Steps to replicate the issue (include links if applicable):

I created an OAuth2 consumer:
https://meta.wikimedia.org/wiki/Special:OAuthManageConsumers/70f9cff79286006878999171321d3ec5

I call the OAuth2 endpoint:
https://meta.wikimedia.org/w/rest.php/oauth2/authorize?response_type=code&response_type=code&client_id=70f9cff79286006878999171321d3ec5&state=<SOMETHING>&redirect_uri=https%3A%2F%2Fgulp.toolforge.org%2Fauth%2Fauthorized&scope=identify

What happens?:

Client authentication failed (e.g., unknown client, no client authentication included, or unsupported authentication method)

What should have happened instead?:
Authorize button for the tool