Week 2

Unfamiliar codebases

Not breaking things

Feature isolation and boundaries

Practice

Assignment

Back to Track

The single most common way an agent breaks an existing project: you ask for a feature, and it "helpfully" edits shared code — a utility, a config, a service other parts depend on. Your feature works; three other screens break. This page is about preventing exactly that.

Isolate the feature

Keep new code in its own files and modules. If your feature needs a helper, create it next to the feature — don't rewrite the shared one to fit your case. Spreading a feature across the whole project multiplies what can break and makes your diff hard to review.

Set boundaries in the prompt — explicitly

The agent doesn't know what's off-limits unless you say so. Make limits part of the task:

• "Work only inside src/features/profile/."
• "Do not modify anything in src/shared/ or any config files. If you think a shared change is needed — stop and ask first."
• "If a file outside the feature folder needs to change, list it and wait for my approval."

A good stop looks like this: the agent halts before writing any code, explains why the task can't be done cleanly within the boundary, and offers options — including, sometimes, a "technically-inside-the-folder" workaround that it flags as a bad idea. That last part matters: the boundary protects the architecture, not the folder. An agent (or a developer) that sneaks around the spirit of a rule while respecting its letter is doing more damage than one that asks.

Boundaries that repeat on every task belong in CLAUDE.md (Week 1) — that's exactly what its "Do not" section is for. In a shared repo, this protects the whole team, not just you.

Touching shared code = mandatory manual review

Sometimes the shared change is genuinely needed. Then the rule is: you review it yourself, line by line, deliberately. Not a skim — an actual read with the question "what else depends on this?" Ask the agent: "what other parts of the project use this function/config? list the call sites." Then check the key ones.

This is where "what worked yesterday broke today" almost always comes from — and where your review earns its keep.

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