Lab 4 — Plan mode: review the plan before any edit

Start from: app/ at tag lab03-done, with the Lab 0 setup loaded (see Working in app/).

Goal: for a feature that touches several files, have the harness produce a plan with no write tools available, change the plan, and only then let it edit. Afterwards, compare what it planned with what it actually did.

1. Plan without editing

Branch first, so approving the plan is cheap to undo:

git switch -c feature/discount-codes
claude --permission-mode plan

(In a running session, Shift+Tab cycles the mode to plan mode too.)

Add discount codes to orders:
- codes live in a new discounts.csv: code,kind,value,expires (kind is percent or cents)
- SAVE10 = 10 percent, FLAT500 = 500 cents off, plus one already-expired code
- `python cli.py total P1:2 --code SAVE10` applies it; expired or unknown codes are rejected with a clear message
- a discount can never make the total negative
Plan this change.

Observe: Claude reads files and maybe runs read-only commands, but cannot edit. It ends with a plan and asks how to proceed.

2. Steer the plan

Don’t approve yet. Reply with feedback such as:

Changes: use a dataclass for discount codes, no new dependencies, apply the discount before tax, and list every test you will add.

Observe: the plan is revised. Save a copy of the final plan, for example by copying it into PLAN-discount-codes.md yourself, or Ctrl+G to open it in your editor if offered.

3. Approve and implement

Approve the plan. Choose the option that auto-accepts edits only if you are happy with the plan. Let it implement and run the tests.

4. Compare plan vs reality

git diff --stat main

Check against the plan: same files? Every promised test present? Anything it did that the plan didn’t mention?

When you’re satisfied, commit (or ask Claude to commit) on the branch and merge into main:

git switch main ; git merge --no-ff feature/discount-codes

Record

• Tools used in plan mode vs after approval.
• Differences between the final plan and git diff --stat.
• Whether your feedback (dataclass, before-tax) actually landed in code.

Checkpoint

python -m pytest -q
git tag lab04-done