Capsule is a free, open-source application that lets anyone build web tools and package them into a single .capsule file containing the user interface, media assets, and an embedded SQLite database. It is currently available for macOS 12 Monterey and later, Windows 10 and later, and Linux distributions including Ubuntu, Debian, and Fedora, with a browser-based preview mode and native iOS and Android support forthcoming.
A .capsule file is a self-contained package. When opened, the application runs inside an isolated webview governed by a strict Content Security Policy that prevents access to the local filesystem and unauthorized network connections. The user interface is written in standard HTML, CSS, and JavaScript, and all data is stored in standard SQLite tables inside the same file. This means recipients can open a shared .capsule file on any supported platform without installing additional software—simply double-clicking the file launches the app with its data intact.
Because everything lives in one file, Capsule apps can be shared the way documents are sent: via AirDrop, email, or messaging apps. When multiple people edit separate copies of a .capsule file through cloud services like Dropbox or iCloud, Capsule batches writes into atomic transactions to keep files intact and includes a three-way merge tool to inspect changes and reconcile differences safely.
Capsule is not the only approach to local web app storage. The Origin Private File System (OPFS) is a browser API that provides private, high-performance file access within a web page's origin. Unlike Capsule's bundled-file approach, OPFS allows web applications to read and write files directly in the browser's private storage without packaging them into a standalone file. It is faster than the File System Access API because it avoids the security checks and permission prompts that the latter requires, and it offers synchronous calls that can run inside web workers without blocking the main thread. OPFS is subject to browser storage quota restrictions, and clearing site data deletes the OPFS contents.
Capsule's bundled approach makes apps trivially shareable and fully offline-ready, but it also means every state change requires redistributing the entire file. Browser-native storage APIs like OPFS avoid this friction by keeping state persistent on the device, but they lack Capsule's portability and sandboxed execution model. Both approaches serve different use cases, and the choice depends on whether the priority is document-like sharing or persistent local state.
Capsule is free and open source. Desktop versions are available for macOS, Windows, and Linux. A browser-based web version allows quick previews but cannot open or save files locally. Native iOS and Android support is forthcoming.