How do vibe coders know their changes won’t break the unhappy paths?

With vibe coding, it’s incredibly easy to get a feature working for the happy path.

But I’m more curious about the flows that aren’t obvious from the code you’re currently working on.

A change might look perfectly safe within one repo, but what if it affects:

• An edge case handled by another service or repository

• An older flow that isn’t documented in the current codebase

• A downstream API or integration

• A workflow that the AI simply has no context about

How do you know you haven’t broken something that is completely outside the context of the repo you’re currently vibe coding in?

Do you rely on tests, AI-generated test cases, manual verification, staging, production monitoring, or something else?