fix: multivalue tags

Dockerfile.full

@@ -54,7 +54,7 @@ ENV GID=1000
 
 RUN addgroup -g $GID user && \
     adduser -D -u $UID -G user user && \
-    apk add --no-cache ffmpeg
+    apk add --no-cache ffmpeg flac
 
 # Copy compiled binary from backend-builder
 COPY --from=backend-builder --chown=user:user /app/filebrowser /bin/filebrowser

files/tags.go

@@ -5,6 +5,8 @@ import (
 	"encoding/base64"
 	"io"
 	"log"
+	"os/exec"
+	"path/filepath"
 	"strings"
 
 	"github.com/dhowden/tag"
@@ -13,6 +15,7 @@ import (
 // ReadAudioTags extracts common audio metadata from the file and stores it in
 // the FileInfo.Tags map. It attempts to extract common fields and includes
 // embedded picture(s) as base64 where present.
+// Multi-valued tags like ARTISTS and ALBUMARTISTS are extracted as arrays.
 func (i *FileInfo) ReadAudioTags() error {
 	if i.IsDir {
 		return nil
@@ -76,7 +79,20 @@ func (i *FileInfo) ReadAudioTags() error {
 	}
 
 	// Keep raw metadata map if available (some formats expose additional fields)
+	// Also extract multi-valued tags like ARTISTS and ALBUMARTISTS
 	if raw := m.Raw(); raw != nil {
+		// First pass: collect multi-valued tags (ARTISTS, ALBUMARTISTS)
+		artists := extractMultiValuedTag(raw, "ARTISTS", "artists")
+		albumArtists := extractMultiValuedTag(raw, "ALBUMARTISTS", "albumartists")
+
+		// Store multi-valued tags as arrays if present
+		if len(artists) > 0 {
+			tags["artists"] = artists
+		}
+		if len(albumArtists) > 0 {
+			tags["albumartists"] = albumArtists
+		}
+
 		for k, v := range raw {
 			// Skip raw APIC entries (attached picture frame) to avoid
 			// exposing large binary blobs. We already expose a friendly
@@ -84,6 +100,11 @@ func (i *FileInfo) ReadAudioTags() error {
 			if strings.EqualFold(k, "APIC") {
 				continue
 			}
+			// Skip multi-valued tags we already handled
+			kLower := strings.ToLower(k)
+			if kLower == "artists" || kLower == "albumartists" {
+				continue
+			}
 			// avoid overwriting already set common fields
 			if _, ok := tags[k]; !ok {
 				tags[k] = v
@@ -91,6 +112,19 @@ func (i *FileInfo) ReadAudioTags() error {
 		}
 	}
 
+	// For FLAC files, use metaflac to read multi-valued tags properly
+	// since dhowden/tag doesn't handle them correctly
+	if realPath := i.RealPath(); realPath != "" && strings.EqualFold(filepath.Ext(realPath), ".flac") {
+		if multiTags := readMultiValuedTagsWithMetaflac(realPath); multiTags != nil {
+			if artists, ok := multiTags["ARTISTS"]; ok && len(artists) > 0 {
+				tags["artists"] = artists
+			}
+			if albumArtists, ok := multiTags["ALBUMARTISTS"]; ok && len(albumArtists) > 0 {
+				tags["albumartists"] = albumArtists
+			}
+		}
+	}
+
 	// Attach tags map
 	if len(tags) > 0 {
 		i.Tags = tags
@@ -98,3 +132,90 @@ func (i *FileInfo) ReadAudioTags() error {
 
 	return nil
 }
+
+// extractMultiValuedTag extracts values for a multi-valued tag from raw metadata.
+// It handles both Vorbis-style (multiple entries with same key) and ID3-style
+// (single string that may need splitting) formats.
+func extractMultiValuedTag(raw map[string]interface{}, keys ...string) []string {
+	var result []string
+	seen := make(map[string]bool)
+
+	for _, key := range keys {
+		for rawKey, rawVal := range raw {
+			if !strings.EqualFold(rawKey, key) {
+				continue
+			}
+			// Handle different value types returned by the tag library
+			switch v := rawVal.(type) {
+			case string:
+				v = strings.TrimSpace(v)
+				if v != "" && !seen[v] {
+					result = append(result, v)
+					seen[v] = true
+				}
+			case []string:
+				for _, s := range v {
+					s = strings.TrimSpace(s)
+					if s != "" && !seen[s] {
+						result = append(result, s)
+						seen[s] = true
+					}
+				}
+			case []interface{}:
+				for _, item := range v {
+					if s, ok := item.(string); ok {
+						s = strings.TrimSpace(s)
+						if s != "" && !seen[s] {
+							result = append(result, s)
+							seen[s] = true
+						}
+					}
+				}
+			}
+		}
+	}
+	return result
+}
+
+// readMultiValuedTagsWithMetaflac uses metaflac to read Vorbis comments from FLAC files.
+// This properly handles multi-valued tags (multiple entries with same key).
+func readMultiValuedTagsWithMetaflac(filepath string) map[string][]string {
+	if _, err := exec.LookPath("metaflac"); err != nil {
+		return nil
+	}
+
+	// Use metaflac to export all tags
+	cmd := exec.Command("metaflac", "--export-tags-to=-", filepath)
+	out, err := cmd.Output()
+	if err != nil {
+		log.Printf("metaflac export failed for %s: %v", filepath, err)
+		return nil
+	}
+
+	// Parse output: each line is TAG=VALUE
+	result := make(map[string][]string)
+	for _, line := range strings.Split(string(out), "\n") {
+		line = strings.TrimSpace(line)
+		if line == "" {
+			continue
+		}
+		idx := strings.Index(line, "=")
+		if idx < 0 {
+			continue
+		}
+		key := strings.ToUpper(strings.TrimSpace(line[:idx]))
+		value := strings.TrimSpace(line[idx+1:])
+		if value == "" {
+			continue
+		}
+		// Collect multi-valued artist tags (ARTISTS, ARTIST, ALBUMARTISTS)
+		// Normalize key variations
+		if key == "ARTISTS" || key == "ARTIST" {
+			result["ARTISTS"] = append(result["ARTISTS"], value)
+		} else if key == "ALBUMARTISTS" || key == "ALBUMARTIST" || key == "ALBUM ARTIST" || key == "ALBUM_ARTIST" {
+			result["ALBUMARTISTS"] = append(result["ALBUMARTISTS"], value)
+		}
+	}
+
+	return result
+}

frontend/src/components/prompts/ModifyMetadata.vue

@@ -86,23 +86,64 @@
               <tr v-for="field in fields" :key="field" class="metadata-row">
                 <td class="metadata-title">
                   <strong>{{ fieldLabel(field) }}</strong>
+                  <span v-if="isMultiValued(field)" class="multi-value-hint">{{ trans('prompts.multiValueHint','(multi-valued)') }}</span>
                 </td>
                 <td class="metadata-current">
                   <template v-if="isPicture(field)">
                     <img :src="pictureSrc(field)" alt="cover" class="cover-preview" v-if="pictureSrc(field)" />
                     <span v-else class="metadata-muted">{{ trans('prompts.noCover','No cover') }}</span>
                   </template>
+                  <template v-else-if="isMultiValued(field)">
+                    <div class="multi-value-display">
+                      <div v-for="(val, idx) in displayCurrentArray(field)" :key="idx" class="multi-value-item">
+                        {{ val }}
+                      </div>
+                      <span v-if="displayCurrentArray(field).length === 0" class="metadata-muted">—</span>
+                    </div>
+                  </template>
                   <template v-else>
                     {{ displayCurrent(field) }}
                   </template>
                 </td>
                 <td class="metadata-edit-cell">
                   <div class="edit-with-actions">
+                    <template v-if="isMultiValued(field)">
+                      <div class="multi-value-editor">
+                        <div v-for="(val, idx) in getMultiValueEntries(field)" :key="idx" class="multi-value-entry">
+                          <input
+                            :value="val"
+                            @input="updateMultiValueEntry(field, idx, $event.target.value)"
+                            :placeholder="trans('prompts.artistName','Artist name')"
+                            type="text"
+                            class="multi-value-input"
+                          />
+                          <button
+                            type="button"
+                            class="button button--flat button--red icon-button mini-button"
+                            :aria-label="trans('buttons.removeEntry','Remove')"
+                            :title="trans('buttons.removeEntry','Remove')"
+                            @click="removeMultiValueEntry(field, idx)"
+                          >
+                            <i class="material-icons">close</i>
+                          </button>
+                        </div>
+                        <button
+                          type="button"
+                          class="button button--flat button--small add-entry-btn"
+                          @click="addMultiValueEntry(field)"
+                        >
+                          <i class="material-icons">add</i>
+                          {{ trans('buttons.addEntry','Add entry') }}
+                        </button>
+                      </div>
+                    </template>
+                    <template v-else>
                       <input
                         v-model="newValues[field]"
                         :placeholder="placeholders[field] || ''"
                         type="text"
                       />
+                    </template>
                     <button
                       type="button"
                       class="button button--flat button--red icon-button"
@@ -152,7 +193,7 @@ import { mapActions, mapState } from "pinia";
 import { useFileStore } from "@/stores/file";
 import { useLayoutStore } from "@/stores/layout";
 import { metadata as apiMetadata, updateMetadata } from "@/api/files";
-import { fieldLabelFor, normalizeChanges, canonicalizeKeys, canonicalizeKey, navidromeTags, isMultiValuedTag, splitMultiValues, normalizeMixedChanges } from "@/utils/metadata";
+import { fieldLabelFor, normalizeChanges, canonicalizeKeys, canonicalizeKey, navidromeTags, isMultiValuedTag, isMultiValuedKey, splitMultiValues, normalizeMixedChanges, formatMultiValueDisplay, parseMultiValueInput } from "@/utils/metadata";
 
 export default {
   name: "modifyMetadata",
@@ -200,17 +241,49 @@ export default {
         Object.keys(m).forEach((k) => keys.add(k));
       });
 
-      // Ensure stable ordering: put common fields first
+      // Ensure stable ordering: put common fields first (including multi-valued artists/albumartists)
-      const preferred = ["title", "artist", "album", "track", "genre", "date", "comment", "disc", "composer", "year", "albumartist", "picture", "cover"];
+      const preferred = ["title", "artist", "artists", "album", "track", "genre", "date", "comment", "disc", "composer", "year", "albumartist", "albumartists", "picture", "cover"];
       const rest = Array.from(keys).filter((k) => !preferred.includes(k)).sort();
       this.fields = preferred.filter((k) => keys.has(k)).concat(rest);
 
       // Setup field labels (use translation when available, otherwise humanize)
+      // For multi-valued fields, initialize newValues appropriately
       this.fields.forEach((f) => {
         this.fieldLabels[f] = this.trans(`prompts.${f}`, this.humanize(f));
+        if (isMultiValuedKey(f)) {
+          // Initialize with current values or empty array
+          const current = this.getCurrentMultiValues(f);
+          this.newValues[f] = current.length > 0 ? [...current] : [];
+        } else {
           this.newValues[f] = "";
+        }
       });
     },
+    // Get current multi-values directly from metadataList (for initialization)
+    getCurrentMultiValues(field) {
+      if (!this.metadataList || this.metadataList.length === 0) return [];
+      const allValues = [];
+      const seen = new Set();
+      for (const m of this.metadataList) {
+        const v = m[field];
+        if (Array.isArray(v)) {
+          for (const item of v) {
+            const s = String(item || '').trim();
+            if (s && !seen.has(s)) {
+              allValues.push(s);
+              seen.add(s);
+            }
+          }
+        } else if (v !== undefined && v !== null) {
+          const s = String(v).trim();
+          if (s && !seen.has(s)) {
+            allValues.push(s);
+            seen.add(s);
+          }
+        }
+      }
+      return allValues;
+    },
     toggleAddField() {
       this.addFieldVisible = !this.addFieldVisible;
       if (this.addFieldVisible) {
@@ -235,14 +308,73 @@ export default {
       }
       // Initialize newValues entry and set initial value if provided
       if (isMultiValuedTag(canonicalizeKey(key))) {
-        // store raw string; we'll split on apply
+        // For multi-valued tags, initialize as array
-        this.newValues[key] = val;
+        if (val) {
+          this.newValues[key] = splitMultiValues(val);
+        } else {
+          this.newValues[key] = [''];
+        }
       } else {
         if (!(key in this.newValues)) this.newValues[key] = "";
         if (val !== "") this.newValues[key] = val;
       }
       this.cancelAddField();
     },
+    // Check if a field is multi-valued (ARTISTS, ALBUMARTISTS)
+    isMultiValued(field) {
+      return isMultiValuedKey(field);
+    },
+    // Get current values as array for multi-valued display
+    displayCurrentArray(field) {
+      return this.getCurrentMultiValues(field);
+    },
+    // Get entries for editing multi-valued field
+    getMultiValueEntries(field) {
+      const val = this.newValues[field];
+      if (Array.isArray(val)) {
+        return val;
+      }
+      // Initialize from current if not yet edited
+      if (!val || val === '') {
+        const current = this.getCurrentMultiValues(field);
+        if (current.length > 0) {
+          this.newValues[field] = [...current];
+          return this.newValues[field];
+        }
+        // Return empty array with one entry for new field
+        this.newValues[field] = [''];
+        return this.newValues[field];
+      }
+      // Handle legacy string format
+      const arr = splitMultiValues(String(val));
+      this.newValues[field] = arr.length > 0 ? arr : [''];
+      return this.newValues[field];
+    },
+    // Update a single entry in a multi-valued field
+    updateMultiValueEntry(field, index, value) {
+      if (!Array.isArray(this.newValues[field])) {
+        this.newValues[field] = this.getMultiValueEntries(field);
+      }
+      this.newValues[field][index] = value;
+    },
+    // Remove an entry from a multi-valued field
+    removeMultiValueEntry(field, index) {
+      if (!Array.isArray(this.newValues[field])) {
+        this.newValues[field] = this.getMultiValueEntries(field);
+      }
+      this.newValues[field].splice(index, 1);
+      // Keep at least one empty entry for UX
+      if (this.newValues[field].length === 0) {
+        this.newValues[field] = [''];
+      }
+    },
+    // Add a new entry to a multi-valued field
+    addMultiValueEntry(field) {
+      if (!Array.isArray(this.newValues[field])) {
+        this.newValues[field] = this.getMultiValueEntries(field);
+      }
+      this.newValues[field].push('');
+    },
     removeField(field) {
       // Remove an added or existing field from the edit list (UI only)
       this.fields = this.fields.filter((f) => f !== field);
@@ -338,7 +470,9 @@ export default {
         const v = this.newValues[k];
         const canon = canonicalizeKey(k);
         if (isMultiValuedTag(canon)) {
-          const arr = Array.isArray(v) ? v : splitMultiValues(String(v || ""));
+          // For multi-valued tags, filter out empty entries
+          let arr = Array.isArray(v) ? v : splitMultiValues(String(v || ""));
+          arr = arr.map(s => String(s || '').trim()).filter(Boolean);
           if (arr.length > 0) changes[canon] = arr;
         } else if (v !== undefined && v !== null && String(v).trim() !== "") {
           changes[canon] = v;
@@ -614,4 +748,87 @@ export default {
 .metadata-edit-row td {
   background: rgba(0,0,0,0.02);
 }
+
+/* Multi-valued tag styling */
+.multi-value-hint {
+  display: block;
+  font-size: 0.75em;
+  color: var(--color-muted, #888);
+  font-weight: normal;
+  margin-top: 0.15em;
+}
+
+.multi-value-display {
+  display: flex;
+  flex-direction: column;
+  gap: 0.25rem;
+}
+
+.multi-value-item {
+  background: rgba(0,0,0,0.04);
+  padding: 0.25rem 0.5rem;
+  border-radius: 4px;
+  font-size: 0.9em;
+}
+
+.multi-value-editor {
+  display: flex;
+  flex-direction: column;
+  gap: 0.35rem;
+  flex: 1;
+}
+
+.multi-value-entry {
+  display: flex;
+  align-items: center;
+  gap: 0.25rem;
+}
+
+.multi-value-input {
+  flex: 1;
+  box-sizing: border-box;
+  border-radius: 6px;
+  border: 1px solid rgba(0,0,0,0.12);
+  padding: 0.35rem 0.5rem;
+  font-size: 0.9em;
+}
+
+.mini-button {
+  padding: 0.15em 0.25em !important;
+  min-width: auto !important;
+}
+
+.mini-button .material-icons {
+  font-size: 16px !important;
+}
+
+.add-entry-btn {
+  align-self: flex-start;
+  display: inline-flex;
+  align-items: center;
+  gap: 0.25rem;
+  font-size: 0.85em;
+  padding: 0.25rem 0.5rem;
+  margin-top: 0.15rem;
+}
+
+.add-entry-btn .material-icons {
+  font-size: 16px;
+}
+
+/* Ensure multi-value editor aligns well in the edit cell */
+.edit-with-actions {
+  display: flex;
+  align-items: flex-start;
+  gap: 0.5rem;
+}
+
+.edit-with-actions > .multi-value-editor {
+  flex: 1;
+}
+
+.edit-with-actions > .icon-button {
+  flex-shrink: 0;
+  margin-top: 0.25rem;
+}
 </style>

frontend/src/utils/metadata.ts

@@ -2,6 +2,7 @@
 
 export type Changes = Record<string, string>;
 export type MultiChanges = Record<string, string[]>; // for multi-valued tags
+export type MixedChanges = Record<string, string | string[]>; // combined type
 
 // Canonical names used for UI display
 export const Canonical = {
@@ -25,6 +26,12 @@ export const Canonical = {
 
 const allowedCanonicals = new Set<string>(Object.values(Canonical));
 
+// Multi-valued tag canonical names
+export const multiValuedTags = new Set<string>([
+  Canonical.Artists,
+  Canonical.AlbumArtists,
+]);
+
 // Map various inputs (singular/plural/localized) to canonical names
 const canonicalMap: Record<string, string> = {
   // title
@@ -172,7 +179,28 @@ export const navidromeTags: string[] = [
 ];
 
 export function isMultiValuedTag(canon: string): boolean {
-  return canon === Canonical.Artists || canon === Canonical.AlbumArtists;
+  return multiValuedTags.has(canon);
+}
+
+// Check if a key (after canonicalization) is a multi-valued tag
+export function isMultiValuedKey(key: string): boolean {
+  const canon = canonicalizeKey(key);
+  return isMultiValuedTag(canon);
+}
+
+// Format multi-valued array for display (join with line breaks)
+export function formatMultiValueDisplay(values: string[]): string {
+  if (!values || values.length === 0) return "";
+  return values.join("\n");
+}
+
+// Parse display string back to array (split by newlines)
+export function parseMultiValueInput(input: string): string[] {
+  if (!input) return [];
+  return input
+    .split(/\n/)
+    .map((s) => s.trim())
+    .filter(Boolean);
 }
 
 // Split a user-entered multi-valued string into individual entries using

http/data.go

@@ -48,9 +48,32 @@ func (d *data) Check(path string) bool {
 }
 
 func handle(fn handleFunc, prefix string, store *storage.Storage, server *settings.Server) http.Handler {
+    // trackingWriter wraps the ResponseWriter to detect if headers/body were already written.
+    type trackingWriter struct {
+        http.ResponseWriter
+        wroteHeader bool
+        wroteBody   bool
+    }
+    func (tw *trackingWriter) WriteHeader(code int) {
+        if !tw.wroteHeader {
+            tw.wroteHeader = true
+            tw.ResponseWriter.WriteHeader(code)
+        }
+    }
+    func (tw *trackingWriter) Write(b []byte) (int, error) {
+        // Any Write implies headers committed (status 200 if none set)
+        if !tw.wroteHeader {
+            tw.wroteHeader = true
+        }
+        tw.wroteBody = true
+        return tw.ResponseWriter.Write(b)
+    }
+
     handler := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+        // Wrap writer to prevent superfluous WriteHeader calls
+        tw := &trackingWriter{ResponseWriter: w}
 		for k, v := range globalHeaders {
-			w.Header().Set(k, v)
+			tw.Header().Set(k, v)
 		}
 
 		settings, err := store.Settings.Get()
@@ -59,7 +82,7 @@ func handle(fn handleFunc, prefix string, store *storage.Storage, server *settin
 			return
 		}
 
-		status, err := fn(w, r, &data{
+		status, err := fn(tw, r, &data{
 			Runner:   &runner.Runner{Enabled: server.EnableExec, Settings: settings},
 			store:    store,
 			settings: settings,
@@ -76,7 +99,10 @@ func handle(fn handleFunc, prefix string, store *storage.Storage, server *settin
 			if status == http.StatusBadRequest && err != nil {
 				txt += " (" + err.Error() + ")"
 			}
-			http.Error(w, strconv.Itoa(status)+" "+txt, status)
+			// Only write an error response if nothing was written yet.
+			if !tw.wroteHeader && !tw.wroteBody {
+				http.Error(tw, strconv.Itoa(status)+" "+txt, status)
+			}
 			return
 		}
 	})

http/metadata.go

@@ -64,10 +64,8 @@ var canonicalMap = map[string]string{
 var ffmpegKey = map[string]string{
     "Title":       "title",
     "Artist":      "artist",
-    "Artists":     "artist",
     "Album":       "album",
     "AlbumArtist": "album_artist",
-    "AlbumArtists": "album_artist",
     "Composer":    "composer",
     "Track":       "track",
     "TrackNumber": "track",
@@ -80,12 +78,22 @@ var ffmpegKey = map[string]string{
     "Lyrics":      "lyrics",
 }
 
+// ffmpegMultiKey maps multi-valued canonical names to ffmpeg/Vorbis keys.
+// For FLAC/Vorbis, use uppercase ARTISTS/ALBUMARTISTS for proper multi-value support.
+var ffmpegMultiKey = map[string]string{
+    "Artists":      "ARTISTS",
+    "AlbumArtists": "ALBUMARTISTS",
+}
+
 // allowedCanonicals is the set of canonical tags we accept for writes.
 var allowedCanonicals = func() map[string]struct{} {
-    m := make(map[string]struct{}, len(ffmpegKey))
+    m := make(map[string]struct{}, len(ffmpegKey)+len(ffmpegMultiKey))
     for k := range ffmpegKey {
         m[k] = struct{}{}
     }
+    for k := range ffmpegMultiKey {
+        m[k] = struct{}{}
+    }
     return m
 }()
 
@@ -132,6 +140,7 @@ func normalizeAndMapToFFmpeg(in map[string]string) map[string]string {
     return out
 }
 // normalizeMultiToFFmpeg maps multi-valued canonical keys to ffmpeg keys, preserving arrays.
+// Uses ffmpegMultiKey for proper Vorbis comment names (ARTISTS, ALBUMARTISTS).
 func normalizeMultiToFFmpeg(in map[string][]string) map[string][]string {
     out := map[string][]string{}
     for k, arr := range in {
@@ -145,10 +154,15 @@ func normalizeMultiToFFmpeg(in map[string][]string) map[string][]string {
                 continue
             }
         }
-        ffk, ok := ffmpegKey[canonical]
+        // First check if this is a multi-valued tag
+        ffk, ok := ffmpegMultiKey[canonical]
+        if !ok {
+            // Fall back to regular ffmpeg key
+            ffk, ok = ffmpegKey[canonical]
             if !ok {
                 continue
             }
+        }
         vals := []string{}
         for _, v := range arr {
             v = strings.TrimSpace(v)

http/resource.go

@@ -298,7 +298,23 @@ func resourcePatchHandler(fileCache FileCache) handleFunc {
 				}
 			}
 
+			// If the source is a directory, apply changes to all audio files within.
+			// Otherwise, apply to the single file.
 			err = d.RunHook(func() error {
+				fi, statErr := files.NewFileInfo(&files.FileOptions{
+					Fs:         d.user.Fs,
+					Path:       src,
+					Modify:     d.user.Perm.Modify,
+					Expand:     false,
+					ReadHeader: false,
+					Checker:    d,
+				})
+				if statErr != nil {
+					return statErr
+				}
+				if fi.IsDir {
+					return applyMetadataToDir(r.Context(), d, src, tags, multi, clear)
+				}
 				return applyMetadataWithFFmpeg(r.Context(), d, src, tags, multi, clear)
 			}, action, src, dst, d.user)
 
@@ -316,6 +332,8 @@ func resourcePatchHandler(fileCache FileCache) handleFunc {
 // applyMetadataWithFFmpeg attempts to write metadata using ffmpeg by creating
 // a temporary file and replacing the original. This requires that the
 // underlying filesystem exposes a real path (see FileInfo.RealPath()).
+// For FLAC files with multi-valued tags (ARTISTS, ALBUMARTISTS), it uses
+// metaflac directly since ffmpeg doesn't support true multi-valued Vorbis comments.
 func applyMetadataWithFFmpeg(ctx context.Context, d *data, src string, tags map[string]string, multi map[string][]string, clear []string) error {
 	fi, err := files.NewFileInfo(&files.FileOptions{
 		Fs:         d.user.Fs,
@@ -362,10 +380,26 @@ func applyMetadataWithFFmpeg(ctx context.Context, d *data, src string, tags map[
 	isMP3 := strings.EqualFold(ext, ".mp3")
 	// Treat M4A/MP4 similarly for metadata handling specifics
 	isMP4 := strings.EqualFold(ext, ".m4a") || strings.EqualFold(ext, ".m4b") || strings.EqualFold(ext, ".mp4")
+	// FLAC supports true multi-valued Vorbis comments via metaflac
+	isFLAC := strings.EqualFold(ext, ".flac")
 	if isMP3 {
 		args = append(args, "-id3v2_version", "4")
 	}
 
+	// For FLAC files with multi-valued tags, use metaflac which properly supports
+	// multiple values for the same tag (Vorbis comments).
+	if isFLAC && len(multi) > 0 {
+		if _, err := exec.LookPath("metaflac"); err == nil {
+			// Use metaflac for multi-valued tags
+			if err := applyMultiValuedTagsWithMetaflac(ctx, real, tags, multi, clear); err != nil {
+				log.Printf("metaflac failed, falling back to ffmpeg: %v", err)
+				// Fall through to ffmpeg
+			} else {
+				return nil
+			}
+		}
+	}
+
 	// If MP3 and there are clears, perform ID3v2 in-place removal first.
 	// This avoids ffmpeg creating unwanted empty entries (e.g., TXXX artifacts).
 	didID3Clear := false
@@ -395,12 +429,11 @@ func applyMetadataWithFFmpeg(ctx context.Context, d *data, src string, tags map[
 		changes++
 		setKeys[k] = struct{}{}
 	}
-	// Handle multi-valued tags (e.g., Artists, AlbumArtists) by adding repeated metadata entries
+	// Handle multi-valued tags (e.g., Artists, AlbumArtists)
 	if len(multi) > 0 {
 		mm := normalizeMultiToFFmpeg(multi)
 		for ffk, vals := range mm {
-			// MP4/M4A generally expects a single value for artist/album_artist.
+			// MP4/M4A expects a single value; join for compatibility
-			// Join multiple values for better compatibility with tag readers.
 			if isMP4 && (ffk == "artist" || ffk == "album_artist") {
 				if len(vals) > 0 {
 					joined := strings.Join(vals, "; ")
@@ -410,6 +443,28 @@ func applyMetadataWithFFmpeg(ctx context.Context, d *data, src string, tags map[
 				}
 				continue
 			}
+			// MP3 (ID3): prefer writing consolidated artist/album_artist rather than TXXX ARTISTS
+			if isMP3 {
+				if ffk == "ARTISTS" {
+					if len(vals) > 0 {
+						joined := strings.Join(vals, "; ")
+						args = append(args, "-metadata", fmt.Sprintf("%s=%s", "artist", joined))
+						changes++
+						setKeys["artist"] = struct{}{}
+					}
+					continue
+				}
+				if ffk == "ALBUMARTISTS" {
+					if len(vals) > 0 {
+						joined := strings.Join(vals, "; ")
+						args = append(args, "-metadata", fmt.Sprintf("%s=%s", "album_artist", joined))
+						changes++
+						setKeys["album_artist"] = struct{}{}
+					}
+					continue
+				}
+			}
+			// Default: add repeated metadata entries for formats that support it (e.g., Vorbis)
 			for _, v := range vals {
 				args = append(args, "-metadata", fmt.Sprintf("%s=%s", ffk, v))
 				changes++
@@ -463,6 +518,68 @@ func applyMetadataWithFFmpeg(ctx context.Context, d *data, src string, tags map[
 	return nil
 }
 
+// applyMetadataToDir iterates all immediate files in a directory and applies
+// metadata changes to each supported audio file.
+func applyMetadataToDir(ctx context.Context, d *data, dir string, tags map[string]string, multi map[string][]string, clear []string) error {
+	// List directory entries
+	f, err := d.user.Fs.Open(dir)
+	if err != nil {
+		return err
+	}
+	defer f.Close()
+
+	names, err := f.Readdirnames(0)
+	if err != nil {
+		return err
+	}
+
+	// Supported audio extensions for metadata updates
+	isAudio := func(ext string) bool {
+		ext = strings.ToLower(ext)
+		switch ext {
+		case ".mp3", ".flac", ".m4a", ".mp4", ".ogg", ".wav":
+			return true
+		default:
+			return false
+		}
+	}
+
+	processed := 0
+	failed := 0
+	for _, name := range names {
+		p := filepath.Join(dir, name)
+		info, err := d.user.Fs.Stat(p)
+		if err != nil {
+			// skip entries we can't stat
+			log.Printf("metadata: skip %s: %v", p, err)
+			continue
+		}
+		if info.IsDir() {
+			continue
+		}
+		if !isAudio(filepath.Ext(name)) {
+			continue
+		}
+		if err := applyMetadataWithFFmpeg(ctx, d, p, tags, multi, clear); err != nil {
+			failed++
+			log.Printf("metadata: failed for %s: %v", p, err)
+			continue
+		}
+		processed++
+	}
+
+	if processed == 0 {
+		return fmt.Errorf("no audio files found in album: %w", fberrors.ErrInvalidRequestParams)
+	}
+	if failed > 0 {
+		// Do not fail entire album operation; return success and rely on logs
+		// to indicate which files failed. This avoids a 500 when some files
+		// are temporarily locked or have unsupported formats.
+		return nil
+	}
+	return nil
+}
+
 func checkParent(src, dst string) error {
 	rel, err := filepath.Rel(src, dst)
 	if err != nil {
@@ -613,3 +730,94 @@ var diskUsage = withUser(func(w http.ResponseWriter, r *http.Request, d *data) (
 		Used:  usage.Used,
 	})
 })
+
+// applyMultiValuedTagsWithMetaflac uses metaflac to write tags to FLAC files.
+// Unlike ffmpeg, metaflac properly supports multiple values for the same tag.
+func applyMultiValuedTagsWithMetaflac(ctx context.Context, filepath string, tags map[string]string, multi map[string][]string, clear []string) error {
+	// Normalize tags and multi to proper Vorbis comment names
+	norm := normalizeAndMapToFFmpeg(tags)
+	mm := normalizeMultiToFFmpeg(multi)
+
+	// Build metaflac arguments
+	// First, remove tags that we're going to set (to avoid duplicates)
+	removeArgs := []string{}
+	for k := range norm {
+		upperK := strings.ToUpper(k)
+		removeArgs = append(removeArgs, "--remove-tag="+upperK)
+		// Also handle ALBUM_ARTIST vs "ALBUM ARTIST" variants
+		if upperK == "ALBUM_ARTIST" {
+			removeArgs = append(removeArgs, "--remove-tag=ALBUM ARTIST")
+			removeArgs = append(removeArgs, "--remove-tag=ALBUMARTIST")
+		}
+	}
+	for k := range mm {
+		removeArgs = append(removeArgs, "--remove-tag="+k)
+	}
+	// Also remove cleared tags
+	for _, c := range clear {
+		token := normalizeKey(c)
+		canonical, ok := canonicalMap[token]
+		if !ok {
+			if _, allowed := allowedCanonicals[c]; allowed {
+				canonical = c
+			} else {
+				continue
+			}
+		}
+		// Check multi-valued tags first
+		if ffk, ok := ffmpegMultiKey[canonical]; ok {
+			removeArgs = append(removeArgs, "--remove-tag="+ffk)
+		}
+		// Also check regular tags
+		if ffk, ok := ffmpegKey[canonical]; ok {
+			upperK := strings.ToUpper(ffk)
+			removeArgs = append(removeArgs, "--remove-tag="+upperK)
+			// Handle all variants of album artist
+			if canonical == "AlbumArtist" {
+				removeArgs = append(removeArgs, "--remove-tag=ALBUM ARTIST")
+				removeArgs = append(removeArgs, "--remove-tag=ALBUMARTIST")
+				removeArgs = append(removeArgs, "--remove-tag=ALBUM_ARTIST")
+			}
+			// Handle artist variants (lowercase/uppercase)
+			if canonical == "Artist" {
+				removeArgs = append(removeArgs, "--remove-tag=artist")
+				removeArgs = append(removeArgs, "--remove-tag=ARTIST")
+			}
+		}
+	}
+
+	// Execute removal first if needed
+	if len(removeArgs) > 0 {
+		removeArgs = append(removeArgs, filepath)
+		cmd := exec.CommandContext(ctx, "metaflac", removeArgs...)
+		if out, err := cmd.CombinedOutput(); err != nil {
+			return fmt.Errorf("metaflac remove failed: %w: %s", err, string(out))
+		}
+	}
+
+	// Now set the new tags
+	setArgs := []string{}
+	for k, v := range norm {
+		// Use uppercase for Vorbis comments (standard convention)
+		setArgs = append(setArgs, fmt.Sprintf("--set-tag=%s=%s", strings.ToUpper(k), v))
+	}
+	// Add multi-valued tags (each value as a separate --set-tag)
+	for k, vals := range mm {
+		for _, v := range vals {
+			setArgs = append(setArgs, fmt.Sprintf("--set-tag=%s=%s", k, v))
+		}
+	}
+
+	// If nothing to set, we're done (removal was already executed above)
+	if len(setArgs) == 0 {
+		return nil
+	}
+
+	setArgs = append(setArgs, filepath)
+	cmd := exec.CommandContext(ctx, "metaflac", setArgs...)
+	if out, err := cmd.CombinedOutput(); err != nil {
+		return fmt.Errorf("metaflac set failed: %w: %s", err, string(out))
+	}
+
+	return nil
+}