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If ffmpeg.wasm works in development but stalls or fails after deployment, check the browser boundary, worker and core asset requests, cross-origin isolation for multithread builds, and the awaited virtual-filesystem workflow. Start with the single-thread core unless your workload needs multithreading and your site can support its security headers and cross-origin requirements.
1. The ffmpeg.wasm code is running on the server
ffmpeg.wasm is a browser-based WebAssembly and JavaScript tool. Its main package starts a Web Worker, and the project’s current installation guidance targets browser use; the FAQ says Node.js support was discontinued before version 0.12.0. In Next.js, do not initialize or call the worker-backed API during server rendering or from a server component. The project overview describes the worker architecture, and the installation guide covers browser support and package setup.
A client component is not a guarantee that browser globals exist at every point in its lifecycle. Avoid touching window, Worker, or an FFmpeg instance at module evaluation or during server rendering. Initialize after the component mounts or in a browser-triggered action, keep one reusable instance rather than creating a worker on every render, and expose loading and error states in the UI.
Clues and checks
- Look for prerender or build errors,
window is not defined, worker-construction errors, or a loading interface that never starts. - Confirm the initialization path runs in the browser. Check the browser console as well as the Next.js server terminal.
- Handle a rejected
load()promise visibly instead of leaving the interface in a permanent loading state.
2. The worker or core asset request fails
The worker loads and initializes a separate FFmpeg core. A deployment can therefore fail if an asset was not included or served, the configured URL is wrong, a base path changes the expected location, or the host returns a blocked or invalid response. These are deployment-specific possibilities, not behavior that occurs in every Next.js build. The installation guide explains that the worker-spawning package cannot be imported from a CDN in the documented way and recommends installing packages and hosting them on the app’s server in most cases: ffmpeg.wasm installation.
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Check the actual requests
- Open the browser’s DevTools Network panel before calling
load(), then trigger loading. - Find the worker and each core asset request. Check that the requested URL is the expected origin and path, and inspect the status and response content type.
- Review the browser console for worker errors, failed fetches, CORS failures, or Content Security Policy blocks. A UI that appears to have advanced is not proof that the core initialized.
Install the supported packages with npm or Yarn, serve assets at stable URLs, and configure an explicit core URL if your app’s layout or deployment path requires one. Keep the wrapper and core versions aligned. The configuration field and bundler syntax depend on the ffmpeg.wasm version: consult the matching version’s API and example rather than copying a 0.11.x snippet into a 0.12-or-later app. The project maintains a separate documentation branch for 0.11.x: ffmpeg.wasm documentation.
3. A multithread core is missing cross-origin isolation
The official framework example index labels its Next.js example as the single-thread version. The project distinguishes @ffmpeg/core from @ffmpeg/core-mt; the multithread core uses additional workers. Threaded WebAssembly depends on shared memory, which browsers restrict to secure, cross-origin-isolated pages. The examples index and the overview describe the project’s examples and core options.
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MDN states that a document must be in a secure context and cross-origin isolated to use shared memory. Check this in the browser console with self.crossOriginIsolated; a value of false means the page is not isolated. See MDN’s SharedArrayBuffer reference.
Enable isolation only if the app can support it
To enable a threaded build, serve the relevant page with these response headers:
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Cross-Origin-Opener-Policy: same-originCross-Origin-Embedder-Policy: require-corp
web.dev explains the headers, how to verify isolation with self.crossOriginIsolated, and how COEP affects cross-origin resources: Cross-origin isolation guide. In Next.js, the headers() configuration in next.config.js can set response headers for matched routes; confirm that your hosting adapter and CDN preserve them. See Next.js headers configuration.
After enabling COEP, verify that every required cross-origin worker and core asset meets the applicable CORS or CORP policy. Isolation can also affect popup communication and resource loading. If OAuth popups, payment windows, embeds, or third-party assets make those constraints impractical, use the single-thread core rather than applying COEP across the site without checking dependencies.
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4. The async job or virtual filesystem flow is incomplete
ffmpeg.wasm runs work in a worker, and most API calls are asynchronous. The documented workflow puts input into the core’s virtual filesystem before processing and reads the result back afterward. A conversion may appear silent if a promise is not awaited, a rejection is swallowed, the command uses a different path from the one written, or output is read before execution finishes. A resolved execution promise alone does not prove that the expected file exists. See the overview and the API reference.
Trace one job from input to output
- Await
load()and surface any rejection. - Write the selected input into the virtual filesystem, recording the exact filename and path.
- Await
exec()with an argument array. Capture FFmpeg log and progress events, and inspect the returned exit status. - Read the output only after execution completes. Check the exact output path in the virtual filesystem before treating the job as successful.
- Convert the returned bytes into a browser download, and present distinct UI states for loading the core, reading input, transcoding, writing output, success, and error.
When the job fails, compare the input path written with the path passed to FFmpeg, then compare the command’s output filename with the path read afterward. Keep exceptions, logs, progress, and exit information visible while diagnosing instead of catching and discarding failures.
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Choose the core that fits the deployment
Single-thread execution is the compatibility baseline: it avoids the extra shared-memory and cross-origin-isolation requirements of the multithread core. Multithreading may improve speed, but it also uses more CPU and memory and is described by the project as less stable. The FAQ characterizes the speed difference as “around 2x speed comparing to single thread”; that is the project’s approximate general claim, not a guaranteed benchmark for a particular file, device, or app. The same FAQ says WebAssembly is slower than native FFmpeg. See the ffmpeg.wasm FAQ.
Base the choice on the workload and deployment, not a speed assumption. Consider compatibility, expected performance for your files, CPU and memory limits on user devices, and whether the site can safely enable isolation without breaking cross-origin resources or popup integrations. The project FAQ currently states a 2 GB maximum input-file size as a hard WebAssembly limit; that is not a promise that files near the limit will process reliably on every device. Check the FAQ and the package version you deploy. For very large files or server-side jobs, browser processing may be the wrong architecture.
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