diff --git a/roadscore/prototype/prepare_playback_video.py b/roadscore/prototype/prepare_playback_video.py
new file mode 100644
index 0000000000..c314eb691b
--- /dev/null
+++ b/roadscore/prototype/prepare_playback_video.py
@@ -0,0 +1,93 @@
+"""Build an optional full-resolution software-decode cache from local route assets."""
+import argparse
+from fractions import Fraction
+import hashlib
+import json
+import os
+from pathlib import Path
+import shutil
+import subprocess
+
+
+def probe(path, ffprobe):
+  args=[ffprobe,'-v','error','-select_streams','v:0','-count_frames','-show_entries',
+        'stream=codec_name,width,height,r_frame_rate,avg_frame_rate,nb_read_frames,duration','-of','json',str(path)]
+  return json.loads(subprocess.check_output(args))['streams'][0]
+
+
+def nominal_rate(value):
+  exact=Fraction(value);rate=exact.limit_denominator(1001)
+  if rate<=0 or abs(float(rate-exact))>1e-5:
+    raise ValueError('Cannot establish a stable source frame rate')
+  return rate
+
+
+def validate_output(source, output, timestamps):
+  rate=nominal_rate(source['r_frame_rate']);count=int(source['nb_read_frames'])
+  if output['codec_name']!='h264' or int(output['nb_read_frames'])!=count:
+    raise ValueError('Output frame count or codec differs')
+  if any(output[key]!=source[key] for key in ('width','height')):
+    raise ValueError('Resolution differs')
+  if Fraction(output['avg_frame_rate'])!=rate:
+    raise ValueError('Output frame rate differs')
+  tolerance=1/90000+1e-6
+  if len(timestamps)!=count or any(abs(t-i/float(rate))>tolerance for i,t in enumerate(timestamps)):
+    raise ValueError('Output timestamps are not aligned with source frame indices')
+  if abs(float(output['duration'])-count/float(rate))>tolerance:
+    raise ValueError('Output duration differs')
+  return {'source_frames':count,'output_frames':count,'width':source['width'],'height':source['height'],
+          'frame_order_verified':True,'timing_verified':True,'rate':str(rate),
+          'duration':count/float(rate),'verification':'one-to-one sequential transcode; no frame filters, passthrough timing, no B frames; decoded counts and every output PTS checked'}
+
+
+def build(parent, ffmpeg='ffmpeg', ffprobe='ffprobe', reserve_bytes=1024**3):
+  parent=Path(parent).resolve();original=json.loads((parent/'cache_manifest.json').read_text())
+  dest=parent/'playback';dest.mkdir(exist_ok=True)
+  marker=dest/'.acquiring'
+  if marker.exists():raise RuntimeError('Playback cache already being built or needs inspection')
+  with marker.open('x') as stream:stream.write(str(os.getpid()))
+  manifest={'schema':'roadscore-playback-cache-v1','route':original['route'],'complete':False,'files':[]}
+  try:
+    for item in original['files']:
+      rel=Path(item['path'])
+      if rel.is_absolute() or '..' in rel.parts or len(rel.parts)!=2:raise ValueError('Invalid source path')
+      source=parent/rel;before=source.stat()
+      if before.st_size!=item['bytes']:raise ValueError('Original manifest size mismatch')
+      target_rel=rel.with_name('fcamera.ts') if rel.name=='fcamera.hevc' else rel
+      target=dest/target_rel;target.parent.mkdir(exist_ok=True)
+      entry={'source':str(rel),'path':str(target_rel),'source_bytes':before.st_size,'source_mtime_ns':before.st_mtime_ns}
+      if rel.name=='fcamera.hevc':
+        info=probe(source,ffprobe);rate=nominal_rate(info['r_frame_rate'])
+        if target.exists():raise RuntimeError('Derived video already exists; inspect before rebuilding')
+        if shutil.disk_usage(dest).free<reserve_bytes+before.st_size*3:
+          raise RuntimeError('Insufficient disk reserve for next video segment')
+        temporary=target.with_suffix('.partial.ts')
+        subprocess.run([ffmpeg,'-v','error','-nostdin','-threads','2','-r',str(rate),'-i',str(source),
+          '-an','-c:v','libx264','-threads','2','-preset','ultrafast','-tune','zerolatency','-crf','20',
+          '-pix_fmt','yuv420p','-profile:v','baseline','-x264-params',
+          'keyint=20:min-keyint=20:scenecut=0:bframes=0:ref=1:cabac=0','-fps_mode','passthrough',
+          '-bsf:v',f'h264_metadata=tick_rate={rate*2}:fixed_frame_rate_flag=1',
+          '-muxdelay','0','-muxpreload','0','-f','mpegts',str(temporary)],check=True)
+        temporary.rename(target)
+        out=probe(target,ffprobe)
+        stamps=json.loads(subprocess.check_output([ffprobe,'-v','error','-select_streams','v:0',
+          '-show_packets','-show_entries','packet=pts_time','-of','json',str(target)]))['packets']
+        entry['video_validation']=validate_output(info,out,[float(x['pts_time']) for x in stamps])
+        with target.open('rb') as stream:entry['sha256']=hashlib.file_digest(stream,'sha256').hexdigest()
+        print(f"Validated {rel.parent}: {entry['video_validation']['output_frames']} frames",flush=True)
+      elif not target.exists():target.symlink_to(source)
+      after=source.stat()
+      if (before.st_size,before.st_mtime_ns)!=(after.st_size,after.st_mtime_ns):raise RuntimeError('Original changed during conversion')
+      entry['bytes']=target.stat().st_size;manifest['files'].append(entry)
+    manifest['complete']=True
+    temporary=dest/'playback_manifest.json.tmp';temporary.write_text(json.dumps(manifest,indent=2)+'\n')
+    temporary.replace(dest/'playback_manifest.json')
+  finally:
+    marker.unlink()
+  return dest
+
+
+if __name__=='__main__':
+  p=argparse.ArgumentParser(description=__doc__);p.add_argument('route_cache')
+  p.add_argument('--ffmpeg',default='ffmpeg');p.add_argument('--ffprobe',default='ffprobe')
+  a=p.parse_args();print(build(a.route_cache,a.ffmpeg,a.ffprobe))
diff --git a/roadscore/prototype/test_prepare_playback_video.py b/roadscore/prototype/test_prepare_playback_video.py
new file mode 100644
index 0000000000..31fcbe926b
--- /dev/null
+++ b/roadscore/prototype/test_prepare_playback_video.py
@@ -0,0 +1,17 @@
+import unittest
+from roadscore.prototype.prepare_playback_video import nominal_rate,validate_output
+
+class PlaybackValidationTest(unittest.TestCase):
+  def test_nominal_camera_rate(self):
+    self.assertEqual(nominal_rate('1000000000/50000001'),20)
+  def test_complete_and_partial_segments(self):
+    for n in (1200,573):
+      source={'r_frame_rate':'20/1','nb_read_frames':str(n),'width':1928,'height':1208}
+      out={**source,'codec_name':'h264','avg_frame_rate':'20/1','duration':str(n/20)}
+      self.assertEqual(validate_output(source,out,[i/20 for i in range(n)])['output_frames'],n)
+      for key,value in [('nb_read_frames',str(n-1)),('width',1280),('avg_frame_rate','40/1'),('duration','1')]:
+        with self.assertRaises(ValueError):validate_output(source,{**out,key:value},[i/20 for i in range(n)])
+      with self.assertRaises(ValueError):validate_output(source,out,[i/20+.05 for i in range(n)])
+      with self.assertRaises(ValueError):validate_output(source,out,list(reversed([i/20 for i in range(n)])))
+
+if __name__=='__main__':unittest.main()