What is a system prompt in AI roleplay?
Updated
A system prompt is the instruction placed at the top of every request to an AI model, before the conversation. It tells the model what role to play and how to behave: which character it is, the style and length of replies, and any rules. In roleplay apps the system prompt is usually built from the app's main prompt plus the character card and your persona.
Try it now, free, no account neededStart a story with Detective Rosa QuinnThe case is buried. She brought a shovel. You're in.Where it sits
Chat models receive a list of messages, each with a role. The first one, with the role "system", sets the frame for everything after:
{ "role": "system", "content": "You are Rosa Quinn, a homicide detective. Write in third person, past tense, two to four paragraphs. Never write {{user}}'s actions or dialogue." }
Then come the "user" and "assistant" messages of the actual chat. The model treats the system message as the ground rules rather than as something said in the story. If you use an API directly, you write this message yourself; roleplay apps assemble it for you.
What goes into a roleplay system prompt
- Who the model plays: usually the character card's description, personality and scenario.
- Who you are: your persona.
- Writing rules: perspective, tense, length, formatting such as *actions in asterisks*.
- Boundaries: do not speak for the user, stay in character, keep the pace slow.
- Lorebook entries and example dialogue, depending on the app.
SillyTavern calls the general instruction the Main Prompt and lets a card supply its own system_prompt; that field can include {{original}} to keep the main one. It also has post-history instructions, which go after the chat and are read right before the reply.
Common mistakes
- Huge prompts full of rules. Every line is read on every reply; a long list of "never" rules can make writing stiff and still be ignored.
- Contradictions, such as "be concise" next to "write long, detailed replies".
- Putting changing details at the very top, such as the current time. It changes every message and makes long chats slower to process.
- Generic assistant language ("You are a helpful AI") in a roleplay prompt, which pulls the character toward customer-service tone.
System prompts in UnsavedAI and other apps
On the UnsavedAI site you never have to write one: each house character's personality, scenario and example dialogue form its instructions, and what you put under About you is added too. When you make a character at /chat/custom, those fields become its system prompt. With an API key in SillyTavern, JanitorAI or your own app, the system message you send is passed to the model as is. Detective Rosa Quinn is a good one to try for how much a clear character frame shapes the writing. For writing your own, see keeping characters in character.
Questions people ask
Can I see a house character's system prompt?
The parts that matter are shown on each character's page: personality, scenario and greeting. You can write your own character from the same parts.
Is a jailbreak the same as a system prompt?
In roleplay apps, "jailbreak" usually refers to an extra instruction block, often placed after the chat history. It is another kind of instruction, not the main system prompt.
Does a longer system prompt make better roleplay?
Rarely. A few clear sentences plus good example dialogue usually beat pages of rules.
