lesson-learned — Claude Code skill: what it does, how to install

---
name: lesson-learned
description: "Analyze recent code changes via git history and extract software engineering lessons. Use when the user asks 'what is the lesson here?', 'what can I learn from this?', 'engineering takeaway', 'what did I just learn?', 'reflect on this code', or wants to extract principles from recent work."
---

# 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.

| Scope | Git Commands | When to Use |
|-------|-------------|-------------|
| Feature branch | `git log main..HEAD --oneline` + `git diff main...HEAD` | User is on a non-main branch (default) |
| Last N commits | `git log --oneline -N` + `git diff HEAD~N..HEAD` | User specifies a range, or on main (default N=5) |
| Specific commit | `git show <sha>` | User references a specific commit |
| Working changes | `git diff` + `git diff --cached` | User says "what about these changes?" before committing |

**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`. Be specific -- quote actual code, reference actual file names and line changes.

## Phase 4: Present the Lesson

Use this template:

```markdown
## 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):

```markdown
---

### Also worth noting: [Principle Name]

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

## What NOT to Do

| Avoid | Why | Instead |
|-------|-----|---------|
| Listing every principle that vaguely applies | Overwhelming and generic | Pick the 1-2 most relevant |
| Analyzing files that were not changed | Scope creep | Stick to the diff |
| Ignoring commit messages | They contain intent that diffs miss | Read them as primary context |
| Abstract advice disconnected from the code | Not actionable | Always reference specific files/lines |
| Negative-only feedback | Demoralizing | Lead with what works, then suggest improvements |
| More than 3 lessons | Dilutes the insight | One well-grounded lesson beats seven vague ones |

## 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.