Wrangler's unstable_startWorker()

Last updated Jul 27, 2026 Caution

unstable_startWorker() is deprecated. Cloudflare recommends using the createTestHarness() API, which provides a harness specifically designed for integration testing.

The unstable_startWorker() API exposes the internals of the Wrangler dev server, and allows you to customize how it runs. Compared to using Miniflare directly for testing, you can pass in a Wrangler configuration file, and it will automatically load the configuration for you.

This example uses node:test, but should apply to any testing framework:

import assert from "node:assert";
import test, { after, before, describe } from "node:test";
import { unstable_startWorker } from "wrangler";

describe("worker", () => {
    let worker;

    before(async () => {
        worker = await unstable_startWorker({ config: "wrangler.json" });
    });

    test("hello world", async () => {
        assert.strictEqual(
            await (await worker.fetch("http://example.com")).text(),
            "Hello world",
        );
    });

    after(async () => {
        await worker.dispose();
    });
});

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