Grok Video Edit

Grok (xAI) video editing

Use Grok for edits that require generative changes to the footage itself — recoloring objects, adding physical effects (smoke, fire, glow), restyling. For compositing a known asset (logo, text, PNG/animated graphic) onto video, use the video-overlay skill instead — it's deterministic and cheaper.

Requires XAI_API_KEY in .env.

Golden rule: always trim the target region first

grok-edit full_video.mp4 chunks the entire video even with --splice-into. Never send the full video. Workflow:

1. Find exact boundaries. Run detect with --step 1 --cell-w 384 inside the rough window to locate the frame-exact first/last frames of the subject. Trim to those exact boundaries so Grok doesn't reimagine irrelevant context.

   uv run video-agent trim source.mp4 --start X --end Y -o segment.mp4
   

2. Edit the segment:

   uv run video-agent grok-edit segment.mp4 --prompt "..." -o edited.mp4
   

3. Mux original audio back (Grok outputs video only, and drops ~0.2–0.3s of frames):

   ffmpeg -i edited.mp4 -i segment.mp4 -map 0:v -map 1:a -c copy -shortest out.mp4
   

   Always -shortest — without it the audio outlasts the shorter video track and everything downstream desyncs.
4. Final join with filter_complex concat (re-encodes — kills keyframe freeze + drift):

   ffmpeg -i a.mp4 -i b.mp4 \
     -filter_complex "[0:v][0:a][1:v][1:a]concat=n=2:v=1:a=1[v][a]" \
     -map "[v]" -map "[a]" -c:v h264_videotoolbox -b:v 8M -c:a aac -ar 44100 out.mp4
   

   Do NOT use stream-copy concat after a Grok-edited clip — non-keyframe seams cause freeze frames.

The --splice-into flag automates chunking, base64 upload, concat, and audio stitch, but still only use it on an already-trimmed region.

Chunking & seams

• GROK_MAX_SECONDS = 8.0 (in code): each chunk is edited independently — Grok reimagines the scene fresh per chunk. Shorter chunks = more seams/style jumps. Default 8s. Do NOT go below 3s.
• Split chunks at natural scene cuts, not arbitrary intervals. Use the detect grid to find cuts, trim each shot separately. A chunk spanning a shot change will have the second shot restyled differently.
• Chunks are downscaled to 720p/2Mbps (gRPC 4MB limit). AV1 sources handled automatically (PyAV decode → h264 chunks).
• Grok re-encodes at ~24fps regardless of source fps, and the output is a few frames shorter than the input. On a high-fps source (e.g. 120fps) the edited segment will not match the surrounding clips. Normalize fps in the final concat — add fps=<orig> to each input in the filter_complex ([1:v]fps=120,setsar=1,...), or the seam will stutter / the durations won't line up. Never stream-copy a Grok segment back into a higher-fps clip.

Effects with aftermath — don't pop at the seam

If an added effect leaves visual aftermath that outlives the action it came from, it will still be on screen at the trim boundary. A hard cut back to the untouched clip makes it vanish in one frame — a jarring pop. Two fixes:

• Extend the edited region to cover the aftermath so the effect resolves within Grok's footage. Trade-off: a longer region means Grok reimagines more frames (more chance of subject/character drift), so only extend as far as needed.
• Dissolve the seam with a short xfade (≈0.25–0.4s) so the effect fades instead of popping. Because both sides are the same subject in nearly the same pose, the dissolve reads as the effect clearing, not as a transition.
• You can xfade freely wherever the audio is silent. xfade shortens the video by the overlap, which desyncs muxed audio — but only if speech crosses the seam. When the effect lands in a silent stretch, dissolve as much as you like and re-mux the original audio with -shortest. Use speech-segments (filler-removal skill) to see which stretches are silent.

Character / consistency drift

Grok cannot maintain character consistency between chunks. The system constraint (_GROK_CONSTRAINT) is generic — keep it that way. Pass character/subject descriptions in the user prompt, not the constraint.

Prompt tips

• Say what to change, not what to keep — Grok handles positive instructions better than negative constraints.
• Avoid incidental words like "shockwave rings", "portal effects" unless you want them — Grok adds them literally.
• For color changes, name the exact source color to replace and the exact target color.
• For added effects, describe direction and physics, not just the effect — where it originates, which way it travels, how it ends (e.g. rises then drifts, radiates outward, settles). Grok renders the motion you specify and invents motion you leave unsaid.

Parallelize independent edits

If a video needs multiple separate Grok edits (e.g. two different moments), trim each region and run the grok-edit calls in background simultaneously — they're independent network jobs. Mux audio and assemble once both complete.

Audio

Grok outputs video only. For final assembly always rebuild audio from the original source (extract with -vn -c:a copy, then mux). Never rely on PyAV-trimmed audio for sync.