Cobalt is an open-source application platform for Kobo e-readers that enables apps to run on devices like the Kobo Clara BW. It includes a launcher, a signed App Store, a Rust SDK, and a runtime that isolates each app in its own unprivileged process. Installation occurs once via USB, after which apps install, update, and remove over Wi-Fi. A reboot returns the device to its stock reader. Apps are static ARM binaries with verified signatures before launch. The platform supports various apps including arXiv, Sudoku, Morse, Gutenbird, Hacker News, Feeds, Daily Brief, Sidekick, Terminal, and more. The SDK simplifies app development with declarative UI definitions, e-ink refresh planning, and capability-gated resource access. The Store delivers signed packages independently of platform updates, with each app merging via pull request triggering automated builds and signing. Safety measures ensure device writes match exact hardware/firmware profiles, and the project is not affiliated with Rakuten Kobo.

The Cobalt runtime verifies catalogs, packages, manifests, and binaries before execution. Apps request resources rather than accessing them directly, with network, storage, audio, frontlight, and Wi-Fi capabilities gated. The UI toolkit provides e-ink optimized controls, layouts, typography, and states. Simulators offer browser and runtime testing with layout diagnostics. State management uses atomic per-app keyed storage. Publishing apps requires running on actual hardware, with contributors opening PRs containing proof of functionality.

Installation involves cloning the repository, connecting a supported device over USB, and running setup commands. The complete walkthrough includes recovery steps. Contributing apps follows standard pull request workflows with unit tests, layout tests, and simulator verification. Porting to other Kobo models is encouraged but requires prior discussion.

Cobalt prioritizes safety by not replacing Kobo's boot chain and maintaining recovery-safe transactions. Only the Clara BW profile has been hardware-tested, with other models requiring reviewed profiles before installation.