denoland/deno · error · TypeError

Invalid WebAssembly content type

Error message

Invalid WebAssembly content type

What it means

WebAssembly.compileStreaming/instantiateStreaming in Deno validate the response Content-Type: it must equal 'application/wasm' after lowercasing, otherwise the compiler would be fed bytes of unknown type. The check is skipped for file:// URLs because file fetches carry no Content-Type. Note the comparison is exact, so parameters such as 'application/wasm; charset=utf-8' are rejected too.

Source

Thrown at ext/fetch/26_fetch.js:1027

  try {
    const res = webidl.converters["Response"](
      source,
      "Failed to execute 'WebAssembly.compileStreaming'",
      "Argument 1",
    );

    // 2.3.
    // The spec is ambiguous here, see
    // https://github.com/WebAssembly/spec/issues/1138. The WPT tests expect
    // the raw value of the Content-Type attribute lowercased. We ignore this
    // for file:// because file fetches don't have a Content-Type.
    if (!StringPrototypeStartsWith(res.url, "file://")) {
      const contentType = res.headers.get("Content-Type");
      if (
        typeof contentType !== "string" ||
        StringPrototypeToLowerCase(contentType) !== "application/wasm"
      ) {
        throw new TypeError("Invalid WebAssembly content type");
      }
    }

    // 2.5.
    if (!res.ok) {
      throw new TypeError(
        `Failed to receive WebAssembly content: HTTP status code ${res.status}`,
      );
    }

    // Pass the resolved URL to v8.
    op_wasm_streaming_set_url(rid, res.url);

    if (res.body !== null) {
      // 2.6.
      // Rather than consuming the body as an ArrayBuffer, this feeds each chunk
      // to the streaming compiler as soon as it's available. Instead of reading
      // the body chunk-by-chunk in JS and calling `op_wasm_streaming_feed` once

View on GitHub (pinned to 89f33cbef2)

Solutions

1. Serve the module with exactly 'Content-Type: application/wasm' and no parameters
2. Fall back to non-streaming compilation: WebAssembly.instantiate(await res.arrayBuffer(), imports)
3. If you cannot change headers, fetch the bytes first, verify manually, then compile from the buffer

Example fix

// before
const { instance } = await WebAssembly.instantiateStreaming(fetch("/lib.wasm")); // Invalid WebAssembly content type

// after (option 1: fix server header to application/wasm)
// after (option 2: compile from buffer)
const res = await fetch("/lib.wasm");
if (!res.ok) throw new Error(`HTTP ${res.status}`);
const { instance } = await WebAssembly.instantiate(await res.arrayBuffer());

function isWasmResponseType(res) {
  const ct = res.headers.get("content-type");
  return typeof ct === "string" && ct.toLowerCase().split(";")[0].trim() === "application/wasm";
}

function canStreamCompile(res: Response): boolean {
  const ct = res.headers.get("content-type");
  return res.ok && ct !== null && ct.toLowerCase().startsWith("application/wasm");
}

async function instantiateAnyhow(url, imports) {
  const res = await fetch(url);
  if (!res.ok) throw new Error(`HTTP ${res.status}`);
  try {
    return await WebAssembly.instantiateStreaming(Promise.resolve(res), imports);
  } catch (err) {
    if (err instanceof TypeError && err.message === "Invalid WebAssembly content type") {
      return await WebAssembly.instantiate(await res.arrayBuffer(), imports); // non-streaming fallback
    }
    throw err;
  }
}

When it happens

Trigger: A server responding with Content-Type: text/plain, application/octet-stream, no Content-Type header, or application/wasm with appended parameters (charset) when the wasm module is fetched for instantiateStreaming; happens for http(s) URLs but not file://.

Common situations: Static file servers that guess MIME types and mark .wasm as application/octet-stream; dev servers without a wasm MIME mapping; CDNs/frameworks that append charset to every text-ish response; proxy layers stripping headers.

Related errors

• Failed to receive WebAssembly content: HTTP status code ${re
• Body can not be decoded as form data
• Missing content type

AI-assisted analysis of denoland/deno@89f33cbef2 (2026-08-16). Data as JSON: /api/errors/fa704bd53dabe5db. Report an issue: GitHub.