Turn outside input into a dependable contract before it reaches the rest of the build.
“Reject broken input” is one deliberate move within the “Build the data boundary” phase. Start by naming the result you expect to see before changing the build. A visible target keeps the work small and testable.
Work at the nearest useful boundary. Read the current state, make one clear change, and inspect the surface that should respond. Keep the connection between cause and effect short enough to explain.
Run the expected path, then disturb one assumption. A professional result makes both success and failure legible, and leaves the next move obvious.