Endpoint
The body is an OpenAI Decisions API request: an input and a list of typed questions. The gateway resolves model through the virtual key, then forwards the body to the OpenAI provider bound to the key, with the resolved model and the provider’s API key in place of the virtual key. The answer comes back as OpenAI wrote it.
Request
Response
Which providers serve it
Only an OpenAI provider. A key with no OpenAI provider answers 400 model_provider_not_bound. The request goes to the provider’s base URL: with none set it goes to https://api.openai.com, and with https://eu.api.openai.com (with or without /v1) it goes to OpenAI’s EU endpoint.
Metering and tracing
Budgets are checked before the call; a key over its budget answers 402 budget_exceeded and the provider is not called. usage.input_tokens and usage.output_tokens are charged at the model’s price, on the key’s spend and budgets.
The trace span records the model, provider, status, latency, token usage and cost. It never records the input, the questions or the answers.
Guardrails are separate from tracing. When the key has guardrails, the request body goes to the guardrail check and its evaluator can record what it scores. For private input, use a key without guardrails. See Guardrails boundaries.
End-user attribution
The X-LangWatch-End-User-Id header names the end user for spend and per-user budgets. Without it, the body’s safety_identifier is used.
Streaming
This endpoint does not stream.
Errors
Codes this endpoint raises before the provider is called: missing_model, model_not_allowed, model_provider_not_bound, bad_request (all 400), budget_exceeded (402) and payload_too_large (413, a body over the gateway’s request size cap). The checks every gateway route runs can also answer, depending on the key’s settings: end_user_required, rate_limited, policy_violation, guardrail_blocked, guardrail_upstream_unavailable. The full table is on Errors.
A provider’s own error is forwarded with its status and body. When a call runs long enough for a keep-alive byte, the response carries X-LangWatch-Heartbeat-Active: true and the status is already 200; a later error then arrives in the body, so read the body for an error key.Last modified on October 30, 2026
