If you have the choice, change teams or companies as soon as possible. Life is too short to spend it paying off someone else's debt. If moving is not an optio

If you have the choice, change teams or companies as soon as possible. Life is too short to spend it paying off someone else's debt. If moving is not an option, then follow the parent's advice. Add all the missing tests as fast as you can. Explain what you are doing to your manager. And set up CI to email your team whenever anyone checks in breaking changes. This will motivate your teammate to start running your tests. If they are particularly narrow-minded, they will be unhappy about this. If this happens, reconsider your decision to stay. Even after you write a lot of tests, coverage will be low. You will likely code some bugs that negatively impact the business. I urge you to find a mentor inside the company, a senior engineer or engineering manager, and show them code and explain the technical debt to them. Try not to blame your teammate for the situation, just explain to your mentor that you are concerned about introducing bugs. Ask them to keep your discussion confidential. Your mentor can back you up when your bugs negatively impact the business.