dotnet/runtime · error · Error

Failed to load Webcil module '${virtualPath}'. HTTP status:

Error message

Failed to load Webcil module '${virtualPath}'. HTTP status: ${(res as Response)?.status} ${(res as Response)?.statusText}

What it means

Thrown by checkWebcilResponse() (src/native/libs/Common/JavaScript/host/assets.ts:83) when the awaited Response for a Webcil asset is falsy or res.ok === false. instantiateWebcilModule awaits this check before allocating the payload buffer, so a failed fetch surfaces as an HTTP status rather than a cryptic WebAssembly error.

Source

Thrown at src/native/libs/Common/JavaScript/host/assets.ts:86

            instance = instantiated.instance;
        } else {
            const data = await res.arrayBuffer();
            const instantiated = await WebAssembly.instantiate(data, imports);
            instance = instantiated.instance;
        }
        finishWebcilInstance(instance, payloadPtr, payloadSize, tableEntries, virtualPath);
    } catch (err) {
        // Instantiation failed after the payload buffer was allocated; free it to avoid leaking
        // unmanaged memory. (A grown R2R table cannot be shrunk back, but a failed R2R instantiate is fatal.)
        _ems_._free(payloadPtr as any);
        throw err;
    }
}

async function checkWebcilResponse(webcilPromise: Promise<Response>, virtualPath: string): Promise<Response> {
    const res = await webcilPromise;
    if (!res || res.ok === false) {
        throw new Error(`Failed to load Webcil module '${virtualPath}'. HTTP status: ${(res as Response)?.status} ${(res as Response)?.statusText}`);
    }
    return res;
}

// Allocates a 16-byte-aligned buffer for the Webcil payload. The pointer is heap memory that
// outlives the stack frame, so it can be passed as the imageBase import.
function allocWebcilPayload(payloadSize: number): number {
    const sp = _ems_.stackSave();
    try {
        const ptrPtr = _ems_.stackAlloc(sizeOfPtr);
        if (_ems_._posix_memalign(ptrPtr as any, 16, payloadSize)) {
            throw new Error("posix_memalign failed for Webcil payload");
        }
        return _ems_.HEAPU32[ptrPtr as any >>> 2];
    } finally {
        _ems_.stackRestore(sp);
    }
}

View on GitHub (pinned to 60108ba66e)

Solutions

  1. Open the failing virtualPath URL directly in the browser/network tab and confirm it returns 200 with Content-Type application/wasm.
  2. Verify the deployed wwwroot/_framework contains the .wasm file and the application base / CDN prefix matches.
  3. If using a custom loadBootResourceCallback, return a Response that resolves ok:true (and the correct URL).
  4. Fix CORS so the response is usable (correct headers, no opaque no-cors response).

Example fix

// before
loadBootResource: (type, name, uri) => uri.replace('/cdn/', '/wrong/') // -> 404

// after
loadBootResource: (type, name, uri) => uri // return a resolvable URL
Defensive patterns

Strategy: retry

Validate before calling

async function probeWebcil(url: string): Promise<boolean> {
  const r = await fetch(url, { method: 'HEAD' });
  return r.ok;
}

Try / catch

try {
  await instantiateWebcilModule(webcilPromise, memory, virtualPath, tableSize, payloadSize);
} catch (err) {
  if (/Failed to load Webcil module/.test(String((err as Error).message))) {
    // inspect the HTTP status embedded in the message; redeploy the asset or fix CORS
  }
  throw err;
}

Prevention

When it happens

Trigger: fetchWebcil() resolves the webcilPromise via loadResource, then instantiateWebcilModule calls checkWebcilResponse. A 404 for the .wasm file, a CDN error (5xx), a CORS block producing an opaque response, or a custom loadBootResourceCallback returning a failed Response all trigger this.

Common situations: Asset not published/deployed (404); wrong application base / CDN path; CORS misconfiguration returning an error response; integrity check failure promoted to a non-OK response; a custom loadBootResourceCallback returning the wrong URL.

Related errors

AI-assisted analysis of dotnet/runtime@60108ba66e (2026-08-10). Data as JSON: /api/errors/c9d8f9bdffb56510. Report an issue: GitHub.