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Typst is open source. It can be used as a cli tool https://github.com/typst/typst

There's also an LSP (Tinymist.) My Typst workflow is all local in VSCode, with the source markup in the left pane and a live preview on the right.

Is the format "open" as well?

I’m curious what you mean. It’s a language with an open source implementation.

Others can and do implement parts of the language - e.g. Pandoc implements a parser and evaluates Typst.