Prompt Formatter Online
Format prompts into OpenAI, Anthropic, and Gemini message structures
{
"model": "gpt-4o",
"messages": [
{
"role": "system",
"content": ""
},
{
"role": "user",
"content": ""
}
]
}~39 estimated tokens · use the Token Counter for an exact count
Format differences
OpenAI
system / user / assistant all in messages[]
Anthropic
system is a top-level field; conversation in messages[]
Gemini
system_instruction at root; assistant renamed to model; content in parts[]
About this tool
Prompt Formatter Online is a free browser-based tool for structuring AI prompts into the correct message format for OpenAI (GPT-4o), Anthropic (Claude), and Google Gemini APIs. Build a conversation with system, user, and assistant turns, then export the exact JSON payload ready to paste into your API call or codebase.
Each provider uses a slightly different message structure. OpenAI uses a flat messages array with a role field. Anthropic separates the system prompt from the conversation turns and uses a top-level system field. Gemini uses contents with parts arrays and renames the assistant role to model. This tool handles all those differences automatically — just write your prompt and switch providers.
All formatting runs locally in your browser. Nothing is sent to any server.
Common use cases
- Build a multi-turn prompt and export it for OpenAI, Claude, or Gemini with one click.
- Convert a prompt from one provider format to another when switching APIs.
- Quickly scaffold the correct JSON structure for an API call without looking up docs.
- Test different system prompt variations by editing and re-exporting.
- Build few-shot prompts with user/assistant example turns.
- Format prompts for use with the Anthropic SDK, OpenAI SDK, or Google GenAI SDK.
- Create a clean, readable plain-text version of a multi-turn conversation.
- Share a prompt structure with teammates in a standard JSON format.
Frequently Asked Questions
What is the difference between system, user, and assistant roles?
The system message sets the AI's behavior, persona, and constraints. User messages are the human's inputs. Assistant messages are the AI's previous responses — used to provide few-shot examples or to continue an existing conversation.
How does Anthropic handle the system prompt differently?
Anthropic's API takes the system prompt as a separate top-level field (system) rather than as a message in the messages array. This tool automatically moves system messages to the correct location when exporting in Anthropic format.
Why does Gemini use "model" instead of "assistant"?
Google's Gemini API uses "model" as the role name for AI responses instead of "assistant". The formatter handles this rename automatically.
Can I have multiple system messages?
For Anthropic and Gemini, multiple system messages are concatenated into a single system field. For OpenAI, all messages including multiple system messages are included in the messages array as-is.
What is the Raw Text format?
Raw Text produces a human-readable version of the conversation with clearly labeled [SYSTEM], [USER], and [ASSISTANT] sections — useful for documentation, sharing with non-technical stakeholders, or storing prompts in plain text files.
Is my prompt content sent to a server?
No. All formatting runs locally in your browser using JavaScript. Your prompts never leave your device.
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