I didn't test it with curl because I was assuming it would be used on the frontend, but as afn mentioned, I think it should work correctly if you add -g as shown below: curl -g -v 'https://echoserver.dev/server?query={"headers":[["foo","bar"...}'

Also, I am using render.com for hosting, and render.com uses Cloudflare internally, so it seems that related headers are also included. However, I believe this should not be a problem for testing purposes.