I've generally had better luck when using it on new projects/repos. When working on a large existing repo it's very important to give it good context/links/pointers to how things currently work/how they should work in that repo.

Also - claude (~the best coding agent currently imo) will make mistakes, sometimes many of them - tell it to test the code it writes and make sure it's working - I've generally found its pretty good at debugging/testing and fixing it's own mistakes.