Error 'unsupported value' when using Foundry Agent with a Model Gateway Connection and Reasoning Model

[bradveng]

Technical Feedback

Error occurs within the Foundry agent playground when using the configuration below which suggests the Foundry Agent playground can not be used with model gateway connections and GPT-5.1 (or any model with reasoning parameters).

Error: unsupported_valueUnsupported value: 'none' is not supported with the 'gpt-5.1-2025-11-13' model. Supported values are: 'low', 'medium', and 'high'

Configuration:
-Foundry has no local model and instead the agent uses a model gateway connection.
-Standalone APIM instance as AI gateway.
-Backend is an Azure Open AI service with GPT 5.1 2025-11-13

When using a model gateway connection within the Foundry agent playground there appears to be a bug that the reasoning parameter is not enabled for GPT-5.1 (when using an MGC) that results in this error.
When using a local GPT-5.1 model within Foundry there is no error.

Desired Outcome

Foundry agent playground supports using reasoning models (reasoning parameters) when using a model agteway connection.

Current Workaround

Unknown