diff --git a/services/search/pkg/mapping/fieldindex.go b/services/search/pkg/mapping/fieldindex.go
new file mode 100644
index 0000000000..5bc5d21b66
--- /dev/null
+++ b/services/search/pkg/mapping/fieldindex.go
@@ -0,0 +1,36 @@
+package mapping
+
+import (
+	"reflect"
+	"strings"
+)
+
+// FieldNameIndex maps a lowercased field path to the real field name for every
+// field of t, recursing into nested facets (photo.cameraMake, ...). Names come
+// from json tags, so it is backend-neutral; the query layer resolves KQL keys
+// case-insensitively against it.
+func FieldNameIndex(t reflect.Type, overrides map[string]FieldOpts) map[string]string {
+	out := map[string]string{}
+	var walk func(t reflect.Type, prefix string)
+	walk = func(t reflect.Type, prefix string) {
+		_ = walkFields(t, func(fi fieldInfo) error {
+			name := fi.Name
+			if prefix != "" {
+				name = prefix + "." + fi.Name
+			}
+			out[strings.ToLower(name)] = name
+
+			fieldType := overrides[name].Type
+			if fieldType == "" {
+				fieldType = inferType(fi.GoField.Type)
+			}
+			// recurse into nested facets; time.Time is a struct too but a leaf.
+			if sub := structType(fi.GoField.Type); sub != nil && fieldType != TypeDatetime {
+				walk(sub, name)
+			}
+			return nil
+		})
+	}
+	walk(t, "")
+	return out
+}
diff --git a/services/search/pkg/mapping/fieldindex_test.go b/services/search/pkg/mapping/fieldindex_test.go
new file mode 100644
index 0000000000..d0e47dcb2e
--- /dev/null
+++ b/services/search/pkg/mapping/fieldindex_test.go
@@ -0,0 +1,79 @@
+package mapping_test
+
+import (
+	"reflect"
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+
+	"github.com/opencloud-eu/opencloud/services/search/pkg/mapping"
+	"github.com/opencloud-eu/opencloud/services/search/pkg/search"
+)
+
+func resourceFieldIndex(testing.TB) map[string]string {
+	return mapping.FieldNameIndex(
+		reflect.TypeFor[search.Resource](),
+		search.Resource{}.SearchFieldOverrides(),
+	)
+}
+
+// resolve looks up the lowercased key in the index, falling back to the key.
+func resolve(idx map[string]string, key string) string {
+	if v, ok := idx[strings.ToLower(key)]; ok {
+		return v
+	}
+	return key
+}
+
+func TestFieldNameIndex_TopLevelCaseInsensitive(t *testing.T) {
+	idx := resourceFieldIndex(t)
+	for in, want := range map[string]string{
+		"rootid": "RootID", "ROOTID": "RootID", "RootID": "RootID",
+		"name": "Name", "NAME": "Name",
+		"mimetype": "MimeType", "MimeType": "MimeType",
+		"tags": "Tags", "favorites": "Favorites",
+		"mtime": "Mtime", "parentid": "ParentID", "id": "ID",
+	} {
+		require.Equalf(t, want, resolve(idx, in), "resolve(%q)", in)
+	}
+}
+
+// Facet sub-fields are lowerCamelCase in the index (from libregraph json tags);
+// the derived index resolves them case-insensitively.
+func TestFieldNameIndex_FacetsCaseInsensitive(t *testing.T) {
+	idx := resourceFieldIndex(t)
+	for in, want := range map[string]string{
+		// case-insensitive: same field, different casings
+		"photo.cameramake": "photo.cameraMake",
+		"photo.CAMERAMAKE": "photo.cameraMake",
+		// a representative sub-field across each facet
+		"photo.takendatetime": "photo.takenDateTime",
+		"audio.artist":        "audio.artist",
+		"audio.albumartist":   "audio.albumArtist",
+		"image.width":         "image.width",
+		"location.latitude":   "location.latitude",
+	} {
+		require.Equalf(t, want, resolve(idx, in), "resolve(%q)", in)
+	}
+}
+
+func TestFieldNameIndex_UnknownPassesThrough(t *testing.T) {
+	idx := resourceFieldIndex(t)
+	require.Equal(t, "nope.field", resolve(idx, "nope.field"))
+	require.Equal(t, "custom", resolve(idx, "custom"))
+}
+
+// All top-level fields are covered from one derived source, so both backends
+// resolve them the same way.
+func TestFieldNameIndex_CoversAllTopLevelFields(t *testing.T) {
+	idx := resourceFieldIndex(t)
+	for in, want := range map[string]string{
+		"rootid": "RootID", "path": "Path", "id": "ID", "name": "Name",
+		"size": "Size", "mtime": "Mtime", "type": "Type",
+		"content": "Content", "hidden": "Hidden", "tags": "Tags",
+		"favorites": "Favorites",
+	} {
+		require.Equalf(t, want, resolve(idx, in), "derived should cover %q", in)
+	}
+}