An agent building more than you asked for: extra files, options and layers for a small job. Each addition is reasonable on its own, and the total is a bigger thing to carry for as long as the code lives.
Code costs an agent almost nothing to write, so nothing in its loop pushes back on size. That force has to come from you, which is why pstack and ub-mode, two published sets of agent instructions, both carry a rule about smallness.
Say it to your agent
“Smallest change that makes this test pass. No new files, no new dependencies, no config options. If you think more is needed, stop and tell me why before you write it.”