diff --git a/cmd/minstrel/main.go b/cmd/minstrel/main.go
index 04cfe6ff..285f00ff 100644
--- a/cmd/minstrel/main.go
+++ b/cmd/minstrel/main.go
@@ -245,6 +245,7 @@ func run() error {
 	}, cfg.Events, cfg.Recommendation, cfg.Storage.DataDir, cfg.Branding, coverEnricher, coverSettings, scanner, scanCfg, scheduler)
 	srv.Bus = bus
 	srv.PlaylistScheduler = playlistScheduler
+	srv.StreamSecret = cfg.StreamSecret
 	httpServer := &http.Server{
 		Addr:              cfg.Server.Address,
 		Handler:           srv.Router(),
diff --git a/internal/api/api.go b/internal/api/api.go
index 59bef6a8..4a644661 100644
--- a/internal/api/api.go
+++ b/internal/api/api.go
@@ -28,7 +28,7 @@ import (
 // Mount attaches /api/* handlers to r. Public endpoints (login) are outside
 // RequireUser; everything else is gated by the middleware. The events writer
 // is shared with the Subsonic mount so /rest/scrobble feeds the same store.
-func Mount(r chi.Router, pool *pgxpool.Pool, logger *slog.Logger, events *playevents.Writer, recCfg config.RecommendationConfig, lidarrCfg *lidarrconfig.Service, lidarrReqs *lidarrrequests.Service, lidarrQuar *lidarrquarantine.Service, tracksSvc *tracks.Service, playlistsSvc *playlists.Service, coverEnricher *coverart.Enricher, coverSettings *coverart.SettingsService, scanner *library.Scanner, scanCfg library.RunScanConfig, scheduler *library.Scheduler, dataDir string, sender mailer.Sender, bus *eventbus.Bus, playlistScheduler *playlists.Scheduler) {
+func Mount(r chi.Router, pool *pgxpool.Pool, logger *slog.Logger, events *playevents.Writer, recCfg config.RecommendationConfig, lidarrCfg *lidarrconfig.Service, lidarrReqs *lidarrrequests.Service, lidarrQuar *lidarrquarantine.Service, tracksSvc *tracks.Service, playlistsSvc *playlists.Service, coverEnricher *coverart.Enricher, coverSettings *coverart.SettingsService, scanner *library.Scanner, scanCfg library.RunScanConfig, scheduler *library.Scheduler, dataDir string, sender mailer.Sender, bus *eventbus.Bus, playlistScheduler *playlists.Scheduler, streamSecret []byte) {
 	rng := rand.New(rand.NewSource(rand.Int63()))
 	h := &handlers{
 		pool: pool, logger: logger, events: events, recCfg: recCfg,
@@ -47,6 +47,7 @@ func Mount(r chi.Router, pool *pgxpool.Pool, logger *slog.Logger, events *playev
 		mailer:            sender,
 		eventbus:          bus,
 		playlistScheduler: playlistScheduler,
+		streamSecret:      streamSecret,
 	}
 
 	r.Route("/api", func(api chi.Router) {
@@ -97,6 +98,11 @@ func Mount(r chi.Router, pool *pgxpool.Pool, logger *slog.Logger, events *playev
 			authed.Get("/home/index", h.handleGetHomeIndex)
 			authed.Post("/events", h.handleEvents)
 			authed.Get("/events/stream", h.handleEventsStream)
+			// UPnP / Sonos cast slice: issue a short-lived HMAC stream URL
+			// the speaker can fetch without the user's session. See the
+			// design at
+			// docs/superpowers/specs/2026-06-03-android-output-picker-upnp-design.md.
+			authed.Post("/cast/stream-token", h.handleCastStreamToken)
 			authed.Post("/likes/tracks/{id}", h.handleLikeTrack)
 			authed.Delete("/likes/tracks/{id}", h.handleUnlikeTrack)
 			authed.Post("/likes/albums/{id}", h.handleLikeAlbum)
diff --git a/internal/api/cast_token.go b/internal/api/cast_token.go
new file mode 100644
index 00000000..5fdf14f8
--- /dev/null
+++ b/internal/api/cast_token.go
@@ -0,0 +1,83 @@
+package api
+
+import (
+	"net/http"
+	"strconv"
+	"time"
+
+	"git.fabledsword.com/bvandeusen/minstrel/internal/apierror"
+)
+
+const (
+	castTokenMinExpSeconds = 60
+	castTokenMaxExpSeconds = 86400 // 24h
+	castTokenDefaultExp    = 21600 // 6h
+)
+
+type castTokenRequest struct {
+	TrackID    string `json:"trackId"`
+	ExpSeconds int    `json:"expSeconds,omitempty"`
+}
+
+type castTokenResponse struct {
+	Token string `json:"token"`
+	Exp   int64  `json:"exp"`
+	URL   string `json:"url"`
+}
+
+// handleCastStreamToken issues a short-lived HMAC stream token for the
+// given trackId. Authenticated via the standard session cookie / bearer.
+//
+// The returned URL is a fully-formed stream URL (token + exp embedded
+// as query params) that the client passes verbatim to a UPnP / Sonos
+// device's AVTransport.SetAVTransportURI call — those devices cannot
+// carry the user's session, so the signed query string is the only way
+// they can fetch the bytes.
+//
+// expSeconds is clamped to [60, 86400]; default 21600 (6h) — long enough
+// to play through any typical track without re-minting mid-playback.
+//
+// Part of the output-picker UPnP slice. See
+// docs/superpowers/specs/2026-06-03-android-output-picker-upnp-design.md.
+func (h *handlers) handleCastStreamToken(w http.ResponseWriter, r *http.Request) {
+	if _, ok := requireUser(w, r); !ok {
+		return
+	}
+	var req castTokenRequest
+	if !decodeBody(w, r, &req) {
+		return
+	}
+	trackUUID, ok := parseUUID(req.TrackID)
+	if !ok || !trackUUID.Valid {
+		writeErr(w, apierror.BadRequest("invalid_track_id", "trackId must be a UUID"))
+		return
+	}
+	expSec := clampExpSeconds(req.ExpSeconds)
+	exp := time.Now().Add(time.Duration(expSec) * time.Second).Unix()
+	token := SignStreamToken(h.streamSecret, req.TrackID, exp)
+
+	scheme := "http"
+	if r.TLS != nil {
+		scheme = "https"
+	}
+	url := scheme + "://" + r.Host + "/api/tracks/" + req.TrackID +
+		"/stream?token=" + token + "&exp=" + strconv.FormatInt(exp, 10)
+
+	writeJSON(w, http.StatusOK, castTokenResponse{Token: token, Exp: exp, URL: url})
+}
+
+// clampExpSeconds applies the [60, 86400] window with a 6h default for
+// non-positive inputs. Extracted so it doesn't bloat the handler's
+// detekt-equivalent line count and so the test can exercise edges directly.
+func clampExpSeconds(v int) int {
+	if v <= 0 {
+		return castTokenDefaultExp
+	}
+	if v < castTokenMinExpSeconds {
+		return castTokenMinExpSeconds
+	}
+	if v > castTokenMaxExpSeconds {
+		return castTokenMaxExpSeconds
+	}
+	return v
+}
diff --git a/internal/api/cast_token_test.go b/internal/api/cast_token_test.go
new file mode 100644
index 00000000..0687b57d
--- /dev/null
+++ b/internal/api/cast_token_test.go
@@ -0,0 +1,152 @@
+package api
+
+import (
+	"bytes"
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+	"time"
+)
+
+const testTrackUUID = "11111111-1111-1111-1111-111111111111"
+
+func TestCastStreamToken_HappyPath(t *testing.T) {
+	h, pool := testHandlers(t)
+	user := seedUser(t, pool, "alice", "hunter2", false)
+	h.streamSecret = []byte("cast-token-test-secret")
+
+	body, err := json.Marshal(castTokenRequest{
+		TrackID:    testTrackUUID,
+		ExpSeconds: 3600,
+	})
+	if err != nil {
+		t.Fatalf("marshal: %v", err)
+	}
+	req := httptest.NewRequest(http.MethodPost, "/api/cast/stream-token", bytes.NewReader(body))
+	req.Header.Set("Content-Type", "application/json")
+	req = withUser(req, user)
+	w := httptest.NewRecorder()
+
+	h.handleCastStreamToken(w, req)
+
+	if w.Code != http.StatusOK {
+		t.Fatalf("status = %d, want 200; body=%s", w.Code, w.Body.String())
+	}
+	var resp castTokenResponse
+	if err := json.NewDecoder(w.Body).Decode(&resp); err != nil {
+		t.Fatalf("decode: %v", err)
+	}
+	if resp.Token == "" || resp.Exp == 0 || resp.URL == "" {
+		t.Fatalf("empty fields in response: %+v", resp)
+	}
+	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") {
+		t.Fatalf("URL missing stream path: %s", resp.URL)
+	}
+	if !VerifyStreamToken(h.streamSecret, testTrackUUID, resp.Exp, resp.Token) {
+		t.Fatal("returned token does not verify")
+	}
+}
+
+func TestCastStreamToken_RejectsBadUUID(t *testing.T) {
+	h, pool := testHandlers(t)
+	user := seedUser(t, pool, "alice", "hunter2", false)
+	h.streamSecret = []byte("cast-token-test-secret")
+
+	body, err := json.Marshal(castTokenRequest{TrackID: "not-a-uuid"})
+	if err != nil {
+		t.Fatalf("marshal: %v", err)
+	}
+	req := httptest.NewRequest(http.MethodPost, "/api/cast/stream-token", bytes.NewReader(body))
+	req.Header.Set("Content-Type", "application/json")
+	req = withUser(req, user)
+	w := httptest.NewRecorder()
+
+	h.handleCastStreamToken(w, req)
+
+	if w.Code != http.StatusBadRequest {
+		t.Fatalf("status = %d, want 400; body=%s", w.Code, w.Body.String())
+	}
+}
+
+func TestCastStreamToken_RejectsUnauthenticated(t *testing.T) {
+	h, _ := testHandlers(t)
+	h.streamSecret = []byte("cast-token-test-secret")
+
+	body, err := json.Marshal(castTokenRequest{TrackID: testTrackUUID})
+	if err != nil {
+		t.Fatalf("marshal: %v", err)
+	}
+	req := httptest.NewRequest(http.MethodPost, "/api/cast/stream-token", bytes.NewReader(body))
+	req.Header.Set("Content-Type", "application/json")
+	// NO withUser — handler must reject via requireUser.
+	w := httptest.NewRecorder()
+
+	h.handleCastStreamToken(w, req)
+
+	if w.Code != http.StatusUnauthorized {
+		t.Fatalf("status = %d, want 401; body=%s", w.Code, w.Body.String())
+	}
+}
+
+func TestCastStreamToken_ClampsExpSeconds(t *testing.T) {
+	h, pool := testHandlers(t)
+	user := seedUser(t, pool, "alice", "hunter2", false)
+	h.streamSecret = []byte("cast-token-test-secret")
+
+	// Request 1 second (below min 60), expect clamp to 60s.
+	body, err := json.Marshal(castTokenRequest{
+		TrackID:    testTrackUUID,
+		ExpSeconds: 1,
+	})
+	if err != nil {
+		t.Fatalf("marshal: %v", err)
+	}
+	before := time.Now().Unix()
+	req := httptest.NewRequest(http.MethodPost, "/api/cast/stream-token", bytes.NewReader(body))
+	req.Header.Set("Content-Type", "application/json")
+	req = withUser(req, user)
+	w := httptest.NewRecorder()
+
+	h.handleCastStreamToken(w, req)
+
+	if w.Code != http.StatusOK {
+		t.Fatalf("status = %d, want 200; body=%s", w.Code, w.Body.String())
+	}
+	var resp castTokenResponse
+	if err := json.NewDecoder(w.Body).Decode(&resp); err != nil {
+		t.Fatalf("decode: %v", err)
+	}
+	// Allow a small jitter window around the clamp target (60s).
+	delta := resp.Exp - before
+	if delta < 55 || delta > 70 {
+		t.Fatalf("expected ~60s expiry after clamp, got delta=%ds", delta)
+	}
+}
+
+func TestClampExpSeconds(t *testing.T) {
+	cases := []struct {
+		name string
+		in   int
+		want int
+	}{
+		{"zero defaults to 6h", 0, castTokenDefaultExp},
+		{"negative defaults to 6h", -1, castTokenDefaultExp},
+		{"below min clamps up", 30, castTokenMinExpSeconds},
+		{"exact min passes through", castTokenMinExpSeconds, castTokenMinExpSeconds},
+		{"mid-range passes through", 3600, 3600},
+		{"exact max passes through", castTokenMaxExpSeconds, castTokenMaxExpSeconds},
+		{"above max clamps down", castTokenMaxExpSeconds + 1, castTokenMaxExpSeconds},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			if got := clampExpSeconds(tc.in); got != tc.want {
+				t.Fatalf("clampExpSeconds(%d) = %d, want %d", tc.in, got, tc.want)
+			}
+		})
+	}
+}
diff --git a/internal/config/config.go b/internal/config/config.go
index 986dad6d..e28d4432 100644
--- a/internal/config/config.go
+++ b/internal/config/config.go
@@ -1,8 +1,12 @@
 package config
 
 import (
+	"crypto/rand"
+	"encoding/base64"
 	"fmt"
+	"log"
 	"os"
+	"path/filepath"
 	"strconv"
 	"strings"
 
@@ -19,6 +23,14 @@ type Config struct {
 	Events         EventsConfig         `yaml:"events"`
 	Recommendation RecommendationConfig `yaml:"recommendation"`
 	Branding       BrandingConfig       `yaml:"branding"`
+	// StreamSecret is the base64-decoded HMAC key used to sign UPnP /
+	// Sonos stream URLs (see internal/api/stream_token.go and
+	// docs/superpowers/specs/2026-06-03-android-output-picker-upnp-design.md).
+	// Sourced from MINSTREL_STREAM_SECRET (base64-encoded), with a
+	// per-machine fallback persisted at <Storage.DataDir>/stream_secret.
+	// Not serialized to YAML — operator-machine-scoped runtime state, not
+	// a user-facing setting. Never log the contents.
+	StreamSecret []byte `yaml:"-"`
 }
 
 type ServerConfig struct {
@@ -137,9 +149,83 @@ func Load(path string) (Config, error) {
 		}
 	}
 	applyEnv(&cfg)
+	if err := resolveStreamSecret(&cfg); err != nil {
+		return cfg, err
+	}
 	return cfg, nil
 }
 
+// streamSecretBytes is the raw HMAC-key length; 64 bytes gives 512 bits
+// of entropy, more than enough for HMAC-SHA256, and lands at a 86-char
+// base64-url-no-padding string that fits cleanly in a single .env line.
+const streamSecretBytes = 64
+
+// streamSecretFile is the basename for the per-machine persisted fallback
+// under <Storage.DataDir>. Kept as a constant so tests assert against the
+// exact path and operators can find it during backup planning.
+const streamSecretFile = "stream_secret"
+
+// resolveStreamSecret populates cfg.StreamSecret using, in order:
+//  1. MINSTREL_STREAM_SECRET env var (operator-supplied, base64-url
+//     encoded — accepts either padded or raw form).
+//  2. <Storage.DataDir>/stream_secret if present (auto-generated on a
+//     previous boot; survives restarts so signed URLs stay valid).
+//  3. Auto-generate streamSecretBytes random bytes, persist them to that
+//     file at 0600, then use them.
+//
+// Logs (only) when an auto-generation happens so the operator can spot
+// it during first-boot. Never logs the contents.
+//
+// The loader runs before slog is initialized in cmd/minstrel/main.go;
+// log.Printf is the project's pre-logger convention.
+func resolveStreamSecret(cfg *Config) error {
+	if raw := strings.TrimSpace(os.Getenv("MINSTREL_STREAM_SECRET")); raw != "" {
+		decoded, err := decodeBase64Secret(raw)
+		if err != nil {
+			return fmt.Errorf("decode MINSTREL_STREAM_SECRET: %w", err)
+		}
+		cfg.StreamSecret = decoded
+		return nil
+	}
+	if cfg.Storage.DataDir == "" {
+		return fmt.Errorf("stream secret: empty storage.data_dir; " +
+			"set MINSTREL_STREAM_SECRET or storage.data_dir")
+	}
+	path := filepath.Join(cfg.Storage.DataDir, streamSecretFile)
+	if buf, err := os.ReadFile(path); err == nil && len(buf) > 0 {
+		decoded, err := decodeBase64Secret(strings.TrimSpace(string(buf)))
+		if err != nil {
+			return fmt.Errorf("decode %s: %w", path, err)
+		}
+		cfg.StreamSecret = decoded
+		return nil
+	}
+	raw := make([]byte, streamSecretBytes)
+	if _, err := rand.Read(raw); err != nil {
+		return fmt.Errorf("auto-gen stream secret: %w", err)
+	}
+	encoded := base64.RawURLEncoding.EncodeToString(raw)
+	if err := os.MkdirAll(cfg.Storage.DataDir, 0o755); err != nil {
+		return fmt.Errorf("ensure data_dir for stream secret: %w", err)
+	}
+	if err := os.WriteFile(path, []byte(encoded), 0o600); err != nil {
+		return fmt.Errorf("persist stream secret: %w", err)
+	}
+	log.Printf("config: auto-generated MINSTREL_STREAM_SECRET (persisted to %s)", path)
+	cfg.StreamSecret = raw
+	return nil
+}
+
+// decodeBase64Secret accepts base64-url with OR without padding; the
+// stream-secret bytes are opaque so callers shouldn't have to know which
+// encoder produced their string. Returns an error on any other shape.
+func decodeBase64Secret(s string) ([]byte, error) {
+	if buf, err := base64.RawURLEncoding.DecodeString(s); err == nil {
+		return buf, nil
+	}
+	return base64.URLEncoding.DecodeString(s)
+}
+
 func applyEnv(cfg *Config) {
 	if v, ok := os.LookupEnv("MINSTREL_SERVER_ADDRESS"); ok {
 		cfg.Server.Address = v
diff --git a/internal/config/config_test.go b/internal/config/config_test.go
index 7768f3c9..06afe2fa 100644
--- a/internal/config/config_test.go
+++ b/internal/config/config_test.go
@@ -1,11 +1,23 @@
 package config
 
 import (
+	"encoding/base64"
 	"os"
 	"path/filepath"
+	"strings"
 	"testing"
 )
 
+// stubStreamSecret keeps existing tests focused on the fields they
+// assert against by short-circuiting the auto-generation path (which
+// would otherwise write ./data/stream_secret in the test workspace).
+// New tests that exercise the resolver directly do NOT use this.
+func stubStreamSecret(t *testing.T) {
+	t.Helper()
+	t.Setenv("MINSTREL_STREAM_SECRET",
+		base64.RawURLEncoding.EncodeToString([]byte("test-stream-secret-stub-1234567890")))
+}
+
 func TestDefault(t *testing.T) {
 	cfg := Default()
 	if cfg.Server.Address != ":4533" {
@@ -17,6 +29,7 @@ func TestDefault(t *testing.T) {
 }
 
 func TestLoadYAML(t *testing.T) {
+	stubStreamSecret(t)
 	dir := t.TempDir()
 	path := filepath.Join(dir, "config.yaml")
 	content := []byte(`server:
@@ -46,6 +59,7 @@ log:
 }
 
 func TestLoadMissingFileReturnsDefaults(t *testing.T) {
+	stubStreamSecret(t)
 	cfg, err := Load(filepath.Join(t.TempDir(), "does-not-exist.yaml"))
 	if err != nil {
 		t.Fatalf("Load: %v", err)
@@ -56,6 +70,7 @@ func TestLoadMissingFileReturnsDefaults(t *testing.T) {
 }
 
 func TestEnvOverrides(t *testing.T) {
+	stubStreamSecret(t)
 	t.Setenv("MINSTREL_SERVER_ADDRESS", ":8080")
 	t.Setenv("MINSTREL_DATABASE_URL", "postgres://env")
 	t.Setenv("MINSTREL_LOG_LEVEL", "WARN")
@@ -80,6 +95,7 @@ func TestEnvOverrides(t *testing.T) {
 }
 
 func TestLibraryEnvOverrides(t *testing.T) {
+	stubStreamSecret(t)
 	t.Setenv("MINSTREL_LIBRARY_SCAN_PATHS", "/music:/other::/third")
 	t.Setenv("MINSTREL_LIBRARY_SCAN_ON_STARTUP", "true")
 
@@ -102,6 +118,7 @@ func TestLibraryEnvOverrides(t *testing.T) {
 }
 
 func TestSubsonicEnvOverride(t *testing.T) {
+	stubStreamSecret(t)
 	t.Setenv("MINSTREL_SUBSONIC_ALLOW_PLAINTEXT_PASSWORD", "true")
 	cfg, err := Load("")
 	if err != nil {
@@ -113,6 +130,7 @@ func TestSubsonicEnvOverride(t *testing.T) {
 }
 
 func TestLibraryYAMLLoads(t *testing.T) {
+	stubStreamSecret(t)
 	dir := t.TempDir()
 	path := filepath.Join(dir, "config.yaml")
 	content := []byte(`library:
@@ -147,6 +165,7 @@ func TestBrandingDefaults(t *testing.T) {
 }
 
 func TestBrandingYAMLOverride(t *testing.T) {
+	stubStreamSecret(t)
 	dir := t.TempDir()
 	path := filepath.Join(dir, "config.yaml")
 	content := []byte(`branding:
@@ -168,7 +187,81 @@ func TestBrandingYAMLOverride(t *testing.T) {
 	}
 }
 
+func TestStreamSecret_EnvOverride(t *testing.T) {
+	want := []byte("hello-stream-secret-from-env-32b")
+	t.Setenv("MINSTREL_STREAM_SECRET", base64.RawURLEncoding.EncodeToString(want))
+	t.Setenv("MINSTREL_STORAGE_DATA_DIR", t.TempDir())
+	cfg, err := Load("")
+	if err != nil {
+		t.Fatalf("Load: %v", err)
+	}
+	if string(cfg.StreamSecret) != string(want) {
+		t.Fatalf("StreamSecret mismatch: got %q want %q", cfg.StreamSecret, want)
+	}
+}
+
+func TestStreamSecret_AutoGenPersistsToDataDir(t *testing.T) {
+	os.Unsetenv("MINSTREL_STREAM_SECRET")
+	dataDir := t.TempDir()
+	t.Setenv("MINSTREL_STORAGE_DATA_DIR", dataDir)
+
+	cfg, err := Load("")
+	if err != nil {
+		t.Fatalf("Load: %v", err)
+	}
+	if len(cfg.StreamSecret) != streamSecretBytes {
+		t.Fatalf("StreamSecret len = %d, want %d", len(cfg.StreamSecret), streamSecretBytes)
+	}
+	path := filepath.Join(dataDir, streamSecretFile)
+	buf, err := os.ReadFile(path)
+	if err != nil {
+		t.Fatalf("expected persisted secret at %s: %v", path, err)
+	}
+	decoded, err := base64.RawURLEncoding.DecodeString(strings.TrimSpace(string(buf)))
+	if err != nil {
+		t.Fatalf("persisted secret is not raw-url-base64: %v", err)
+	}
+	if string(decoded) != string(cfg.StreamSecret) {
+		t.Fatal("persisted file does not match returned secret")
+	}
+	info, err := os.Stat(path)
+	if err != nil {
+		t.Fatalf("stat: %v", err)
+	}
+	// 0600 — never let other users read the HMAC key.
+	if perm := info.Mode().Perm(); perm != 0o600 {
+		t.Fatalf("perm = %o, want 0600", perm)
+	}
+}
+
+func TestStreamSecret_LoadsPersistedOnSecondBoot(t *testing.T) {
+	os.Unsetenv("MINSTREL_STREAM_SECRET")
+	dataDir := t.TempDir()
+	t.Setenv("MINSTREL_STORAGE_DATA_DIR", dataDir)
+
+	first, err := Load("")
+	if err != nil {
+		t.Fatalf("Load #1: %v", err)
+	}
+	second, err := Load("")
+	if err != nil {
+		t.Fatalf("Load #2: %v", err)
+	}
+	if string(first.StreamSecret) != string(second.StreamSecret) {
+		t.Fatal("second Load got a different secret — file fallback didn't fire")
+	}
+}
+
+func TestStreamSecret_RejectsMalformedEnvValue(t *testing.T) {
+	t.Setenv("MINSTREL_STREAM_SECRET", "not!base64!!!")
+	t.Setenv("MINSTREL_STORAGE_DATA_DIR", t.TempDir())
+	if _, err := Load(""); err == nil {
+		t.Fatal("expected error on malformed MINSTREL_STREAM_SECRET")
+	}
+}
+
 func TestBrandingEnvOverride(t *testing.T) {
+	stubStreamSecret(t)
 	t.Setenv("MINSTREL_BRANDING_APP_NAME", "Office Music")
 	t.Setenv("MINSTREL_BRANDING_DESCRIPTION", "Office tunes.")
 	cfg, err := Load("")
diff --git a/internal/server/server.go b/internal/server/server.go
index cf381871..b402d2af 100644
--- a/internal/server/server.go
+++ b/internal/server/server.go
@@ -89,6 +89,12 @@ type Server struct {
 	// PUT /api/me/timezone and POST /api/auth/register can call
 	// Refresh synchronously.
 	PlaylistScheduler *playlists.Scheduler
+	// StreamSecret is the HMAC key used by /api/cast/stream-token to
+	// mint signed UPnP / Sonos stream URLs and by /api/tracks/{id}/stream
+	// to verify them. Sourced from config.Config.StreamSecret. Tests that
+	// leave it nil leave the cookie path intact and reject all signed
+	// tokens (HMAC of empty key matches nothing a client could mint).
+	StreamSecret []byte
 }
 
 func New(logger *slog.Logger, pool *pgxpool.Pool, scanner ScanTrigger, subCfg subsonic.Config, eventsCfg config.EventsConfig, recCfg config.RecommendationConfig, dataDir string, brandingCfg config.BrandingConfig, coverEnricher *coverart.Enricher, coverSettings *coverart.SettingsService, libraryScanner *library.Scanner, scanCfg library.RunScanConfig, scheduler *library.Scheduler) *Server {
@@ -138,7 +144,7 @@ func (s *Server) Router() http.Handler {
 		if bus == nil {
 			bus = eventbus.New()
 		}
-		api.Mount(r, s.Pool, s.Logger, writer, s.RecommendationCfg, lidarrCfg, lidarrReqs, lidarrQuar, tracksSvc, playlistsSvc, s.CoverEnricher, s.CoverSettings, s.LibraryScanner, s.ScanCfg, s.Scheduler, s.DataDir, smtpSender, bus, s.PlaylistScheduler)
+		api.Mount(r, s.Pool, s.Logger, writer, s.RecommendationCfg, lidarrCfg, lidarrReqs, lidarrQuar, tracksSvc, playlistsSvc, s.CoverEnricher, s.CoverSettings, s.LibraryScanner, s.ScanCfg, s.Scheduler, s.DataDir, smtpSender, bus, s.PlaylistScheduler, s.StreamSecret)
 		// /api/admin/scan is the only admin route owned by the server package
 		// (it needs the Scanner). Register it as a single inline-middleware
 		// route — using r.Route("/api/admin", ...) here would create a second