AI Video Prompt Builder
Write start-frame and 4-shot video prompts block by block, check them against safety and length rules, and copy captions for five platforms. Free, no sign-up, bring your own AI key if you want AI to write.
Free · No sign-upHow to use AI Video Prompt Builder
- 1. Fill the Builder blocks (character, location, outfit, light, camera) or start from the generic presets. The coloured preview on the right shows which block wrote which words.
- 2. Choose the On-screen text kind (title, POV, question, beat words, price reveal, place stamp, count, item tags, rule or Save end card) and fill its fields; then check the Checks panel: banned words, length limits (2,000 and 2,500 characters), @tag in the video prompt and weak camera plans each come with a Fix button.
- 3. Need many at once? Use Import (one idea per line, a CSV, or full script blocks) and AI writer with your own Gemini or Groq key. No key? Templates fill every card.
- 4. Open Cards, copy the image prompt, video prompt, hook or a platform caption, or export a .txt (script format) or .csv file.
GrabCast vs the prompt helpers inside Kling and Runway
Video generators ship their own prompt helpers. They are good at their own model's quirks. Here is an honest look at where this builder is different, and where it is not.
| GrabCast Prompt Builder | Built-in generator helpers | |
|---|---|---|
| Price | Free, no sign-up. AI writing uses your own key. | Usually tied to an account and the generator's credits. |
| Structure | Editable blocks (character, location, outfit, light, camera) and a 4-shot plan with 13 safe camera moves, coloured by block. | Typically one box that rewrites your sentence. |
| Safety lint | Banned words, length limits, @tag misuse, static cameras, with one-click fixes. | The generator tells you after you spend a credit. |
| Captions | Hook plus TikTok, Instagram, Facebook, Threads and YouTube fields with each platform's lengths. | Prompt only. |
| Your own AI | Bring a free Gemini or Groq key; batch many ideas at once. | Their model only. |
| Where it falls short | It does not generate video and cannot know a generator's newest rules. A built-in helper knows its own model better. | Locked to one generator. |
Frequently asked questions
Is the AI Video Prompt Builder free?
Yes. The builder, safety lint, import and export are free with no sign-up. If you want AI to write the prompts, it uses your own API key, so any cost is billed to your own provider account. A free Gemini or Groq key is enough for most people.
Which video generators does it work with?
It writes plain-English prompts for any image-to-video workflow: Kling, Veo, Runway, Sora-style tools and others. It does not generate video itself and cannot guarantee that a given generator accepts a prompt, because each platform changes its rules.
What does the safety lint check?
It flags words that most generators block (such as nude, see-through, sheer or monochrome), accents and other scripts when English-only is on, an @element tag in the video prompt, prompts over 2,000 characters (image) or 2,500 (video), static cameras and fewer than three camera moves. Many issues have a one-click Fix.
Where does my text go?
While you build, everything stays in your browser. Import, template fill, AI writing and export send only the text you are working on to GrabCast. AI writing then uses your connected key, or a key stored only in this browser. Characters you save while signed in are private to your account.
Can I use my own character or Kling Element?
Yes. Save a character (identity text and an optional @element tag) once and apply it to every prompt. The tag is added to the start-frame prompt only, and the lint blocks it from the video prompt.
What kinds of on-video text can I plan?
Ten kinds, each with its goal: title (save), POV (share), question (comment), beat words (stop), price reveal (comment), place stamp (follow), count 1-4 (watch), item tags (save), rule X / V (save) and a Save end card (save). Pick one opener plus at most one more text moment per video, and the Checks panel enforces each kind's limits (for example a question ends with ?, a place stamp is up to 24 characters, an end card starts with Save). The kinds are exported as script lines you can import into other tools.
What are the captions for?
Each card has a hook for on-video text plus platform fields: TikTok, Instagram, Facebook, Threads and YouTube. Lengths follow each platform's limits, with an AI-generated disclosure line. In the GrabCast Scheduler you can send them straight to a draft.