Refactoring legacy code is fraught enough when you limit it to inputs & outputs. But almost all legacy code has side effects, too. HTTP, logging, database access, etc. etc. And any of those side effects can prevent a tool like Suture (or, say, Scientist) from working as intended.

Build some kind of tripwire alarm that will raise while mid-surgery/observation to notify the user that side effects are happening (raise, log, stderr?)

This should probably be its own gem.