dotnet/runtime · error · Error

This browser doesn't support fetch API. Please use a modern…

Error message

This browser doesn't support fetch API. Please use a modern browser. See also https://learn.microsoft.com/aspnet/core/blazor/supported-platforms

What it means

verifyEnvironment() throws this browser-specific message when globalThis.fetch or globalThis.AbortController are missing in a non-Node environment. It points the developer to the Blazor supported-platforms doc. It means the host browser is too old to provide the Fetch API the .NET HttpClient relies on.

Solutions

  1. Require a modern, supported browser per the Blazor supported-platforms doc (Edge, Chrome, Firefox, Safari recent versions).
  2. If you must support older browsers, load a fetch + AbortController polyfill (e.g. whatwg-fetch, abortcontroller-polyfill) before booting the runtime.
  3. Detect the missing API up front and show a user-facing 'browser not supported' message instead of letting HttpClient throw.
  4. For embedded WebViews, update the WebView runtime (WebView2 Evergreen) so a recent Chromium engine provides fetch.

Example fix

// before: legacy browser, no polyfill

// after: load polyfill then boot
import 'abortcontroller-polyfill/dist/polyfill-patch-fetch';
// (whatwg-fetch) then start dotnet
Defensive patterns

Strategy: validation

Validate before calling

// Feature-detect fetch in the browser before allowing HttpClient usage.
function isFetchSupported(): boolean {
  return typeof globalThis.fetch === "function" && typeof globalThis.AbortController === "function";
}

if (!isFetchSupported()) {
  // load a polyfill (whatwg-fetch + abortcontroller-polyfill) or show 'unsupported' UI
}

Type guard

function supportsFetchApi(): boolean {
  return typeof globalThis.fetch === "function" && typeof globalThis.AbortController === "function";
}

Try / catch

null

Prevention

When it happens

Trigger: Running the .NET WASM runtime (Blazor WASM or standalone) in a legacy browser (old Safari, IE, embedded WebView) that lacks window.fetch or AbortController. httpCreateController()/httpFetch() trigger verifyEnvironment().

Common situations: Users on Internet Explorer or very old Safari/Chrome; embedded WebViews in older desktop apps (old WebView2/Edge legacy); a polyfill that should have been loaded wasn't, so fetch is undefined; strict CSP blocking the fetch polyfill script.

Related errors

AI-assisted analysis of dotnet/runtime@60108ba66e (2026-08-10). Data as JSON: /api/errors/abeaa716e747fe43. Report an issue: GitHub.

Appendix: source

Thrown at src/native/libs/System.Runtime.InteropServices.JavaScript.Native/interop/http.ts:20

// The .NET Foundation licenses this file to you under the MIT license.

import BuildConfiguration from "consts:configuration";

import type { VoidPtr, ControllablePromise } from "./types";

import { wrapAsCancelablePromise } from "./cancelable-promise";
import { ENVIRONMENT_IS_NODE } from "./per-module";
import { assertJsInterop } from "./utils";
import { MemoryViewType, Span } from "./marshaled-types";
import { dotnetLogger, dotnetAssert } from "./cross-module";


function verifyEnvironment() {
    if (typeof globalThis.fetch !== "function" || typeof globalThis.AbortController !== "function") {
        const message = ENVIRONMENT_IS_NODE
            ? "Please install `node-fetch` and `node-abort-controller` npm packages to enable HTTP client support."
            : "This browser doesn't support fetch API. Please use a modern browser. See also https://learn.microsoft.com/aspnet/core/blazor/supported-platforms";
        throw new Error(message);
    }
}

function commonAsserts(controller: HttpController) {
    if (BuildConfiguration !== "Debug") {
        return;
    }
    assertJsInterop();
    dotnetAssert.check(controller, "expected controller");
}

export function httpSupportsStreamingRequest(): boolean {
    // Detecting streaming request support works like this:
    // If the browser doesn't support a particular body type, it calls toString() on the object and uses the result as the body.
    // So, if the browser doesn't support request streams, the request body becomes the string "[object ReadableStream]".
    // When a string is used as a body, it conveniently sets the Content-Type header to text/plain;charset=UTF-8.
    // So, if that header is set, then we know the browser doesn't support streams in request objects, and we can exit early.
    // Safari does support streams in request objects, but doesn't allow them to be used with fetch, so the duplex option is tested, which Safari doesn't currently support.

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