diff --git a/internal/api/api.go b/internal/api/api.go
index 4a644661..e95a26a1 100644
--- a/internal/api/api.go
+++ b/internal/api/api.go
@@ -65,6 +65,10 @@ func Mount(r chi.Router, pool *pgxpool.Pool, logger *slog.Logger, events *playev
 		// design at
 		// docs/superpowers/specs/2026-06-03-android-output-picker-upnp-design.md.
 		api.With(auth.OptionalUser(pool, logger)).Get("/tracks/{id}/stream", h.handleGetStream)
+		// Extension-bearing alias so Sonos's URL probe can identify the
+		// audio format from the path. The {ext} param is consumed by chi
+		// and ignored by the handler (which keys off {id}). See task #610.
+		api.With(auth.OptionalUser(pool, logger)).Get("/tracks/{id}/stream.{ext}", h.handleGetStream)
 
 		api.Group(func(authed chi.Router) {
 			authed.Use(auth.RequireUser(pool))
diff --git a/internal/api/cast_token.go b/internal/api/cast_token.go
index e638308f..5f3f3046 100644
--- a/internal/api/cast_token.go
+++ b/internal/api/cast_token.go
@@ -57,6 +57,33 @@ func mimeForFormat(format string) string {
 	}
 }
 
+// extForFormat maps the tracks.file_format column to a path-safe file
+// extension. Sonos firmware gates duration probing on the URL path
+// extension (Content-Type header alone is insufficient) -- without a
+// recognizable extension, Sonos reports TrackDuration=0 and seeks
+// trigger auto-advance because every position past 0 looks past-the-
+// end. Defaults to "mp3" for unknown formats. See task #610.
+func extForFormat(format string) string {
+	switch strings.ToLower(strings.TrimSpace(format)) {
+	case "mp3", "mpeg":
+		return "mp3"
+	case "flac":
+		return "flac"
+	case "aac":
+		return "aac"
+	case "m4a", "mp4":
+		return "m4a"
+	case "ogg", "vorbis":
+		return "ogg"
+	case "opus":
+		return "opus"
+	case "wav", "wave":
+		return "wav"
+	default:
+		return "mp3"
+	}
+}
+
 // handleCastStreamToken issues a short-lived HMAC stream token for the
 // given trackId. Authenticated via the standard session cookie / bearer.
 //
@@ -114,8 +141,12 @@ func (h *handlers) handleCastStreamToken(w http.ResponseWriter, r *http.Request)
 	if h := r.Header.Get("X-Forwarded-Host"); h != "" {
 		host = h
 	}
+	// Include the file extension in the path so Sonos's URL probe sees a
+	// recognizable audio file. Without it, Sonos reports TrackDuration=0
+	// and seeks past 0s land "after the end" -> early track-skip.
 	url := scheme + "://" + host + "/api/tracks/" + req.TrackID +
-		"/stream?token=" + token + "&exp=" + strconv.FormatInt(exp, 10)
+		"/stream." + extForFormat(track.FileFormat) +
+		"?token=" + token + "&exp=" + strconv.FormatInt(exp, 10)
 
 	writeJSON(w, http.StatusOK, castTokenResponse{
 		Token: token,