diff --git a/cmd/wstracker-probe/main.go b/cmd/wstracker-probe/main.go
index 660e297..7eecaa5 100644
--- a/cmd/wstracker-probe/main.go
+++ b/cmd/wstracker-probe/main.go
@@ -1,11 +1,19 @@
-// wstracker-probe — connects to a WebSocket BitTorrent tracker, advertises
-// a fake info_hash, and reports whether the announce succeeds.
+// wstracker-probe — connects to a WebSocket BitTorrent tracker and either
+// (a) advertises a fake info_hash to verify announce signalling, or
+// (b) seeds a real file via the WebTorrent protocol so a browser
+// webtorrent.js client can fetch it for end-to-end verification.
 //
-// Usage:
+// Modes:
 //
-//	go run ./cmd/wstracker-probe -tracker wss://tracker.torrentclaw.com
+//	wstracker-probe -tracker wss://tracker.torrentclaw.com
+//	    Announces a random info_hash; exits 0 on TrackerAnnounceSuccessful.
 //
-// Exit code 0 on TrackerAnnounceSuccessful, 1 on timeout/error.
+//	wstracker-probe -tracker wss://… -seed /path/to/file.mp4
+//	    Builds a single-file torrent in memory, seeds forever, prints the
+//	    magnet (with the WSS tracker injected). Ctrl-C to stop.
+//
+// Useful for browser ↔ unarr e2e — point a webtorrent.js page at the
+// printed magnet and the player should pull pieces via WebRTC data channel.
 package main
 
 import (
@@ -14,42 +22,44 @@ import (
 	"flag"
 	"fmt"
 	"log"
+	"net/url"
 	"os"
+	"os/signal"
+	"path/filepath"
+	"syscall"
 	"time"
 
 	alog "github.com/anacrolix/log"
 	"github.com/anacrolix/torrent"
+	"github.com/anacrolix/torrent/bencode"
+	"github.com/anacrolix/torrent/metainfo"
 	"github.com/anacrolix/torrent/storage"
 	"github.com/pion/webrtc/v4"
 )
 
 func main() {
 	tracker := flag.String("tracker", "wss://tracker.torrentclaw.com", "WSS tracker URL to probe")
-	timeout := flag.Duration("timeout", 30*time.Second, "max wait for successful announce")
+	timeout := flag.Duration("timeout", 30*time.Second, "max wait for successful announce (ignored in -seed mode)")
+	seedPath := flag.String("seed", "", "path to a file to seed (single-file torrent). When set, runs forever instead of exiting on first announce.")
 	flag.Parse()
 
+	if *seedPath != "" {
+		runSeeder(*seedPath, *tracker)
+		return
+	}
+
+	runProbe(*tracker, *timeout)
+}
+
+// runProbe — single random-hash announce, exits on success/error/timeout.
+func runProbe(trackerURL string, timeout time.Duration) {
 	tmp, err := os.MkdirTemp("", "wstracker-probe-*")
 	if err != nil {
 		log.Fatalf("temp dir: %v", err)
 	}
 	defer os.RemoveAll(tmp)
 
-	cfg := torrent.NewDefaultClientConfig()
-	cfg.DataDir = tmp
-	cfg.DefaultStorage = storage.NewMMap(tmp)
-	cfg.Seed = false
-	cfg.NoUpload = false
-	cfg.DisableTCP = true
-	cfg.DisableUTP = true
-	cfg.DisableIPv6 = true
-	cfg.NoDHT = true
-	cfg.NoDefaultPortForwarding = true
-	cfg.ListenPort = 0
-	cfg.Logger = alog.Default.FilterLevel(alog.Critical)
-	cfg.DisableWebtorrent = false
-	cfg.ICEServerList = []webrtc.ICEServer{
-		{URLs: []string{"stun:stun.l.google.com:19302"}},
-	}
+	cfg := baseClientConfig(tmp)
 
 	annSuccess := make(chan struct{}, 1)
 	annError := make(chan error, 1)
@@ -91,8 +101,8 @@ func main() {
 	if _, err := rand.Read(ih[:]); err != nil {
 		log.Fatalf("random info_hash: %v", err)
 	}
-	magnet := fmt.Sprintf("magnet:?xt=urn:btih:%x&tr=%s", ih, *tracker)
-	fmt.Printf("[probe] tracker=%s info_hash=%x timeout=%s\n", *tracker, ih, *timeout)
+	magnet := fmt.Sprintf("magnet:?xt=urn:btih:%x&tr=%s", ih, trackerURL)
+	fmt.Printf("[probe] tracker=%s info_hash=%x timeout=%s\n", trackerURL, ih, timeout)
 
 	t, err := client.AddMagnet(magnet)
 	if err != nil {
@@ -100,7 +110,7 @@ func main() {
 	}
 	defer t.Drop()
 
-	ctx, cancel := context.WithTimeout(context.Background(), *timeout)
+	ctx, cancel := context.WithTimeout(context.Background(), timeout)
 	defer cancel()
 
 	select {
@@ -111,7 +121,148 @@ func main() {
 		fmt.Printf("[probe] FAIL — tracker announce error: %v\n", err)
 		os.Exit(1)
 	case <-ctx.Done():
-		fmt.Printf("[probe] FAIL — timeout after %s\n", *timeout)
+		fmt.Printf("[probe] FAIL — timeout after %s\n", timeout)
 		os.Exit(2)
 	}
 }
+
+// runSeeder — builds a single-file torrent for the given path, adds it to
+// a WebTorrent-enabled client, and seeds until SIGINT/SIGTERM.
+func runSeeder(filePath, trackerURL string) {
+	abs, err := filepath.Abs(filePath)
+	if err != nil {
+		log.Fatalf("resolve seed path: %v", err)
+	}
+	st, err := os.Stat(abs)
+	if err != nil {
+		log.Fatalf("stat seed file: %v", err)
+	}
+	if st.IsDir() {
+		log.Fatalf("-seed currently supports a single file, not a directory: %s", abs)
+	}
+
+	dataDir := filepath.Dir(abs)
+
+	// Build single-file torrent metadata.
+	info := metainfo.Info{
+		PieceLength: chooseSeedPieceLength(st.Size()),
+		Name:        filepath.Base(abs),
+	}
+	if err := info.BuildFromFilePath(abs); err != nil {
+		log.Fatalf("build info from file: %v", err)
+	}
+	infoBytes, err := bencode.Marshal(info)
+	if err != nil {
+		log.Fatalf("marshal info: %v", err)
+	}
+
+	mi := &metainfo.MetaInfo{
+		InfoBytes:    infoBytes,
+		AnnounceList: metainfo.AnnounceList{{trackerURL}},
+		CreatedBy:    "wstracker-probe",
+	}
+	ih := mi.HashInfoBytes()
+
+	cfg := baseClientConfig(dataDir)
+	cfg.Seed = true
+
+	cfg.Callbacks.StatusUpdated = append(
+		cfg.Callbacks.StatusUpdated,
+		func(e torrent.StatusUpdatedEvent) {
+			switch e.Event { //nolint:exhaustive
+			case torrent.TrackerConnected:
+				if e.Error != nil {
+					fmt.Printf("[seed] tracker connect FAILED: %v\n", e.Error)
+				} else {
+					fmt.Printf("[seed] tracker connected: %s\n", e.Url)
+				}
+			case torrent.TrackerAnnounceSuccessful:
+				fmt.Printf("[seed] tracker announce OK: %s ih=%s\n", e.Url, e.InfoHash)
+			case torrent.TrackerAnnounceError:
+				fmt.Printf("[seed] tracker announce ERROR: %s err=%v\n", e.Url, e.Error)
+			case torrent.TrackerDisconnected:
+				fmt.Printf("[seed] tracker disconnected: %s err=%v\n", e.Url, e.Error)
+			}
+		},
+	)
+
+	client, err := torrent.NewClient(cfg)
+	if err != nil {
+		log.Fatalf("create torrent client: %v", err)
+	}
+	defer client.Close()
+
+	t, err := client.AddTorrent(mi)
+	if err != nil {
+		log.Fatalf("add torrent: %v", err)
+	}
+	t.DownloadAll()
+
+	dn := url.QueryEscape(info.Name)
+	enc := url.QueryEscape(trackerURL)
+	magnet := fmt.Sprintf("magnet:?xt=urn:btih:%s&dn=%s&tr=%s", ih.HexString(), dn, enc)
+
+	fmt.Printf("[seed] file=%s size=%d bytes piece_length=%d\n", abs, st.Size(), info.PieceLength)
+	fmt.Printf("[seed] info_hash=%s\n", ih.HexString())
+	fmt.Printf("[seed] magnet=%s\n", magnet)
+	fmt.Println("[seed] seeding via WebRTC. Ctrl-C to stop.")
+
+	stop := make(chan os.Signal, 1)
+	signal.Notify(stop, syscall.SIGINT, syscall.SIGTERM)
+	statTicker := time.NewTicker(5 * time.Second)
+	defer statTicker.Stop()
+
+	for {
+		select {
+		case <-statTicker.C:
+			s := t.Stats()
+			fmt.Printf("[seed] peers=%d uploaded=%d bytes seeders=%d leechers=%d\n",
+				s.ActivePeers, s.BytesWrittenData.Int64(),
+				s.ConnectedSeeders, s.ActivePeers-s.ConnectedSeeders)
+		case <-stop:
+			fmt.Println("[seed] stopping")
+			return
+		}
+	}
+}
+
+// baseClientConfig — shared anacrolix client config for both modes.
+// WebTorrent is the only transport enabled; TCP/uTP/DHT/IPv6 are disabled
+// to keep the moving parts to the minimum required for a WSS-only test.
+func baseClientConfig(dataDir string) *torrent.ClientConfig {
+	cfg := torrent.NewDefaultClientConfig()
+	cfg.DataDir = dataDir
+	cfg.DefaultStorage = storage.NewMMap(dataDir)
+	cfg.NoUpload = false
+	cfg.DisableTCP = true
+	cfg.DisableUTP = true
+	cfg.DisableIPv6 = true
+	cfg.NoDHT = true
+	cfg.NoDefaultPortForwarding = true
+	cfg.ListenPort = 0
+	cfg.Logger = alog.Default.FilterLevel(alog.Critical)
+	cfg.DisableWebtorrent = false
+	cfg.ICEServerList = []webrtc.ICEServer{
+		{URLs: []string{"stun:stun.l.google.com:19302"}},
+		{URLs: []string{"stun:stun1.l.google.com:19302"}},
+	}
+	return cfg
+}
+
+// chooseSeedPieceLength picks a sane piece size for a given file size.
+// Mirrors the libtorrent / qBittorrent ladder so the resulting torrent
+// is interoperable with mainstream clients.
+func chooseSeedPieceLength(size int64) int64 {
+	switch {
+	case size < 4*1024*1024: // < 4 MiB
+		return 16 * 1024 // 16 KiB
+	case size < 64*1024*1024: // < 64 MiB
+		return 64 * 1024 // 64 KiB
+	case size < 512*1024*1024: // < 512 MiB
+		return 256 * 1024 // 256 KiB
+	case size < 4*1024*1024*1024: // < 4 GiB
+		return 1024 * 1024 // 1 MiB
+	default:
+		return 4 * 1024 * 1024 // 4 MiB
+	}
+}