So, the issue was actually that the

requires_compatibilities

attribute is a string in my code and needs to be a list. 🤦‍♂️

BTW - if you run

mypy

or use an IDE tool that does type checking in the background - that should catch issues like this for you automatically, as the Pulumi Python SDKs do have type annotations. https://www.pulumi.com/docs/reference/pkg/aws/ecs/taskdefinition/#requires_compatibilities_python