Phone-to-phone file transfer with no network, built overnight with animated QR codes

AI coding tools built this overnight: file transfer between two phones with no Wi-Fi, no Bluetooth and no network at all, using nothing but a screen and a camera.

One phone rapidly flashes animated QR codes. The other points its camera at that screen and rebuilds the file from the frames. In the project’s own words: no network path between the devices, no app, no pairing, no permissions beyond the camera.

The numbers

The documented throughput is roughly 129 KB/s goodput in the proof of concept, about 128 KB/s handheld and around 186 KB/s when the phones are propped. That moves a 2 MB image in seconds.

The clever part: error handling with no back-channel

A one-way channel has no back-channel, so the receiver cannot ask for a frame it missed. The system uses Luby transform fountain coding: each frame is the XOR of a pseudorandom subset of blocks, so frames can arrive blurred, duplicated or out of order, plenty can be dropped, and the file still reconstructs once enough data lands.

That is the engineering trick that turns a flickering screen into a usable data link. The sender just keeps streaming; the receiver just keeps watching until the math closes.

Built in one night

The origin story, as the developer tells it: he wanted a cached MP3 player to avoid ads and poor streaming quality, needed phone-to-phone transfers without shared internet, and built the working prototype in a single night with Claude Code. It is open source as Decimen Optical Transfer, published on GitHub under the handle bashalarmistalt.

Worth noting that optical file transfer is not new. The Reddit thread filled up with prior implementations going back decades. What is new is the throughput, and how fast one person got from idea to working code.

If the prototype takes one night now, the interesting question is which ideas stop being too small to bother building.