When sending a non-simple CORS request (e.g. some JSON via sendBeacon), a "cache-control" header is added to the preflight request when it is fetched by a Service Worker. Since this is unexpected for most of the servers, the sendBeacon request fails in that case. I implemented a simple test page that reproduces the issue here: https://falling-bolt-17.app.baqend.com/ The test page registers a simple Service Worker which listens to fetch requests and tries to fetch them. It also includes the following script tag that sends the beacon request: <script> // dummy data var data = '{"currency":"EUR","is_logged_in":false,"producer_properties":{"common":{"sent_by":"","timestamp":"2022-02-15T07:02:51.078Z","producer_id":"","tracking_method":"collector","application_version":""},"web_frontend":{"platform":"desktop","user_agent":"","view_id":"","container_name":"","visitor_id":"","domain":"","url":"","referrer_url":""}}}'; // create blob with data for beachon var blob = new Blob([data], { type: 'application/vnd.gyg.v2+json' }); // send beacon request navigator.sendBeacon('https://collector.getyourguide.com/track/HomePageRequest', blob); </script> Ways to reproduce: 1. Go to https://falling-bolt-17.app.baqend.com/ 2. Open JavaScript console 3. Refresh the page 4. Check the request headers of the 'HomePageRequest' request on the network tab and notice that it failed and that a "cache-control" header with the value "max-age=0" was added The error can also be seen in the Service Worker console (Develop -> Service Workers). Is there any fix for this issue?