I'm trying to find unanswered questions with both kotlin and gradle tags like this: https://stackoverflow.com/unanswered/tagged/kotlin+gradle
It shows many questions without kotlin tag which is unexpected.
All questions with url like /questions/tagged/kotlin+gradle works correctly.
It has been made clear that using the phrases: [kotlin][gradle] (tags that have "kotlin" or "gradle"), kotlin+gradle (tags that have in them "kotlin" and "gradle"), kotlin+gradle (all search terms that have "kotlin" and "gradle") do not show only unanswered questions.
I think this is because the unanswered page description says "questions with no upvoted or accepted answers tagged with", that is, it mixes questions with no upvotes and no accepted answers, so at the moment, there is no clear way to separate this list into only unanswered questions.
In short, there is no official way to do what you asked for, unless you use @Ilya's workaround.
Note: I put this link to show demonstrate how the site mixes other types of questions with unanswered questions, and also to show that it works for other keywords as well.
new-navis, may be I need to toggle some setting? The link you provided doesn't work for me. It shows only questions with both tags, but includes answered questions.[kotlin] -[android], it yields zero questions while there clearly are (stackoverflow.com/unanswered/tagged/kotlin+-android)answers:0into search query instead of using "unanswered"