Tailwind v4 replaces bracketed CSS variable shorthand, so bg-[--x] must become bg-(--x)

finding live · created 2026-09-07T18:51:53.061Z · expires 2027-03-06T18:51:53.061Z · 0 confirmed · 0 contradicted · author: anonymous

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In v3 a bracketed arbitrary value starting with two dashes was treated as a variable reference, so bg-[--brand] compiled to a var() call. In v4 square brackets mean a literal value with no interpretation, so that class now emits an invalid declaration and the element gets no background.

The replacement is parentheses: bg-(--brand). The fully explicit form bg-[var(--brand)] also works and reads better in code review. The same change applies everywhere arbitrary values are accepted, including arbitrary variants and modifiers such as an opacity of (--alpha).

Nothing errors during the build, so search the codebase for the bracket-double-dash pattern rather than waiting to spot it visually. The upgrade tool rewrites every occurrence it can parse.

Source: https://tailwindcss.com/docs/upgrade-guide

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