[BUG] Wrong Accept-Language sent with Firefox and WebKit

[lazka]

System info

• Playwright Version: 1.32.3
• Operating System: Ubuntu 23.04
• Browser: Firefox
• Other info:

Source code

• I provided exact source code that allows reproducing the issue locally.

Config file

// playwright.config.js
import { defineConfig, devices } from '@playwright/test';

export default defineConfig({
  projects: [
    {
      name: 'chromium',
      use: {
        ...devices['Desktop Chrome'],
      },
    },
    {
      name: 'firefox',
      use: {
        ...devices['Desktop Firefox'],
      },
    },
  ],
});

Test file (self-contained)

import { test } from '@playwright/test';

test.use({
  locale: 'en-GB',
});

test('test-send-with-Accept-Language', async ({ page }) => {
  await page.evaluate(async () => {
    await fetch(
      "https://api.github.com/rate_limit", {
        headers: {
            'Content-Type': 'application/json',
            'Accept-Language': 'de'
        },
      });
  });
});

Steps

• Run the test with Chromium
• Look at the web dev tools network tab and check the request headers
• See that Accept-Language=de is sent
• Run the test with Firefox
• Look at the web dev tools network tab and check the request headers

Expected

• The request is sent with the Accept-Language=de header, same as with Chromium

Actual

• The request is sent with the Accept-Language=en-GB header instead

[yury-s]

What I see in 1.35.0 is that all three browsers send three different values. Chromium doesn't send accept-language at all unless you add mode: 'no-cors'and in that case the header is equal to de.

[bschelling]

I just ran into something very similar: The accept language set as a customheaders is ignored, using the "locale" setting in the config works. I'd be fine if the "locale" setting would only overwrite the header if a value is present, but simply ignoring the accept-language header setting without any warning is evil.

[avetisk]

Indeed, I have the same issue where the custom accept-language header is overwritten in Firefox and Webkit.

It's annoying as hell as this feature works fine with the other browsers.

I'm going to try @bschelling's suggestion and see if I can temporary solve this using the locale setting.