LESSONS.md

@@ -142,6 +142,8 @@ During write-skill refactor, an "open thread" note (about a deferred research st
 
 Three separate times in one PR (#85), a check reported success because it had silently not run. (1) Vale's `text.frontmatter.description` scope stops matching once the value is a multi-line YAML block scalar — the style most skills here use — so a repo-wide sweep returned 0 alerts across 49 files and was read as a clean repo. (2) Five of six rules were `level: warning`, but Vale's exit code keys on `error` alone and pre-commit hides output from passing hooks, so those rules were invisible and blocked nothing for two review rounds while the ADR described them as "enforcing immediately." (3) `.vale.ini`'s globs matched no file outside `plugins/`, so Vale printed "0 files" and exited 0, which both audit skills read as "no findings" and used to skip their own judgment passes. Each time the green result was worse than no check at all, because it was cited as positive evidence of cleanliness. Fix: for any new check, prove it fails before trusting that it passes — run it against a deliberately-bad fixture, confirm the failure, then run the real corpus. Where a check can scan zero inputs, assert on the input count, not just the exit code. **[graduated → core/instructions/testing.md]** (4th instance below, kept for audit trail).
 
+**5th instance (2026-08-09, PR #85 round 6):** `tests/test-vale-hooks-consumer.sh` asserted `grep -c "VagueWording" >= 2` across the *combined* output of both shipped Vale hooks, and the SKILL.md fixture alone raised two alerts — so one working hook satisfied the threshold and the agent hook could be disabled entirely (glob retargeted to match nothing) while the suite still reported `3 passed` under the message "both hooks flatten and flag". The `Skipped` guard did not catch it: the hook still *matched* the file, Vale simply linted nothing, reported `0 errors in 1 file`, and exited 0, which pre-commit renders as `Passed`. The general shape: **an assertion that aggregates over N subjects proves nothing about any individual subject** — a total is satisfiable by a proper subset. Fix: attribute each signal to its source before asserting (alerts are now filed by path, with a distinct trigger token per fixture so one hook's alert cannot be credited to another), and assert per subject. Corollary technique, now standing practice for any check whose failure mode is silence: run the mutation sweep in *reverse* as well — neuter each assertion in turn and confirm exactly one test case fails. Applied to `check-vale-style-sync.sh` it exposed two assertions bound to no failing case at all, one of them masked by a stronger check that ran first.
+
 **4th instance (2026-08-09, ADR-0014):** splitting the single root `.vale.ini` into two skill-scoped copies (skill-audit: `SKILL.md` only; agent-audit: agent files only) meant a single retargeted pre-commit hook pointed at agent-audit's copy alone would have silently scanned 0 `SKILL.md` files and exited 0 — caught only because the full corpus was dry-run against both the old and new config and the outputs diffed before the old config was deleted, not because any test asserted on file counts. Standing practice going forward: when a Vale (or any linter) config that serves multiple file-glob scopes is split or moved, dry-run the full corpus through both the old and new config and diff the outputs before removing the superseded source — a hook silently scanning 0 files looks identical to a clean pass.
 
 ## 2026-08-08 — One signal, two consumers, no named distinction
@@ -159,3 +161,7 @@ The root `.pre-commit-hooks.yaml` shipped Vale hooks whose `entry:` carried a `-
 ## 2026-08-09 — Deleting a token from a shared artifact breaks whatever parses it, silently
 
 Dropping the `--config` argument from `.pre-commit-hooks.yaml` was the right fix, but `scripts/check-release-needed.sh` derived its release-relevant path list by scanning those same `entry:` lines for `--config` and taking the target's `dirname` — that parse was the only thing giving the bundled `.vale.ini` and its sibling `styles/` tree release coverage. With the token gone the loop simply never fired: no error, no failing test, no warning, just a path list that shrank from six entries to four and lost both `assets/vale/` trees. Consequence: a change to a Vale *rule* could land on `main` without demanding a release tag, leaving external consumers pinned to an old `rev:` with stale rules — the exact drift the gate exists to prevent. It surfaced only because the agent making the change reported it as a suspected side effect of its own edit, and was confirmed by diffing the derived path list before and after. Fix: before removing a token from an artifact more than one script reads, grep for everything that *parses* the artifact, not just everything that consumes its documented purpose. The smell to watch for is a loop that builds a list, where an empty or short list is indistinguishable from a correct one — assert on the expected members, so a derivation whose input vanished fails loudly instead of quietly covering less.
+
+## 2026-08-09 — A documented impossibility is a claim, not a constraint
+
+`vale-wrap.sh` flattens multi-line YAML `description:` scalars so Vale's `text.frontmatter.description` scope keeps matching. Its last-resort branch rewrote ASCII `'` to U+2019, justified at the emission site and in review as "the single combination no YAML scalar can carry verbatim" — an accepted-by-design residual, documented and test-covered, which is exactly why nobody retested it. The claim was false: a `|-` literal block with one indented content line carries `'`, `"`, `\` and `: ` verbatim, keeps the scope alive, and the wrapper's own header docstring already said literal blocks were unaffected. The cost of the unexamined claim was a silent underlint on 12 of 54 in-scope files — any rule whose token contained an apostrophe simply never fired, and the covering test (case 20) pinned only "the scope stays alive", so it passed either way. Fix: when a residual is accepted because something is "impossible", write down the specific claim in a falsifiable form and test *that*, not the workaround built on top of it. The tell here was that the residual and its justification were documented in the same breath by the same author — documentation records a belief, and a belief adjacent to a workaround is the one most worth attacking. Related: an assertion written to cover an accepted residual tends to assert the residual's *presence* rather than the behaviour it costs; case 20b asserted the scope survived flattening, never that a rule matching the rewritten characters still fired.