Four places ffmpeg.wasm fails silently in a Next.js app (and the fixes)

I shipped four browser-only video tools with ffmpeg.wasm: trim, compress, video-to-GIF and MP3 extraction. Files never leave the browser, nothing to install. Trim · Compress · GIF · MP3 (Korean UI, but the buttons are obvious) Getting there, I hit four walls. Every one of them surfaced as a single "conversion failed" line in the UI and nothing in the console . Writing them down for the next person. Stack: Next.js App Router + webpack, @ffmpeg/ffmpeg 0.12, self-hosted core. 1. webpack hijacks the dynamic import inside the worker @ffmpeg/ffmpeg spawns its worker like this: new Worker ( new URL ( " ./worker.js " , import . meta . url ), { type : " module " }); webpack recognises the pattern and bundles the worker. Fine. But it also rewrites the import(coreURL) inside that worker to go through its own module loader. The core URL arrives at runtime as a blob: URL, which webpack's loader has never heard of, so it dies with Cannot find module 'blob:...' . The error is thrown inside the worker, so the main-thread console stays empty. Fix: keep the worker out of the bundle. Copy node_modules/@ffmpeg/ffmpeg/dist/esm/worker.js to public/ffmpeg/<version>/lib/ and pass it via classWorkerURL in load() . Now the untouched worker runs. 2. classWorkerURL needs the origin Passing a path like /ffmpeg/0.12.x/lib/worker.js is not enough. The library resolves it with new URL(classWorkerURL, import.meta.url) , and inside the bundle import.meta.url is a build-time file:///C:/... path. So it goes looking for file:///C:/ffmpeg/... and fails. const BASE = `/ffmpeg/ ${ FFMPEG_VERSION } ` ; await ffmpeg . load ({ coreURL : ` ${ location . origin }${ BASE } /core/ffmpeg-core.js` , wasmURL : ` ${ location . origin }${ BASE } /core/ffmpeg-core.wasm` , classWorkerURL : ` ${ location . origin }${ BASE } /lib/worker.js` , }); Prefix location.origin and it works. 3. You cannot build a GIF palette with -vf For decent GIF quality you run palettegen first and paletteuse second. Doing it in one pass needs