Koboshi
Koboshi is a vocabulary app for people who live in more than one language. You look up a word once, and Koboshi shows you what it means across the languages you already know, saves it, and brings it back for review right before you would forget it, on a spaced-repetition schedule (FSRS, the same family of scheduling that serious flashcard systems use). That is the whole product in one sentence. This post is about why it exists, and the story is quite personal.
The moment that made it click
“Ah, THAT word” moment does not always come from the same language.
When I look up a Spanish word and see its meaning, sometimes the click comes from the English meaning. Sometimes English does nothing for me and the Urdu meaning lands it instantly. Sometimes it is the Japanese. The language that unlocks a word is not predictable, so an app that shows you the meaning in one language is not enough for me.
Koboshi shows the meaning across your languages, side by side. And that produces a quiet second effect I did not fully appreciate until I lived with it: while you study the new language, your weaker languages get exercised too. Every lookup is a small rep for them. One stone, two birds: 一石二鳥, as we say in Japanese. The research points the same way, from several directions at once: a language you grew up speaking is dormant, not lost, and actively recalling it is what wakes it up.
The heritage speaker part is personal
I grew up in Pakistan. I speak Urdu like a native, because I am one: the rhythm, the accent, the jokes, all of it. But if I am honest about my Urdu vocabulary, it sits around A2, maybe B1. Fluent mouth, thin dictionary.
Part of that is just how Urdu works day to day: we swap English words into Urdu sentences constantly and they fit, so you can be a full member of the society without ever needing the harder native vocabulary. Plenty of educated people around me had the same tendency, just with a bit more schooling behind their word choices than mine.
That combination, sounding native while missing the vocab, has a name: being a heritage speaker. Koboshi is built for heritage speakers as much as for classroom learners.
Who this is actually for
If you already hold two or more languages and are adding another, Koboshi was built around your exact situation: the new language gets the serious spaced-repetition treatment, and your existing languages ride along, getting stronger instead of rusting.
And if one of your languages is a heritage language you would like to finally give the vocabulary it deserves, this is for you. Surprise your friends and family in other parts of the world in six months.
However, even if you do not have a heritage language, you could still use Koboshi, and it should be nearly as helpful as it is for the heritage speakers. You still get to learn a new language while strengthening the other you already know. Even if you are learning the first foreign language, it is still useful for you to use it as a learning engine and your personal dictionary.
The real origin story: a ChatGPT project that kept forgetting
Before Koboshi existed, my system was a ChatGPT project. Every new word I met went into the chat. Most of them came from Duolingo: it tracks the words its lessons introduce, but the listening exercises kept handing me words that never showed up in its tracking, and I wanted to keep those.
It worked, for about two or three months at a time. Then the chat would fill up. A conversation has a memory limit (the context window), and when it fills, the oldest content quietly falls out. Words I had carefully saved were just gone. So I would start the next chat in the same project and begin the pile again.
ChatGPT will happily offer to export everything into a spreadsheet, and that fixes nothing: the memory limit is still there, and no one in 2026 is learning vocabulary out of a spreadsheet. What I needed was not storage. I needed to record a word in the same motion as meeting it, and to be able to learn it in the same place, with review that happens by schedule instead of by willpower.
Building that had recently become possible for one person. So I built it. Koboshi is the tool I wished existed during those three-month cycles: look it up, see it across your languages, it saves itself, and it comes back to quiz you right before you would have lost it. The name comes from 起き上がり小法師, the little Japanese doll that rights itself every time it falls, and from 取りこぼし, the things you let slip while trying to hold on. Words you looked up and lost. That is exactly what this app rescues. Cool Gaijin stuff right?
Where it goes from here
Koboshi is young, and it gets better every week. If the situation I described is yours, try it with a handful of words you met today.
