Things are not that bad. While the whole thing is kind of a monolith, there are at least broad sections that split code into logical sections. With some practic

Things are not that bad. While the whole thing is kind of a monolith, there are at least broad sections that split code into logical sections. With some practice, it is possible to easily narrow down changes and any issues to 10-20 LOC. Later, breaking code by them could be a starting place for refactoring. In the end, the decision is manually reviewed. There's also a log that tells how and why the code had arrived at the numbers. So, if you know that client cannot be X but the log says X then you know that there's a bug for example. This code was around before I joined. It started small, but more and more requirements were added until it became like that. Not much effort went into structuring it. Now it's my responsibility, and I'm trying to make it better for me and anyone who will have to deal with it in the future.