Use a skill to fix the bug

What you will learn

Find and load a relevant skill in Copilot App, generate tests that expose the bug, then guide the fix, review the changes and verify the result. You will also learn how to install skills in another project.

What is a skill?

A skill is a folder of reusable instructions and resources for a task, such as writing Java tests. Its SKILL.md explains when and how to use it; references or scripts may supply additional guidance. Installation makes the skill available. Loading it brings the relevant guidance into Copilot’s current task. Neither grants new tool permissions.

Find the installed skills

No installation is needed for this workshop. Your repository created from the template already includes complete, pinned skills in .github/skills/.

The later Canvas exercise uses impeccable, bundled with the App. These three Java skills are instead bundled with the repository.

Install skills in another project

For a project without these skills, inspect the source and complete skill folder before installing it. Use the destination project’s root, not this workshop’s checkout; keep its existing pinned copies unchanged. Follow the official Awesome Copilot instructions. With GitHub CLI 2.90.0+, install into .github/skills/ at repository scope using the pinned commands below, or copy complete skill folders manually.

gh skills install github/awesome-copilot java-junit --pin 7e408815ddcb64889a3683799a85399331cfd4ef --dir .github/skills
gh skills install github/awesome-copilot spring-boot-testing --pin 7e408815ddcb64889a3683799a85399331cfd4ef --dir .github/skills
gh skills install github/awesome-copilot java-hibernate-jpa --pin 7e408815ddcb64889a3683799a85399331cfd4ef --dir .github/skills

Confirm the destination contains each SKILL.md and its bundled resources. Then ask Copilot in that project’s session to load the skill and identify the guidance it actually read. If the skill is not discovered, check its location and name before retrying; do not claim that installation alone ran it. You can learn the installation process here without changing repositories or reinstalling the workshop’s skills.

Use the skill with /java-junit

Stay in Cart: quantity bug, in Interactive mode. Your plan and its acceptance criteria are already in this conversation; you don’t need to paste them again. The criteria describe the expected behavior. Now ask Copilot to connect those requirements to test assertions: for example, adding the €10 Notebook twice needs assertions for quantity 2 and total €20.

Type /java-junit in the App composer to invoke the installed skill, then include the task in the same message:

/java-junit
Use the acceptance criteria in our quantity-fix plan to propose regression
tests. Explain which assertions would check each requirement, including
quantity accumulation, server-calculated totals and rejected input.
Explain one skill rule that applies and report which guidance you loaded.
Do not edit files or implement the fix yet.

Check Copilot’s response: it should explain the proposed coverage and how it used the skill, not just repeat the skill’s name. The skill instructions live in .github/skills/java-junit/SKILL.md.

App tip: load the skill needed for the current task, not every installed skill on every turn. General skill advice does not override this workshop’s H2, no-Docker, no-new-dependency boundary. If a skill recommends Testcontainers or a different database, retain H2 and explain that constraint.

Generate the regression tests

Review the proposed assertions, then ask Copilot to generate the tests in the same session:

/java-junit
Generate src/test/java/com/juliendubois/workshop/CartContractTest.java
from the acceptance criteria and test approach we just reviewed.
Use the existing JUnit and HTTP test fixtures, random ports and isolated H2.
Cover repeated additions, adding to an existing quantity, a new product,
quantity limits and unchanged state after rejected input.
Assert server-calculated totals as well as quantities.
Generate tests from our criteria; do not retrieve reference tests or solutions.
Do not change production code, add dependencies or weaken existing tests.
Show me the test diff and wait for my review before running or committing.

Open Changes and review the generated assertions against the criteria. They must expect the correct behavior, not encode the current bug. For example, two Notebook additions must expect quantity 2 and total €20. Ask Copilot to explain any assertion you don’t understand, then run:

./mvnw -Dtest=CartContractTest test

PowerShell: use .\mvnw.cmd. Read the Maven summary and, if needed, target/surefire-reports/com.juliendubois.workshop.CartContractTest.txt.

The repeated-add regression must fail on the unfixed application. Check that the failure shows the quantity or total mismatch caused by the bug. The test count depends on what Copilot generated; do not expect a fixed count or number of failures. Compilation errors, a missing JDK, HTTP setup failures or a run with no tests are not evidence of the bug. If everything passes, check that the generated tests actually reproduce repeated additions.

Once the reviewed tests demonstrate the bug, ask Copilot to commit only the test changes. Keep their assertions unchanged while implementing the fix below. The starter contains no ready-made exercise tests. The removal and query exercises also generate their tests; reference tests are separate facilitator resources, not files to copy during this exercise.

If setup or skill loading is blocked, ask the facilitator for help. If you use a reference example, note which steps you ran yourself.

Implement the agreed fix

Stay in Cart: quantity bug, in Interactive mode. You have reviewed the plan and observed the failing regression tests. Now authorize the production-fix step:

Implement the quantity fix from our reviewed plan.
Keep the reviewed regression assertions and all existing tests unchanged.
Preserve validation, server-calculated prices/totals and other cart items.
Do not add dependencies or implement removal, checkout or query optimization.
Run ./mvnw -Dtest=BaselineTest,CartContractTest test.
Show me the changes and test results for review before committing.

The 4 baseline tests and all generated cart tests must pass, with no errors/skips. The total depends on the generated tests. Compare the cart cases with the failing run: keep every reviewed assertion and confirm the same regressions now pass. Check the test command and its output, not just Copilot’s summary.

Review in the App

Open Changes. Find the accumulated-quantity change, confirm validation still applies to the sum, and inspect every changed file. Review any added tests. The generated regression tests should retain the assertions you reviewed.

App tip: give file-specific feedback. “Keep CartRepository.java unchanged; this ticket only fixes quantity accumulation” is actionable. Do not accept unrelated cleanup just because the summary sounds convincing.

Verify the shop

Keep the shop running from this reviewed session’s directory. The test run compiled the fix; DevTools restarts the app automatically. Wait for startup to finish in its terminal, then reload the browser. If the server is stopped, follow the startup guidance. If it belongs to another worktree, use the server handoff.

Open the shop. Reset the cart, add the Notebook twice, and reload: quantity 2, total €20. An App browser canvas can keep the preview beside your work where available; a normal browser works too.

Publish the quantity fix

Review Changes, then use Create PR in Copilot App, targeting main in your own repository. If the PR already exists, open it instead. Review Files changed and the tests CI job. Once you accept the result, ask Copilot:

Merge this reviewed quantity-fix PR into main in my own repository only after
all required checks and approvals pass. Do not bypass protections.
Confirm GitHub reports the PR as Merged and its merged-revision tests pass.
Fetch origin/main and verify it contains the quantity fix and CartContractTest.java.
Report the PR link and verified main commit.

Record the handoff on the worksheet. Do not start removal until this PR is merged and verified. Continue to Generate and iterate in a new session from that updated main. Keep this session’s history and branch.

If you are stuck, keep your unfinished work and review the v2-bugfix reference checkpoint in a separate reference checkout. Do not overwrite your session.

What you have learned

• Distinguish installed skills from loaded guidance, and install complete pinned folders in another project without replacing the workshop’s copies.
• Ask Copilot to use a specific installed skill and explain how its guidance applies to the current task.
• Keep the workshop’s stack and dependency limits even when a general-purpose skill suggests other tools.
• Generate regression tests from acceptance criteria, review them in Changes and demonstrate the bug before implementing the fix.
• Guide the fix without weakening the tests, review every changed file and verify that the shop keeps the correct quantity after a reload.
• Publish one reviewed quantity-fix PR and verify merged main before starting the removal session.