PrivateCloudComputeLanguageModel fails to respond

I am trying out the new PrivateCloudComputeLanguageModel but can't get it to work. When I call session.respond it throws the following error:

Error Domain=FoundationModels.LanguageModelError Code=-1 "The operation couldn’t be completed. (FoundationModels.LanguageModelError error -1.)" UserInfo={NSMultipleUnderlyingErrorsKey=(
         "Error Domain=FoundationModels.LanguageModelError Code=-1 \"(null)\" UserInfo={NSMultipleUnderlyingErrorsKey=(\n    \"Error Domain=ModelManagerServices.ModelManagerError Code=1046 \\\"(null)\\\" UserInfo={NSMultipleUnderlyingErrorsKey=(\\n)}\"\n)}"
     ), NSLocalizedDescription=The operation couldn’t be completed. (FoundationModels.LanguageModelError error -1.)}

Maybe error code 1046 means something, but I can't find a mention of it in the docs.

My set-up:

• macOS Golden Gate on a MacBook Pro M1
• Xcode 27.0.0 beta 1, calling the model as part of a test on a iPhone 17 simulator with iOS 27 beta 1.

I do have the Private Cloud Compute entitlement (removing it triggers a fatalError). model.isAvailable returns true. I tried logging into iCloud on both the macOS host as well as the simulator, but no difference.

Relevant code:

• calling the model: https://github.com/Thomvis/Construct/blob/feature/foundation-models/Sources/MechMuse/FoundationModels.swift#L20
• the test to run to trigger the issue: https://github.com/Thomvis/Construct/blob/feature/foundation-models/App/UnitTests/DescribeCombatantsEvaluation.swift#L78