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Confirmed that it's my S3 remote state that's not configured correctly, ignore the AWS SDK bit.

I think in this particular case you just need to export the aws region environment variable 

Good hint. It was actually AWS_PROFILE that was needed, that provides the AWS_REGION to the state-accessing code. I'd forgotten about it since I was moving from <http://app.pulumi.com|app.pulumi.com> and just had PULUMI_ACCESS_TOKEN set.