Only on GPT (never was) and more recently Claude, and it still can be simulated with structured outputs for the purpose above.
Even if it's not supported somewhere (e.g. z.ai API which isn't mature enough and has neither assistant prefills nor actual structured outputs), it's still better and more seamless than using the default user/assistant scaffolding for role alternation.
Even if it's not supported somewhere (e.g. z.ai API which isn't mature enough and has neither assistant prefills nor actual structured outputs), it's still better and more seamless than using the default user/assistant scaffolding for role alternation.