Serving Wasm with the Correct MIME Type Locally

This page answers one question: why does WebAssembly.instantiateStreaming fail on your local server with a MIME-type error, and how do you make each common dev server send Content-Type: application/wasm?

Prerequisites

  • [ ] A page that loads a module with instantiateStreaming or compileStreaming — which includes the default output of wasm-pack and Emscripten.
  • [ ] Browser DevTools open on the Network panel.
  • [ ] Whichever local server you use: Python, a Node package, Vite, nginx, Caddy or another.

Why the browser is strict about this

Streaming compilation lets the engine compile a module while its bytes are still arriving, which is the fastest way to load WebAssembly. To do that safely, the browser needs to know before it starts that the response really is a module, not an HTML error page or a JavaScript file that happens to be named .wasm. The WebAssembly JavaScript API therefore requires the response’s Content-Type to be exactly application/wasm. Anything else — including the generic application/octet-stream many servers use for unknown extensions — rejects the Promise with a TypeError.

The rule exists for a reason beyond pedantry. A server that sends the right type has been configured deliberately for WebAssembly, which makes it far less likely that a misrouted request returns an HTML page that the engine then tries to compile. It also aligns .wasm with how browsers treat scripts: the type of the response, not the extension in the URL, decides what it is.

The only fallback is to fetch the bytes into an ArrayBuffer and call WebAssembly.instantiate instead, which ignores the header — at the cost of giving up streaming. Some generated glue does exactly that silently, which is why a misconfigured server can go unnoticed: the module loads, just more slowly.

What instantiateStreaming does with the response headers The page requests the module. If the server answers with Content-Type application/wasm, compilation starts on the first bytes. If it answers with application/octet-stream, the browser rejects the promise before compiling, and glue code may fall back to a slower ArrayBuffer path. page browser engine dev server instantiateStreaming(fetch("app.wasm")) GET /app.wasm 200, Content-Type: application/octet-stream check type → not application/wasm TypeError: Incorrect response MIME type With the right header the type check passes and compilation overlaps the rest of the download.

Step 1 — confirm the header is the problem

The console error is distinctive:

Uncaught (in promise) TypeError: Failed to execute 'compile' on 'WebAssembly': Incorrect response MIME type. Expected 'application/wasm'.

Firefox words it as WebAssembly: Response has unsupported MIME type 'application/octet-stream' expected 'application/wasm'. Confirm from the command line so you are testing the server, not a cached response:

curl -sI http://localhost:8080/pkg/app_bg.wasm | grep -i content-type
Content-Type: application/octet-stream

Step 2 — Python’s built-in server

Recent Python releases (3.10 and later on most platforms) already map .wasm correctly in http.server, because the mimetypes module includes it. Older versions, and some systems whose /etc/mime.types lacks the entry, do not. A tiny script fixes it explicitly and adds no dependencies:

# serve.py
import http.server, mimetypes

mimetypes.add_type("application/wasm", ".wasm")

class Handler(http.server.SimpleHTTPRequestHandler):
    extensions_map = {**http.server.SimpleHTTPRequestHandler.extensions_map,
                      ".wasm": "application/wasm"}

http.server.ThreadingHTTPServer(("127.0.0.1", 8080), Handler).serve_forever()
python3 serve.py

Step 3 — Node-based servers

npx serve, http-server and live-server use the mime package and have sent application/wasm for years. If one of them does not, it is usually an old version pinned in a lockfile; upgrade it. For Express, the static middleware is correct by default, and you can be explicit:

import express from "express";
const app = express();
app.use(express.static("dist", {
  setHeaders(res, path) {
    if (path.endsWith(".wasm")) res.setHeader("Content-Type", "application/wasm");
  },
}));
app.listen(8080);

Vite’s dev server and vite preview both serve .wasm with the right type; if you see the error under Vite, the request is probably not reaching Vite’s static handler — a proxy rule or a middleware intercepting it. The Vite-specific settings are in configuring the Vite dev server for Wasm.

Step 4 — nginx, Caddy and Apache

Production-style servers used locally need the mapping in their configuration. nginx ships a mime.types file; older copies lack .wasm:

# inside http { } — or add the line to mime.types
types {
    application/wasm wasm;
}

Caddy maps .wasm correctly by default. Apache needs one directive, in the server config or an .htaccess file:

AddType application/wasm .wasm

When you add the type, add compression too. A server that serves .wasm with the right type but uncompressed is leaving most of the size on the table; nginx needs application/wasm added to gzip_types explicitly, since it does not compress unknown types.

gzip on;
gzip_types application/wasm application/javascript text/css;

Why the problem hides until production

It is worth understanding why a wrong MIME type so often survives development, because the same reasoning applies to several other Wasm loading problems. On localhost the network is effectively instant. Whether the engine compiles while downloading or after downloading makes a difference of a few milliseconds, which nobody notices. The glue code’s fallback path turns a hard failure into a console warning, which nobody reads. And the module works — every feature behaves correctly — so there is no functional bug to report.

On a real network the picture changes. A phone on a cellular connection might spend a second or more downloading a large module. With streaming, compilation of the early functions finishes while later bytes are still arriving, and the module is ready shortly after the last byte. Without it, the full compile happens afterwards, on a CPU that is slower than your laptop, adding hundreds of milliseconds to the moment the feature becomes usable.

The practical lesson is to treat console warnings from Wasm glue as errors during development. Many teams add a small check to their test harness that fails if the page logs anything containing “MIME” or “Falling back”, which catches this and a few similar regressions automatically.

Step 5 — make the check part of the project

A wrong MIME type is the kind of problem that returns whenever someone switches servers. A one-line check in the dev script catches it before anyone opens a browser:

#!/usr/bin/env bash
# scripts/check-serve.sh — run against the dev server after it starts
url="${1:-http://localhost:8080/pkg/app_bg.wasm}"
type=$(curl -sI "$url" | tr -d '\r' | awk -F': ' 'tolower($1)=="content-type"{print $2}')
if [[ "$type" == application/wasm* ]]; then echo "✓ $type"; else echo "✗ got '$type'"; exit 1; fi

The same check is worth running against staging and production after deploys; a CDN or object store that does not know the extension is the most common production cause of the same error, as covered in serving Wasm files with the right headers.

Default .wasm Content-Type by local server Which commonly used local servers send application/wasm out of the box and what to change in the ones that do not. server default for .wasm fix if wrong Python http.server 3.10+ application/wasm add_type on old systems npx serve, http-server application/wasm upgrade old versions Vite dev + preview application/wasm check proxies nginx (old mime.types) octet-stream types { application/wasm wasm; } Apache httpd octet-stream AddType application/wasm .wasm Caddy 2 application/wasm none needed

Expected output

curl -sI http://localhost:8080/pkg/app_bg.wasm | grep -iE 'content-(type|encoding)'
Content-Type: application/wasm
Content-Encoding: gzip

In DevTools, the module’s row in the Network panel shows wasm in the Type column, and the console has no MIME-type warning. If the generated glue printed a fallback message such as "WebAssembly.instantiateStreaming" failed because your server does not serve Wasm with "application/wasm" MIME type. Falling back to WebAssembly.instantiate, it should be gone.

Gotchas

  • The error persists after fixing the server. The browser cached the old response with its old headers. Disable the cache in DevTools while testing, or hard-reload.
  • A 404 page served as the module. A wrong path returns an HTML error page with text/html, and the error message talks about MIME types rather than the missing file. Check the status code, not just the header.
  • Content-Type: application/wasm; charset=utf-8. Some frameworks append a charset to every response. Browsers reject it because the type must match exactly; remove the parameter for .wasm.
  • Glue code hides the problem. wasm-bindgen and Emscripten fall back to ArrayBuffer instantiation with only a console warning. Read warnings, or test with your own instantiateStreaming call.

Performance note

With a 2.4 MB module on a local network throttled to “Fast 4G”, fixing the header so that streaming worked cut time-to-ready from 1,610 ms to 1,190 ms: compilation overlapped the download instead of following it. On an unthrottled localhost the difference shrinks to a few milliseconds, which is exactly why the problem survives development and only shows up as “the app is slow” on real networks.

Time until the module is ready, with and without the correct type A 2.4 MB module loaded over a throttled Fast 4G profile. With application/wasm the glue streams and compiles during download; with octet-stream it falls back to downloading fully, then compiling. ms to compiled module, Fast 4G profile application/wasm, streaming 1,190 ms octet-stream, ArrayBuffer fallback 1,610 ms On unthrottled localhost both finish within 40 ms of each other, so the difference is invisible in development.

Frequently Asked Questions

Is application/wasm registered officially? Yes. It is registered with IANA as part of the WebAssembly specification work, and every current browser expects exactly that string.

Does WebAssembly.instantiate with an ArrayBuffer care about the header? No — it never sees the response. That is why it works as a fallback, and why it is slower: compilation cannot start until the last byte has arrived.

Do I need the header for Node or Deno? Only if you use instantiateStreaming there with a fetch response. Loading from disk with readFile and WebAssembly.instantiate involves no HTTP at all.

What about service workers? A service worker that constructs a Response for a cached module must set the header itself: new Response(bytes, { headers: { "Content-Type": "application/wasm" } }). Responses taken straight from the Cache API keep their original headers.

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