Preview Video

Preview Video is the most unintentionally honest node in the whole ComfyUI Core Video Nodes pack (Immac/ComfyUI-CoreVideoMocks). Its job in a real video workflow is simple: take a finished video stream and show it in ComfyUI's preview pane so you can eyeball the result. What this mock actually does is write a text file named video_preview.webm to ComfyUI's temp folder, point the UI at it, and let your browser try to play it. The file contains the sentence "Mock video preview content for stream: ...". A .webm that is not a webm.

Which is sort of perfect, because it demonstrates exactly how the preview plumbing works - an output node returning a ui payload with a preview_video item that the frontend renders in a video player - without any of the hard parts. If you're studying the pack as the RFC it claims to be ("A set of mock nodes for RFC purposes"), this is the node to read to understand how a core video node would hand a file to the UI.

The input

Just one: video_stream, a STRING that must be wired from something upstream (force input only - no typing values in). Feed it anything and you get a preview pane that either shows a broken player or a flash of text, depending on how your browser handles a mislabeled file.

Install

cd ComfyUI/custom_nodes
git clone https://github.com/Immac/ComfyUI-CoreVideoMocks

Restart ComfyUI. No dependencies beyond ComfyUI itself - no pip packages, no models, no ffmpeg. Manager works too: search "ComfyUI Core Video Nodes."

The catch

The pack has essentially no community presence - a couple of GitHub stars, no Reddit discussion - because it's a spec sketch, and Preview Video is the clearest proof: it returns no real output, writes no real video, and is only useful as a shape to imitate or a placeholder in a workflow you're designing before the real tools exist.

If you actually want to preview video in ComfyUI today, the tool you want is VideoHelperSuite (VHS), whose preview and playback nodes feed a real player with real frames. This node is a paper cutout of that. Wire it into a real pipeline and you'll get the most famous failure mode in video work: a player window that opens, blinks, and shows you nothing.