How to counter code coverage as only quality metric

Hi all, I am working with a client who is looking to send most of its development work to 3rd parties. Right now the prevailing wisdom is to use code coverage as one measure of success/contractual completeness for the 3rd party. This is something that can cause some negative behaviours (…well all measurements can!). If you are interested in debating this, I just really enjoyed this article by Majd Uddin: Code coverage dos and don’ts

I am looking for counter measures to help hone in on what the client is actually looking to achieve…a uniform, high quality, highly valuable code base produced from vendors around the world.

Right now I am looking into mutation testing as a possible addition to help support a more well rounded approach. Any other ideas?