Build a rules engine: evaluate boolean expressions like `age > 18 and country in ["US","CA"]` over a record
The build behind every access policy and routing rule a customer wants to edit without a deploy. The trap is operator precedence: candidates who tokenize fine still get `a or b and c` wrong, and that's the whole grade.
Updated Sep 2026 · Grounded in real Forward Deployed Engineer interview loops and written to a senior-engineer editorial bar.
The build behind every access policy and routing rule a customer wants to edit without a deploy. The trap is operator precedence: candidates who tokenize fine still get `a or b and c` wrong, and that's the whole grade.
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.
