It is Das Werkzeug.

But funnily enough: Die Werkzeuge (pl) Der Werkzeugkasten = the tool box I just talked to my friend (married to an American, who is utterly confused about masculine, feminine and neuter articles) and she asked her sister, a teacher ... there is no rule, you just have to know and/or guess and hope for the best.

I ran into the same issue just now. I have been pronouncing it vairk tsoykh for years and leo.org offers vark tsoykh as the correct pronunciation. Huh?