diff --git a/resources/statefulsensor/statefulsensor.go b/resources/statefulsensor/statefulsensor.go
index 16c2137..76e5236 100644
--- a/resources/statefulsensor/statefulsensor.go
+++ b/resources/statefulsensor/statefulsensor.go
@@ -196,18 +196,43 @@ func (s *statefulSensor) Status(ctx context.Context) (map[string]interface{}, er
 	}, nil
 }
 
-// DoCommand supports a "set" command that replaces the value the sensor holds.
+// DoCommand supports two commands:
 //
-// Example:
+//   - "set" replaces the entire value the sensor holds:
+//     {"set": {"temperature": 72.5, "unit": "F"}}
+//   - "merge" overlays the given keys onto the existing value, leaving keys not
+//     mentioned untouched: {"merge": {"usage": 0}}
 //
-//	{"set": {"temperature": 72.5, "unit": "F"}}
-//
-// The provided object becomes the sensor's value, is persisted to disk, and is
-// returned by Readings.
+// In both cases the result is persisted to disk and returned by Readings.
 func (s *statefulSensor) DoCommand(ctx context.Context, cmd map[string]interface{}) (map[string]interface{}, error) {
+	if raw, ok := cmd["merge"]; ok {
+		patch, ok := raw.(map[string]interface{})
+		if !ok {
+			return nil, fmt.Errorf("%q must be an object mapping keys to values, got %T", "merge", raw)
+		}
+
+		s.mu.Lock()
+		// Guard against a nil map (sensor never set, empty state file): writing to
+		// a nil map panics, so initialize before overlaying the patch.
+		if s.value == nil {
+			s.value = make(map[string]interface{}, len(patch))
+		}
+		for k, v := range patch {
+			s.value[k] = v
+		}
+		s.mu.Unlock()
+
+		if err := s.saveToFile(); err != nil {
+			return nil, err
+		}
+
+		s.logger.Infof("merged %+v", patch)
+		return map[string]interface{}{"merge": "ok"}, nil
+	}
+
 	raw, ok := cmd["set"]
 	if !ok {
-		return nil, fmt.Errorf("unsupported command: expected a %q key", "set")
+		return nil, fmt.Errorf("unsupported command: expected a %q or %q key", "set", "merge")
 	}
 
 	value, ok := raw.(map[string]interface{})
diff --git a/resources/statefulsensor/statefulsensor_test.go b/resources/statefulsensor/statefulsensor_test.go
index cf5e048..716984d 100644
--- a/resources/statefulsensor/statefulsensor_test.go
+++ b/resources/statefulsensor/statefulsensor_test.go
@@ -127,6 +127,63 @@ func TestStatus(t *testing.T) {
 	}
 }
 
+func TestMergeUpdatesOneKeyAndPreservesOthers(t *testing.T) {
+	ctx := context.Background()
+	path := filepath.Join(t.TempDir(), "state.json")
+	s := newTestSensor(t, path)
+
+	// Seed the full coffee-machine state.
+	full := map[string]interface{}{
+		"usage":          11.5,
+		"regular_grinds": 9.0,
+		"decaf_grinds":   3.0,
+		"cleanings":      11.0,
+	}
+	if _, err := s.DoCommand(ctx, map[string]interface{}{"set": full}); err != nil {
+		t.Fatalf("DoCommand set: %v", err)
+	}
+
+	// Reset only usage, as a "filled water tank" button would.
+	if _, err := s.DoCommand(ctx, map[string]interface{}{"merge": map[string]interface{}{"usage": 0.0}}); err != nil {
+		t.Fatalf("DoCommand merge: %v", err)
+	}
+
+	got, err := s.Readings(ctx, nil)
+	if err != nil {
+		t.Fatalf("Readings: %v", err)
+	}
+	if got["usage"] != 0.0 {
+		t.Fatalf("usage = %v, want 0 after merge", got["usage"])
+	}
+	// The whole point of merge: the other counters are untouched.
+	if got["regular_grinds"] != 9.0 || got["decaf_grinds"] != 3.0 || got["cleanings"] != 11.0 {
+		t.Fatalf("merge clobbered other keys: %+v", got)
+	}
+}
+
+func TestMergeOnNilValueDoesNotPanic(t *testing.T) {
+	ctx := context.Background()
+	path := filepath.Join(t.TempDir(), "state.json")
+	// A state file containing the literal "null" unmarshals to a nil map, which
+	// would panic on write without the guard in DoCommand.
+	if err := os.WriteFile(path, []byte("null"), 0o600); err != nil {
+		t.Fatalf("write state file: %v", err)
+	}
+	s := newTestSensor(t, path)
+
+	if _, err := s.DoCommand(ctx, map[string]interface{}{"merge": map[string]interface{}{"usage": 0.0}}); err != nil {
+		t.Fatalf("DoCommand merge on nil value: %v", err)
+	}
+
+	got, err := s.Readings(ctx, nil)
+	if err != nil {
+		t.Fatalf("Readings: %v", err)
+	}
+	if got["usage"] != 0.0 {
+		t.Fatalf("usage = %v, want 0", got["usage"])
+	}
+}
+
 func TestDoCommandRejectsUnknownCommand(t *testing.T) {
 	ctx := context.Background()
 	s := newTestSensor(t, filepath.Join(t.TempDir(), "state.json"))