In that case, you should have already gotten the validation via

expected_num_files

Oh! Are you reusing the same sources field for both the synthetic target and normal target? Instead, you can have the synthetic target subclass and override

expected_num_files

. Your rules still use

DockerSingleSourceField

, and they work just fine with the subclass

SyntheticDockerSingleSourceField

The Target API was designed that way precisely for situations like this