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 llmtr-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 (Chat Completions spelling; translated to text.format)
text	object	no	Structured output (Responses spelling): { "format": { "type": "json_schema", ... } }
stream	boolean	no	SSE streaming. The provider stream is proxied through as it arrives

Basic example

curl "$LLMTR_BASE_URL/v1/responses" \
  -H "Authorization: Bearer llmtr-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 llmtr-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.

Streaming

With stream: true the gateway proxies the provider's SSE stream through as it arrives: tokens reach you while the model is still generating, rather than after the response is complete.

curl https://llmtr.com/v1/responses \
  -H "Authorization: Bearer llmtr-your_key" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "openai/gpt-5.3-codex",
    "input": "Explain cursor-based pagination.",
    "stream": true
  }'

The stream uses OpenAI Responses event names:

event: response.created
data: {"type":"response.created","response":{"id":"resp_xxx","model":"openai/gpt-5.3-codex"}}

event: response.output_text.delta
data: {"type":"response.output_text.delta","delta":"Cursor"}

event: response.completed
data: {"type":"response.completed","response":{"status":"completed","usage":{...}}}

data: [DONE]

Notes:

• response.model inside event payloads is rewritten to the canonical LLMTR model ID, so it matches what a non-streaming response returns.
• usage arrives only on the response.completed event. Billing is derived from that block, so streaming and non-streaming cost the same.
• Failures before the stream opens return a normal HTTP status code. Failures after it opens are reported inside the stream as an {"error":{...}} payload, because the 200 status line has already been sent.
• SSE comment lines (: keepalive) are emitted periodically while waiting. Every compliant SSE parser ignores them.

Structured output

Both spellings are accepted. The Chat Completions response_format field is translated to the Responses API's text.format:

curl https://llmtr.com/v1/responses \
  -H "Authorization: Bearer llmtr-your_key" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "openai/gpt-5.3-codex",
    "input": "Produce a weather JSON for Istanbul.",
    "text": {
      "format": {
        "type": "json_schema",
        "name": "weather",
        "strict": true,
        "schema": {
          "type": "object",
          "properties": { "city": { "type": "string" }, "tempC": { "type": "number" } },
          "required": ["city", "tempC"],
          "additionalProperties": false
        }
      }
    }
  }'

If you send both response_format and text.format, text.format wins.

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.

xAI Grok

The recommended endpoint for xai/grok-4.3, xai/grok-4.20-multi-agent, xai/grok-4.20-0309-reasoning, and xai/grok-4.20-0309-non-reasoning is /v1/responses:

curl https://llmtr.com/v1/responses \
  -H "Authorization: Bearer llmtr-your_key" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "xai/grok-4.3",
    "input": "Simplify a TypeScript service function."
  }'

For compatibility, /v1/chat/completions requests are also accepted; the gateway internally routes them through Responses and returns the OpenAI Chat Completions shape. LLMTR sends store:false for these models. store:true, previous_response_id, instructions, and xAI server-side tools are rejected in this release. If you need a system prompt, send a first system or developer message through input or messages.

For Grok 4.3, input above 200K tokens is not supported and those requests return pricing_unverified. If xAI does not report an actual cost for a text request, the request is not billed and returns provider_usage_missing even if the provider request succeeded — you are not charged in that case. On Grok 4.20 Multi-Agent, reasoning.effort controls agent count, so high and xhigh may consume more tokens.

For the full Grok family, including image, video, TTS, and STT examples, see xAI Grok models.

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.