Constraint
A limit on how the job may be done, rather than on what it must do: no new libraries, under 200 milliseconds, use the database we already have.
An agent takes whichever route looks most natural, and it has no idea what your project ruled out last year. Constraints are cheap to state and expensive to discover during review. Once one matters twice, turn it into a check.
Say it to your agent
“No new dependencies. Use the Map we already have, not a database table. It must handle 1,000 events a second on one server. If you can't meet one of these, tell me which and why before writing code.”