diff --git a/custom_components/localthings/registry/by_type/refrigerator.py b/custom_components/localthings/registry/by_type/refrigerator.py
index 8e4f684..bd3e52c 100644
--- a/custom_components/localthings/registry/by_type/refrigerator.py
+++ b/custom_components/localthings/registry/by_type/refrigerator.py
@@ -13,6 +13,7 @@ REGISTRY = DeviceRegistry(
             fridge.STATUS_LOCK,
             fridge.DOOR_ALERT,
             common.WATER_FILTER,
+            fridge.AIR_FILTER,
             dishwasher.DIAGNOSIS,
             fridge.ICEMAKER_NIGHTTIME,
             fridge.FLEX_ZONE,
diff --git a/custom_components/localthings/registry/capabilities/fridge.py b/custom_components/localthings/registry/capabilities/fridge.py
index 3182738..52a42e5 100644
--- a/custom_components/localthings/registry/capabilities/fridge.py
+++ b/custom_components/localthings/registry/capabilities/fridge.py
@@ -25,7 +25,7 @@ from ..entities import (
     SwitchDesc,
     TimeDesc,
 )
-from .common import normalize_temp_unit
+from .common import int_or_none, normalize_temp_unit
 
 # Display names for the beverage zone, flex zone, ice type, and
 # ice-making-status enums below live in translations/en.json, keyed by the
@@ -266,6 +266,39 @@ DOOR_ALERT = Capability(
 )
 
 
+# Internal deodorizing filter (issue #318, TP1X_REF_21K). Same
+# filterUsage/filterStatus field pair as common.WATER_FILTER, but
+# filterUsage here is already a 0-100 percentage with no filterCapacity to
+# divide by (confirmed by filterStatus=="wash" at filterUsage=="100") --
+# 'air_'-prefixed keys so a fridge with both a water and an air filter gets
+# two distinct entities rather than a unique_id collision.
+AIR_FILTER = Capability(
+    href="/filter/airdustfilter/vs/0",
+    poll_tier="cold",
+    entities=(
+        SensorDesc(
+            key="air_filter_usage",
+            field="x.com.samsung.da.filterUsage",
+            unit="%",
+            state_class="measurement",
+            icon="mdi:air-filter",
+            entity_category="diagnostic",
+            value_fn=int_or_none,
+        ),
+        SensorDesc(
+            key="air_filter_status",
+            field="x.com.samsung.da.filterStatus",
+            device_class="enum",
+            options=("normal", "wash", "replace"),
+            translation_key="filter_status",
+            icon="mdi:air-filter",
+            entity_category="diagnostic",
+            value_fn=lambda v: v.lower() if isinstance(v, str) else v,
+        ),
+    ),
+)
+
+
 def _status_lock_write(field):
     return lambda p, rep, href=None: (
         ["status", "lock", "vs", "0"],
diff --git a/tests/fixtures/golden/refrigerator_tp1x_ref_21k_airfilter.json b/tests/fixtures/golden/refrigerator_tp1x_ref_21k_airfilter.json
new file mode 100644
index 0000000..0acf308
--- /dev/null
+++ b/tests/fixtures/golden/refrigerator_tp1x_ref_21k_airfilter.json
@@ -0,0 +1,36 @@
+{
+  "state_keys": [
+    "ai_energy_level",
+    "air_filter_status",
+    "air_filter_usage",
+    "alarm_code",
+    "auto_door_opener",
+    "brightness_level",
+    "cabinet_light_dim",
+    "cabinet_light_switch",
+    "cooler_setpoint",
+    "cooler_temperature",
+    "day_brightness",
+    "door_alert",
+    "door_cooler_open",
+    "door_freezer_open",
+    "energy_kwh",
+    "energy_saved_kwh",
+    "firmware_update",
+    "freezer_setpoint",
+    "freezer_temperature",
+    "fridge_sound",
+    "ice_night_mode",
+    "icemaker_one_enabled",
+    "icemaker_one_making_status",
+    "night_end",
+    "night_start",
+    "power_energy_kwh",
+    "power_watts",
+    "rapid_freezing",
+    "rapid_fridge",
+    "selfcheck_error",
+    "selfcheck_result",
+    "selfcheck_status"
+  ]
+}
diff --git a/tests/fixtures/refrigerator_tp1x_ref_21k_airfilter_device.json b/tests/fixtures/refrigerator_tp1x_ref_21k_airfilter_device.json
new file mode 100644
index 0000000..ca6ef30
--- /dev/null
+++ b/tests/fixtures/refrigerator_tp1x_ref_21k_airfilter_device.json
@@ -0,0 +1,623 @@
+{
+  "device0": [
+    {
+      "rt": [
+        "x.com.samsung.devcol",
+        "oic.wk.col"
+      ],
+      "if": [
+        "oic.if.baseline",
+        "oic.if.ll",
+        "oic.if.b"
+      ]
+    },
+    {
+      "href": "/alarms/vs/0",
+      "rep": {
+        "href": "/alarms/vs/0",
+        "rt": [
+          "x.com.samsung.da.alarms"
+        ],
+        "if": [
+          "oic.if.baseline",
+          "oic.if.a"
+        ]
+      }
+    },
+    {
+      "href": "/bespoke/vs/0",
+      "rep": {
+        "x.com.samsung.da.BespokeProduct": "On",
+        "href": "/bespoke/vs/0"
+      }
+    },
+    {
+      "href": "/cabinet/light/enhanced/vs/0",
+      "rep": {
+        "light.control.status": "On",
+        "level.brightness.daytime": "100",
+        "level.brightness.nighttime": "33",
+        "night.starttime": "2026-08-07T12:00:00",
+        "night.duration.minute": "540",
+        "timezone.offset": "+09:00",
+        "href": "/cabinet/light/enhanced/vs/0",
+        "rt": [
+          "x.com.samsung.da.light.enhanced"
+        ],
+        "if": [
+          "oic.if.baseline",
+          "oic.if.a"
+        ]
+      }
+    },
+    {
+      "href": "/cabinet/light/total/vs/0",
+      "rep": {
+        "x.com.samsung.da.lightLevel": "100",
+        "x.com.samsung.da.lightResolution": "3",
+        "x.com.samsung.da.lightControl.off.include": "Off",
+        "x.com.samsung.da.lightControl": "Off",
+        "x.com.samsung.da.lightControl.hide": "true",
+        "light.dimming.status": "On",
+        "href": "/cabinet/light/total/vs/0",
+        "rt": [
+          "x.com.samsung.da.cabinetlight"
+        ],
+        "if": [
+          "oic.if.baseline",
+          "oic.if.a"
+        ]
+      }
+    },
+    {
+      "href": "/configuration/vs/0",
+      "rep": {
+        "x.com.samsung.da.region": "",
+        "x.com.samsung.da.countryCode": "",
+        "href": "/configuration/vs/0"
+      }
+    },
+    {
+      "href": "/connectionconfig/vs/0",
+      "rep": {
+        "autoReconnectionMinVersion": "1.0",
+        "autoReconnection": "true",
+        "autoReconnectionProtocolType": [
+          "helper_hotspot",
+          "ble_ocf"
+        ],
+        "supportedWiFiAuthType": [
+          "OPEN",
+          "WEP",
+          "WPA-PSK",
+          "WPA2-PSK",
+          "SAE"
+        ],
+        "supportedWiFiCryptoType": [
+          "TKIP",
+          "AES",
+          "WEP-64",
+          "WEP-128"
+        ],
+        "supportedWiFiFreq": [
+          "2.4G"
+        ],
+        "calmConnectionCare": {
+          "version": "1.0",
+          "role": [
+            "things"
+          ]
+        }
+      }
+    },
+    {
+      "href": "/defrost/prediction/vs/0",
+      "rep": {
+        "ai.cooling.care": "Off",
+        "href": "/defrost/prediction/vs/0"
+      }
+    },
+    {
+      "href": "/dginformation/vs/0",
+      "rep": {
+        "enrolmentstatus": "Unknown",
+        "devicestate": "Unknown",
+        "lockstatus": "Normal",
+        "nextduedate": "",
+        "workingminutes": 0,
+        "paymentinfo": {
+          "emiplan": "Unknown",
+          "currency": "Unknown",
+          "totalemi": 0,
+          "totalemipaid": 0
+        }
+      }
+    },
+    {
+      "href": "/door/cooler/0",
+      "rep": {
+        "openState": "Close",
+        "href": "/door/cooler/0",
+        "rt": [
+          "oic.r.door"
+        ],
+        "if": [
+          "oic.if.baseline",
+          "oic.if.s"
+        ]
+      }
+    },
+    {
+      "href": "/door/freezer/0",
+      "rep": {
+        "openState": "Close",
+        "href": "/door/freezer/0",
+        "rt": [
+          "oic.r.door"
+        ],
+        "if": [
+          "oic.if.baseline",
+          "oic.if.s"
+        ]
+      }
+    },
+    {
+      "href": "/doors/vs/0",
+      "rep": {
+        "x.com.samsung.da.items": [
+          {
+            "x.com.samsung.da.openState": "Close",
+            "x.com.samsung.da.id": "0",
+            "x.com.samsung.da.description": "Door"
+          },
+          {
+            "x.com.samsung.da.openState": "Close",
+            "x.com.samsung.da.id": "1",
+            "x.com.samsung.da.description": "Door"
+          }
+        ],
+        "href": "/doors/vs/0"
+      }
+    },
+    {
+      "href": "/drlc/vs/0",
+      "rep": {
+        "x.com.samsung.da.drlcLevel": "2",
+        "x.com.samsung.da.override": "Not_Supported",
+        "x.com.samsung.da.durationminutes": "1441",
+        "x.com.samsung.da.start": "2026-08-07T00:00:41Z",
+        "x.com.samsung.da.realSaving": "On",
+        "href": "/drlc/vs/0"
+      }
+    },
+    {
+      "href": "/energy/ailevel/vs/0",
+      "rep": {
+        "aiLevel": "1",
+        "supportedAiLevel": [
+          "1",
+          "2"
+        ],
+        "href": "/energy/ailevel/vs/0"
+      }
+    },
+    {
+      "href": "/energy/consumption/vs/0",
+      "rep": {
+        "x.com.samsung.da.cumulativeConsumption": "25344",
+        "x.com.samsung.da.instantaneousPower": "48",
+        "x.com.samsung.da.cumulativePower": "413385",
+        "x.com.samsung.da.cumulativeSavedPower": "50694",
+        "x.com.samsung.da.cumulativeUnit": "Wh",
+        "x.com.samsung.da.instantaneousPowerUnit": "W",
+        "href": "/energy/consumption/vs/0",
+        "x.com.samsung.da.cumulativeDateUTC": "1786020360"
+      }
+    },
+    {
+      "href": "/file/information/vs/0",
+      "rep": {
+        "x.com.samsung.timeoffset": "+09:00",
+        "x.com.samsung.supprtedtype": 1,
+        "href": "/file/information/vs/0"
+      }
+    },
+    {
+      "href": "/filter/airdustfilter/vs/0",
+      "rep": {
+        "x.com.samsung.da.filterUsage": "100",
+        "x.com.samsung.da.filterUsageResolution": "1",
+        "x.com.samsung.da.filterResetType": [
+          "washable"
+        ],
+        "x.com.samsung.da.filterStatus": "wash",
+        "href": "/filter/airdustfilter/vs/0"
+      }
+    },
+    {
+      "href": "/icemaker/nighttime/vs/0",
+      "rep": {
+        "ice.night.status": "On",
+        "ice.night.starttime": "2026-08-07T12:00:00",
+        "ice.night.duration": "540",
+        "ice.night.timezone": "+09:00",
+        "href": "/icemaker/nighttime/vs/0",
+        "rt": [
+          "x.com.samsung.da.ice.night"
+        ],
+        "if": [
+          "oic.if.baseline",
+          "oic.if.a"
+        ]
+      }
+    },
+    {
+      "href": "/icemaker/one/vs/0",
+      "rep": {
+        "x.com.samsung.da.iceMaker.name": "ICE_MAKER",
+        "x.com.samsung.da.iceMaker.state": "On",
+        "x.com.samsung.da.iceType.desired": "NORMAL",
+        "x.com.samsung.da.iceMaker.iceMakingStatus": "ICESTATUS_STOP",
+        "x.com.samsung.da.iceMaker.type": "toggle",
+        "href": "/icemaker/one/vs/0",
+        "rt": [
+          "x.com.samsung.da.icemaker"
+        ],
+        "if": [
+          "oic.if.baseline",
+          "oic.if.a"
+        ]
+      }
+    },
+    {
+      "href": "/icemaker/status/vs/0",
+      "rep": {
+        "x.com.samsung.da.iceMaker": "On",
+        "href": "/icemaker/status/vs/0"
+      }
+    },
+    {
+      "href": "/information/vs/0",
+      "rep": {
+        "x.com.samsung.da.modelNum": "TP1X_REF_21K|70664141|0000033C011913114100000041FB5F00",
+        "x.com.samsung.da.description": "TP1X_REF_21K",
+        "x.com.samsung.da.serialNum": "**REDACTED**",
+        "x.com.samsung.da.otnDUID": "**REDACTED**",
+        "x.com.samsung.da.diagDumpType": "file",
+        "x.com.samsung.da.diagEndPoint": "SSM",
+        "x.com.samsung.da.diagLogType": [
+          "errCode",
+          "dump"
+        ],
+        "x.com.samsung.da.diagMnid": "0AJT",
+        "x.com.samsung.da.diagSetupid": "RR7",
+        "x.com.samsung.da.diagProtocolType": "BLE_OCF",
+        "x.com.samsung.da.diagMinVersion": "3.0",
+        "x.com.samsung.da.diagTsId": "DA01",
+        "x.com.samsung.da.items": [
+          {
+            "x.com.samsung.da.id": "0",
+            "x.com.samsung.da.description": "WiFi Module",
+            "x.com.samsung.da.type": "Software",
+            "x.com.samsung.da.number": "260618",
+            "x.com.samsung.da.newVersionAvailable": "0"
+          },
+          {
+            "x.com.samsung.da.id": "1",
+            "x.com.samsung.da.description": "Micom",
+            "x.com.samsung.da.type": "Firmware",
+            "x.com.samsung.da.number": "2605071D, 24120906, 26040701, FFFFFFFF, 25082208, FFFFFFFF",
+            "x.com.samsung.da.newVersionAvailable": "0"
+          }
+        ],
+        "href": "/information/vs/0"
+      }
+    },
+    {
+      "href": "/mode/vs/0",
+      "rep": {
+        "x.com.samsung.da.supportedModes": [
+          "HOMECARE_WIZARD_V2",
+          "ENERGY_REPORT_MODEL",
+          "18K_REF_OUTDOOR_CONTROL_V2"
+        ],
+        "href": "/mode/vs/0",
+        "rt": [
+          "x.com.samsung.da.mode"
+        ],
+        "if": [
+          "oic.if.baseline",
+          "oic.if.a"
+        ]
+      }
+    },
+    {
+      "href": "/otninformation/vs/0",
+      "rep": {
+        "x.com.samsung.da.target": "Micom",
+        "x.com.samsung.da.newVersionAvailable": "false",
+        "x.com.samsung.da.newVersionNo": "26040701",
+        "x.com.samsung.da.currentVersionInfo": "10000000",
+        "otnStatus": "None",
+        "flashingProgress": "0",
+        "otnTarget": "inverter",
+        "otnCompleteDate": "noHistory",
+        "scheduledTime": "None",
+        "swVersionInfo": {
+          "platform": "Tizen Lite",
+          "oneUiVersion": "7.0 Refrigerator",
+          "osVersion": "4.0"
+        },
+        "otnList": [
+          {
+            "type": "WIFI",
+            "modelId": "A-RFWW-TP1-24-T4-RE1",
+            "versions": [
+              "20260618"
+            ],
+            "visVersion": "260618"
+          },
+          {
+            "type": "Micom",
+            "modelId": "823070664141FFFFFFFF",
+            "versions": [
+              "2605071D",
+              "FFFFFFFF"
+            ],
+            "visVersion": "260507"
+          },
+          {
+            "type": "Micom",
+            "modelId": "823070664041FFFFFFFF",
+            "versions": [
+              "24120906",
+              "FFFFFFFF"
+            ],
+            "visVersion": "241209"
+          },
+          {
+            "type": "Micom",
+            "modelId": "02307066414170664041",
+            "versions": [
+              "2605071D",
+              "24120906"
+            ],
+            "visVersion": "260507"
+          },
+          {
+            "type": "Micom",
+            "modelId": "023070680641FFFFFFFF",
+            "versions": [
+              "26040701",
+              "FFFFFFFF"
+            ],
+            "visVersion": "260407"
+          },
+          {
+            "type": "Micom",
+            "modelId": "023070668841FFFFFFFF",
+            "versions": [
+              "25082208",
+              "FFFFFFFF"
+            ],
+            "visVersion": "250822"
+          }
+        ]
+      }
+    },
+    {
+      "href": "/quickcontrol/info/vs/0",
+      "rep": {
+        "supportedVersion": "1.0"
+      }
+    },
+    {
+      "href": "/realtimenotiforclient/vs/0",
+      "rep": {
+        "x.com.samsung.da.timeforshortnoti": "0",
+        "x.com.samsung.da.periodicnotisubscription": "true",
+        "href": "/realtimenotiforclient/vs/0"
+      }
+    },
+    {
+      "href": "/refrigeration/vs/0",
+      "rep": {
+        "x.com.samsung.da.rapidFridge": "Off",
+        "x.com.samsung.da.rapidFreezing": "Off",
+        "href": "/refrigeration/vs/0",
+        "rt": [
+          "x.com.samsung.da.fridge"
+        ],
+        "if": [
+          "oic.if.baseline",
+          "oic.if.a"
+        ]
+      }
+    },
+    {
+      "href": "/rm/control/vs/0",
+      "rep": {
+        "minPeriod": "9000",
+        "href": "/rm/control/vs/0"
+      }
+    },
+    {
+      "href": "/runningmode/vs/0",
+      "rep": {
+        "x.com.samsung.da.runningMode": 0,
+        "href": "/runningmode/vs/0"
+      }
+    },
+    {
+      "href": "/selfcheck/vs/0",
+      "rep": {
+        "x.com.samsung.da.supportedActions": [
+          "Start"
+        ],
+        "x.com.samsung.da.status": "Ready",
+        "x.com.samsung.da.result": "Success",
+        "x.com.samsung.da.error": [
+          "ErrorCode_None"
+        ],
+        "href": "/selfcheck/vs/0"
+      }
+    },
+    {
+      "href": "/settings/sound/alert/door/vs/0",
+      "rep": {
+        "alert.door": "1",
+        "supportedAlert.door": [
+          "1",
+          "2",
+          "3",
+          "4"
+        ],
+        "href": "/settings/sound/alert/door/vs/0",
+        "rt": [
+          "x.com.samsung.alert.door"
+        ],
+        "if": [
+          "oic.if.baseline",
+          "oic.if.a"
+        ]
+      }
+    },
+    {
+      "href": "/status/lock/vs/0",
+      "rep": {
+        "x.com.samsung.da.device.sound": "On",
+        "x.com.samsung.da.preciseCooling": "On",
+        "x.com.samsung.da.doorAlarmSound": "On",
+        "cleaning.status": "On",
+        "cleaning.type": "SPI_AND_UV",
+        "href": "/status/lock/vs/0",
+        "rt": [
+          "x.com.samsung.da.lockstatus"
+        ],
+        "if": [
+          "oic.if.baseline",
+          "oic.if.a"
+        ]
+      }
+    },
+    {
+      "href": "/temperature/current/cooler/0",
+      "rep": {
+        "temperature": 2.0,
+        "range": [
+          1.0,
+          7.0
+        ],
+        "units": "C",
+        "href": "/temperature/current/cooler/0",
+        "rt": [
+          "oic.r.temperature"
+        ],
+        "if": [
+          "oic.if.baseline",
+          "oic.if.a"
+        ]
+      }
+    },
+    {
+      "href": "/temperature/current/freezer/0",
+      "rep": {
+        "temperature": -19.0,
+        "range": [
+          -23.0,
+          -15.0
+        ],
+        "units": "C",
+        "href": "/temperature/current/freezer/0",
+        "rt": [
+          "oic.r.temperature"
+        ],
+        "if": [
+          "oic.if.baseline",
+          "oic.if.a"
+        ]
+      }
+    },
+    {
+      "href": "/temperature/desired/cooler/0",
+      "rep": {
+        "temperature": 2.0,
+        "range": [
+          1.0,
+          7.0
+        ],
+        "units": "C",
+        "href": "/temperature/desired/cooler/0",
+        "rt": [
+          "oic.r.temperature"
+        ],
+        "if": [
+          "oic.if.baseline",
+          "oic.if.a"
+        ]
+      }
+    },
+    {
+      "href": "/temperature/desired/freezer/0",
+      "rep": {
+        "temperature": -19.0,
+        "range": [
+          -23.0,
+          -15.0
+        ],
+        "units": "C",
+        "href": "/temperature/desired/freezer/0",
+        "rt": [
+          "oic.r.temperature"
+        ],
+        "if": [
+          "oic.if.baseline",
+          "oic.if.a"
+        ]
+      }
+    },
+    {
+      "href": "/temperatures/vs/0",
+      "rep": {
+        "temperature.unit.control": "true",
+        "x.com.samsung.da.items": [
+          {
+            "x.com.samsung.da.id": "0",
+            "x.com.samsung.da.description": "Freezer",
+            "x.com.samsung.da.desired": "-19",
+            "x.com.samsung.da.current": "-19",
+            "x.com.samsung.da.maximum": "-15",
+            "x.com.samsung.da.minimum": "-23",
+            "x.com.samsung.da.unit": "Celsius"
+          },
+          {
+            "x.com.samsung.da.id": "1",
+            "x.com.samsung.da.description": "Fridge",
+            "x.com.samsung.da.desired": "2",
+            "x.com.samsung.da.current": "2",
+            "x.com.samsung.da.maximum": "7",
+            "x.com.samsung.da.minimum": "1",
+            "x.com.samsung.da.unit": "Celsius"
+          }
+        ],
+        "href": "/temperatures/vs/0"
+      }
+    },
+    {
+      "href": "/timezone/vs/0",
+      "rep": {
+        "timezoneid": "Asia/Seoul",
+        "offset": "+09:00",
+        "DST": "OFF"
+      }
+    },
+    {
+      "href": "/wirelessinfo/vs/0",
+      "rep": {
+        "macaddressWiFi": "**REDACTED**",
+        "macaddressBLE": "**REDACTED**",
+        "connectedApSsid": "eomkim_IoT"
+      }
+    }
+  ]
+}
diff --git a/tests/test_golden_regression.py b/tests/test_golden_regression.py
index 0522d4b..e84d0ea 100644
--- a/tests/test_golden_regression.py
+++ b/tests/test_golden_regression.py
@@ -277,6 +277,24 @@ def test_registry_reproduces_golden_state_keys_for_tp1x_ref_21k_eu():
     )
 
 
+def test_registry_reproduces_golden_state_keys_for_tp1x_ref_21k_airfilter():
+    """TP1X_REF_21K, air-filter-equipped variant (issue #318) -- reports
+    /filter/airdustfilter/vs/0 (internal deodorizing filter), the one
+    unbound href on this dump. Unlike airconditioner.AIR_FILTER's
+    filterUsage/filterCapacity pair, this board's filterUsage is already a
+    0-100 percentage with no filterCapacity to divide by."""
+    from tests.conftest import _load_device
+
+    resources = _load_device("refrigerator_tp1x_ref_21k_airfilter")
+    golden = json.loads((GOLDEN / "refrigerator_tp1x_ref_21k_airfilter.json").read_text())
+    state_keys = _new_state_keys("refrigerator_tp1x_ref_21k_airfilter", resources)
+    assert set(state_keys) == set(golden["state_keys"]), (
+        f"state_keys mismatch:\n"
+        f"  extra:   {sorted(set(state_keys) - set(golden['state_keys']))}\n"
+        f"  missing: {sorted(set(golden['state_keys']) - set(state_keys))}"
+    )
+
+
 def test_registry_reproduces_golden_state_keys_for_range_hood():
     from tests.conftest import _load_device
 
diff --git a/tests/test_refrigerator_tp1x_ref_21k_airfilter_capabilities.py b/tests/test_refrigerator_tp1x_ref_21k_airfilter_capabilities.py
new file mode 100644
index 0000000..4a05fe2
--- /dev/null
+++ b/tests/test_refrigerator_tp1x_ref_21k_airfilter_capabilities.py
@@ -0,0 +1,33 @@
+"""Tests for the TP1X_REF_21K air-filter-equipped variant (issue #318).
+
+The dump's only unbound href was /filter/airdustfilter/vs/0, this board's
+internal deodorizing filter -- same filterUsage/filterStatus field pair as
+common.WATER_FILTER, but filterUsage here is already a 0-100 percentage
+(no filterCapacity to divide by, unlike airconditioner.AIR_FILTER).
+"""
+
+from custom_components.localthings.registry.adapter import flatten
+from custom_components.localthings.registry.by_type import resolve
+from custom_components.localthings.registry.discovery import discover
+from tests.conftest import _load_device
+
+
+def _bound():
+    resources = _load_device("refrigerator_tp1x_ref_21k_airfilter")
+    reg = resolve(resources)
+    return discover(resources, reg.capabilities, reg.pattern_capabilities), resources
+
+
+def test_no_unbound_hrefs():
+    _, resources = _bound()
+    unbound = []
+    reg = resolve(resources)
+    discover(resources, reg.capabilities, reg.pattern_capabilities, log=unbound.append)
+    assert unbound == []
+
+
+def test_air_filter_usage_and_status():
+    bound, resources = _bound()
+    state = flatten(bound, resources)
+    assert state["air_filter_usage"] == 100
+    assert state["air_filter_status"] == "wash"