Always use strict test doubles (instance_double, class_double) instead of basic doubles or OpenStruct in tests. Strict doubles provide compile-time checks that prevent tests from passing when the mocked interface changes, improving test maintainability and catching integration issues early. Example - Instead of: ```ruby auth_stub = double("vault auth", aws_iam: nil) dummy = OpenStruct.new(stdout: output, exitstatus: 0) ``` Use: ```ruby auth_stub = instance_double("VaultAuth", aws_iam: nil) shell_out = instance_double(Mixlib::ShellOut, stdout: output, exitstatus: 0, error?: false ) ``` This approach: - Ensures mocks accurately reflect the real interfaces - Catches interface changes during test execution - Makes dependencies explicit in test code - Prevents tests from silently passing with invalid assumptions