Control reasoning

Use reasoning effort to adjust how much work a supported model does before answering. Higher effort can increase latency and output-token cost. Omit the control to use the model’s default.

curl --fail-with-body https://api.impossibl.com/v1/chat/completions \
  -H "Authorization: Bearer $IMPOSSIBL_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "openai/gpt-5-mini",
    "messages": [{"role": "user", "content": "Compare two ways to deduplicate events in a queue."}],
    "reasoning_effort": "medium",
    "max_completion_tokens": 2048
  }'

Chat Completions accepts none, minimal, low, medium, high, and xhigh. The gateway translates the value for the model and serving route: a supported effort level, a reasoning-token budget, or an on/off control. Models without a working reasoning control may ignore it; none is not a universal guarantee of zero reasoning tokens. Keep enough output budget for both reasoning and the visible answer. A small budget can end in finish_reason: "length" with little or no answer text.

Use the Responses API

Send reasoning.effort and max_output_tokens instead. Responses forwards effort on OpenAI-shaped backends; it does not provide the same cross-provider translation as Chat Completions:

{
  "model": "openai/gpt-5-mini",
  "input": "Compare two ways to deduplicate events in a queue.",
  "reasoning": {"effort": "medium"},
  "max_output_tokens": 2048
}

reasoning.summary accepts auto, concise, or detailed and is forwarded to backends that support it. It does not guarantee a summary in the gateway response. reasoning.context is accepted for client compatibility but has no effect. Responses remains stateless; send conversation history in input.

Read usage and preserve tool context

When reported by the backend, Chat Completions includes usage.completion_tokens_details.reasoning_tokens. It is a subset of completion_tokens; do not add it again when calculating usage.Some backends return reasoning_content with the assistant message or streaming delta. Keep the complete assistant message when sending tool results back: reasoning content and opaque signatures can be required for the next turn. See tool calling.

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