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SekaiBuilder

At Web Summit 2026

Builder
of worlds.

I’m Francesco, a software developer who ships real products. I build apps that make learning stick, keep work organized, and leave your data on your device.

See my products Available nowWork with me
Catch cats and climb the Web Summit ranking世界

Products

Three apps people use to learn and stay organized. Mobile versions are on the way.

KaTrainer on a phone: home screen, hiragana flashcards, and quiz settings

Japanese · Web app

KaTrainer

Learn to read Japanese from zero. Master hiragana, katakana, and your first N5 kanji with smart flashcards, quizzes that adapt to your mistakes, and animated stroke order.

Coming soon to the App Store and Google Play

Try KaTrainer free
Postik: sticky notes that stick to the exact spot

Productivity · macOS

Postik

Sticky notes that stick to the exact spot: a window, a browser tab, even a single line of text. Come back and your note is waiting. Everything stays on your Mac.

Coming soon to the App Store and Google Play

Get Postik
A Kioku study session: a Japanese flashcard with review buttons

Spaced repetition · Web app

Kioku Closed beta

Remember anything for good. Kioku schedules every review at the right moment, imports your Anki decks in one step, and keeps working offline.

Coming soon to the App Store and Google Play

Request beta access

Tools and experiments

Spectator summary: no failures, one decision only you can make

Developer tool · VS Code and CLI

Spectator

Explains any codebase back to you in plain words. It maps your backend endpoint by endpoint and sorts every finding into proven failures, risks, and decisions only you can make. No AI, no cloud.

GitHub ↗
Exodus self-test: every secret masked before it leaves the machine

Privacy · Open source

Exodus

A local firewall for AI. It masks API keys, card numbers, and personal data before a prompt leaves your machine, then restores them in the reply.

GitHub ↗
The SonicRouter mixer with a volume slider for each app

Audio · macOS

SonicRouter

Volume and audio output for each app on your Mac. Send music to your speakers and calls to your headphones, right from the menu bar.

AI agents · Open source

Paradax

A team of local AI agents that produce media and check their own work before handing it over.

GitHub ↗
  • PrivacyMac

    Blur your screen in one keystroke when someone looks over your shoulder.

    macOS
  • Sintony

    Listen to the same song together on Apple Music, each from your own account.

    Mac and iPhone

I’m Francesco.

I build software end to end: product idea, interface, backend, security, and release. I start from problems I know firsthand, like learning kanji or losing track of a note, and turn them into tools people use every day.

Open to freelance projects and full-time roles.

Let’s build something.

Have a product idea, a role to fill, or questions about one of my apps? Write to me.

contact@sekaibuilder.com

Handmade: plain HTML, CSS, and JavaScript, no trackers, and a strict security policy. The cat in the intro travels through 56 art styles.