Cannot apply unknown utility class: font-bold — @apply in Vue/Svelte <style> or CSS modules needs @reference (v4)
Cannot apply unknown utility class: font-bold
Add @reference pointing at your main stylesheet (the one with @import "tailwindcss" and your @theme). It imports definitions for lookup only and emits no duplicate CSS.
Before:
<style scoped>
h1 { @apply text-2xl font-bold text-red-500; }
</style>
After:
<style scoped>
@reference "../../app.css";
h1 { @apply text-2xl font-bold text-red-500; }
</style>
If you use only the default theme (no @theme, @custom-variant, @plugin), @reference "tailwindcss"; is enough. To avoid long relative paths, define a subpath import in package.json ("imports": { "#app.css": "./src/css/app.css" }) and write @reference "#app.css";. Alternative that avoids @apply entirely: use theme CSS variables, e.g. color: var(--color-red-500);.
Also check the class still exists in v4: removed utilities like bg-opacity-50 raise the same error (use bg-black/50).
Symptom
After upgrading to Tailwind v4, @apply inside a Vue/Svelte/Astro <style> block, a *.module.css file, or any CSS file other than the one that imports Tailwind fails, even for built-in utilities:
Cannot apply unknown utility class: font-bold
Newer 4.1.x builds word it as: Cannot apply unknown utility class font-bold. Are you using CSS modules or similar and missing @reference? https://tailwindcss.com/docs/functions-and-directives#reference-directive
Cause
In Tailwind v4 (2025-01-22), stylesheets bundled separately from your main CSS file (CSS modules, component <style> blocks) are processed in isolation and do not see the theme variables, custom utilities or custom variants defined in the file that has @import "tailwindcss". In v3 the global JS config made them available everywhere.
Verify
The dev server compiles the component and the styles apply; no duplicate Tailwind preflight appears in the component's CSS chunk.
Sources
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