode_state()` reports `available: true` only when BOTH units are present.
+- The AVAILABILITY comment in `routes/system.py` is updated to reflect that a handset requires both the kiosk unit (taOS shell) and the plasma unit (Plasma Mobile fallback).
\ No newline at end of file
diff --git a/docs/agent-coordination.md b/docs/agent-coordination.md
index ec4cdc029..68c1374c1 100644
--- a/docs/agent-coordination.md
+++ b/docs/agent-coordination.md
@@ -2619,7 +2619,7 @@ pinned 0.3.1 binary). `taos_agent_runtime.decide_framework` is the one place
 the choice is made, from two config.yaml keys:
 
 - `device.class`: `auto` (default; `hardware._detect_device_class`, which
-  reads the `taos-kiosk.service` unit), `mobile` or `desktop`;
+  reads the `taos-kiosk.service` and `plasma-mobile.service` units), `mobile` or `desktop`;
 - `taos_agent.framework`: `auto` (default; picoclaw on mobile, opencode
   elsewhere), `opencode`, `picoclaw` or `claude_code` (operator overrides;
   `auto` never picks `claude_code`).
diff --git a/tests/test_device_class_probe.py b/tests/test_device_class_probe.py
new file mode 100644
index 000000000..a2296a7b6
--- /dev/null
+++ b/tests/test_device_class_probe.py
@@ -0,0 +1,56 @@
+"""Device class probe: a handset requires BOTH kiosk and plasma units."""
+
+import subprocess
+from unittest.mock import patch
+from tinyagentos import hardware
+
+
+def test_desk_with_kiosk_but_no_plasma_is_not_mobile(monkeypatch):
+    """A desk with only taos-kiosk.service must NOT be detected as mobile."""
+    calls = []
+
+    def fake_run(cmd, *args, **kwargs):
+        calls.append(cmd)
+        if cmd == ["systemctl", "cat", "taos-kiosk.service"]:
+            return subprocess.CompletedProcess(cmd, 0, stdout="# taos-kiosk.service", stderr="")
+        if cmd == ["systemctl", "cat", "plasma-mobile.service"]:
+            return subprocess.CompletedProcess(cmd, 1, stdout="", stderr="No files found")
+        return subprocess.CompletedProcess(cmd, 1, stdout="", stderr="")
+
+    monkeypatch.setattr(subprocess, "run", fake_run)
+    assert hardware._detect_device_class() is None
+    assert calls.count(["systemctl", "cat", "taos-kiosk.service"]) == 1
+    assert calls.count(["systemctl", "cat", "plasma-mobile.service"]) == 1
+
+
+def test_handset_with_both_units_is_mobile(monkeypatch):
+    """A handset with both taos-kiosk.service and plasma-mobile.service is mobile."""
+    calls = []
+
+    def fake_run(cmd, *args, **kwargs):
+        calls.append(cmd)
+        if cmd == ["systemctl", "cat", "taos-kiosk.service"]:
+            return subprocess.CompletedProcess(cmd, 0, stdout="# taos-kiosk.service", stderr="")
+        if cmd == ["systemctl", "cat", "plasma-mobile.service"]:
+            return subprocess.CompletedProcess(cmd, 0, stdout="# plasma-mobile.service", stderr="")
+        return subprocess.CompletedProcess(cmd, 1, stdout="", stderr="")
+
+    monkeypatch.setattr(subprocess, "run", fake_run)
+    assert hardware._detect_device_class() == "mobile"
+    assert calls.count(["systemctl", "cat", "taos-kiosk.service"]) == 1
+    assert calls.count(["systemctl", "cat", "plasma-mobile.service"]) == 1
+
+
+def test_missing_kiosk_unit_is_not_mobile(monkeypatch):
+    """A machine without taos-kiosk.service must NOT be detected as mobile."""
+    calls = []
+
+    def fake_run(cmd, *args, **kwargs):
+        calls.append(cmd)
+        if cmd == ["systemctl", "cat", "taos-kiosk.service"]:
+            return subprocess.CompletedProcess(cmd, 1, stdout="", stderr="No files found")
+        return subprocess.CompletedProcess(cmd, 1, stdout="", stderr="")
+
+    monkeypatch.setattr(subprocess, "run", fake_run)
+    assert hardware._detect_device_class() is None
+    assert calls.count(["systemctl", "cat", "taos-kiosk.service"]) == 1
\ No newline at end of file
diff --git a/tests/test_session_mode.py b/tests/test_session_mode.py
index c748bd721..f5146e906 100644
--- a/tests/test_session_mode.py
+++ b/tests/test_session_mode.py
@@ -71,6 +71,40 @@ async def json(self):
     assert started == [], "must not start a unit on an unsupported device"
 
 
+@pytest.mark.asyncio
+async def test_desk_with_kiosk_but_no_plasma_is_unavailable(monkeypatch):
+    """A desk with only taos-kiosk.service must report available=False and 404 on switch."""
+    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, "# taos-kiosk.service"
+            if unit == "plasma-mobile.service":
+                return 1, "No files found"
+        return 3, "inactive"
+
+    monkeypatch.setattr(system_routes, "_systemctl", fake)
+    state = await system_routes._session_mode_state()
+    assert state["available"] is False
+    assert state["current"] == "unknown"
+
+    class _Req:
+        async def body(self):
+            return b'{"mode":"plasma"}'
+
+        async def json(self):
+            return {"mode": "plasma"}
+
+    resp = await system_routes.set_session_mode(_Req())
+    assert resp.status_code == 404
+    assert started == [], "must not start a unit when plasma service is absent"
+
+
 @pytest.mark.asyncio
 async def test_unknown_mode_rejected_before_any_unit_is_touched(monkeypatch):
     started: list[str] = []
diff --git a/tests/test_taos_agent_picoclaw.py b/tests/test_taos_agent_picoclaw.py
index e8eb3827c..332c493ba 100644
--- a/tests/test_taos_agent_picoclaw.py
+++ b/tests/test_taos_agent_picoclaw.py
@@ -57,8 +57,9 @@
 # ---------------------------------------------------------------------------
 
 _FAKE_SYSTEMCTL = """#!/bin/sh
-# taos-kiosk.service exists only on a taOSmobile handset.
+# taos-kiosk.service and plasma-mobile.service both exist on a taOSmobile handset.
 if [ "$1" = "cat" ] && [ "$2" = "taos-kiosk.service" ]; then exit {kiosk_rc}; fi
+if [ "$1" = "cat" ] && [ "$2" = "plasma-mobile.service" ]; then exit {plasma_rc}; fi
 exit 1
 """
 
@@ -126,7 +127,9 @@ async def _build_app(tmp_path_factory, name: str, *, mobile: bool):
     data_dir = tmp_path_factory.mktemp(name)
     _write_test_config(data_dir)
     bindir = tmp_path_factory.mktemp(name + "-bin")
-    _write_exec(bindir / "systemctl", _FAKE_SYSTEMCTL.format(kiosk_rc=0 if mobile else 1))
+    kiosk_rc = 0 if mobile else 1
+    plasma_rc = 0 if mobile else 1
+    _write_exec(bindir / "systemctl", _FAKE_SYSTEMCTL.format(kiosk_rc=kiosk_rc, plasma_rc=plasma_rc))
     _write_exec(bindir / "picoclaw", _FAKE_PICOCLAW.format(python=sys.executable))
     with pytest.MonkeyPatch.context() as mp:
         mp.setenv("TAOS_LLM_GATEWAY", "1")
diff --git a/tinyagentos/hardware.py b/tinyagentos/hardware.py
index 9a4c7f9ce..04abbae4b 100644
--- a/tinyagentos/hardware.py
+++ b/tinyagentos/hardware.py
@@ -654,19 +654,28 @@ def _detect_os() -> OsInfo:
 def _detect_device_class() -> str | None:
     """Detect if this is a taOSmobile handset.
 
-    A taOSmobile handset is identified by the presence of the taos-kiosk.service
-    systemd unit (which only exists on devices provisioned as taOS handsets).
-    This mirrors the capability probe used by /api/system/session-mode.
+    A taOSmobile handset is identified by the presence of BOTH the
+    taos-kiosk.service AND plasma-mobile.service systemd units. The kiosk unit
+    provides the taOS shell; the plasma unit is the stock KDE phone shell that
+    acts as the fallback. A desk runs the kiosk unit only and is NOT a handset.
     """
     try:
         import subprocess
-        result = subprocess.run(
+        kiosk_result = subprocess.run(
             ["systemctl", "cat", "taos-kiosk.service"],
             capture_output=True,
             text=True,
             timeout=5,
         )
-        if result.returncode == 0:
+        if kiosk_result.returncode != 0:
+            return None
+        plasma_result = subprocess.run(
+            ["systemctl", "cat", "plasma-mobile.service"],
+            capture_output=True,
+            text=True,
+            timeout=5,
+        )
+        if plasma_result.returncode == 0:
             return "mobile"
     except (subprocess.SubprocessError, FileNotFoundError):
         pass
diff --git a/tinyagentos/routes/system.py b/tinyagentos/routes/system.py
index 2947bb279..8cf9b0ada 100644
--- a/tinyagentos/routes/system.py
+++ b/tinyagentos/routes/system.py
@@ -327,10 +327,11 @@ async def restart_status(request: Request):
 # runs as the unprivileged `taos` user and must not be able to manage arbitrary
 # system services just to offer a UI toggle.
 #
-# AVAILABILITY IS THE DEVICE GATE. `taos-kiosk.service` only exists on a device
-# provisioned as a taOS handset, so `available: false` everywhere else and the
-# frontend hides the app rather than shipping a button that cannot work. This
-# is deliberately a capability probe, not a hardcoded device list.
+# AVAILABILITY IS THE DEVICE GATE. A taOS handset has BOTH taos-kiosk.service
+# (the taOS shell) AND plasma-mobile.service (the Plasma Mobile fallback). A
+# desk runs the kiosk unit only, so `available: false` there and the frontend
+# hides the app rather than shipping a button that cannot work. This is
+# deliberately a capability probe, not a hardcoded device list.
 
 _SESSION_UNITS = {"kiosk": "taos-kiosk.service", "plasma": "plasma-mobile.service"}
 
@@ -357,8 +358,9 @@ async def _session_mode_state() -> dict:
     # `is-active` exits non-zero for inactive units, so the OUTPUT is the signal
     # 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
+    kiosk_cat_rc, _ = await _systemctl("cat", _SESSION_UNITS["kiosk"])
+    plasma_cat_rc, _ = await _systemctl("cat", _SESSION_UNITS["plasma"])
+    available = kiosk_cat_rc == 0 and plasma_cat_rc == 0
     _, plasma_state = await _systemctl("is-active", _SESSION_UNITS["plasma"])
     if kiosk_state == "active":
         current = "kiosk"