A Caption API for Webinars: Turn Every Recording into Captioned Clips Automatically

Every webinar ends the same way: an hour of good material lands in a folder as a single MP4, and the plan to clip it later quietly dies. The recordings pile up because the work after the webinar, scrubbing the timeline, pulling clips, captioning them, is manual and slow. The fix is not more discipline. It is removing the manual step.

caption.sh handles the captioning part of that pipeline as an API call. Cut your webinar recording into clips however you like, a simple ffmpeg trim works fine, then POST each clip and get back a new MP4 with word-timed, styled captions burned in. The captions come out in the animated, word-by-word styles that perform on LinkedIn, YouTube Shorts, and TikTok, which is where webinar clips actually get watched.

The pipeline

# 1. trim a highlight out of the recording (30s - 2:30)
ffmpeg -ss 30 -to 150 -i webinar.mp4 -c copy clip.mp4

# 2. upload somewhere addressable, then caption it
curl -X POST https://api.caption.sh/v1/videos \
  -H "Authorization: Bearer $CAPTION_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "source_url": "https://cdn.example.com/clip.mp4",
    "options": {
      "font": "inter",
      "position": "bottom"
    }
  }'

# 3. poll GET /v1/jobs/{id} until status is done,
#    then GET /v1/jobs/{id}/result (a 302 to the signed MP4 URL)

There are no webhooks to configure and no editor to open. You poll the job until it completes and download the result. Because the captions are burned into the pixels, they render identically in every player and feed, with no subtitle support required on the other end.

Why this works for webinars specifically

• Webinars are speech-dense, which is exactly what word-level timing makes watchable. Viewers decide in the first seconds whether a clip is worth their time, and moving captions carry that decision.
• The cost is a known number: $0.10 per minute, prepaid. Captioning every clip from an hour-long webinar costs a few dollars, not an afternoon of an editor's time.
• The style is identical across every clip. Set the font and animation once and the whole series looks like one brand, not a stack of one-off exports.

If your team runs webinars even monthly, the recordings are the cheapest content you own. A trim script plus one API call per clip turns them into a steady stream of captioned shorts. Get an API key at caption.sh and run one clip from your last webinar through it.