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OpenAI ↔ Anthropic Format Converter

Convert chat messages, tool definitions and responses between the OpenAI and Anthropic API formats in either direction, with format auto-detection.

OpenAI ↔ Anthropic Format Converter

Direction:
Source (OpenAI format)
Converted (Anthropic format)

Conversion ready

{
  "messages": [
    {
      "role": "user",
      "content": [
        {
          "type": "text",
          "text": "What is the capital of France?"}
      ]
    },
    {
      "role": "assistant",
      "content": [
        {
          "type": "text",
          "text": "The capital of France is Paris."}
      ]
    },
    {
      "role": "user",
      "content": [
        {
          "type": "text",
          "text": "And Germany?"}
      ]
    }
  ],
  "system": "You are a helpful assistant."}

Conversion reference

Messages

role/content strings vs content blocks, system message handling, tool_calls vs tool_use blocks

Tools

function.parameters vs input_schema, type: "function" wrapper

Response

choices[0].message vs content blocks, finish_reason vs stop_reason, usage field mapping

Updated . Provided as is. Check the output before you rely on it in production.

How to use OpenAI ↔ Anthropic Format Converter

  1. 1

    Convert OpenAI function_calling format to Anthropic tools

    Paste OpenAI function schema (name, description, parameters.properties). Tool converts to Anthropic format (input_schema with properties and required fields). Deploy converted schema to Claude API.

  2. 2

    Convert Anthropic tool results to OpenAI format

    Paste Anthropic tool_result (tool_use_id, content). Tool reformats as OpenAI assistant message with tool_call_id and function result. Useful for multi-LLM pipelines.

  3. 3

    Migrate agent codebase between API providers

    Have many functions defined in OpenAI format? Paste all schemas, convert in bulk. Replace in your codebase with Anthropic equivalents. Reduces manual refactoring.

  4. 4

    Validate format compatibility for multi-LLM systems

    If using multiple LLMs (GPT + Claude), convert both to a neutral JSON schema. Check that conversions are semantically equivalent. Ensures consistent behavior across providers.

  5. 5

    Handle nested objects and required field conversion

    Complex schemas with nested objects convert differently. Converter handles required[] arrays and nested properties. Verify output matches your API expectations before deploying.

Questions and answers

What's different between OpenAI and Anthropic message formats?
OpenAI uses {role, content} where content is a string. Anthropic uses {role, content} where content is an array of blocks like [{type: 'text', text: '...'}]. System messages and tool calls also differ significantly.
Does this convert tool/function definitions?
Yes. It converts OpenAI function tools ({type: 'function', function: {name, description, parameters}}) to Anthropic tools ({name, description, input_schema}) and back. MCP tool definitions are not converted.
What about features that don't have equivalents?
It maps roles, system prompts, text and tool-call blocks, tool definitions and stop reasons, and warns about unknown roles, missing tool names and unmapped finish_reason values. Fields with no counterpart, such as cache_control or logprobs, are not carried over, so review them by hand.
Can it auto-detect which format I'm pasting?
Yes, the converter detects OpenAI vs Anthropic format based on JSON structure (e.g., presence of 'choices' indicates OpenAI response) and sets the conversion direction automatically.
Is this tool private?
Yes, all conversion happens in your browser. Your API messages and tool definitions are not sent to external services. No data leaves your device.
For AI agents: how to call this tool

Machine-readable contract, endpoints and examples. Humans can ignore this section.

Best Path For Builders

Browser workflow

Runs instantly in the browser with private local processing and copy/export-ready output.

Browser Workflow

This tool is optimized for instant in-browser execution with local data handling. Run it here and copy/export the output directly.

/api-format-converter/

For automation planning, fetch the canonical contract at /api/tool/api-format-converter.json.