diff --git a/custom_components/localthings/registry/capabilities/washer.py b/custom_components/localthings/registry/capabilities/washer.py
index 97d26a4..594adf1 100644
--- a/custom_components/localthings/registry/capabilities/washer.py
+++ b/custom_components/localthings/registry/capabilities/washer.py
@@ -310,7 +310,10 @@ def _add_wash_set_write(mask):
 
 
 def _add_wash_alarm_write(p, rep, href=None):
-    if p not in ("On", "Off") or not rep.get("x.com.samsung.da.options"):
+    # Gated on the mask being readable, like the per-moment writes: a device
+    # reporting a wider mask than these three bits would otherwise have it
+    # truncated to 7 here, silently dropping a moment it supports.
+    if p not in ("On", "Off") or _add_wash_mask(rep, "AddWashSet") is None:
         return None
     return _add_wash_set_write(0b111 if p == "On" else 0)
 
@@ -320,7 +323,10 @@ def _add_wash_bit_switch(key, icon, bit):
 
     The mask is the only state, so switching the last moment off lands on 0
     and takes the alarm with it, and switching one on from 0 turns the alarm
-    back on.
+    back on. The corollary is that switching the master off and on again
+    writes 7, resetting a rinse-only selection to all three moments -- the
+    appliance remembers no previous subset either, so there is nothing to
+    restore.
     """
 
     def read(rep):
@@ -445,6 +451,10 @@ WASHER_COURSE = Capability(
         _add_wash_bit_switch("add_wash_alarm_rinse", "mdi:water", 0),
         _add_wash_bit_switch("add_wash_alarm_final_rinse", "mdi:water-check", 1),
         _add_wash_bit_switch("add_wash_alarm_spin", "mdi:sync", 2),
+        # On at rest: an idle washer reports AddWashAvailable_7 and the mask
+        # only empties as the cycle consumes each moment. This says the cycle
+        # permits AddWash, not that laundry can go in now -- that is
+        # add_wash_indicator.
         BinarySensorDesc(
             key="add_wash_available",
             icon="mdi:tshirt-crew-outline",
diff --git a/tests/test_golden_regression.py b/tests/test_golden_regression.py
index 5a3d805..63e8b4b 100644
--- a/tests/test_golden_regression.py
+++ b/tests/test_golden_regression.py
@@ -390,9 +390,12 @@ def test_registry_reproduces_golden_state_keys_for_washer_dryer_combo():
 
 
 def test_registry_reproduces_golden_state_keys_for_washer_ww6500():
-    """DA_WM_A51_20 front-loader, routed by /oic/d's oic.d.washer rather
-    than a board token. Reports the AddWash tokens and an empty
-    /wm/editcourse/vs/0, so its cycle list comes from supportedOptions."""
+    """DA_WM_A51_20 front-loader, typed solely by the WW consumer prefix in
+    its /information/vs/0 description: A51 is not a board token, so with the
+    description blanked this device resolves to nothing and drops to the
+    unknown-device fallback. That is the fragile route this test pins.
+    Reports the AddWash tokens and an empty /wm/editcourse/vs/0, so its cycle
+    list comes from supportedOptions."""
     from tests.conftest import _load_device
 
     resources = _load_device("washer_ww6500")
diff --git a/tests/test_washer_add_wash.py b/tests/test_washer_add_wash.py
index 305bb0c..24138b0 100644
--- a/tests/test_washer_add_wash.py
+++ b/tests/test_washer_add_wash.py
@@ -86,6 +86,17 @@ class TestAlarmMasterSwitch:
         desc = _desc("add_wash_alarm", SwitchDesc)
         assert _write(desc, "On", {}) is None
 
+    @pytest.mark.parametrize("raw", ["15", "On", "-1"])
+    def test_refuses_to_write_over_a_mask_it_cannot_read(self, raw):
+        """A device reporting a wider mask must not have it truncated to 7.
+        The master is gated exactly like the per-moment writes, so an
+        unrecognized mask leaves every AddWash switch read-only."""
+        desc = _desc("add_wash_alarm", SwitchDesc)
+        rep = _rep(f"AddWashSet_{raw}")
+        assert desc.rep_fn(rep) is None
+        assert _write(desc, "On", rep) is None
+        assert _write(desc, "Off", rep) is None
+
 
 class TestAlarmMomentSwitches:
     @pytest.mark.parametrize("key,bit", BITS.items())
@@ -197,6 +208,17 @@ class TestAgainstTheWW6500Dump:
     """DA_WM_A51_20_COMMON_WW6500, captured with the alarm off, the course
     permitting all three moments and the lamp dark."""
 
+    def test_types_only_by_the_description_consumer_prefix(self):
+        """A51 is not a board token, so the WW prefix in the description is
+        the only thing routing this device -- see the golden test's docstring.
+        Pinned here so the claim can't quietly stop being true."""
+        from custom_components.localthings.registry.by_type import for_device_by_model
+
+        info = _load_device("washer_ww6500")["/information/vs/0"]
+        model = info["x.com.samsung.da.modelNum"]
+        assert for_device_by_model(model, info["x.com.samsung.da.description"]).name == "washer"
+        assert for_device_by_model(model, "") is None
+
     def test_no_unbound_hrefs(self):
         resources = _load_device("washer_ww6500")
         reg = resolve(resources)