Why it matters
/about and /about/ are different URLs; serving both creates duplicates that split links and crawl budget.
How to fix it
Pick one form (Next.js default: no slash, via 308) and make the other permanently redirect. Keep canonicals, sitemap and links on the same form.
Example
// next.config.ts — default is trailingSlash: false (/about/ → 308 → /about)
const nextConfig: NextConfig = { trailingSlash: false }References
More seo rules
void seo reports 60 rules in this category. What search engines need from the raw response: status codes, titles and descriptions in <head>, self-referencing canonicals, one h1, crawlable links, Open Graph, icons, robots.txt and a sitemap with real dates.
img-alt-qualityAlt text is a filename, a generic word, or overly longlinks-not-crawlableNavigation uses elements crawlers can't followlink-textLinks with generic or empty textbroken-linksInternal links return 4xx/5xx (or go through redirects)hreflang-invalidhreflang uses invalid codes or relative URLshreflang-not-reciprocalhreflang set isn't self-referencing and reciprocalog-missingRequired Open Graph tags missing (og:title, og:type, og:image, og:url)og-incompleteog:description / og:site_name missing, or og:url ≠ canonical