Liquid / Shopify theme lint: POST {code}, get the bugs with line numbers. Deterministic — never rendered, no AI. Catches unbalanced/misnested blocks, unknown tags/filters with did-you-mean, the JS-isms every dev writes in Liquid (&&, !, ternaries, arithmetic in output, parens in conditions), deprecated img_url/include, and FULL {% schema %} validation: setting types, duplicate ids, range/select rules, blocks, presets. POST bare schema JSON to test just a schema. Max 128 KB.