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LLM API Payload Builder

FmtDev LLM API Payload Builder is a free, browser-based tool that visually construct and export valid JSON payloads for OpenAI/Claude API calls. Test system prompts and parameters instantly. It runs entirely on your device with zero data transmission, making it safe for proprietary code and sensitive content.

How to use

How to use API Payload Builder

  1. 1

    Select your target model and set model parameters.

  2. 2

    Enter the system instructions and the user prompt.

  3. 3

    The JSON payload is generated automatically in the right panel.

  4. 4

    Copy the payload to use in your API integration.

FAQ

Which APIs are supported?
We follow the Chat Completions standard compatible with OpenAI, Anthropic, and Gemini.

Standardizing LLM API Requests

Eliminating Payload Errors

Constructing nested JSON objects for LLM APIs manually often leads to syntax errors and malformed requests. A payload builder ensures structure validity before testing your inference loops.

Local Prompt Engineering

Drafting prompts locally prevents accidental leakage of proprietary business logic or system instructions into third-party cloud tools before making the actual API call. Read our full manifesto on why we prioritize local data orchestration: Optimizing Developer Workflow Security