Zero-cost architecture
PrepAI is built on a "zero-cost" architecture: it deliberately uses only free-tier services and client-side processing, so the app can stay free to use without anyone footing a usage-based bill. Understanding this explains both why PrepAI is free and why certain features work the way they do.
The principle
Every part of PrepAI is chosen so that ordinary usage costs nothing:
| Concern | How PrepAI handles it | Why it's free |
|---|---|---|
| Running your code | A sandboxed Web Worker in your browser | No server-side compiler farm to pay for. |
| Database & auth | Firebase free tier (Firestore + Auth) | Generous free quotas cover normal use. |
| AI features | Google Gemini free tier | Free-tier model calls, with a bring-your-own-key option for self-hosters. |
| File storage | FilesHub (free file API) | Avoids paid object storage. |
| Charts & tools | Client-side (D3, pdf.js, image libs) | Processing happens on your device. |
Client-first by design
The most important decision is that your code and your files are processed on your device, not on a server:
- Coding drills run in a Web Worker — private and free.
- Developer tools (PDF-to-Word, image compression, formatters) process everything locally.
- PDF exports are generated in the browser.
This isn't just a cost decision; it's a privacy benefit. There's no server that sees your solutions.
What this means for features
The architecture shapes the product honestly:
- No remote code judge. Grading is done against each drill's expected output and tests, locally — there's no farm compiling submissions in 40 languages.
- AI is free-tier. The AI interviewer is genuinely useful but bounded by free-tier limits; PrepAI doesn't pretend a paid model is running behind it.
- Storage uses FilesHub, not paid buckets. Uploaded assets go through a free file API.
Sustainability
A free-tier architecture is sustainable precisely because it doesn't scale into a usage bill. Optional Pro and Ultimate plans add convenience features for people who want to support the project, but the core experience — practice, mock interviews, reviews, analytics, and tools — is free.
FAQ
Is PrepAI really free? Yes, the core experience is free. Optional paid plans add conveniences.
Where does my code run? In a Web Worker on your device. It is not uploaded.
Does the AI cost the maintainer money? No — it uses Google Gemini's free tier, and self-hosters bring their own free key.
What stores my data? Firebase's free tier for account data and progress; FilesHub for any file uploads. See Privacy & data.