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reasoning.summary is an optional Responses API field that asks the model to return a summary of its reasoning. Not every reasoning model supports it — this error means the model named in the message does not accept the field, so the request is rejected.

Two things are worth checking separately. First, the model id: gpt-5.3-codex-spark is not in the CodeGateway model catalog. The Codex model we serve is gpt-5.3-codex, and it is Responses-API-only — send it to POST /v1/responses, not to /v1/chat/completions. If you call CodeGateway with the model still set to gpt-5.3-codex-spark, you get a 400 Unsupported model response instead, which is how you can tell the two failures apart.

Second, the parameter: drop reasoning.summary from the request body. The field is optional, and requests that omit it are unaffected. If you need reasoning summaries, check the target model's own API reference first — availability differs by model and snapshot. When a tool adds the field for you, look for the corresponding reasoning or summary option in its configuration and turn it off, or switch to a model whose reference documents reasoning summaries.