[Bug]: When routes are enabled, Service Worker fetch requests always receive a `cache-control` header in WebKit

Version

1.61.1

Steps to reproduce

Clone my repo at https://github.com/achingbrain/safari-service-worker-playwright-fetch-headers

npm i
npm run test

Expected behavior

When a service worker is installed it intercepts all browser requests, either from the address bar or from window.fetch running on a page.

When Playwright routes are enabled, it disables HTTP caching. In Safari it seems to do this by sending a cache-control: no-cache header.

If a browser request is made via window.fetch, custom headers can be specified, including cache-control.

In Chromium, specifying a cache-control header on a window.fetch request intended to be intercepted by a service worker works as expected, the service worker sees the correct cache-control value.

In WebKit it doesn't, the service worker always sees cache-control: no-cache, regardless of what the user specified.

I would expect WebKit to match the Chromium behaviour and allow specifying cache-control values.

Actual behavior

Service workers running in WebKit always receive cache-control: no-cache, regardless of what the user specified.

Additional context

The test in my application this affects ensures that the cache-control header is handled correctly by the service worker. It's impossible for this to pass if the test cannot control the cache-control header value.

It also has a catch-all route configured to ensure the test doesn't accidentally make requests to the actual internet which creates the conflict described above.

---

Additionally, WebKit always receives accept-language: en-US and the value cannot be overridden, it Chromium it can. This also seems like a bug.

The accept header seems to work as expected in Chromium and WebKit.

Environment

System:
    OS: macOS 26.5.1
    CPU: (16) arm64 Apple M4 Max
    Memory: 4.51 GB / 64.00 GB
  Binaries:
    Node: 24.13.1 - /Users/alex/.nvm/versions/node/v24.13.1/bin/node
    npm: 11.8.0 - /Users/alex/.nvm/versions/node/v24.13.1/bin/npm
    pnpm: 11.4.0 - /Users/alex/.nvm/versions/node/v24.13.1/bin/pnpm
    Deno: 2.7.12 - /Users/alex/.nvm/versions/node/v24.13.1/bin/deno
  IDEs:
    VSCode: 1.128.0 - /usr/local/bin/code
  Languages:
    Bash: 3.2.57 - /bin/bash
  npmPackages:
    @playwright/test: ^1.61.1 => 1.61.1