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System Prompt Builder
Create a reusable system prompt for a custom GPT, Claude project, chatbot or AI agent: persona, goals, rules, refusals, tone and response format.
🔒 Processed locally in your browser — we don’t upload your input.
How to use the System Prompt Builder
A system prompt is the standing instruction that shapes every reply an AI assistant gives. Custom GPTs, Claude Projects, Gemini Gems, customer-support chatbots and API-based agents all rely on one. This builder walks you through the pieces a solid system prompt needs: the assistant’s name and persona, its primary goal, the knowledge it can assume, hard rules it must follow, topics it should decline, its tone of voice, and the default format of its answers.
The output is organised in short labelled blocks, which keeps instructions readable for both humans and models and makes later edits simple. Rules are written as a numbered list because models follow enumerated constraints more reliably than prose. A built-in “if unsure” rule is added by default, telling the assistant to ask a clarifying question or admit uncertainty instead of inventing facts, which is one of the most effective ways to reduce hallucinations.
Use the result directly in the “Instructions” box of a custom GPT, the system field of the OpenAI, Anthropic or Gemini APIs, or the configuration of open-source chat UIs. The builder runs offline in your browser and stores nothing, so you can safely draft prompts for internal business tools.
Frequently asked questions
What is a system prompt?
It is the hidden standing instruction given to a chat model before the conversation, defining its role, rules and style.
Where do I paste it?
In the Instructions field of a custom GPT, the system message of an API call, or your chatbot platform’s settings.
How long should a system prompt be?
Usually 150–600 words. Longer prompts work, but keep rules concise and non-contradictory.
Is anything sent to a server?
No, it is generated locally in your browser.
About this tool
Create a reusable system prompt for a custom GPT, Claude project, chatbot or AI agent: persona, goals, rules, refusals, tone and response format. People also look for: system prompt generator, custom gpt instructions, ai assistant prompt, chatbot system prompt.
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