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The VS Code extension

Live preview for .hmd knowledge bases: source on the left, the rendered card on the right, keeping up as you type. It is the one place where a card looks like a card while you are writing it — embeds stay embeds, links resolve as you type, and a link to nothing is red.

Install from the VS Marketplace Open VSX

ext install hypermarkdown.hmd-vsc-ext

Cursor, Windsurf, and VSCodium install from Open VSX; stock VS Code from the marketplace. Either way, open a .hmd file and click the ⚡ at the top right of the editor.

The extension previewing a card: source on the left, rendered card on the right, with resolved links, a table, a callout, and a d2 diagram.

What it gives you

  • A live preview that renders from the unsaved buffer and scroll-syncs with the editor in both directions.
  • Embeds as cards — labelled with the card and fragment they came from, collapsible, and clicking through to the card they came from rather than the one they are in. A preview that flattens an embed into anonymous prose has hidden the only thing worth seeing.
  • Red links, with an action that writes the missing card.
  • Backlinks for the card you are on, link and embed edges listed apart.
  • The same diagnostics as hmd lint in the Problems panel, under the same HMD001–HMD016 rule IDs.
  • Math, callouts, and D2 diagrams, and .hmd as its own language with its own highlighting.

No Python required

The extension carries its own implementation of the format — the parser, the resolver, and the renderer are TypeScript, running in the editor. Nothing has to be installed to see a card rendered: no interpreter to find, no virtualenv to activate, and no subprocess between a keystroke and the preview.

The hmd CLI stays canonical. Where the two could disagree they are checked against a shared conformance corpus on every push, so the preview and the published site are the same reading of the same file. Completion and the rest of the language-server features are not in the extension yet; they arrive with the Python language server, and the preview will keep rendering without it.

Diagrams are the one exception to nothing to install: a d2 fence draws when d2 is on your PATH or Docker is available, and shows its source when neither is.

This is a preview release

The preview surface is built and gated by tests. The graph tab is specified and unstarted, and HMD017 — the rule about a published card linking to a private one — is not ported yet, so the extension never reports it. The listing carries a preview flag until both land.

The full list of divergences from the CLI, each with its reason, is in the extension's README, what changed is in its changelog, and bugs go to the issue tracker.