Marko 6 language

Distilled from the full Marko 6 docs (reference, explanation, guide, tutorial; Marko 5 / class API intentionally excluded). This file = rules agents get wrong + quick reference. Full detail lives in references/. When something here doesn't match observed behavior, the live docs win — Marko moves fast: index at https://markojs.com/llms.txt; any page fetches as markdown by appending .md (e.g. https://markojs.com/docs/reference/core-tag.md).

Marko 6 only — never write Marko 5

Reject on sight: class { onCreate/onMount... } blocks, $ const x = ... scriptlets, <if(cond)>/<for(x in y)> paren args, string handlers on-click("handleClick"), state/this.state, <include>/<invoke>, components/ discovery, Marko.Component types. Marko 6 equivalents: <let>/<const>/<script>/<lifecycle>, <if=cond>, <for|x| of=y>, onClick() {}, tags/ discovery, export interface Input.

Checklist for any new or edited .marko file

• Marko 6 syntax only (list above).
• State first: no <script>/<lifecycle> where <let>/<const>/controllable natives (value:=) do the job; <script> only for genuine browser effects.
• Handlers are functions (onClick() {} / onClick=fn), never strings; (event, element) args, no currentTarget.
• export interface Input declared; typecheck with mtc (@marko/type-check) — tsc skips .marko and reports success.
• Ship minimal client JS: prefer HTML/CSS (<details>, <dialog>, :has, popover) over scripted behavior; only reactive parts ship anyway.
• Keep components small; split by concern, not by technology (bundle size is unaffected by splitting).
• SSR-safe: nothing unserializable in <let>; $signal/element refs only in <script>/<lifecycle>/handlers.
• Immutable updates (x = x.concat()), by= on reorderable <for>s, <show> vs <if> chosen deliberately.