Hmm, maybe both...
How do you set your custom user agent?
Do you use OkHttp integration or just Glide?

There has been some changes to user agent handling, see 3.6.1 release notes and v3.6.0...v3.6.1.
As far as I can see every precaution has been made (isUserAgentDefault) to prevent concatenating two User-Agent headers, which is what you have if I see it correctly.

Hello Róbert,

I use just Glide, the new feature from 3.6.0 release. My usage:

Glide.with(context)
    .load(HttpHeader.getUrl(source))
    .into(ViewHolder.Image);

HttpHeader class:

public class HttpHeader {

    public static final String USER_AGENT = "Mozilla/5.0 (Windows NT 6.3; WOW64; rv:37.0) Gecko/20100101 Firefox/37.0";
    public static final String APP_AGENT = "XYZ";

    public static GlideUrl getUrl(String url) {
        if (TextUtils.isEmpty(url)) {
            return null;
        }
        else return new GlideUrl(url, new LazyHeaders.Builder()
                .addHeader("User-Agent", USER_AGENT)
                .addHeader("App-agent", APP_AGENT)
                .build());
    }
}

@sjudd confirmed, it's a bug, it looks like the isUserAgentDefault is always initialized false: 487d9c5#diff-0d69682540c80f964a21f69e247fbb99R120

LazyHeaders headers = new Builder()
        .addHeader("User-Agent", "My user agent")
        .addHeader("App-agent", "My app agent")
        .build();

System.out.println(headers);
System.out.println(headers.getHeaders());

3.6.0:

LazyHeaders{headers={App-agent=[StringHeaderFactory{value='My app agent'}], User-Agent=[StringHeaderFactory{value='My user agent'}]}}
{App-agent=My app agent, User-Agent=My user agent}

3.6.1:

LazyHeaders{headers={App-agent=[StringHeaderFactory{value='My app agent'}], Accept-Encoding=[StringHeaderFactory{value='identity'}], User-Agent=[StringHeaderFactory{value='Dalvik/1.6.0 (Linux; U; Android 4.4.2; GT-I9505 Build/KOT49H)'}, StringHeaderFactory{value='My user agent'}]}}
{Accept-Encoding=identity, App-agent=My app agent, User-Agent=Dalvik/1.6.0 (Linux; U; Android 4.4.2; GT-I9505 Build/KOT49H),My user agent}

@TomasValenta workaround is really simple if you want to use Glide 3.6.1 (addHeader -> setHeader):

LazyHeaders headers = new Builder()
        .setHeader("User-Agent", "My user agent")
        .addHeader("App-agent", "My app agent")
        .build();

Thank you Róbert!

Urk, thanks for reporting this, I'll take look!

@sjudd This looks like it's been fixed on your 3.0 branch and has been also merged to bumptech/glide 3.0 branch, but not yet released (scheduled for 3.6.2 or 3.7.0). I thought merging a "fixes #???" commit will close the issue, but it seems that only works on master or the default branch. Closing this manually because I see the commit on the right branch.

Confirmed fixed. Thank you!