Responses API

The LLMTR gateway supports OpenAI’s /v1/responses endpoint. The GPT-5 Codex family (gpt-5-codex, gpt-5.1-codex, gpt-5.1-codex-max, gpt-5.1-codex-mini, gpt-5.2-codex, gpt-5.3-codex) is only available through this endpoint; sending these models to /v1/chat/completions returns 400 endpoint_mismatch.

When to use it

• When you need reasoning effort control (low / medium / high / xhigh)
• When you use models that benefit from cached input
• For models that only support the Responses endpoint

For classic chat completion flows keep using /v1/chat/completions.

Request

POST /v1/responses
Authorization: Bearer sk_your_key
Content-Type: application/json

Body parameters

Field	Type	Required	Description
model	string	yes	Canonical model ID (e.g. openai/gpt-5.1-codex). Optional suffix: openai/gpt-5.3-codex:max
messages or input	array | string	yes	OpenAI message format or raw input
instructions	string	no	System prompt equivalent
reasoning	object	no	{ "effort": "low" | "medium" | "high" | "xhigh", "summary": "auto" | "concise" | "detailed" }
max_output_tokens	integer	no	Output token cap
temperature	number	no	Within the model’s supported range
tools	array	no	Function calling (where supported)
tool_choice	string | object	no	auto, none, or specific tool
response_format	object	no	Structured output
stream	boolean	no	SSE streaming (coming soon)

Basic example

curl https://llmtr.com/v1/responses \
  -H "Authorization: Bearer sk_your_key" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "openai/gpt-5.1-codex:high",
    "messages": [
      {"role": "user", "content": "Refactor: make this function pure"}
    ]
  }'

The same request with the body field:

curl https://llmtr.com/v1/responses \
  -H "Authorization: Bearer sk_your_key" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "openai/gpt-5.1-codex",
    "reasoning": { "effort": "high" },
    "messages": [
      {"role": "user", "content": "Refactor: make this function pure"}
    ]
  }'

Response

{
  "id": "resp_xxx",
  "object": "response",
  "status": "completed",
  "model": "gpt-5.1-codex",
  "output": [
    {
      "type": "message",
      "role": "assistant",
      "content": [
        { "type": "output_text", "text": "Here is the refactored version..." }
      ]
    }
  ],
  "usage": {
    "input_tokens": 142,
    "input_tokens_details": { "cached_tokens": 24 },
    "output_tokens": 318,
    "output_tokens_details": { "reasoning_tokens": 256 }
  }
}

status may be completed, incomplete, or failed.

Usage and cost notes

• input_tokens_details.cached_tokens shows how much of the prompt came from cache.
• output_tokens_details.reasoning_tokens shows reasoning-related token usage.
• Total cost follows the model’s own pricing; the platform margin is applied only when topping up credit.
• Confirm the current model price in the dashboard or catalog before production use.

Reasoning effort

Reasoning levels are detailed on a separate page: Reasoning effort.

Error codes

HTTP	error.type	Meaning
400	invalid_request_error	Invalid parameter / unknown suffix
400	endpoint_mismatch	Model requires /v1/responses but /v1/chat/completions was called (or vice versa)
400	unsupported_capability	Model does not support this reasoning level or modality
401	auth_error	Invalid API key
402	insufficient_balance	Insufficient balance
429	rate_limit_exceeded	RPM/TPM limit exceeded
502	provider_error	Upstream provider error

For details see the Errors section.