How to Build Resumable S3 Multipart Uploads in Next.js Without Overloading Your Server

A developer guide published on DEV Community outlines a three-step pattern for handling large file uploads in Next.js App Router without routing file bytes through the server. The approach uses AWS S3 multipart uploads combined with presigned URLs, so the browser uploads file chunks directly to S3 while the Next.js Route Handler only processes metadata and enforces validation policies. Files are split into 5–10MB chunks on the client side, uploaded via XMLHttpRequest to enable real-time progress tracking, and limited to 3–6 concurrent transfers to avoid network strain. Each successfully uploaded part's ETag and part number must be recorded — ideally in IndexedDB — to support resumable uploads after interruptions or page refreshes. The server finalizes the multipart upload only after all parts are confirmed, keeping memory usage low and making the solution suitable for serverless hosting environments.