why

Explain why code exists by finding the session that produced it. ## Steps 1. **Call `why_did_this_change`** with the user's target as `target`: - A file → `src/cache.ts` - A specific line → `src/cache.ts:142` (pins the commit via git blame) - A commit → a sha, tag, or `HEAD~2` - A topic → free text (searches sessions in this repo, no git) The tool is read-only: it never writes to the repository. 2. **Read the evidence.** The result carries the resolved `commit` (subject, author time, files, trailers; `null` for the free-text form) and ranked `sessions`. Trust `confidence`: `files+time` (the session edited the committed files inside its window) is stronger than `time-only` (same repo and window, no file overlap). Two flags change how you read the commit: `commit.merge: true` means this is the merge that landed the change, not the commit that wrote it — the reasoning lives in the sessions, not the merge subject. A non-empty `unlandedAttempts` (file form only) means sessions touched this file but no commit ever landed from them — a possible abandoned attempt. 3. **Synthesize a short answer** from each session's `excerpts` and `headline`. Explain the reasoning and any abandoned approaches — the part git does not record. Do not invent rationale the excerpts do not support. 4. **Cite each session** you used: its tool, date (`startedAt`), and headline. Distinguish `files+time` matches from `time-only` matches so the user can weigh them. 5. **Check closed PRs before concluding "fixed".** When the question is "was this fixed" or "does this bug still exist", git history alone is not proof: a fix may have been tried and rejected without leaving a commit. If the repo has a GitHub remote and `gh` is available, run `gh search prs --state closed -- <terms>` (or `gh pr list --search "<file>" --state closed`) and report a closed-unmerged attempt alongside any merged fix. Skip silently when `gh` or a GitHub remote is absent. 6. **Offer a deep-dive.** If the user wants more, call `get_session_messages` with a session's `filePath` (use an `excerpts[].msgIndex` as the offset) to read the full exchange. ## Guidelines - Empty `sessions` is a real answer: say no session correlates, rather than guessing. - A `time-only` match is a weak signal — present it as "around the same time", not as cause. - Surface commit `trailers` (e.g. Co-Authored-By) when present; they annotate authorship. - Lead with the why, then the citations. Keep it to a few sentences unless asked for more.