diff --git a/positronic/offboard/frame_codec_cost.py b/positronic/offboard/frame_codec_cost.py
new file mode 100644
index 000000000..21350711f
--- /dev/null
+++ b/positronic/offboard/frame_codec_cost.py
@@ -0,0 +1,175 @@
+"""Measure what one observation window costs per image codec: bytes, encode time, decode time.
+
+Replays a recorded episode's cameras through the rig-side bound, then encodes each temporal-stack
+window as one JPEG per frame, which the wire carries, and as one h264 GOP. JPEG encodes
+single-threaded through ``encode_jpeg``; h264 encodes with x264's own frame threading.
+
+Usage
+  python -m positronic.offboard.frame_codec_cost --episode <dir holding <camera>.mp4>
+  python -m positronic.offboard.frame_codec_cost --episode <dir> --bound 640x180 --json rows.json
+"""
+
+import argparse
+import dataclasses
+import io
+import json
+import pathlib
+import statistics
+import time
+
+import av
+import numpy as np
+
+from positronic.policy.codec import RestrictImageSize
+from positronic.utils.serialization import FRAMES, encode_jpeg, unpack
+
+
+@dataclasses.dataclass(frozen=True)
+class Jpeg:
+    """One JPEG per frame, at the quality the wire itself encodes at."""
+
+    name = 'jpeg'
+
+    def cost(self, window: np.ndarray) -> tuple[int, float, float]:
+        """Bytes, encode ms and decode ms for one window."""
+        start = time.perf_counter()
+        marker = encode_jpeg(window)
+        encode_ms = 1000 * (time.perf_counter() - start)
+
+        start = time.perf_counter()
+        unpack(marker)
+        decode_ms = 1000 * (time.perf_counter() - start)
+        return sum(len(buf) for buf in marker[FRAMES]), encode_ms, decode_ms
+
+
+@dataclasses.dataclass(frozen=True)
+class H264:
+    """One x264 setting, over the whole window as a single GOP."""
+
+    preset: str
+    crf: int
+
+    @property
+    def name(self) -> str:
+        return f'h264 {self.preset} crf{self.crf}'
+
+    def cost(self, window: np.ndarray) -> tuple[int, float, float]:
+        """Bytes, encode ms and decode ms for one window."""
+        height, width = window.shape[1:3]
+        buffer = io.BytesIO()
+        start = time.perf_counter()
+        with av.open(buffer, 'w', format='mp4') as container:
+            stream = container.add_stream('libx264', rate=15)
+            stream.width, stream.height, stream.pix_fmt = width, height, 'yuv420p'
+            # One self-contained GOP with no lookahead: a request carries its own window and waits on it.
+            stream.options = {'preset': self.preset, 'crf': str(self.crf), 'tune': 'zerolatency', 'g': str(len(window))}
+            for frame in window:
+                container.mux(stream.encode(av.VideoFrame.from_ndarray(frame, format='rgb24')))
+            container.mux(stream.encode(None))
+        encode_ms = 1000 * (time.perf_counter() - start)
+        payload = buffer.getvalue()
+
+        start = time.perf_counter()
+        with av.open(io.BytesIO(payload), 'r') as container:
+            for frame in container.decode(video=0):
+                frame.to_ndarray(format='rgb24')
+        return len(payload), encode_ms, 1000 * (time.perf_counter() - start)
+
+
+Codec = Jpeg | H264
+
+
+@dataclasses.dataclass(frozen=True)
+class Cost:
+    """What one window of one camera cost under one codec."""
+
+    codec: str
+    camera: str
+    window: int
+    kib: float
+    encode_ms: float
+    decode_ms: float
+
+
+def bounded_frames(mp4: pathlib.Path, bound: RestrictImageSize, rate_hz: float) -> list[np.ndarray]:
+    """Every frame the stack samples, through the rig's own bound."""
+    key = 'image'
+    with av.open(str(mp4), 'r') as container:
+        recorded_rate = container.streams.video[0].average_rate
+        if recorded_rate is None:
+            raise ValueError(f'{mp4} declares no frame rate, so the sampled frames cannot be chosen')
+        step = max(1, round(float(recorded_rate) / rate_hz))
+        frames = []
+        for index, frame in enumerate(container.decode(video=0)):
+            if index % step:
+                continue
+            frames.append(bound.encode({key: frame.to_ndarray(format='rgb24')})[key])
+    return frames
+
+
+def costs(frames: list[np.ndarray], camera: str, depth: int, codecs: list[Codec], limit: int) -> list[Cost]:
+    """One row per codec per window, over the windows the episode holds."""
+    starts = list(range(0, len(frames) - depth + 1))[: limit or None]
+    rows = []
+    for window_index, start in enumerate(starts):
+        window = np.stack(frames[start : start + depth])
+        for codec in codecs:
+            size, encode_ms, decode_ms = codec.cost(window)
+            rows.append(Cost(codec.name, camera, window_index, size / 1024, encode_ms, decode_ms))
+    return rows
+
+
+def _row(label: str, group: list[Cost], windows: list[int]) -> str:
+    """One printed line: the median over windows of what the group cost in each window."""
+    per_window = [[row for row in group if row.window == window] for window in windows]
+    return (
+        f'{label:>16}  '
+        f'{statistics.median(sum(row.kib for row in w) for w in per_window):8.0f}  '
+        f'{statistics.median(sum(row.encode_ms for row in w) for w in per_window):10.1f}  '
+        f'{statistics.median(sum(row.decode_ms for row in w) for w in per_window):10.1f}'
+    )
+
+
+def report(rows: list[Cost], depth: int) -> None:
+    """Median cost per codec, per camera and summed over the cameras one request carries."""
+    names = list(dict.fromkeys(row.codec for row in rows))
+    cameras = list(dict.fromkeys(row.camera for row in rows))
+    windows = sorted({row.window for row in rows})
+    print(f'\n{depth} frames, {len(windows)} windows, {len(cameras)} cameras\n')
+    print(f'{"codec":>22}  {"camera":>16}  {"KiB":>8}  {"encode ms":>10}  {"decode ms":>10}')
+    for name in names:
+        of_codec = [row for row in rows if row.codec == name]
+        for camera in cameras:
+            print(f'{name:>22}  {_row(camera, [row for row in of_codec if row.camera == camera], windows)}')
+        print(f'{name:>22}  {_row("every camera", of_codec, windows)}')
+
+
+def main() -> int:
+    parser = argparse.ArgumentParser()
+    parser.add_argument('--episode', required=True, type=pathlib.Path, help='directory holding <camera>.mp4')
+    parser.add_argument('--cameras', default='image.exterior,image.wrist')
+    parser.add_argument('--frames', type=int, default=25, help='temporal-stack depth')
+    parser.add_argument('--rate', type=float, default=15.0, help='stack sampling rate, Hz')
+    parser.add_argument('--bound', default='1024x288', help='WxH rig-side bound')
+    parser.add_argument('--x264', default='ultrafast:20,veryfast:20', help='comma-separated preset:crf')
+    parser.add_argument('--windows', type=int, default=100, help='windows per camera, 0 for every one')
+    parser.add_argument('--json', type=pathlib.Path, help='write every row here')
+    args = parser.parse_args()
+
+    bound = RestrictImageSize(*(int(side) for side in args.bound.lower().split('x')))
+    codecs: list[Codec] = [Jpeg()]
+    codecs += [H264(spec.split(':')[0], int(spec.split(':')[1])) for spec in args.x264.split(',')]
+    rows: list[Cost] = []
+    for camera in args.cameras.split(','):
+        frames = bounded_frames(args.episode / f'{camera}.mp4', bound, args.rate)
+        print(f'{camera}: {len(frames)} sampled frames at {frames[0].shape[1]}x{frames[0].shape[0]}')
+        rows += costs(frames, camera, args.frames, codecs, args.windows)
+
+    report(rows, args.frames)
+    if args.json:
+        args.json.write_text(json.dumps([dataclasses.asdict(row) for row in rows]))
+    return 0
+
+
+if __name__ == '__main__':
+    raise SystemExit(main())
diff --git a/positronic/utils/serialization.py b/positronic/utils/serialization.py
index 09948c22a..e7552dd8f 100644
--- a/positronic/utils/serialization.py
+++ b/positronic/utils/serialization.py
@@ -26,7 +26,7 @@
 _DATA = b'data'
 _DTYPE = b'dtype'
 _SHAPE = b'shape'
-_FRAMES = b'frames'
+FRAMES = b'frames'  # the wire's own name for the per-frame JPEGs
 _NDIM = b'ndim'
 
 # JPEG quality for images on the wire. A single HD frame — and especially a (T, H, W, 3) stack — is many
@@ -46,12 +46,12 @@ def encode_jpeg(image: np.ndarray) -> dict[bytes, Any]:
         buf = io.BytesIO()
         PilImage.fromarray(np.ascontiguousarray(frame, dtype=np.uint8)).save(buf, format='JPEG', quality=_JPEG_QUALITY)
         bufs.append(buf.getvalue())
-    return {_JPEG: True, _FRAMES: bufs, _NDIM: int(image.ndim)}
+    return {_JPEG: True, FRAMES: bufs, _NDIM: int(image.ndim)}
 
 
 def _decode_jpeg(marker: dict) -> np.ndarray:
     """Inverse of ``encode_jpeg``: decode per-frame JPEGs and restore the original shape."""
-    frames = np.stack([np.asarray(PilImage.open(io.BytesIO(buf))) for buf in marker[_FRAMES]])
+    frames = np.stack([np.asarray(PilImage.open(io.BytesIO(buf))) for buf in marker[FRAMES]])
     return frames if marker[_NDIM] == 4 else frames[0]