The Flow Builder Debugger is an interactive run inside Flow Builder that shows the path a flow takes and how resource values change, so you can confirm the flow before activation or see exactly where a failed run broke. Flow Tests automatically simulate a flow with sample data you define and check assertions, which are the outcomes that must be true for the test to pass, without saving that data to the database. Use the Debugger when you need to watch one run or run as a specific user; use Flow Tests when you need repeatable, data-safe checks you can rerun as the flow changes.