Lesson Learned

Extract specific, grounded software engineering lessons from actual code changes. Not a lecture -- a mirror. Show the user what their code already demonstrates.

Before You Begin

Load the principles reference first.

1. Read

   references/se-principles.md

   to have the principle catalog available
2. Optionally read

   references/anti-patterns.md

   if you suspect the changes include areas for improvement
3. Determine the scope of analysis (see Phase 1)

Do not proceed until you've loaded at least

se-principles.md

Phase 1: Determine Scope

Ask the user or infer from context what to analyze.

git log main..HEAD --oneline

git diff main...HEAD

git log --oneline -N

git diff HEAD~N..HEAD

git show <sha>

git diff

git diff --cached

Default behavior:

• If on a feature branch: analyze branch commits vs main
• If on main: analyze the last 5 commits
• If the user provides a different scope, use that

Phase 2: Gather Changes

1. Run

   git log

   with the determined scope to get the commit list and messages
2. Run

   git diff

   for the full diff of the scope
3. If the diff is large (>500 lines), use

   git diff --stat

   first, then selectively read the top 3-5 most-changed files
4. Read commit messages carefully -- they contain intent that raw diffs miss
5. Only read changed files. Do not read the entire repo.

Phase 3: Analyze

Identify the dominant pattern -- the single most instructive thing about these changes.

Look for:

• Structural decisions -- How was the code organized? Why those boundaries?
• Trade-offs made -- What was gained vs. sacrificed? (readability vs. performance, DRY vs. clarity, speed vs. correctness)
• Problems solved -- What was the before/after? What made the "after" better?
• Missed opportunities -- Where could the code improve? (present gently as "next time, consider...")

Map findings to specific principles from

references/se-principles.md

Phase 4: Present the Lesson

Use this template:

## Lesson: [Principle Name]

**What happened in the code:**
[2-3 sentences describing the specific change, referencing files and commits]

**The principle at work:**
[1-2 sentences explaining the SE principle]

**Why it matters:**
[1-2 sentences on the practical consequence -- what would go wrong without this, or what goes right because of it]

**Takeaway for next time:**
[One concrete, actionable sentence the user can apply to future work]

If there is a second lesson worth noting (maximum 2 additional):

---

### Also worth noting: [Principle Name]

**In the code:** [1 sentence]
**The principle:** [1 sentence]
**Takeaway:** [1 sentence]

What NOT to Do

Conversation Style

• Reflective, not prescriptive. Use the user's own code as primary evidence.
• Never say "you should have..." -- instead use "the approach here shows..." or "next time you face this, consider..."
• If the code is good, say so. Not every lesson is about what went wrong. Recognizing good patterns reinforces them.
• If the changes are trivial (a single config tweak, a typo fix), say so honestly rather than forcing a lesson. "These changes are straightforward -- no deep lesson here, just good housekeeping."
• Be specific. Generic advice is worthless. Every claim must point to a concrete code change.