Twitter’s URL shortener and HTTP/2

Thanks to a unittest failing on webmentiond today I stumbled upon a weird issue: The default HTTP client in Go is running into a 404 error when trying to resolve a URL. Weirdly enough the same request went through with cURL even using the same HTTP headers. Eventually I tried it with enabling/disable HTTP/2 support and lo-and-behold, all of a sudden I got 200 status codes again.

For some reason and out of the blue changed its behaviour regarding HTTP/2 requests:

package main

import (


func TestTwitterH2(t *testing.T) {
	h1client := http.Client{}
	// Using Transport.TLSNextProto you can disable HTTP/2 support:
	tr := http.Transport{
		TLSNextProto: make(map[string]func(string, *tls.Conn) http.RoundTripper),
	h1client.Transport = &tr
	h2client := http.Client{}

	req, _ := http.NewRequest(http.MethodGet, "", nil)

	t.Run("h1", func(t *testing.T) {
		resp, err := h1client.Do(req)
		require.NoError(t, err)
		require.Equal(t, 200, resp.StatusCode)

	t.Run("h2", func(t *testing.T) {
		resp, err := h2client.Do(req)
		require.NoError(t, err)
		require.Equal(t, 404, resp.StatusCode)

In the h2 case I get a 404 error while in the h1 one I get 200. I haven’t yet found out what’s the difference here compared to running curl --http2 , though.

If anyone has some details about what’s going on there, please let me know 🙂