diff --git a/internal/api/cast_token_test.go b/internal/api/cast_token_test.go
index 0687b57d..526f50ed 100644
--- a/internal/api/cast_token_test.go
+++ b/internal/api/cast_token_test.go
@@ -10,15 +10,21 @@ import (
 	"time"
 )
 
-const testTrackUUID = "11111111-1111-1111-1111-111111111111"
+// nonExistentTrackUUID is used by tests that exercise paths which don't
+// require the track to actually exist (auth/UUID-shape rejection).
+const nonExistentTrackUUID = "11111111-1111-1111-1111-111111111111"
 
 func TestCastStreamToken_HappyPath(t *testing.T) {
 	h, pool := testHandlers(t)
 	user := seedUser(t, pool, "alice", "hunter2", false)
+	artist := seedArtist(t, pool, "Artist")
+	album := seedAlbum(t, pool, artist.ID, "Album", 0)
+	track := seedTrack(t, pool, album.ID, artist.ID, "Song", 1, 180_000)
+	trackID := uuidToString(track.ID)
 	h.streamSecret = []byte("cast-token-test-secret")
 
 	body, err := json.Marshal(castTokenRequest{
-		TrackID:    testTrackUUID,
+		TrackID:    trackID,
 		ExpSeconds: 3600,
 	})
 	if err != nil {
@@ -44,10 +50,16 @@ func TestCastStreamToken_HappyPath(t *testing.T) {
 	if !strings.Contains(resp.URL, "token="+resp.Token) {
 		t.Fatalf("URL missing token query: %s", resp.URL)
 	}
-	if !strings.Contains(resp.URL, "/api/tracks/"+testTrackUUID+"/stream") {
+	if !strings.Contains(resp.URL, "/api/tracks/"+trackID+"/stream") {
 		t.Fatalf("URL missing stream path: %s", resp.URL)
 	}
-	if !VerifyStreamToken(h.streamSecret, testTrackUUID, resp.Exp, resp.Token) {
+	// Stream URL must carry a file extension so Sonos's URL probe can
+	// identify the audio format (see task #610). Track seeded above is
+	// .flac via seedTrack's default file_format.
+	if !strings.Contains(resp.URL, "/stream.flac?") {
+		t.Fatalf("URL missing file-extension segment: %s", resp.URL)
+	}
+	if !VerifyStreamToken(h.streamSecret, trackID, resp.Exp, resp.Token) {
 		t.Fatal("returned token does not verify")
 	}
 }
@@ -77,7 +89,7 @@ func TestCastStreamToken_RejectsUnauthenticated(t *testing.T) {
 	h, _ := testHandlers(t)
 	h.streamSecret = []byte("cast-token-test-secret")
 
-	body, err := json.Marshal(castTokenRequest{TrackID: testTrackUUID})
+	body, err := json.Marshal(castTokenRequest{TrackID: nonExistentTrackUUID})
 	if err != nil {
 		t.Fatalf("marshal: %v", err)
 	}
@@ -96,11 +108,15 @@ func TestCastStreamToken_RejectsUnauthenticated(t *testing.T) {
 func TestCastStreamToken_ClampsExpSeconds(t *testing.T) {
 	h, pool := testHandlers(t)
 	user := seedUser(t, pool, "alice", "hunter2", false)
+	artist := seedArtist(t, pool, "Artist")
+	album := seedAlbum(t, pool, artist.ID, "Album", 0)
+	track := seedTrack(t, pool, album.ID, artist.ID, "Song", 1, 180_000)
+	trackID := uuidToString(track.ID)
 	h.streamSecret = []byte("cast-token-test-secret")
 
 	// Request 1 second (below min 60), expect clamp to 60s.
 	body, err := json.Marshal(castTokenRequest{
-		TrackID:    testTrackUUID,
+		TrackID:    trackID,
 		ExpSeconds: 1,
 	})
 	if err != nil {