Prompt Formatter

Write a conversation once and see the exact request body each provider expects. The three disagree in ways that are easy to get wrong when porting a prompt between them.

System prompt

Conversation

{
  "model": "gpt-4o",
  "messages": [
    {
      "role": "system",
      "content": "You are a technical editor."
    },
    {
      "role": "user",
      "content": "Analyse the following code block…"
    }
  ]
}

OpenAI carries the system prompt as the first entry in messages.

llmtools is a vendor-neutral toolkit for working with large language models — token counting, cost estimation and model comparison, computed in your browser. OpenAI token counts are exact, using the same encoding the model does (o200k_base for GPT-4o, cl100k_base for GPT-4/3.5); Claude and Gemini counts are labelled approximations, since neither publishes a tokenizer that runs client-side. Model pricing is read from OpenRouter’s public catalogue once a day. Nothing you type is ever transmitted anywhere.