Exactly zero, if answering your question as stated. But that is not the point. LLMs struggle with things that require tacit knowledge or are not found in the training set (i.e. haven't been done before). They excel at tasks that are described in the most verbose and explicit way, to the point that it looks humiliating from human POV. And what is more explicit than an existing codebase, especially with tests, comments and documentation lovingly put there by the proud author (remember, those github stars).