Tessera: a letter with no moving parts but you

Tessera writes letters to the future. To your kid at eighteen, to yourself at sixty, to a stranger in 2126. You write on a page that runs entirely on your device, seal it, and out comes a folder of plain files plus a print kit: the letter typeset for paper, a cover sheet, instructions for whoever opens it, and a token sheet you cut in two.

The name is old. A tessera hospitalis was a token of hospitality, a piece of bone or clay broken in half, one half kept by each side, so that generations later the descendants could fit the two halves back together and know each other as friends. Every Tessera letter makes one of those. You keep one half. The other half travels with the sealed letter until someone matches them.

The whole project answers one problem. Most ways of sending a message to the future are promises that die with the company that made them. You hand your letter to a service that swears to email your future self in fourteen years, and somewhere in those fourteen years the startup gets acquired and sunset, or the domain lapses, or the card on file expires and the lifetime plan quietly stops being for life. A letter meant to outlive its author ends up depending on a business outliving its Series A. So I designed Tessera to have as few moving parts as I could manage, and then to be honest about the one part I couldn’t remove.

Paper is the original, not the printout

The instinct is to treat the app as the real thing and the paper as a backup. I did it the other way. The printed kit contains everything: the full letter, the token, the instructions. The digital folder is the convenience copy. If you only keep one, keep the paper. It needs no battery, no format that has to survive a century of software churn, no account that has to still exist. A drawer will do.

That decision changes what “sealing” a letter means. It isn’t encryption. Nothing in Tessera stops a person from opening an envelope early, and I say so plainly on the page. Paper letters have crossed decades for as long as there have been letters, and what carried them was never a lock. It was custody, a promise, and someone deciding to wait. Tessera trusts the same thing, because that’s the thing that actually works.

The format outlives the tool that made it

A Tessera letter is one folder of plain text files: the letter, a machine-readable manifest, a human-readable README that explains itself to whoever finds it, checksums, and the token art. The format is public domain, CC0. The tool that generates it is a zero-dependency static page, MIT-licensed, with no build step and nothing to install. You can fork it, mirror it, or drop the whole thing on a USB stick.

I wanted the answer to “what if Lacunae disappears” to be boring. If my tool is gone in 2040, the folder is still readable by anyone with patience and a text editor, and the format is public enough that someone else can rebuild the opener from the README. No promise in the system depends on a server staying up, a company staying solvent, or me staying alive. The only moving part left is you and the people you trust to carry a letter forward, which is the part that was always going to matter most.

Tessera is a sibling to Perpetūra, my interactive fiction. That project is a story about a feeling that reaches across a very long time, and this one is the actual instrument you can hold. You can try Tessera at tessera-letters.netlify.app.