I've found that it can be hard or expensive for the agent to do "big but simple" refactors in some cases. For example, I recently tasked one with updating all our old APIs to accept a more strongly typed user ID instead of a generic UUID type. No business logic changes, just change the type of the parameter, and in some cases be wary of misleading argument names by lazy devs copy pasting code. This ended up burning through the entire context window of GPT-5-codex and cost the most $ of anything I've ever tasked an agent with.