# `ReqLLM.Providers.OpenAI.ChatAPI`
[🔗](https://github.com/agentjido/req_llm/blob/main/lib/req_llm/providers/openai/chat_api.ex#L1)

OpenAI Chat Completions API driver.

Implements the `ReqLLM.Providers.OpenAI.API` behaviour for OpenAI's Chat Completions endpoint.

## Endpoint

`/v1/chat/completions`

## Supported Models

- GPT-4 family: gpt-4o, gpt-4-turbo, gpt-4
- GPT-3.5 family: gpt-3.5-turbo
- Embedding models: text-embedding-3-small, text-embedding-3-large, text-embedding-ada-002
- Other chat-based models with `"api": "chat"` metadata

## Capabilities

- **Streaming**: Full SSE support with usage tracking via `stream_options`
- **Tools**: Function calling with tool_choice format conversion
- **Embeddings**: Dimension and encoding format control
- **Multi-modal**: Text and image inputs
- **Token limits**: Automatic handling of max_tokens vs max_completion_tokens

## Encoding Specifics

- Converts internal `tool_choice` format to OpenAI's function-based format
- Adds `stream_options: {include_usage: true}` for streaming usage metrics
- Handles reasoning model parameter requirements (max_completion_tokens)
- Supports embedding-specific options (dimensions, encoding_format)

## Decoding

Uses default OpenAI Chat Completions response format:
- Standard message structure with role/content
- Tool calls in OpenAI's native format
- Usage metrics: input_tokens, output_tokens, total_tokens

---

*Consult [api-reference.md](api-reference.md) for complete listing*