- This provider uses the OpenAI Responses or Chat Completions API. Both accept JSON schema `tools` arrays.
- Function tools require `type`, `name`, `description`, and `parameters` as JSON Schema.
- Streaming tool call deltas arrive in `choices[].delta.tool_calls[]` and must be assembled by index.
- Place system instructions in the `messages` array with role `system` for Chat Completions.
