diff --git a/internal/catalog/detail.go b/internal/catalog/detail.go
index a8ab3458..759e57de 100644
--- a/internal/catalog/detail.go
+++ b/internal/catalog/detail.go
@@ -3453,8 +3453,13 @@ func buildVersionSubtitleTracks(file *models.MediaFile) []VersionSubtitleTrack {
 			FileName:        sub.FileName,
 		})
 	}
-	for _, sub := range file.ExternalSubtitles {
+	for i, sub := range file.ExternalSubtitles {
 		tracks = append(tracks, VersionSubtitleTrack{
+			// Combined-index identity: externals occupy 0..n-1 in the playback
+			// selection space (session subtitle_urls, ResolveSubtitlePolicyV3).
+			// Without this every external serialized index 0 and clients keying
+			// on index got duplicates.
+			Index:           i,
 			Language:        sub.Language,
 			Codec:           sub.Format,
 			Title:           firstNonEmpty(sub.Title, filepath.Base(sub.Path)),
diff --git a/internal/catalog/detail_subtitle_tracks_test.go b/internal/catalog/detail_subtitle_tracks_test.go
new file mode 100644
index 00000000..5ddc2c21
--- /dev/null
+++ b/internal/catalog/detail_subtitle_tracks_test.go
@@ -0,0 +1,44 @@
+package catalog
+
+import (
+	"testing"
+
+	"github.com/Silo-Server/silo-server/internal/models"
+)
+
+// External subtitles must carry their combined-index identity (externals
+// occupy 0..n-1 in the playback selection space; see
+// ResolveSubtitlePolicyV3 / session subtitle_urls). Before this test they all
+// serialized the zero value, so any file with two external subs — or one
+// external plus an embedded stream index 0 — published duplicate indexes and
+// clients keying rows on index crashed or selected the wrong track.
+func TestBuildVersionSubtitleTracksAssignsUniqueExternalIndexes(t *testing.T) {
+	file := &models.MediaFile{
+		SubtitleTracks: []models.SubtitleTrack{
+			{Index: 2, Codec: "subrip", Language: "en"},
+			{Index: 3, Codec: "hdmv_pgs_subtitle", Language: "de"},
+		},
+		ExternalSubtitles: []models.ExternalSubtitle{
+			{Path: "/media/movie.en.srt", Format: "srt", Language: "en"},
+			{Path: "/media/movie.nl.srt", Format: "srt", Language: "nl"},
+		},
+	}
+
+	tracks := buildVersionSubtitleTracks(file)
+	if len(tracks) != 4 {
+		t.Fatalf("tracks len = %d, want 4", len(tracks))
+	}
+
+	// Embedded entries keep their ffprobe stream indexes (existing contract).
+	if tracks[0].Index != 2 || tracks[1].Index != 3 {
+		t.Fatalf("embedded indexes = %d,%d, want 2,3", tracks[0].Index, tracks[1].Index)
+	}
+
+	// Externals carry their combined-space ordinal, not the zero value.
+	if tracks[2].Index != 0 || !tracks[2].External {
+		t.Fatalf("first external = index %d external %v, want 0/true", tracks[2].Index, tracks[2].External)
+	}
+	if tracks[3].Index != 1 || !tracks[3].External {
+		t.Fatalf("second external = index %d external %v, want 1/true", tracks[3].Index, tracks[3].External)
+	}
+}