Next.js 15 Upgrade Causes 80% Traffic Loss
A developer is sharing a cautionary tale after their Next.js 15 app lost 80% of its traffic overnight following a production deployment. The root causes were subtle breaking changes in routing, misconfigured SSR leading to missing meta tags, and outdated plugin compatibility, creating major crawl issues for search engines.
Next.js 15, released in October 2024, introduced several significant breaking changes, moving to support React 19 RC by default in the App Router. A core change is a shift in caching behavior, where `fetch` requests, GET Route Handlers, and client-side navigations are no longer cached by default, requiring developers to opt-in for caching. One of the most impactful changes is that request-specific APIs like `cookies`, `headers`, and `searchParams` are now asynchronous. This architectural shift requires developers to update their data-fetching patterns, though Vercel has provided codemods to help automate this migration process. The reported SEO issues often stem from how Server-Side Rendering (SSR) handles metadata. Developers have encountered problems where meta tags, crucial for social media crawlers, are not included in the initial server-rendered HTML, leading to poor indexing. This can happen if the metadata is streamed after the initial response, which many crawlers do not wait for. Compatibility issues with the broader ecosystem have also surfaced. The update to React 19 RC has caused peer dependency conflicts with popular libraries like Shadcn UI. Additionally, some plugins have failed during the build process due to a mismatch with the underlying Speedy Web Compiler (SWC) runtime used by Next.js 15.
