Temperature parameter not supported in GPT-5 models

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Summary

Issue: The temperature parameter appears disabled or has no effect when using GPT-5 models in Braintrust prompts. Cause: Whether GPT-5 models accept the temperature parameter depends on the reasoning effort. GPT-5.1 and later support temperature only when reasoning effort is set to none. At any higher reasoning effort, and for older GPT-5 models (gpt-5, gpt-5-mini, gpt-5-nano) and GPT-5 Pro, the API doesn’t accept temperature, so Braintrust removes it. Resolution: To use temperature with a GPT-5 model, select GPT-5.1 or later and set Reasoning effort to none. If you need both a higher reasoning effort and temperature control, switch to a model that supports temperature at any reasoning effort.

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Resolution steps

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Use temperature with a GPT-5 model

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Step 1: Select GPT-5.1 or later

In the prompt editor, choose GPT-5.1 or a later GPT-5 model. Older GPT-5 models and GPT-5 Pro don’t expose a none reasoning effort, so they never accept temperature.

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Step 2: Set reasoning effort to none

In the model parameters, set Reasoning effort to none. The temperature parameter is available only at this setting. Raising the reasoning effort removes the temperature setting.

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Step 3: Configure the temperature value

Set your desired temperature value (0.0 to 2.0) in the model parameters.

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If you need a higher reasoning effort and temperature control

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Step 1: Switch to a compatible model

Change your prompt to a model that supports temperature at any reasoning effort, such as GPT-4.1.

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Step 2: Configure the temperature value

Set your desired temperature value in the model parameters.

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Additional information

This reflects OpenAI’s parameter support for each model, not a Braintrust limitation. Refer to OpenAI’s model documentation for complete parameter support details for each model family.