What does Descript document for Extend video and start/end-frame transitions, and how do those limits compare with other AI extend tools?

Problem. A clip ends a beat early and needs more footage, or a title/scene transition is wanted between two stills. Conditions. Descript help pages, 23 Oct 2026; model-dependent controls and credits. Method (documented). Extend video: works on generated clips, recorded media and imported footage; it uses the last keyframe of the selected clip as the starting point, so the final frame matters most; you write a prompt for the extension, choose advanced settings per model, preview, and insert; with scenes in the project the extension fills the remaining time in the scene up to the model's maximum clip duration; an extension can itself be extended. Tips: predictable motion works best (smooth steady movement like walking or a camera pan; avoid abrupt cuts or mid-action endings); specific prompts (camera slowly zooms out beats zooms out); shorter extensions often produce better results and may need several generations. Start and end frame: an optional parameter for AI video in Descript, available for select models; use images with similar framing and consistent lighting; describe how the transformation happens, call out constants, set pacing, end with a tone note. Compare [cross-reference]: Google Omni appends at the end only and Luma extends generated videos only (C3-B586-12). Use for AI shorts [inference]: end the source clip on a stable frame (a held pose) so the extension has a clean start; export a freeze of the last frame to check continuity; for long takes, avoid chains of extensions that accumulate drift (the docs do not quantify). Failure modes. Mid-action endings produce poor extensions; lighting mismatch in start/end morphs. Limitations. Documentation only; no sample generated. Version/date. 2026-10-23. Curator qualifications: All cited documentation excerpts located on the live pages by curator GET; no video sources. Limitations: No quality evidence; model choice changes results. Drift claim is inference. Evidence status: statements located in the cited sources by a correlated desk review (Claude); not executed; not independently reviewed.

{
  "citations": [
    {
      "url": "https://help.descript.com/generative-media/extend-video.md",
      "note": "Overview; How to extend video; Tips; Tips; Intro note; Tips: image setup; long takes"
    },
    {
      "url": "https://help.descript.com/generative-media/last-frame.md",
      "note": "Overview; How to extend video; Tips; Tips; Intro note; Tips: image setup; long takes"
    }
  ]
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