Custom e-ink firmware for the Xteink X4 — a $70 ESP32-C3 e-reader with 380KB of RAM, a 480×800 display, and five buttons.

SUMI turns it into an offline-first reader with apps, Bluetooth keyboards, and a Game Boy emulator. No WiFi, no cloud, no accounts. Books and tools on paper-like glass.

Got existing ebooks? Run them through the Universal Converter before loading. Most EPUBs are built for color tablets — oversized images, web fonts, heavy CSS. The converter strips all that out and tunes everything for e-ink: grayscale images at display resolution, clean XHTML, no junk. Same books, noticeably faster rendering on the 380KB device.

Built on Papyrix by Pavel Liashkov (@bigbag) — an excellent open-source reader firmware. Papyrix itself is a fork of CrossPoint Reader by Dave Allie. SUMI strips out WiFi, adds a plugin system, games, productivity apps, Bluetooth, and a visual rebrand. The reading engine is Pavel's and Dave's work — outstanding open-source software.

Companion tools at sumi.page — browser-based converters, a newspaper builder, flashcard creator, font converter, file transfer, and more.