Same thoughts here. I routinely contract developers to bring my ideas to life. I pay them for this. The code is not the developers, and neither is the credit (unless I agree, as others suggest through providing a letter of reference of some sort). There would be no misunderstanding of "original author" or copyright holder. Neither developer owns the copyright to the works I commission you for. Of course you could say you worked on the code for project X, and you should be able to demonstrate through contacts, SOW, payments, etc. that you did.