diff --git a/internal/cmd/daemon.go b/internal/cmd/daemon.go
index b85c9c2..aa0dd63 100644
--- a/internal/cmd/daemon.go
+++ b/internal/cmd/daemon.go
@@ -451,6 +451,7 @@ func runDaemonStart() error {
 				webrtcRegistry.remove(sess.SessionID)
 				sessCancel()
 			}()
+			tcRuntime := buildTranscodeRuntime(ctx, cfg)
 			runCfg := engine.WebRTCStreamConfig{
 				SessionID:  sess.SessionID,
 				FilePath:   filePath,
@@ -459,6 +460,7 @@ func runDaemonStart() error {
 				ICEServers: engine.BuildICEServers(cfg.Download.WebRTC),
 				Signal:     agentClient,
 				Logger:     stdLogger{},
+				Transcode:  tcRuntime,
 			}
 			log.Printf("[wrtc %s] starting session: %s", agent.ShortID(sess.SessionID), filepath.Base(filePath))
 			if err := engine.RunWebRTCStream(sessCtx, runCfg); err != nil {
diff --git a/internal/cmd/webrtc_session_registry.go b/internal/cmd/webrtc_session_registry.go
index b0ec0b7..a1bf37a 100644
--- a/internal/cmd/webrtc_session_registry.go
+++ b/internal/cmd/webrtc_session_registry.go
@@ -4,6 +4,10 @@ import (
 	"context"
 	"log"
 	"sync"
+
+	"github.com/torrentclaw/unarr/internal/config"
+	"github.com/torrentclaw/unarr/internal/engine"
+	"github.com/torrentclaw/unarr/internal/library/mediainfo"
 )
 
 // webrtcRegistry tracks per-session cancel funcs for active custom WebRTC
@@ -60,3 +64,43 @@ type stdLogger struct{}
 func (stdLogger) Infof(format string, args ...any)  { log.Printf(format, args...) }
 func (stdLogger) Warnf(format string, args ...any)  { log.Printf("WARN: "+format, args...) }
 func (stdLogger) Errorf(format string, args ...any) { log.Printf("ERROR: "+format, args...) }
+
+// buildTranscodeRuntime resolves the ffmpeg/ffprobe binaries + config knobs
+// for the WebRTC streaming pipeline. Failure to resolve a binary returns a
+// runtime with empty paths so engine.RunWebRTCStream falls back to
+// passthrough — the user gets a clearer codec error from the browser than a
+// daemon-side abort.
+func buildTranscodeRuntime(ctx context.Context, cfg config.Config) engine.TranscodeRuntime {
+	if !cfg.Download.Transcode.Enabled {
+		return engine.TranscodeRuntime{Disabled: true}
+	}
+	ffmpegPath, errF := mediainfo.ResolveFFmpeg(cfg.Library.FFmpegPath)
+	ffprobePath, errP := mediainfo.ResolveFFprobe(cfg.Library.FFprobePath)
+	if errF != nil || errP != nil {
+		return engine.TranscodeRuntime{Disabled: true}
+	}
+	hw := engine.HWAccelNone
+	switch cfg.Download.Transcode.HWAccel {
+	case "auto":
+		hw = engine.DetectHWAccel(ctx, ffmpegPath)
+	case "nvenc":
+		hw = engine.HWAccelNVENC
+	case "qsv":
+		hw = engine.HWAccelQSV
+	case "vaapi":
+		hw = engine.HWAccelVAAPI
+	case "videotoolbox":
+		hw = engine.HWAccelVideoToolbox
+	case "none", "":
+		hw = engine.HWAccelNone
+	}
+	return engine.TranscodeRuntime{
+		FFmpegPath:   ffmpegPath,
+		FFprobePath:  ffprobePath,
+		HWAccel:      hw,
+		Preset:       cfg.Download.Transcode.Preset,
+		VideoBitrate: cfg.Download.Transcode.VideoBitrate,
+		AudioBitrate: cfg.Download.Transcode.AudioBitrate,
+		MaxHeight:    cfg.Download.Transcode.MaxHeight,
+	}
+}
diff --git a/internal/config/config.go b/internal/config/config.go
index bb7498c..b84655a 100644
--- a/internal/config/config.go
+++ b/internal/config/config.go
@@ -34,17 +34,32 @@ type AgentConfig struct {
 }
 
 type DownloadConfig struct {
-	Dir              string       `toml:"dir"`
-	PreferredMethod  string       `toml:"preferred_method"`
-	PreferredQuality string       `toml:"preferred_quality"` // "2160p", "1080p", "720p" — hint for auto-selection
-	MaxConcurrent    int          `toml:"max_concurrent"`
-	MaxDownloadSpeed string       `toml:"max_download_speed"` // e.g. "10MB", "500KB", "0" = unlimited
-	MaxUploadSpeed   string       `toml:"max_upload_speed"`   // e.g. "1MB", "0" = unlimited
-	MetadataTimeout  string       `toml:"metadata_timeout"`   // e.g. "1h", "30m", "0" = unlimited (default: "0")
-	StallTimeout     string       `toml:"stall_timeout"`      // e.g. "30m", "1h", "0" = unlimited (default: "30m")
-	ListenPort       int          `toml:"listen_port"`        // fixed port for incoming peer connections (default: 42069, 0 = random)
-	StreamPort       int          `toml:"stream_port"`        // fixed port for streaming HTTP server (default: 11818)
-	WebRTC           WebRTCConfig `toml:"webrtc"`
+	Dir              string          `toml:"dir"`
+	PreferredMethod  string          `toml:"preferred_method"`
+	PreferredQuality string          `toml:"preferred_quality"` // "2160p", "1080p", "720p" — hint for auto-selection
+	MaxConcurrent    int             `toml:"max_concurrent"`
+	MaxDownloadSpeed string          `toml:"max_download_speed"` // e.g. "10MB", "500KB", "0" = unlimited
+	MaxUploadSpeed   string          `toml:"max_upload_speed"`   // e.g. "1MB", "0" = unlimited
+	MetadataTimeout  string          `toml:"metadata_timeout"`   // e.g. "1h", "30m", "0" = unlimited (default: "0")
+	StallTimeout     string          `toml:"stall_timeout"`      // e.g. "30m", "1h", "0" = unlimited (default: "30m")
+	ListenPort       int             `toml:"listen_port"`        // fixed port for incoming peer connections (default: 42069, 0 = random)
+	StreamPort       int             `toml:"stream_port"`        // fixed port for streaming HTTP server (default: 11818)
+	WebRTC           WebRTCConfig    `toml:"webrtc"`
+	Transcode        TranscodeConfig `toml:"transcode"`
+}
+
+// TranscodeConfig controls real-time transcoding for the in-browser player
+// when source codecs aren't browser-decodable (HEVC, AV1, AC3, DTS, etc.).
+// Disabled by default; enabling requires ffmpeg + ffprobe on PATH (or
+// explicit paths via the library config).
+type TranscodeConfig struct {
+	Enabled       bool   `toml:"enabled"`        // master switch
+	HWAccel       string `toml:"hw_accel"`       // "auto" | "none" | "nvenc" | "qsv" | "vaapi" | "videotoolbox"
+	Preset        string `toml:"preset"`         // libx264 preset; "veryfast" by default
+	VideoBitrate  string `toml:"video_bitrate"`  // e.g. "5M"
+	AudioBitrate  string `toml:"audio_bitrate"`  // e.g. "192k"
+	MaxHeight     int    `toml:"max_height"`     // optional downscale cap (e.g. 720)
+	MaxConcurrent int    `toml:"max_concurrent"` // safety cap on simultaneous transcoder processes
 }
 
 // WebRTCConfig opts the daemon into acting as a WebTorrent peer so browsers
@@ -106,6 +121,14 @@ func Default() Config {
 				Trackers:    []string{"wss://tracker.torrentclaw.com"},
 				STUNServers: []string{"stun:stun.l.google.com:19302", "stun:stun1.l.google.com:19302"},
 			},
+			Transcode: TranscodeConfig{
+				Enabled:       true,
+				HWAccel:       "auto",
+				Preset:        "veryfast",
+				VideoBitrate:  "5M",
+				AudioBitrate:  "192k",
+				MaxConcurrent: 2,
+			},
 		},
 		Organize: OrganizeConfig{
 			Enabled: true,
diff --git a/internal/engine/stream_source.go b/internal/engine/stream_source.go
new file mode 100644
index 0000000..98aca74
--- /dev/null
+++ b/internal/engine/stream_source.go
@@ -0,0 +1,354 @@
+package engine
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"io"
+	"os"
+	"path/filepath"
+	"sync"
+	"sync/atomic"
+	"time"
+)
+
+// streamSource abstracts the byte source served over the WebRTC DataChannel.
+// Two implementations:
+//   - diskFileSource — direct passthrough of the on-disk file.
+//   - transcodeSource — ffmpeg writes a fragmented MP4 to a temp file in
+//     real time; reads block briefly when callers ask for bytes ahead of
+//     the writer.
+type streamSource interface {
+	ReadAt(p []byte, off int64) (int, error)
+	// Size returns the currently known size. For transcoded sources this
+	// grows as ffmpeg produces output; on Final() it's the final size.
+	Size() int64
+	// Final reports whether the source size is now stable (passthrough is
+	// always final, transcoder becomes final when ffmpeg exits).
+	Final() bool
+	// EstimatedSize returns the final size we expect to converge on. For
+	// passthrough it's the same as Size(). For transcoder it's a bitrate
+	// × duration estimate so the browser scrubber has something to anchor
+	// on; the real size will differ ±20%.
+	EstimatedSize() int64
+	FileName() string
+	Transcoded() bool
+	Close() error
+}
+
+// ─────────────────────────────────────────────────────────────────────────────
+// disk passthrough
+// ─────────────────────────────────────────────────────────────────────────────
+
+type diskFileSource struct {
+	f    *os.File
+	size int64
+	name string
+}
+
+func newDiskFileSource(path string) (*diskFileSource, error) {
+	f, err := os.Open(path)
+	if err != nil {
+		return nil, fmt.Errorf("stream source: open %s: %w", path, err)
+	}
+	stat, err := f.Stat()
+	if err != nil {
+		f.Close()
+		return nil, fmt.Errorf("stream source: stat %s: %w", path, err)
+	}
+	return &diskFileSource{f: f, size: stat.Size(), name: filepath.Base(path)}, nil
+}
+
+func (d *diskFileSource) ReadAt(p []byte, off int64) (int, error) {
+	return d.f.ReadAt(p, off)
+}
+func (d *diskFileSource) Size() int64          { return d.size }
+func (d *diskFileSource) Final() bool          { return true }
+func (d *diskFileSource) EstimatedSize() int64 { return d.size }
+func (d *diskFileSource) FileName() string     { return d.name }
+func (d *diskFileSource) Transcoded() bool     { return false }
+func (d *diskFileSource) Close() error         { return d.f.Close() }
+
+// ─────────────────────────────────────────────────────────────────────────────
+// transcode source — ffmpeg → tmp file
+// ─────────────────────────────────────────────────────────────────────────────
+
+type transcodeSource struct {
+	tmpPath  string
+	tmpFile  *os.File
+	cmd      *Transcoder
+	name     string
+	estimate int64
+
+	mu        sync.Mutex
+	cond      *sync.Cond
+	size      atomic.Int64
+	final     atomic.Bool
+	failure   error
+	startedAt time.Time
+}
+
+const (
+	// readBlockTimeout caps how long ReadAt waits for bytes that haven't
+	// been transcoded yet before returning EOF/io.ErrUnexpectedEOF. The
+	// pump treats EOF as "respond with whatever we have so far + RangeEnd"
+	// so the browser can re-request once more bytes appear.
+	readBlockTimeout = 30 * time.Second
+)
+
+func newTranscodeSource(
+	ctx context.Context,
+	srcPath string,
+	probe *StreamProbe,
+	action TranscodeAction,
+	opts TranscodeOpts,
+	displayName string,
+) (*transcodeSource, error) {
+	tmpFile, err := os.CreateTemp("", "tc-stream-*.mp4")
+	if err != nil {
+		return nil, fmt.Errorf("transcode source: tmp file: %w", err)
+	}
+	tmpPath := tmpFile.Name()
+	tmpFile.Close()
+
+	args := buildFFmpegArgs(srcPath, opts)
+	// Override -f mp4 pipe:1 with output to our tmp file path (last 3 args).
+	if len(args) >= 3 && args[len(args)-1] == "pipe:1" {
+		args[len(args)-1] = tmpPath
+	}
+
+	// Spawn ffmpeg directly (not via NewTranscoder pipe) so it writes to
+	// disk in real time. We re-use the rest of TranscodeOpts wiring.
+	cmd := &Transcoder{}
+	cmd, err = startTranscoderToFile(ctx, opts.FFmpegPath, args, cmd)
+	if err != nil {
+		os.Remove(tmpPath)
+		return nil, err
+	}
+
+	estimate := estimateOutputSize(probe, opts)
+
+	t := &transcodeSource{
+		tmpPath:   tmpPath,
+		cmd:       cmd,
+		name:      displayName,
+		estimate:  estimate,
+		startedAt: time.Now(),
+	}
+	t.cond = sync.NewCond(&t.mu)
+
+	// Re-open the tmp file for reading; ffmpeg keeps writing to it.
+	rf, err := os.Open(tmpPath)
+	if err != nil {
+		_ = cmd.Close()
+		os.Remove(tmpPath)
+		return nil, fmt.Errorf("transcode source: reopen tmp: %w", err)
+	}
+	t.tmpFile = rf
+
+	go t.watchSize(ctx)
+	go t.watchExit()
+	return t, nil
+}
+
+// watchSize polls the temp file size every 200 ms and wakes any blocked
+// ReadAt callers once new bytes arrive.
+func (t *transcodeSource) watchSize(ctx context.Context) {
+	ticker := time.NewTicker(200 * time.Millisecond)
+	defer ticker.Stop()
+	for {
+		select {
+		case <-ctx.Done():
+			t.mu.Lock()
+			t.cond.Broadcast()
+			t.mu.Unlock()
+			return
+		case <-ticker.C:
+		}
+		if t.final.Load() {
+			t.mu.Lock()
+			t.cond.Broadcast()
+			t.mu.Unlock()
+			return
+		}
+		stat, err := os.Stat(t.tmpPath)
+		if err != nil {
+			continue
+		}
+		current := stat.Size()
+		if current > t.size.Load() {
+			t.size.Store(current)
+			t.mu.Lock()
+			t.cond.Broadcast()
+			t.mu.Unlock()
+		}
+	}
+}
+
+// watchExit waits for ffmpeg to exit and locks in the final size.
+func (t *transcodeSource) watchExit() {
+	err := t.cmd.cmd.Wait()
+	if err != nil && !isExpectedExit(err) {
+		t.mu.Lock()
+		t.failure = fmt.Errorf("ffmpeg exited: %w (%s)", err, t.cmd.Stderr())
+		t.mu.Unlock()
+	}
+	if stat, err := os.Stat(t.tmpPath); err == nil {
+		t.size.Store(stat.Size())
+	}
+	t.final.Store(true)
+	t.mu.Lock()
+	t.cond.Broadcast()
+	t.mu.Unlock()
+}
+
+func isExpectedExit(err error) bool {
+	// Killed by Close() — pion DC closed, that's fine.
+	if err == nil {
+		return true
+	}
+	return false
+}
+
+func (t *transcodeSource) ReadAt(p []byte, off int64) (int, error) {
+	if t.failure != nil {
+		return 0, t.failure
+	}
+	if int64(len(p)) == 0 {
+		return 0, nil
+	}
+	deadline := time.Now().Add(readBlockTimeout)
+
+	for {
+		size := t.size.Load()
+		if off+int64(len(p)) <= size || t.final.Load() {
+			break
+		}
+		// Need to wait for ffmpeg to write more.
+		t.mu.Lock()
+		// Check again under lock to avoid lost wakeup.
+		size = t.size.Load()
+		if off+int64(len(p)) <= size || t.final.Load() {
+			t.mu.Unlock()
+			break
+		}
+		// Wait with timeout via a small sleep loop — sync.Cond doesn't
+		// support timed wait, and a goroutine-per-sleep pattern works fine
+		// for our scale.
+		waited := time.NewTimer(500 * time.Millisecond)
+		done := make(chan struct{})
+		go func() {
+			t.cond.Wait()
+			close(done)
+		}()
+		t.mu.Unlock()
+		select {
+		case <-done:
+		case <-waited.C:
+			t.mu.Lock()
+			t.cond.Broadcast() // wake the goroutine so it can return
+			t.mu.Unlock()
+			<-done
+		}
+		if time.Now().After(deadline) {
+			break
+		}
+	}
+
+	if t.failure != nil {
+		return 0, t.failure
+	}
+
+	n, err := t.tmpFile.ReadAt(p, off)
+	// On growing file ReadAt returns io.EOF when reading past current size.
+	// Convert to io.ErrUnexpectedEOF only when we actually exceeded the
+	// final size; otherwise return n, nil so the pump sends what we have.
+	if err == io.EOF && !t.final.Load() {
+		if n > 0 {
+			return n, nil
+		}
+		return 0, errors.New("transcode source: read timed out waiting for ffmpeg output")
+	}
+	return n, err
+}
+
+func (t *transcodeSource) Size() int64 { return t.size.Load() }
+func (t *transcodeSource) Final() bool { return t.final.Load() }
+func (t *transcodeSource) EstimatedSize() int64 {
+	if t.final.Load() {
+		return t.size.Load()
+	}
+	return t.estimate
+}
+func (t *transcodeSource) FileName() string {
+	// Keep the original extension stripped — output is always fragmented MP4.
+	base := t.name
+	if i := lastIndexByte(base, '.'); i >= 0 {
+		base = base[:i]
+	}
+	return base + ".mp4"
+}
+func (t *transcodeSource) Transcoded() bool { return true }
+func (t *transcodeSource) Close() error {
+	_ = t.cmd.Close()
+	if t.tmpFile != nil {
+		_ = t.tmpFile.Close()
+	}
+	if t.tmpPath != "" {
+		_ = os.Remove(t.tmpPath)
+	}
+	return nil
+}
+
+// estimateOutputSize converts probed bitrate × duration into a byte estimate
+// so the browser scrubber has something to anchor on while transcoding.
+func estimateOutputSize(probe *StreamProbe, opts TranscodeOpts) int64 {
+	if probe == nil || probe.DurationSec <= 0 {
+		return 0
+	}
+	videoKbps := parseBitrateKbps(opts.VideoBitrate, 5000)
+	audioKbps := parseBitrateKbps(opts.AudioBitrate, 192)
+	totalKbps := videoKbps + audioKbps
+	bytesPerSec := int64(totalKbps) * 1000 / 8
+	return int64(probe.DurationSec) * bytesPerSec
+}
+
+// parseBitrateKbps converts ffmpeg-style bitrate strings ("5M", "192k") to
+// kilobits per second. Unknown formats fall back to fallback.
+func parseBitrateKbps(s string, fallback int) int {
+	if s == "" {
+		return fallback
+	}
+	last := s[len(s)-1]
+	num := s
+	mult := 1
+	switch last {
+	case 'k', 'K':
+		num = s[:len(s)-1]
+	case 'M', 'm':
+		num = s[:len(s)-1]
+		mult = 1000
+	default:
+		// already in bps? treat as kbps
+	}
+	v := 0
+	for _, c := range num {
+		if c < '0' || c > '9' {
+			return fallback
+		}
+		v = v*10 + int(c-'0')
+	}
+	if v == 0 {
+		return fallback
+	}
+	return v * mult
+}
+
+func lastIndexByte(s string, c byte) int {
+	for i := len(s) - 1; i >= 0; i-- {
+		if s[i] == c {
+			return i
+		}
+	}
+	return -1
+}
diff --git a/internal/engine/transcoder.go b/internal/engine/transcoder.go
index 45229b2..58f9d09 100644
--- a/internal/engine/transcoder.go
+++ b/internal/engine/transcoder.go
@@ -64,6 +64,26 @@ func NewTranscoder(ctx context.Context, filePath string, opts TranscodeOpts) (*T
 	return t, nil
 }
 
+// startTranscoderToFile spawns ffmpeg with a pre-built argv where the last
+// argument is an output file path (instead of pipe:1). Used by streamSource
+// when we want random-access reads against a growing temp file rather than
+// sequential pipe consumption.
+func startTranscoderToFile(ctx context.Context, ffmpegPath string, args []string, t *Transcoder) (*Transcoder, error) {
+	if ffmpegPath == "" {
+		return nil, fmt.Errorf("transcoder: empty ffmpeg path")
+	}
+	cmd := exec.CommandContext(ctx, ffmpegPath, args...)
+	if t == nil {
+		t = &Transcoder{}
+	}
+	t.cmd = cmd
+	cmd.Stderr = &errWriter{t: t}
+	if err := cmd.Start(); err != nil {
+		return nil, fmt.Errorf("transcoder: start ffmpeg: %w", err)
+	}
+	return t, nil
+}
+
 // Read implements io.Reader.
 func (t *Transcoder) Read(p []byte) (int, error) { return t.out.Read(p) }
 
diff --git a/internal/engine/webrtc_stream.go b/internal/engine/webrtc_stream.go
index 091d42f..3f04b3c 100644
--- a/internal/engine/webrtc_stream.go
+++ b/internal/engine/webrtc_stream.go
@@ -24,7 +24,6 @@ import (
 	"fmt"
 	"io"
 	"math"
-	"os"
 	"path/filepath"
 	"sync"
 	"sync/atomic"
@@ -62,6 +61,25 @@ type WebRTCStreamConfig struct {
 	Signal     *agent.Client
 	// Logger receives diagnostic events; a nil logger swallows everything.
 	Logger StreamLogger
+	// Transcode steers on-the-fly transcoding when source codecs are not
+	// browser-decodable (HEVC/AV1/AC3/DTS). Empty FFmpegPath disables it.
+	Transcode TranscodeRuntime
+}
+
+// TranscodeRuntime carries the resolved ffmpeg/ffprobe paths + tunables so
+// each session can decide whether to passthrough or pipe through ffmpeg.
+type TranscodeRuntime struct {
+	FFmpegPath   string
+	FFprobePath  string
+	HWAccel      HWAccel
+	Preset       string
+	VideoBitrate string
+	AudioBitrate string
+	MaxHeight    int
+	// Disabled forces passthrough for every file even when codecs are not
+	// browser-friendly. Useful when the user explicitly turns transcoding
+	// off in config.
+	Disabled bool
 }
 
 // StreamLogger is an injectable logger so tests can capture events.
@@ -84,6 +102,48 @@ func logger(l StreamLogger) StreamLogger {
 	return l
 }
 
+// buildStreamSource picks between passthrough and transcoded source. ffprobe
+// failure or missing ffmpeg falls back to passthrough — the browser surfaces
+// a clearer codec error than us refusing to start.
+func buildStreamSource(
+	ctx context.Context,
+	abs string,
+	displayName string,
+	cfg WebRTCStreamConfig,
+	log StreamLogger,
+) (streamSource, error) {
+	tc := cfg.Transcode
+	if tc.Disabled || tc.FFmpegPath == "" || tc.FFprobePath == "" {
+		return newDiskFileSource(abs)
+	}
+
+	probe, err := ProbeFile(ctx, tc.FFprobePath, abs)
+	if err != nil {
+		log.Warnf("[wrtc %s] probe failed (%v) — passthrough", agent.ShortID(cfg.SessionID), err)
+		return newDiskFileSource(abs)
+	}
+	action := DecideAction(probe)
+	if action == ActionPassthrough {
+		log.Infof("[wrtc %s] codec passthrough (%s + %s in %s)",
+			agent.ShortID(cfg.SessionID), probe.VideoCodec, probe.AudioCodec, probe.Container)
+		return newDiskFileSource(abs)
+	}
+
+	log.Infof("[wrtc %s] transcoding %s/%s/%s → h264+aac (%s)",
+		agent.ShortID(cfg.SessionID), probe.Container, probe.VideoCodec, probe.AudioCodec, action)
+
+	opts := TranscodeOpts{
+		Action:       action,
+		HWAccel:      tc.HWAccel,
+		Preset:       tc.Preset,
+		VideoBitrate: tc.VideoBitrate,
+		AudioBitrate: tc.AudioBitrate,
+		MaxHeight:    tc.MaxHeight,
+		FFmpegPath:   tc.FFmpegPath,
+	}
+	return newTranscodeSource(ctx, abs, probe, action, opts, displayName)
+}
+
 // RunWebRTCStream blocks until the session ends — either the DataChannel
 // closes, the peer connection drops, or ctx is cancelled. Always returns a
 // non-nil error explaining the termination reason.
@@ -101,24 +161,20 @@ func RunWebRTCStream(ctx context.Context, cfg WebRTCStreamConfig) error {
 	if err != nil {
 		return fmt.Errorf("webrtc_stream: resolve path: %w", err)
 	}
-	file, err := os.Open(abs)
-	if err != nil {
-		return fmt.Errorf("webrtc_stream: open file: %w", err)
-	}
-	defer file.Close()
 
-	stat, err := file.Stat()
+	displayName := cfg.FileName
+	if displayName == "" {
+		displayName = filepath.Base(abs)
+	}
+
+	// Decide passthrough vs transcoding. Probe is best-effort: if ffprobe
+	// is missing or fails we fall back to passthrough (the browser will
+	// surface a clearer error than us guessing wrong).
+	source, err := buildStreamSource(ctx, abs, displayName, cfg, log)
 	if err != nil {
-		return fmt.Errorf("webrtc_stream: stat: %w", err)
-	}
-	fileSize := stat.Size()
-	if cfg.FileSize > 0 && cfg.FileSize != fileSize {
-		log.Warnf("webrtc_stream: declared size %d != actual %d", cfg.FileSize, fileSize)
-	}
-	fileName := cfg.FileName
-	if fileName == "" {
-		fileName = filepath.Base(abs)
+		return fmt.Errorf("webrtc_stream: build source: %w", err)
 	}
+	defer source.Close()
 
 	// 1. Build PeerConnection.
 	api := webrtc.NewAPI()
@@ -188,10 +244,17 @@ func RunWebRTCStream(ctx context.Context, cfg WebRTCStreamConfig) error {
 		}
 	})
 
-	// 2. Drive the SDP exchange.
+	// 2. Drive the SDP exchange. Any error from the loop (browser sent
+	// "bye", signal stream closed, etc.) cancels the session so we don't
+	// dangle on the DC waiting for a peer that's already gone.
 	sdpDone := make(chan error, 1)
 	go func() {
-		sdpDone <- runSDPExchange(sessionCtx, pc, cfg)
+		err := runSDPExchange(sessionCtx, pc, cfg)
+		sdpDone <- err
+		if err != nil && sessionCtx.Err() == nil {
+			log.Infof("[wrtc %s] signal loop ended: %v", agent.ShortID(cfg.SessionID), err)
+			cancelSession()
+		}
 	}()
 
 	// 3. Wait for either SDP error or DataChannel open.
@@ -217,7 +280,7 @@ func RunWebRTCStream(ctx context.Context, cfg WebRTCStreamConfig) error {
 	}
 
 	// 4. Wire up the data channel pump.
-	pump := newDataChannelPump(dc, file, fileSize, fileName, log, cancelSession)
+	pump := newDataChannelPump(dc, source, log, cancelSession)
 	dc.OnOpen(pump.onOpen)
 	dc.OnMessage(pump.onMessage)
 	dc.OnClose(func() {
@@ -346,14 +409,12 @@ func handleSignal(
 	return nil
 }
 
-// dataChannelPump owns the DC + file handle and serves wire-protocol frames.
+// dataChannelPump owns the DC + stream source and serves wire-protocol frames.
 type dataChannelPump struct {
-	dc       *webrtc.DataChannel
-	file     *os.File
-	fileSize int64
-	fileName string
-	log      StreamLogger
-	cancel   context.CancelFunc
+	dc     *webrtc.DataChannel
+	source streamSource
+	log    StreamLogger
+	cancel context.CancelFunc
 
 	// Flow control: writers wait on resumeCh when bufferedAmount goes high.
 	paused   atomic.Bool
@@ -372,17 +433,13 @@ type dataChannelPump struct {
 
 func newDataChannelPump(
 	dc *webrtc.DataChannel,
-	file *os.File,
-	fileSize int64,
-	fileName string,
+	source streamSource,
 	log StreamLogger,
 	cancel context.CancelFunc,
 ) *dataChannelPump {
 	p := &dataChannelPump{
 		dc:       dc,
-		file:     file,
-		fileSize: fileSize,
-		fileName: fileName,
+		source:   source,
 		log:      log,
 		cancel:   cancel,
 		resumeCh: make(chan struct{}, 1),
@@ -395,16 +452,25 @@ func newDataChannelPump(
 }
 
 func (p *dataChannelPump) onOpen() {
+	// Use estimated size for transcoded streams so the browser scrubber has
+	// something to anchor on. Real size is reflected by Range responses as
+	// ffmpeg writes more bytes; the estimate just bootstraps the UI.
+	announceSize := p.source.EstimatedSize()
+	transcoding := p.source.Transcoded()
+	// Seekable=true even for transcoded sources because we read from a tmp
+	// file (random access). Seek backwards just works; seek forward beyond
+	// what ffmpeg has produced will block briefly inside ReadAt.
+	seekable := true
 	hello := wire.HelloPayload{
-		FileSize:    uint64(p.fileSize),
-		Transcoding: false,
-		Seekable:    true,
-		FileName:    p.fileName,
+		FileSize:    uint64(announceSize),
+		Transcoding: transcoding,
+		Seekable:    seekable,
+		FileName:    p.source.FileName(),
 	}
 	payload := wire.EncodeHello(hello)
 	frame := wire.EncodeFrame(wire.Header{
 		Type:     wire.FrameHello,
-		Flags:    wire.HelloFlags(false, true),
+		Flags:    wire.HelloFlags(transcoding, seekable),
 		StreamID: 0,
 		Length:   uint32(len(payload)),
 	}, payload)
@@ -487,19 +553,42 @@ func (p *dataChannelPump) serveRange(streamID uint32, req wire.RangeReqPayload)
 	// Reject offsets above MaxInt64 — uint64→int64 narrowing would wrap to a
 	// negative value and bypass the bounds check, then ReadAt would be called
 	// with a negative offset.
-	if req.Offset > math.MaxInt64 || int64(req.Offset) >= p.fileSize {
+	currentSize := p.source.Size()
+	finalSize := p.source.EstimatedSize()
+	if req.Offset > math.MaxInt64 {
 		p.sendRangeEnd(streamID, 2) // out of range
 		return
 	}
+	// For transcoded streams `currentSize` grows over time; only reject when
+	// the offset is past the *estimated* final size.
+	if int64(req.Offset) >= finalSize && p.source.Final() {
+		p.sendRangeEnd(streamID, 2)
+		return
+	}
 
 	want := int64(req.Length)
 	if req.Length > math.MaxInt64 {
 		want = 0 // treat absurd length as "remainder of file"
 	}
-	remaining := p.fileSize - int64(req.Offset)
+	// "Remainder" target: prefer current known size, fall back to estimate
+	// for transcoded streams so the browser can keep scrolling forward as
+	// ffmpeg produces output.
+	knownEnd := currentSize
+	if p.source.Final() {
+		knownEnd = finalSize
+	}
+	if knownEnd < int64(req.Offset) {
+		knownEnd = int64(req.Offset)
+	}
+	remaining := knownEnd - int64(req.Offset)
 	if want <= 0 || want > remaining {
 		want = remaining
 	}
+	if want <= 0 {
+		// Nothing to serve right now (transcoder hasn't reached this offset).
+		p.sendRangeEnd(streamID, 0)
+		return
+	}
 
 	ctx, cancel := context.WithCancel(context.Background())
 	p.activeMu.Lock()
@@ -527,7 +616,7 @@ func (p *dataChannelPump) serveRange(streamID uint32, req wire.RangeReqPayload)
 		if end-offset < chunkLen {
 			chunkLen = end - offset
 		}
-		n, rerr := p.file.ReadAt(buf[:chunkLen], offset)
+		n, rerr := p.source.ReadAt(buf[:chunkLen], offset)
 		if n > 0 {
 			// EOF on a short read means this is the final chunk — flag it so the
 			// browser doesn't wait for more data before processing RangeEnd.