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Lab 2: Bugfix Engineering

Fix a real bug: a 10% discount on $100 returns a negative total. You will enter three things in your agent; in the terminal, you only prepare the example and approve the result.

With shortcuts​

1. Prepare the example project​

cp -r ~/.rapid-os/examples/bugfix-lab/starter pricer-app && cd pricer-app
git init -b main && git add . && git commit -m "starter"

In Windows PowerShell, copy it with Copy-Item -Recurse $HOME\.rapid-os\examples\bugfix-lab\starter pricer-app. If you installed Rapid OS with pip, the example is in examples/bugfix-lab/starter.

The example includes a failing test (test_regression_expected_discount in tests/test_pricer.py). Delete it before starting: the baseline must pass, and the agent will write its own test to reproduce the bug.

2. Set up the project​

Open your agent in the folder and enter /rapid-init ($rapid-init in Codex). It detects Python and suggests python -m unittest discover as the test command; accept it. You can skip the project documents for this lab.

3. Request the work​

/rapid-bugfix the 10% discount returns a negative total

First, the agent reproduces the bug with a failing test (Rapid OS records that failure as expected evidence). It then fixes the formula and checks that the same test passes.

4. Approve in your terminal​

When the agent finishes, it stops and gives you the exact command. Run it in another terminal, outside the agent, for example:

rapid approve gate.final-verification

Return to the agent and enter /rapid-next to close the run.

Result: The discount is calculated correctly, the regression test passes, and the run ends with a pass verdict.

No shortcuts yet? Install them once with rapid shortcuts install --global --harness claude, or run rapid init --shortcuts inside the folder. Replace claude with your agent. See the shortcuts guide.

Lab details​

Starting state​

pricer-app/
├── pricer.py
└── tests/
    ├── __init__.py
    └── test_pricer.py

The starter version contains this bug:

discount_amount = subtotal * discount_percent # ERROR: must divide by 100.0

A 10% discount on $100 results in -$900.00.

Requirements​

  1. Formula: Calculate the discount as subtotal * (discount_percent / 100.0).
  2. Limits: Validate that discount_percent is in the [0.0, 100.0] range; otherwise, raise ValueError.
  3. Regression: Preserve support for carts with multiple products and quantities.
  4. Verification: Check the baseline with the defective behavior before applying the fix.

Advanced mode​

You do not need this for the lab. The same loop typed by hand in a terminal, with the daily commands (rapid start, next, check, task, approve, finish), for CI or auditing, is covered in the manual walkthrough; the plumbing (rapid evidence add, rapid run gate) does not satisfy test or human gates and the v3 governance loop.