Instead of boasting about tomorrow's plans, what does James 4:15 say we ought to say?

Answer: If the Lord is willing

Since life is a mist that vanishes, James says to say, "If the Lord is willing, we will live and do this or that."

“Instead, you ought to say, “If the Lord is willing, we will live and do this or that.””— James 4:15 (BSB)