This is where we write about how Kool is built, the thinking behind our choices, and the small details we care about.
Unlike the Newsroom, which is for announcements, the blog is for the longer, more honest story: the engineering, the design, the trade-offs.
More soon.
The pieces we needed, cheap edge compute, on-device encryption and good codecs, only lined up recently.
Building this in 2024 would have meant compromises we were not willing to make.
It was called four other things first. Two were taken, one was unpronounceable, and one tested badly in every language we tried.
It was called four other things first. Two were taken, one was unpronounceable, and one tested badly in every language we tried.
A short and slightly embarrassing history.
What we shipped, what we cut, and the week everything was on fire.
Written up honestly, including the parts that did not go well.
Help articles are not documentation you write once and forget. They are where people go when the product has already failed them.
Help articles are not documentation you write once and forget. They are where people go when the product has already failed them.
So we treat the help centre like a feature, with the same review bar.
Most social apps assume five bars. Ours assumes a train tunnel.
Queued sends, optimistic UI, and what happens when the reconnect lands out of order.
We collect the minimum needed to run a social app. No third-party analytics, no advertising trackers, no cross-app tracking.
This post walks through what that means in practice, from passwordless sign-in to short-lived tokens and contextual-only ads.
Passwords are the weakest link in almost every breach, and the worst part of every sign-up flow. So we removed them.
Passwords are the weakest link in almost every breach, and the worst part of every sign-up flow. So we removed them.
Kool uses passwordless sign-in end to end. Nothing to remember, nothing to leak, nothing for us to store badly.
Your life doesn't come in five separate apps. Chat becomes a call, the call becomes a meeting, the meeting ends with a clip everyone shares.
Here's how we thought about collapsing all of that into one identity and one place.
Most product typography apologises for existing. We wanted the opposite: headlines that carry weight and know it.
Anton is condensed, heavy and slightly awkward at small sizes, which is exactly why it works when you give it room.
Dark mode bolted on at the end always looks bolted on. Every colour in Kool is a token, and every token has both values from the moment it is created.
Dark mode bolted on at the end always looks bolted on. Every colour in Kool is a token, and every token has both values from the moment it is created.
Nothing ships until it has been read in both.
Every transition in Kool answers a question: where did this come from, and where did it go.
Animation that does not answer one of those gets cut.
Once for light, once for dark. Optical weight does not survive an inversion, so the same shape needs two masters.
Once for light, once for dark. Optical weight does not survive an inversion, so the same shape needs two masters.
The rules we settled on, and the ones we broke.
Durable Objects give us one authoritative place per conversation. That turns a hard distributed problem into an ordinary one.
How delivery, receipts and retries actually work.
You can keep people safe without reading everything they write.
You can keep people safe without reading everything they write.
Reports, rate limits, reputation and on-device signals, plus the deliberate limits we placed on ourselves.
Going live with friends sounds simple until you have four audio tracks, four video tracks and one viewer on a train.
This is how the live pipeline mixes co-hosts on device without melting the battery.
A badge that means someone paid us is worth nothing. A badge that means this is really them is worth a lot.
A badge that means someone paid us is worth nothing. A badge that means this is really them is worth a lot.
We went with the second one, and this is how verification works underneath.
Connect Gmail, Outlook, Yahoo or any IMAP account and read all of it inside Kool.
The hard part was not the protocols. It was making six very different inboxes feel like one calm list.
A build number that is just the commit count, a lane that does everything, and no human in the release path.
The whole pipeline, and why cadence beats ceremony.
Coins are how value moves around Kool: tips, gifts, boosts and payouts.
Coins are how value moves around Kool: tips, gifts, boosts and payouts.
No gambling mechanics, no loot boxes, no dark patterns. Here is the whole model, written plainly.
Kool runs on Workers at the edge, close to whoever opened the app. No regions to pick, no cold starts to explain away.
A tour of the stack, and the things that got harder because of it.
Face ID to open the chat. Screenshots off. Read receipts off. Nothing synced to the cloud.
Some conversations deserve a door that actually locks.
Contrast, hit targets, reduced motion and screen readers are not a hardening pass at the end.
Contrast, hit targets, reduced motion and screen readers are not a hardening pass at the end.
They are acceptance criteria, and nothing merges without them.
A thousand people used Kool before anyone else did, and they were not gentle.
A thousand people used Kool before anyone else did, and they were not gentle.
The five things they changed our minds about, and the one thing we refused to budge on.
Adding a track to a story is one tap. Making that legal in every country it can be seen in is not.
How the music layer works, and why it sometimes says no.
Creators need numbers to grow. Nobody needs surveillance to get them.
Creators need numbers to grow. Nobody needs surveillance to get them.
We show reach, retention and the moments people replayed, and nothing that identifies a single viewer.
One set of tokens drives the app, the website, the help centre and the status page.
When the accent colour changed, it changed everywhere in an afternoon. That is the whole point.
Keeping a call alive in a floating window while the rest of the app carries on sounds like a checkbox. It is not a checkbox.
A short tour of what breaks, and what we did about it.
The business model is the product decision. Once you sell attention by the gram, every other choice follows.
The business model is the product decision. Once you sell attention by the gram, every other choice follows.
We went with contextual ads and paid features instead, and this is the arithmetic behind that.
87 articles, 11 languages, 957 translations, and a Worker that keeps them all in sync when the English changes.
How the translation pipeline runs itself.