I said partially because it does acknowledge the instructions and will try to follow it partially. It helps a lot if the conversation started with a certain tone, and will try to continue that way.

Start a conversation with GPT5 Fast for cheerful or friendly tones, start with GPT5 Thinking for more serious answers and tones. What model you use later in the chat matters little, but going from Fast to Thinking does show very clearly the more serious tone but will still "wrap it around" your previous funny tone. This is at least what I noticed on Default. GPT5 Fast is pretty much a dumb yes-man, so if you are talking about serious topics but still wants its tone use it for the first message only.