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PromptifyLab

System Prompt Generator

Builds the persistent instruction block for a custom GPT, a Claude Project, a Gemini Gem or an API system parameter. You describe the assistant’s one job, who talks to it, how it should work and what is out of scope; it produces a system prompt sized and formatted for the platform you picked.

Assistants that do one thing well beat assistants that do six things adequately.

System prompt


      


      

    

  


  
Nothing leaves your browser. Internal process details you paste here stay on this device.







How it works


    

  • Pick the destination first — the output format and length advice change with it.
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  • Tight, Standard and Full control length. Long system prompts get loosely followed, so shorter is often better.
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  • Claude plus Full emits XML-tagged sections, which Claude follows more closely.
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  • The Full option adds a failure-modes section — the self-checks that stop an assistant padding or agreeing by default.
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What it does not do


Every tool on this site says this plainly, because a tool that pretends to do more than it does costs you more than it saves.


    

  • It cannot know the failure modes specific to your use. Add those as you find them.
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  • It does not test the prompt. Run it against twenty real inputs before you ship it.
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  • Reference material belongs in knowledge files or retrieval, not in the system prompt.
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Nothing you type here leaves your browser


There is no account, no server and no logging. The whole tool is JavaScript running on your own machine — close the tab and it is gone. You can check: open your browser’s network tab and use it.




Questions people ask




How long should a system prompt be?
As short as does the job. Instruction compliance degrades as instructions accumulate, and you pay for it on every single call.




Where do I paste this?
Custom GPT: Configure → Instructions. Claude Project: Project instructions. Gemini Gem: the Instructions field. API: the system parameter.




Why does my assistant stop following its instructions?
Attention is not uniform across a long conversation. Repeat the one critical constraint in the user message for turns that matter.







    

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    System Prompts
    

    The instruction that ships with every message, written so it is still followed at message forty.
    

        7 prompts
  
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    AI Engineering
    

    Building on models: system prompts, evals, RAG, structured output, cost control.
    

        7 prompts
  
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Free, no sign-up. Last updated 18 September 2026.