The customer's system of record is a legacy GUI/API they can't change. Build an agent on top, where do you put the brittleness?
The most FDE question in the bank: every real deployment hits an unchangeable legacy system. The answer is an architectural principle, concentrate brittleness in one owned layer, plus what to do when the GUI is the only API.
Updated Aug 2026 · Grounded in real Forward Deployed Engineer interview loops and written to a senior-engineer editorial bar.
The most FDE question in the bank: every real deployment hits an unchangeable legacy system. The answer is an architectural principle, concentrate brittleness in one owned layer, plus what to do when the GUI is the only API.
Lead with where the obvious approach breaks, because that is the judgment they are screening for — most candidates jump straight to the happy path and lose the room.
Then walk the failure back through the pipeline in order, naming the one metric the customer's exec sponsor actually cares about before you propose the fix.
