Lesson 9 of 9 · about 45 minutes

Capstone: the full loop

Issue → agent branch → pull request → green CI → main. On your own, start to finish.

Part 1 — Warm up from memory

These questions mix all eight lessons on purpose. Switching between topics is harder than drilling one, and that difficulty is what builds long-term memory. Don't scroll back. Guess, then read the explanation.

Type it, don't pick it

Part 2 — The lab: one task, the whole loop

The task: add a summarize_by_species(df) function to clean.py that returns mean bill length and body mass per species, plus a test. Claude writes it. You run the process.

1. Start clean. In your main penguins-lab folder: git switch main, git pull, git status. Check that .claude/worktrees/ is in .gitignore (Lesson 7).11
2. File the issue. gh issue create --title "Add summary statistics by species" --body "Add summarize_by_species(df) to clean.py returning mean bill_length_mm and body_mass_g per species, with a pytest test." Note the issue number, say #7.12
3. Give the agent its own branch and folder. claude -w add-summary-stats. Claude Code creates .claude/worktrees/add-summary-stats/ on a new branch worktree-add-summary-stats, based on the remote default branch.11 It's a fresh checkout, so install your dependencies there (for example pip install -r requirements.txt).
4. Brief with Explore → Plan → Implement → Commit.13 Paste: "Read issue #7 with gh and look at clean.py and test_clean.py. Don't write code yet — propose a plan." Review the plan, then: "Implement it, run pytest until it passes, then commit with a descriptive message."
5. Review the diff yourself. In the worktree, run git fetch then git diff origin/main...HEAD. The worktree branched from the remote's main, so compare against origin/main. Read every line. Ask Claude to explain anything you couldn't defend in a code review. Note the Co-Authored-By trailer on its commit.14
6. Open the pull request. git push -u origin worktree-add-summary-stats, then gh pr create --title "Add summarize_by_species" --body "Closes #7". The keyword works because the PR targets the default branch.15 (Or ask Claude: "push and open a PR that closes #7". It uses gh pr create.14)
7. Let CI judge. gh pr checks --watch. If it goes red, paste the failure to Claude in the same session, let it fix and push, and watch again. The PR updates by itself.16
8. Merge. On the PR page choose Squash and merge, then Delete branch.6 CLI alternative: gh pr merge --squash --delete-branch. If gh objects that the branch is still checked out in your worktree, do step 9's /exit first. Confirm issue #7 closed itself.
9. Clean up. Type /exit in Claude. If the worktree has commits, Claude asks whether to keep or remove it. Removing deletes the folder and its local branch,11 which is safe now because the work is merged. Back in your main folder: git switch main, git pull, then git worktree list to confirm only your main checkout remains.
10. Prove it. git log --oneline --graph -5 on main shows one squashed commit for the whole task. That's the clean history you were after.

If a leftover local branch survives step 9, git branch -d may say "not fully merged". A squash merge puts a new commit on main, so the branch's original commits aren't reachable from it. Once you've confirmed the PR merged, git branch -D is the right call.36

Stretch: two agents at once

Open two terminals in penguins-lab. Run claude -w add-species-plot in one and claude -w add-island-counts in the other. Brief both, then run each through steps 4–9. If both touch README.md or clean.py, the second PR will hit a merge conflict once the first merges. CI won't even start: "Workflows will not run on pull_request activity if the pull request has a merge conflict."7 Resolve it inside the second worktree:

$ git fetch origin
$ git merge origin/main        # conflict markers appear
# edit the files: keep both changes, delete <<<<<<< ======= >>>>>>>
$ git add README.md
$ git commit                   # seals the merge
$ git push                     # CI re-runs on the PR

Git's docs describe this exact sequence: "Edit the files into shape and git add them… Use git commit… to seal the deal."2 You can also ask the agent in that worktree to do the merge and explain each conflict before resolving it.

Part 3 — Where to go next

You now have the skills. Wisdom comes from using them with other people.

• Ask practitioners. GitHub Community discussions is the official, moderated forum for Actions, pull request, and branch protection questions. The Anthropic Discord is where Claude Code users compare branch and worktree habits. Search the Stack Overflow git tag before asking. Most git questions already have answers there.
• Keep practising. GitHub Skills runs free, hands-on courses inside real repositories: Review pull requests, Resolve merge conflicts, Hello GitHub Actions, and Test with Actions. Learn Git Branching is a bigger version of this course's simulator.
• Make it a habit. For your next real assignment, open an issue first and run the loop above. Keep the Claude workflow card and cheat sheet beside you until you stop needing them.