How to Add Auto-Generated Captions to Clips Without Uploading

Why captions are non-negotiable for shorts

• Muted autoplay. Feeds start videos silent; captions are what make a talk-heavy clip survive its first two seconds.
• Retention. Word-by-word captions give viewers something to track and make jokes and punchlines land even at speed.
• Accessibility. Captions are simply how a meaningful share of your audience watches everything.

For short-form, captions are usually burned in (rendered into the pixels) rather than attached as a subtitle track, because vertical platforms do not reliably show uploaded subtitle files in the feed.

How captions work without a cloud service

The piece that used to require a cloud API is speech-to-text. That changed when OpenAI released the Whisper model openly, and projects like whisper.cpp reimplemented it to run efficiently on consumer CPUs and GPUs. A local caption pipeline is:

1. Extract the clip's audio.
2. Run it through a local Whisper model, producing text with word-level timestamps.
3. Render the words as styled captions synced to those timestamps.
4. Burn the rendered captions into the exported video.

Everything happens on your hardware. Nothing is uploaded, there is no per-minute fee, and unreleased or sensitive footage stays private.

Local caption options on Windows

1. FastClip: transcription and caption burn-in are built into the clip-export workflow, so captioning is a checkbox rather than a separate tool chain.
2. Standalone Whisper tools (whisper.cpp builds, Buzz, and similar front-ends): produce an .srt subtitle file locally. Good when you want the text itself, but you still need an editor or FFmpeg pass to style and burn the captions in.
3. Editors with built-in transcription (DaVinci Resolve, Premiere): powerful styling control, heavier workflow, and in some products the transcription is a cloud feature, so check where the audio goes if privacy matters.

The FastClip caption workflow

1. Import a long video and let FastClip propose highlight candidates (covered in turning a long video into vertical clips).
2. Select the clips to export and enable captions.
3. FastClip runs whisper.cpp on the clip audio locally and burns the captions into the 1080×1920 MP4 at export.

The free beta includes the clean_white caption style; premium caption styles are part of the planned one-time Pro license.

Getting accurate transcriptions

• Clean audio in, clean text out. Whisper is impressively robust, but crosstalk, heavy game audio under speech, and clipped microphones are what produce garbled captions.
• Proper nouns and slang miss most often. Player names, channel in-jokes, and game terms are worth a quick proofread; a wrong name in a burned-in caption cannot be fixed after posting.
• Watch the clip once before posting. Caption errors cluster at exactly the moments that made the clip worth posting: shouting, overlap, chaos.

Limitations

• Burned-in means permanent. Once exported, the captions are pixels. Keep the source clip if you might want a different style later.
• FastClip captions FastClip's clips. It is not a general-purpose subtitle editor for arbitrary videos; for that, a standalone Whisper front-end plus an editor is the better fit.
• English-heavy accuracy. Whisper supports many languages, but accuracy varies; test on your language and audio before committing to a batch.