Your Tailwind v4 upgrade throws 'trying to use tailwindcss directly as a PostCSS plugin'? Here's every cause, the sneaky CI-only variant, and the real fix.
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Route "/[slug]" used `params.slug`. `params` should be awaited before using its properties — here's exactly why Next.js 15+ broke synchronous params, and how to fix every case.
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`params` is a Promise and must be unwrapped with await or React.use() before accessing its properties — the exact fix for Next.js 15 and 16 App Router.
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Next.js turned params and searchParams into Promises. Here's the exact warning text, why v16 makes it crash instead of warn, and how to fix it for good.
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[postcss] It looks like you're trying to use `tailwindcss` directly as a PostCSS plugin. Every cause of this Tailwind v4 error, plus the silent one that hits next.
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