| name | dx-lesson |
| description | Record one finding — a warning or a decision-with-rationale — as an append-only entry in foundation/lessons.md. |
| argument-hint | ["the finding"] |
dx-lesson
Capture one finding so a future change re-reads it instead of relitigating it. A lesson is scar tissue or a load-bearing decision — a rule, not a story. Two flavors, both belong here: cautionary ("this broke because…") and decisional ("we chose X over Y because Z", including a rejected refactor: "don't re-deepen X — it's shallow on purpose because Y"). This is the anteroom to a standard, not an ADR — the framework has no separate decision register.
Guard. foundation/lessons.md must exist (it does after /dx-init). If it is missing, tell the user to run /dx-init first. Read foundation/glossary.md for naming — a one-line habit, no section.
1 — Get the finding
Take the finding from the argument. If nothing was passed, ask what broke or what was decided, and why it matters — one question, then proceed. Don't turn a capture into an interview.
2 — Load the entry shape (invoke dx-references with knowledge-layer)
Use it for the lesson entry shape and to judge whether this is a lesson at all (a one-off finding is fine here; a recurring one is a promotion candidate — see step 4).
3 — Append (never rewrite)
Append to the end of foundation/lessons.md. Never edit, reorder, dedupe, or reformat existing entries — the file is append-only; that friction is the point. One entry, this shape:
## <short title> — <YYYY-MM-DD>
<what happened / what was decided>. **Why:** <the reason it matters>. **How to apply:** <the rule going forward>.
Keep it tight — the title is what future skills scan first.
4 — Flag for promotion if it will recur
If the finding looks like it will keep coming up across changes, note it — that is the trigger for graduating it into a standard via /dx-standards-update. Don't promote here; lessons are the append-only home.
Done when
The entry is the last section of foundation/lessons.md and existing entries are untouched. Then print and stop:
Lesson recorded: foundation/lessons.md — ## <short title>
Next: /dx-standards-update — only if this will recur across changes (promote it to a standard)
Stop. Do not chain into another skill. One capture per invocation — batching invites half-written entries.
