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

Thrown by verifyEnvironment in http.ts when not running on NodeJS (i.e. a browser) and either globalThis.fetch or globalThis.AbortController is missing. Modern browsers all provide both; this fires only on obsolete browsers (old WebView, legacy mobile WebViews, IE) or in unusual embedded web runtimes that lack the Fetch API.

Solutions

Use a supported modern browser (see the linked supported-platforms doc).
If you must support the old engine, polyfill fetch and AbortController with whatwg-fetch / abortcontroller-polyfill before the runtime starts.
Avoid HttpClient in code paths that run on unsupported browsers.

Example fix

// before: no fetch in old WebView
// after: load polyfills before dotnet runtime
import 'whatwg-fetch';
import 'abortcontroller-polyfill/dist/abortcontroller-polyfill-only';

Defensive patterns

Strategy: type-guard

Validate before calling

// before using HttpClient from the wasm runtime in a browser
if (typeof globalThis.fetch !== 'function' || typeof globalThis.AbortController !== 'function') {
  // load whatwg-fetch and abortcontroller-polyfill, or show 'unsupported browser'
}

Type guard

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

Prevention

Feature-detect fetch and AbortController before enabling HTTP scenarios.
Document minimum browser versions to users.
Provide a polyfill bundle for legacy WebView targets.

When it happens

Trigger: Loading the .NET wasm runtime in Internet Explorer or an old WebView that lacks fetch/AbortController; a custom embedded browser shell that did not expose Fetch API; HttpClient used in an environment that intentionally disabled fetch.

Common situations: Targeting legacy browsers; running inside an old Cordova/Capacitor WebView; corporate locked-down browsers.

Related errors

AI-assisted analysis of dotnet/runtime@60108ba66e (2026-08-10). Data as JSON: /api/errors/94ce940277f664be. Report an issue: GitHub.

Appendix: source

Thrown at src/mono/browser/runtime/http.ts:20

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

import BuildConfiguration from "consts:configuration";

import { wrap_as_cancelable_promise } from "./cancelable-promise";
import { ENVIRONMENT_IS_NODE, loaderHelpers, mono_assert } from "./globals";
import { assert_js_interop } from "./invoke-js";
import { MemoryViewType, Span } from "./marshal";
import type { VoidPtr } from "./types/emscripten";
import { ControllablePromise } from "./types/internal";
import { mono_log_debug } from "./logging";


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) {
    assert_js_interop();
    mono_assert(controller, "expected controller");
}

export function http_wasm_supports_streaming_request (): 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.
    // See https://developer.chrome.com/articles/fetch-streaming-requests/
    if (typeof Request !== "undefined" && "body" in Request.prototype && typeof ReadableStream === "function" && typeof TransformStream === "function") {
        let duplexAccessed = false;

View on GitHub (pinned to 60108ba66e)