Frequently asked questions

Yes. DesignDojo is free and open source. There is no subscription, no paywall, and no usage cap. The only cost is whatever your AI provider charges you for tokens — and on cheap models like DeepSeek or Gemini Flash, a full interview session costs roughly $0.0001.

Instead of charging you a subscription that bundles AI usage, DesignDojo asks you to plug in your own API key from OpenAI, Anthropic, Gemini, OpenRouter, or Groq. The key is stored only in your browser — never sent to a DesignDojo server — and AI requests go directly from your browser to the provider you chose.

OpenAI, Anthropic (Claude), Google Gemini, OpenRouter, and Groq are supported out of the box. You can also point DesignDojo at a local model running on Ollama or LM Studio — keys never leave your machine in that mode.

Most providers give free credits to new accounts. OpenRouter and Groq have generous free tiers and very cheap models. If you use a local model via Ollama, it is completely free.

Two libraries: System Design (HLD) covers distributed-systems problems like URL shorteners, news feeds, ride-sharing, ad aggregators, and rate limiters. Low-Level Design (LLD) covers object-oriented design problems like parking lots, chess, vending machines, and file systems.

Every problem ships with a stage-by-stage rubric (requirements, estimation, API design, data model, high-level diagram, deep dives) written to match what senior interviewers actually look for. The tutor checks your answer against the rubric and returns specific must-fix gaps, suggestions, and follow-up questions — not generic praise.

The reading experience — landing page, FAQ, and every problem write-up under /learn — works fully on mobile. The interactive practice mode (whiteboard, code editor, AI tutor panel) needs a wider screen, so we redirect mobile users to the corresponding write-up. Use a laptop or desktop for hands-on practice.

Yes. Your AI key is kept in browser storage and never sent to a DesignDojo server. Your whiteboard, code, and answers are saved in IndexedDB on your own device. We do not run analytics on your interview content.

Absolutely — the project is open source on GitHub. Open an issue with the problem you want added, or submit a PR with a new question JSON and the matching write-up under content/articles.