ReqLLM.Providers.OpenAI.ResponsesAPI (ReqLLM v1.0.0)

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OpenAI Responses API driver for reasoning models.

Implements the ReqLLM.Providers.OpenAI.API behaviour for OpenAI's Responses endpoint, which provides extended reasoning capabilities for advanced models.

Endpoint

/v1/responses

Supported Models

Models with "api": "responses" metadata:

  • o-series: o1, o3, o4, o1-preview, o1-mini
  • GPT-4.1 series: gpt-4.1, gpt-4.1-mini
  • GPT-5 series: gpt-5, gpt-5-preview

Capabilities

  • Reasoning: Extended thinking with explicit reasoning token tracking
  • Streaming: SSE-based streaming with reasoning deltas and usage events
  • Tools: Function calling with responses-specific format
  • Reasoning effort: Control computation intensity (minimal, low, medium, high)
  • Enhanced usage: Separate tracking of reasoning vs output tokens

Encoding Specifics

  • Input messages use input_text content type instead of text
  • Token limits use max_output_tokens instead of max_tokens
  • Tool choice format: {type: "function", name: "tool_name"}
  • Reasoning effort: {effort: "high"} format

Decoding

Non-streaming Responses

Aggregates multiple output segment types:

  • output_text segments → text content
  • reasoning segments (summary + content) → thinking content
  • function_call segments → tool_call parts

Streaming Events

  • response.output_text.delta → text chunks
  • response.reasoning.delta → thinking chunks
  • response.usage → usage metrics with reasoning_tokens
  • response.completed → terminal event with finish_reason
  • response.incomplete → terminal event for truncated responses

Usage Normalization

Extracts reasoning tokens from usage.output_tokens_details.reasoning_tokens and provides:

  • :reasoning_tokens - Primary field (recommended)
  • :reasoning - Backward-compatibility alias (deprecated)