diff --git a/backend/Core/Musify.Application/Albums/Responses/AlbumApplicationResponse.cs b/backend/Core/Musify.Application/Albums/Responses/AlbumApplicationResponse.cs
index c1e6948..0323ded 100644
--- a/backend/Core/Musify.Application/Albums/Responses/AlbumApplicationResponse.cs
+++ b/backend/Core/Musify.Application/Albums/Responses/AlbumApplicationResponse.cs
@@ -1,7 +1,17 @@
+using System.Text.Json.Serialization;
 using Musify.Domain.Entities;
 
 namespace Musify.Application.Albums.Responses
 {
+    // [JsonPolymorphic]/[JsonDerivedType] are required here, not optional: without them
+    // System.Text.Json serializes strictly by the declared CLR type at each call site
+    // (AlbumApplicationResponse, both as a bare return type and nested inside
+    // AlbumSearchItemResponse.Album), so an ExternalAlbumApplicationResponse instance would
+    // silently lose ExternalId/ThumbnailUrl on the wire. This also gives every response a
+    // "source" discriminator field for free, matching AlbumSearchItemResponse.Source.
+    [JsonPolymorphic(TypeDiscriminatorPropertyName = "source")]
+    [JsonDerivedType(typeof(AlbumApplicationResponse), typeDiscriminator: "Local")]
+    [JsonDerivedType(typeof(ExternalAlbumApplicationResponse), typeDiscriminator: "YouTube")]
     public record AlbumApplicationResponse(
         Guid Id,
         string Title,
@@ -14,9 +24,7 @@ namespace Musify.Application.Albums.Responses
         string? LargeImageKeyName,
         DateTime CreatedAt,
         DateTime UpdatedAt,
-        IReadOnlyList<Guid> CoverTrackIds,
-        string? YouTubeAlbumId = null,
-        string? ThumbnailUrl = null)
+        IReadOnlyList<Guid> CoverTrackIds)
     {
         public const int CoverTrackCount = 4;
 
@@ -45,22 +53,12 @@ namespace Musify.Application.Albums.Responses
             int trackCount,
             IReadOnlyList<Guid>? coverTrackIds = null)
         {
-            var externalAlbum = album as ExternalAlbum;
-            return new AlbumApplicationResponse(
-                album.Id,
-                album.Title,
-                album.Description,
-                album.ReleaseYear,
-                album is UserAlbum userAlbum ? userAlbum.OwnerUserId : 0,
-                trackCount,
-                album.Pictures?.SmallName,
-                album.Pictures?.MediumName,
-                album.Pictures?.LargeName,
-                album.CreatedAt,
-                album.UpdatedAt,
-                coverTrackIds ?? [],
-                externalAlbum?.ExternalId,
-                externalAlbum?.ThumbnailUrl);
+            return album switch
+            {
+                ExternalAlbum externalAlbum => ExternalAlbumApplicationResponse.FromEntity(externalAlbum, trackCount, coverTrackIds),
+                UserAlbum userAlbum => FromEntity(userAlbum, trackCount, coverTrackIds),
+                _ => throw new NotSupportedException($"Unsupported album entity type '{album.GetType().Name}'.")
+            };
         }
     }
 }
diff --git a/backend/Core/Musify.Application/Albums/Responses/AlbumSearchItemResponse.cs b/backend/Core/Musify.Application/Albums/Responses/AlbumSearchItemResponse.cs
index 5789edd..40acd5a 100644
--- a/backend/Core/Musify.Application/Albums/Responses/AlbumSearchItemResponse.cs
+++ b/backend/Core/Musify.Application/Albums/Responses/AlbumSearchItemResponse.cs
@@ -14,7 +14,7 @@ namespace Musify.Application.Albums.Responses
         YouTubeAlbumResult? YouTubeAlbum)
     {
         public static AlbumSearchItemResponse FromAlbum(AlbumApplicationResponse album) =>
-            new(album.YouTubeAlbumId is not null ? TrackSource.YouTube : TrackSource.Local, album, null);
+            new(album is ExternalAlbumApplicationResponse ? TrackSource.YouTube : TrackSource.Local, album, null);
 
         public static AlbumSearchItemResponse FromYouTubeAlbum(YouTubeAlbumResult album) =>
             new(TrackSource.YouTube, null, album);
diff --git a/backend/Core/Musify.Application/Albums/Responses/ExternalAlbumApplicationResponse.cs b/backend/Core/Musify.Application/Albums/Responses/ExternalAlbumApplicationResponse.cs
new file mode 100644
index 0000000..a9f12a3
--- /dev/null
+++ b/backend/Core/Musify.Application/Albums/Responses/ExternalAlbumApplicationResponse.cs
@@ -0,0 +1,47 @@
+using Musify.Domain.Entities;
+
+namespace Musify.Application.Albums.Responses
+{
+    public record ExternalAlbumApplicationResponse(
+        Guid Id,
+        string Title,
+        string? Description,
+        int? ReleaseYear,
+        long OwnerUserId,
+        int TrackCount,
+        string? SmallImageKeyName,
+        string? MediumImageKeyName,
+        string? LargeImageKeyName,
+        DateTime CreatedAt,
+        DateTime UpdatedAt,
+        IReadOnlyList<Guid> CoverTrackIds,
+        string ExternalId,
+        string? ThumbnailUrl)
+        : AlbumApplicationResponse(
+            Id, Title, Description, ReleaseYear, OwnerUserId, TrackCount,
+            SmallImageKeyName, MediumImageKeyName, LargeImageKeyName,
+            CreatedAt, UpdatedAt, CoverTrackIds)
+    {
+        public static ExternalAlbumApplicationResponse FromEntity(
+            ExternalAlbum album,
+            int trackCount,
+            IReadOnlyList<Guid>? coverTrackIds = null)
+        {
+            return new ExternalAlbumApplicationResponse(
+                album.Id,
+                album.Title,
+                album.Description,
+                album.ReleaseYear,
+                0,
+                trackCount,
+                album.Pictures?.SmallName,
+                album.Pictures?.MediumName,
+                album.Pictures?.LargeName,
+                album.CreatedAt,
+                album.UpdatedAt,
+                coverTrackIds ?? [],
+                album.ExternalId,
+                album.ThumbnailUrl);
+        }
+    }
+}
diff --git a/web-player/src/routes/+page.svelte b/web-player/src/routes/+page.svelte
index 8b3cbc1..b59f9e9 100644
--- a/web-player/src/routes/+page.svelte
+++ b/web-player/src/routes/+page.svelte
@@ -155,7 +155,9 @@
 	></div>
 	<div class="animate-enter relative flex flex-col gap-2 sm:gap-2.5">
 		<p class="text-sm tracking-[0.14em] text-fg-2 uppercase sm:text-base">{data.greeting}</p>
-		<h1 class="font-display text-3xl leading-[1.05] font-semibold tracking-tight text-fg sm:text-6xl">
+		<h1
+			class="font-display text-3xl leading-[1.05] font-semibold tracking-tight text-fg sm:text-6xl"
+		>
 			Tu música. Sin límites.
 		</h1>
 	</div>
@@ -214,15 +216,15 @@
 			<Rail>
 				{#each albums as album, i (album.id)}
 					<a
-						href={album.youTubeAlbumId
-							? `/albums/external/youtube/${album.youTubeAlbumId}`
+						href={album.source === 'YouTube'
+							? `/albums/external/youtube/${album.externalId}`
 							: `/albums/${album.id}`}
 						class="group/card animate-enter block w-42 shrink-0 rounded-art focus-visible:ring-2 focus-visible:ring-accent focus-visible:ring-offset-2 focus-visible:ring-offset-bg focus-visible:outline-none"
 						style="animation-delay:{Math.min(i, 10) * 45}ms"
 						oncontextmenu={(e) => openAlbumMenu(e, album.id)}
 					>
 						<div class="relative">
-							{#if album.thumbnailUrl}
+							{#if album.source === 'YouTube' && album.thumbnailUrl}
 								<Cover
 									trackId={album.id}
 									src={album.thumbnailUrl}