Testing agent workflows without connecting real accounts

I am building FetchSandbox because I kept running into the same problem: the only way to test if an agent-built workflow actually works is to connect a real account. Real emails send. Real calendar invites go out. Real Slack messages drop. All before you even know if the logic is right. The part that bothered me more was receipts. A success status doesn't tell you if the app did the right thing. So our sandbox goes deeper, it checks the resulting state, it replays webhooks, it catches whether a retry fires a duplicate charge or a duplicate email you'd never see in a test. Right now I'm trying to figure out something: which workflow is actually the hardest to safely test before going live? The incoming email to calendar booking to Slack confirmation is the one I keep hearing about, but I don't know if that's just the loudest voice in the room. If you've built something like this and had to just "yolo and connect real accounts," genuinely curious what broke.