How I built a supersonic AI riddling duel in under 20 Minutes (with zero manual coding)

Can an AI developer agent build complex full-stack logic without constant hand-holding?

To test this, I set up the #NapkinChallenge: take a rough architectural sketch on a paper napkin and turn it into a fully functional web application in under 20 minutes.

The goal: Build "Blaine the Mono's Riddling Competition" — a game where you duel a psychotic, sentient supersonic monorail from Stephen King's The Dark Tower series. If you trip him up with three logic-defying riddles, you save the passengers. If his neural net solves them, you crash at Mach 4.

The result? The AI built the entire application with zero manual lines of code written by me.

Here is how the architecture works, how the backend uses the new @google/genai SDK, and how you can run the challenge yourself.

The Architecture (from sketch to code)

The napkin sketch outlined a straightforward but highly interactive setup:

1. Frontend: A UI built in Next.js (App Router).
2. Backend: A Next.js API route that forwards user riddles to the Gemini API.
3. The Engine: The gemini-3-flash-preview model, prompted to act as Blaine the Mono. It must analyze the riddle and return structured JSON containing its solution, status, and theatrical psychotic commentary.

Key Takeaways

1. Zero-Code Feasibility: Developer agents like Antigravity can now understand visual layouts, bootstrap templates, design customized styling systems, and integrate real-world API clients with robust error boundaries.
2. Styling is a First-Class Citizen: The agent did not just generate functional code; it constructed a bespoke styling sheet (src/app/globals.css) incorporating Orbitron typography, custom CSS scrollbars, CRT overlays, and reactive screen shakes using CSS keyframes.
3. Time to Gameplay: The entire application was built in under 20 minutes, including the time it took to generate the code, test it, and get it running.

Video

See the full video of the process:

Join the #NapkinChallenge 📝

Now it is your turn. Can you match the speed of autonomous generation?

1. Sketch your app concept, system architecture, or database model on a physical paper napkin.
2. Upload the photo to Antigravity, Gemini, or Google AI Studio.
3. Prompt your agent to build the code from the image alone.
4. Share a video or screenshot of your live app using #NapkinChallenge!

Let's see what you can build!