Are your videos private? How browser-based captioning works

Short answer: it depends on the tool. Many caption apps upload your whole video to a server you do not control. A browser-based tool keeps the footage on your device and sends only the audio for transcription.

For a throwaway clip that difference may not matter. For client work, unreleased footage or anything personal, it decides whether a copy of your video exists on someone else's server. Below is what happens to your video when you add captions, and how to check a tool before you trust it with a clip.

Where your video goes when you add captions

With many caption tools, the first step is an upload. The whole video file goes to the company's servers, the captions are generated there, and you download the result.

A full copy of your footage then sits on infrastructure you do not own. What happens to it next depends on a privacy policy most people never read. It may be stored for days, kept in a project library tied to your account, or used to train a model. For a product launch under embargo or a client's footage under an NDA, you should know which.

What browser-based captioning does differently

A browser-based tool does the video work on your machine. Current browsers can decode, draw on and re-encode video inside the tab using an API called WebCodecs. So the heavy part, turning your clip into a captioned MP4, happens locally, and the video file is never uploaded.

One thing does have to leave your device, and it is not the video. To write captions the tool needs the words, and the words are in the audio. So the tool extracts the audio track in your browser and sends only that to a speech model. The audio is a small fraction of the video's size.

Audio versus the whole video

Transcription needs sound, not pixels.

Faces, screen recordings, the street behind you, the unreleased product on the table: none of that is in the audio. When only the audio is sent, every visual part of your clip stays on your device.

Check that a tool works this way rather than assuming it. Plenty of apps that feel fast still upload the full file in the background.

Why this matters

Once a copy of your video is on someone else's server, you are trusting their security, their staff and their retention policy for as long as they keep it. Breaches happen and policies change. The only way to be sure footage does not leak from a server is for it never to reach one.

Cases where that is worth insisting on:

• client or agency work covered by a confidentiality agreement
• unreleased products, announcements or anything under embargo
• personal videos, family footage or clips with other people in them
• screen recordings that show dashboards, messages or internal tools

There is a practical bonus too. Skipping the upload means you do not wait for a slow connection to push a large file before you can start.

How to check any caption tool

You do not have to take a marketing page at its word. Ask 4 questions:

1. Does it make you upload the whole video, or does the video work happen in the browser?
2. What does the privacy policy say about storing footage, and for how long?
3. Does it say what is sent for transcription: the audio only, or the full file?
4. Is your clip saved to a cloud library tied to an account, or does it stay local?

Most cloud tools say plainly that your video is uploaded, because that is how they work. The comparison pages show which popular tools upload to the cloud and which keep processing on your device.

Caption a clip without uploading it

In ReelCaption, the video is processed in your browser and never uploaded. Here is exactly what leaves your device:

• the audio track, extracted in your browser, is sent to Groq, which runs OpenAI's Whisper large-v3 model to transcribe it
• ReelCaption does not store the audio or the transcript on its servers
• the captioned MP4 is rendered in your browser and saved straight to your device

You drop in a clip, edit any word, pick a style, and export a captioned MP4 or an .srt or .vtt file. The privacy policy has the details.

For the full walkthrough from raw clip to finished captions, see how to add captions to a video in your browser.