Prompts aren't boundaries: Why coding agents desperately need a "Change Budget"

We’ve all seen this pattern: you ask an agent to add bounded retry logic to a single client, and it returns a 14-file diff. The retry code is there, but so are changes to dependency injection, a new interface nobody asked for, and edits to bootstrap code that were never part of the request. The agent solved the local task by expanding the scope of the change until the problem disappeared. We usually try to fix this by adding more rules to ⁠AGENTS.md⁠, writing stricter system prompts, or relying on a "human in the loop." But a tired developer reading a 40-file AI-generated diff isn't a control plane. Code generation got incredibly cheap, but review still moves at human speed—and AI code is often much harder to review than a colleague's. Instead of just tweaking instructions, we need a hard structural boundary: a **change budget** that stops scope creep *before* it ever hits a pull request. I wrote a full breakdown on why we need firm boundaries for agent workflows and how to stop small requests from turning into massive, unreviewable edits. *(Link to the article is in the comments!)* Curious to hear from others building or using agents: what strategies are you using to keep your models contained?