@@ -0,0 +1,20 @@
+---
+'@objectstack/spec': patch
+---
+
+refactor(spec): protocol 18's migration step keeps its `rationale` as key-sorted fragments and derives its `conversionIds` — no value changes (#20535)
+
+Nothing a consumer reads changes. `MIGRATIONS_BY_MAJOR[18].rationale` (48,953
+characters), `MIGRATIONS_BY_MAJOR[18].conversionIds` (45 ids, same order) and the
+whole `MIGRATIONS_BY_MAJOR` value are byte-identical to the previous release, and so
+is the rationale `migrate meta` prints for the 17 → 18 hop.
+
+What changed is how the step is written, so two major-18 retirements can be in
+flight at once without conflicting in `packages/spec/src/migrations/registry.ts`:
+
+- The rationale is `STEP18_RATIONALE`, one `{ id, order, text }` fragment per
+  retirement, kept sorted by `id` and rendered by `order`, joined with one space.
+  A retirement adds ONE fragment where its `id` (its D3 semantic entry id) sorts —
+  never at the end — with `order` one more than the highest present.
+- `conversionIds` is read off `CONVERSIONS_BY_MAJOR[18]`, which it copied value
+  for value. A retirement adds its conversion there only.