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If it’s aws resources, you can run LocalStack and point your local service endpoints to the localhost endpoints and create fake resources. Here’s an example of the approach: https://www.pulumi.com/blog/pulumi-and-localstack/ Note that I haven’t personally tested (or even fully read) that post, but I have used Pulumi with LocalStack before and it’s worked reasonably well

LocalStack makes sense.

Or you can leverage mocks in a pure unit testing setup: https://www.pulumi.com/docs/guides/testing/unit/

Or just make resources with something like Random which doesn't create anything that costs

That random package might be quite useful indeed