They have introduced a beta 'Preferences' feature recently under Custom Instructions. I've had good results from this preference setting in GPT:

    Answer concisely when appropriate, more 
    extensively when necessary.  Avoid rhetorical 
    flourishes, bonhomie, and (above all) cliches.  
    Take a forward-thinking view. OK to be mildly 
    positive and encouraging but NEVER sycophantic 
    or cloying.  Above all, NEVER use the phrase 
    "You're absolutely right."

I just copied it into Claude's preferences field, we'll see if it helps.