This is mostly dependent on the agent because the agent sets the system prompt. All coding agents include in the system prompt the instruction to write code, so the model will, unless you tell it not to. But to what extent they do this depends on that specific agent's system prompt, your initial prompt, the conversation context, agent files, etc.

If you were just chatting with the same model (not in an agent), it doesn't write code by default, because it's not in the system prompt.