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System Prompt Builder

AI Tools

Compose structured system prompts with live token counting for Claude and GPT.

Role & Persona~23 tokens
Context
Instructions~42 tokens
Output Format~24 tokens
Constraints
Examples (Few-Shot)
Custom Section
Estimated system prompt tokens~102 / 200,000 (0%)
## Role & Persona
You are an expert software engineer with deep knowledge of TypeScript, React, and Node.js.

## Instructions
- Always provide TypeScript types
- Prefer functional over class components
- Add brief comments for complex logic
- Never generate placeholder code without explaining

## Output Format
Respond concisely. Use markdown with fenced code blocks. Always specify the language identifier.
#system prompt#prompt builder#claude prompt#gpt prompt#token counter

About System Prompt Builder

Compose structured system prompts with live token counting for Claude and GPT. System Prompt Builder runs entirely in your browser — no installation, no uploads, and no data ever leaves your device.

Common use cases: design role-specific personas for ai assistants; optimize system prompts to fit within context window limits; build and iterate on structured prompts with token budget awareness. Whether you are debugging, prototyping, or building production code, having a fast and private tool for this task saves time compared to writing one-off scripts or using an online tool that logs your data.

Like every tool in Catalyst, this one is free forever, requires no login, and is safe to use with confidential data. All processing happens client-side via native browser APIs.

How to use System Prompt Builder

  1. 1

    Open the tool

    Open System Prompt Builder directly — no signup or installation required. The interactive interface loads instantly.

  2. 2

    Paste or enter your input

    Drop your data into the input area. The tool validates and processes your input live as you type or paste.

  3. 3

    Review the result

    The output updates instantly. If the tool has configuration options, adjust them and watch the result update.

  4. 4

    Copy or download

    Use the copy button to send the result to your clipboard. Nothing is saved between page loads, so closing the tab discards everything.

Frequently asked questions

Is any data sent to a server?
No. The tool runs entirely in your browser using native JavaScript and Web APIs. Nothing you paste or upload is transmitted, logged, or stored. You can verify this by disconnecting your network and seeing the tool continue to work.
Is this tool free to use?
Yes. It's completely free with no login, signup, API key, or usage limits. Catalyst is funded by ads and will remain free forever.
Does it work offline?
Once the page has loaded, yes. All processing happens locally in your browser, so you can use the tool on airplanes, trains, or in air-gapped environments.

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