Next.js · silent bug

params.slug is undefined (params is a Promise)

Property 'slug' does not exist on type 'Promise<{ slug: string; }>'.  ·  page renders "undefined"

What it means

Since Next.js 15, params and searchParams are Promises. Code from older tutorials reads params.slug directly. In our Next.js 16.4.3 test build that didn't fail at all: the page just rendered "undefined".

Common causes

• The page is typed { params: { slug: string } } (the old shape), so TypeScript doesn't catch it
• Agents trained on Next.js 13/14 examples
• Same for searchParams, cookies() and headers(): all async now

How to fix it

1. Type params as a Promise and await it in Server Components
2. In a Client Component, unwrap it with React's use(params)
3. Search the codebase for params. and searchParams. without an await

export default async function Page({
  params,
}: {
  params: Promise<{ slug: string }>;
}) {
  const { slug } = await params;
  // …
}

Prompt for Cursor or Claude Code

This is a Next.js 16 app, where params, searchParams, cookies() and headers() are async. Find every page, layout, route handler and generateMetadata that reads them synchronously, type them as Promises and await them (or use() in Client Components). List each file you change.

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