Caught while writing the receiving end. Distinct multipart field names are not sufficient: backends stage an uploaded frame under a name derived from the FILENAME, and our own ComfyUI shim posts to /upload/image with overwrite=true. Both parts were sending initImageFilename(mime) — literally "frame.png" for each — so the second upload would have clobbered the first and BOTH keyframe inputs would have resolved to the same stored image. The failure mode is the worst kind: a clip pinned at both ends to the same frame renders cleanly, returns 200, and looks like the feature not working rather than like a bug. Nothing upstream or downstream would report a fault. writeImagePart now takes the filename stem (frame / frame_last), and the test asserts the two arrive under different filenames. Co-Authored-By: Claude Opus 5 (1M context) <[email protected]> Claude-Session: https://claude.ai/code/session_01PLjgrxvHjm1sJgUu9zBPH9