Week 2

Unfamiliar codebases

Not breaking things

Feature isolation and boundaries

Practice

Assignment

Back to Track

Not breaking things: branch, plan mode, small diffs

Three habits turn agent work from risky to safe on a real codebase. All three work the same in Desktop, VS Code, and the terminal.

1. A separate branch, always

Never let the agent work on main in a shared project. Before any change:

Via the agent: "Create a branch feature/profile-date-format and switch to it." Read the command it proposes before approving.

Via the terminal:

git switch -c feature/profile-date-format

A branch means your experiment can't hurt anyone, and everything you do becomes one reviewable unit.

Watch: Plan mode in the official workflow — explore, plan, only then code.

https://www.youtube.com/watch?v=xJQuF02NAK8&list=PLmWCw1CzcFilebjK89WLb5cAvM8K0cLB3&index=6

2. Plan mode: look before anything touches disk

Claude Code has a mode built exactly for this week: plan mode. In it, the agent reads files and proposes a plan — but makes no edits until you approve.

• Press Shift+Tab during a session to cycle modes (default → acceptEdits → plan). In the CLI the status bar shows ⏸ plan mode; in the Desktop app, pick Plan in the mode selector next to the input — the indicator shows "Plan".
• Use it whenever you're about to change unfamiliar code: "Plan how you would add X" → read the plan → Reject, Revise, or Accept (the buttons under the proposed plan). In an unfamiliar codebase prefer plain Accept over "Accept and auto mode" — auto mode applies the whole plan without stopping for your review at each step.
• Don't be surprised if the agent asks clarifying questions before planning — that's the exploration working. Sometimes those questions catch a real trap before any code exists (e.g., "the field you want to display is overwritten by another process — what should it actually mean?"). Answer them; the plan comes after.

For a beginner in someone else's codebase, plan mode is the seatbelt: you see what the agent intends to touch before it touches anything.

3. Small diffs, reviewed every time

Ask for changes in small units, and read the diff after each one — in VS Code's Source Control panel, or ask the agent: "show me the diff of what you just changed."

A 30-line diff you understand beats a 400-line diff you skim. If a diff surprises you — files you didn't expect, changes you didn't ask for — stop and ask the agent to explain or revert. Respect the project's existing conventions: if the codebase does things a certain way, your change should look like it was written by the same team.

2 files, ~15 lines — a diff you can hold in your head

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