Core

Rheo is built around a plugin architecture. At its center is a core engine that embeds the Typst compiler and provides shared infrastructure – spine resolution, asset management, relative link rewriting, and a development server with live reloading. On top of this core, each output format is implemented as a format plugin.

Format plugins

Typst natively supports PDF and, experimentally, HTML. Rheo’s architecture takes this further: rather than being limited to the formats that Typst itself can produce, Rheo defines a FormatPlugin trait that any output format can implement. The EPUB format, for instance, is not something Typst supports at all – it exists entirely as a Rheo format plugin that converts Typst’s output into a standards-compliant EPUB archive.

Each format plugin declares:

• A compilation target – whether it builds from Typst’s paged (PDF) or HTML output.
• Whether it merges multiple source files into a single output, or produces one output per file.
• Any assets it needs – such as the HTML plugin’s CSS stylesheet and JavaScript entrypoint.
• Optional template files and configuration that are scaffolded when you run rheo init.

The core engine handles the rest: it discovers source files, resolves the spine for each format, copies assets, and invokes each plugin’s compilation in turn. If one format fails to compile a particular file, the others continue uninterrupted.

An open architecture

Our hope is that this plugin architecture will inspire innovation at the format level. Typst is a remarkably expressive document language, and we believe its utility extends well beyond the formats it natively targets.

We have built Rheo as infrastructure for augmented reading research. Reading augmentations – features like hyperlink previews, contextual definitions, interactive visualizations, and explorable explanations – can be implemented either as Typst library functions (at the language level) or as Rheo format plugins (at the compiler level). The existing tooltip example, for instance, delivers an augmentation through a custom Typst function and a JavaScript entrypoint, compiled to the browser via Rheo’s HTML plugin. By making it straightforward to implement new format plugins and to extend existing ones with custom assets, Rheo opens the door to richer ways of reading documents – not just new ways of producing them.

If you are interested in building a format plugin, take a look at the existing plugin implementations for reference. Adding a new plugin to Rheo requires only implementing the FormatPlugin trait and registering it in the CLI – the core engine takes care of everything else.