@@ -0,0 +1,17 @@
+---
+'@objectstack/metadata-core': minor
+'@objectstack/metadata': patch
+---
+
+fix(metadata-core): an artifact declaring `engines.protocol: '^18'` that was built during the v18 prerelease boots on a later runtime that retired one of its shapes — the artifact door replays every retired conversion of protocol 18's step for a floor of `18.0.0`
+
+Clause-②: yes
+
+**What an operator saw.** A compiled artifact (`os build` → `dist/objectstack.json`) that declares `engines.protocol: '^18'` and was built before a protocol-18 retirement landed carries the retired shape, for example a defaulted key that every earlier parse emitted. Booted on a runtime that enforces the retirement, the metadata plugin's startup refused it at the strict parse and the boot rolled back. The artifact door compared the floor `18.0.0` with each retirement's `retiredAfter` stamp, and every retirement that lands during the v18 prerelease is stamped with the last 17.x release, below that floor, so nothing was replayed.
+
+**What an operator sees now.** The same artifact boots. One warning per conversion names it and its site count: `[MetadataPlugin] artifact 'LABEL' declares the first release of protocol 18 (authored engines.protocol floor 18.0.0), which a build from any 18.0.0 prerelease declares too, so it may predate retirements in the conversion step into protocol 18 that this runtime enforces (runtime spec VERSION) — converted N site(s) forward via ADR-0087 conversion 'ID' …`. The artifact file itself is unchanged, so it converts again on every boot until it is rebuilt with `os build`. An artifact built after the retirement boots exactly as before: the replay finds nothing to convert, and nothing is logged.
+
+- **`@objectstack/metadata-core`.** `applyArtifactForwardConversions` opens its window on a third condition. A declared floor of `M.0.0` at or above the runtime label replays every retired entry whose `toMajor` is `M`, except the default flips the door already refuses. An artifact cannot say which of those entries it predates: it carries no field naming the spec or CLI that built it. Each entry rewrites only its own old shape, so the replay changes nothing in an artifact that lacks that shape. `ArtifactForwardConversionVerdict` gains `'converted-major-step'`, so a consumer that switches exhaustively over the verdict adds that arm. `replayedRetirements` still lists only the entries the per-entry rule opened. The rule holds for every 18.0.0 prerelease label and at 18.0.0 itself. From 18.0.1 on, the label half replays the whole chain for such an artifact, as before.
+- **`@objectstack/metadata`.** The artifact door places the new verdict on the open side of its window table and gives it its own clause, so the summary never claims that a `^18` artifact "predates" a runtime labelled below it. The bare-root form-predicate notice uses the same window.
+
+**Unchanged.** The protocol handshake still refuses a `^17` artifact on a protocol-18 runtime and prescribes `objectstack migrate meta --from 17`. A floor above `M.0.0` (such as `^18.0.1`) and a 17.x floor keep their previous verdicts.
@@ -24,6 +24,7 @@ import {
   resolveInstalledSpecVersion,
   type ArtifactConversionNotice,
 } from './artifact-forward-conversion.js';
+import { checkProtocolCompat } from './protocol-handshake.js';
 
 // ── Mirror pin ───────────────────────────────────────────────────────────────
 // `ArtifactConversionNotice` is a structural mirror of the spec root's
@@ -191,7 +192,14 @@ describe('applyArtifactForwardConversions — the versioned window (#12772)', ()
   });
 
   it('REFUSES the amnesty for an artifact authored at a NEWER spec than the runtime', () => {
-    const def = legacyPermissionDefinition('^18.0.0');
+    // "Newer" is a floor past the label AND every `retiredAfter`, and not the
+    // first release of a major. This pin used to spell it `^18.0.0`, which was
+    // hypothetical when it was written; on this tree 18 is the runtime's own
+    // protocol, and a `^18` artifact on a runtime labelled below 18.0.0 is the
+    // v18-prerelease case the major-step half opens for (#23001, block below).
+    const newer = currentSurfaceFloor('17.2.0');
+    expect(newer.endsWith('.0.0'), `${newer} must not be the first release of a major`).toBe(false);
+    const def = legacyPermissionDefinition(`^${newer}`);
     const result = applyArtifactForwardConversions(def, { runtimeSpecVersion: '17.2.0' });
     expect(result.verdict).toBe('authored-current');
     expect(result.definition).toBe(def);
@@ -665,6 +673,190 @@ describe('[#20390] the per-entry window — an artifact built by the last releas
   });
 });
 
+/**
+ * [#23001] The major-step half — a `^18` artifact built during the v18
+ * prerelease boots on a later runtime.
+ *
+ * During the prerelease the protocol constant reads 18 while the package label
+ * still names the last 17.x release, and every retirement landing then is
+ * stamped at or below that label (spec's census). A `^18` floor is 18.0.0:
+ * above every one of those stamps, so the per-entry half never opened for it,
+ * and an artifact built before a prerelease retirement met the tombstone and
+ * rolled the boot back.
+ *
+ * Measured before choosing the rule:
+ * - Reading (i), a build stamp: the artifact carries none. `os build` writes
+ *   the parsed stack, its docs and an optional `runtimeModule`, and nothing
+ *   naming the spec or CLI that built it, so telling "built before this entry"
+ *   apart would need a new artifact field.
+ * - Reading (ii), idempotence: every retired step-18 entry rewrites only its
+ *   own old shape. That is what the door now relies on, and the whole-step
+ *   block below pins both of its directions over every entry's own fixture pair.
+ *
+ * The subject is `declared-index-unique-scope`, a step-18 retirement stamped at
+ * the prerelease label, whose old shape — bare `unique: true` on a declared
+ * index — the strict parse at the door refuses. (#22990's email-template keys,
+ * the case #22994 measured at the boot, join step 18 with their own fixture
+ * pair, and the whole-step pins cover them the day they land.)
+ */
+describe('[#23001] the major-step half — a ^18 artifact built during the v18 prerelease boots on a later runtime', () => {
+  const SUBJECT = 'declared-index-unique-scope';
+
+  /** The subject's old shape under a `^18` floor: what a prerelease build from before the retirement carries. */
+  const builtDuringPrerelease = (protocolRange: string, unique: unknown = true) => ({
+    manifest: {
+      id: 'com.example.prerelease-probe', namespace: 'pre', name: 'prerelease_probe', version: '1.0.0', type: 'app',
+      engines: { protocol: protocolRange },
+    },
+    objects: [{
+      name: 'pre_contact', label: 'Contact',
+      fields: { email: { type: 'text', label: 'Email' } },
+      indexes: [{ name: 'pre_contact_email', fields: ['email'], unique }],
+    }],
+  });
+
+  const issuePaths = (value: unknown): string[] => {
+    const parsed = ObjectStackDefinitionSchema.safeParse(value);
+    return parsed.success ? [] : parsed.error.issues.map((i) => i.path.join('.')).sort();
+  };
+
+  /** The prerelease labels (before and after the first version pass) and GA, each at or below the floor. */
+  const AT_OR_BELOW_THE_FLOOR = ['17.7.0', '18.0.0-next.0', '18.0.0'] as const;
+
+  it('premise: the subject belongs to step 18, the per-entry half alone cannot open it for ^18, and this tree refuses its old shape', () => {
+    const subject = ALL_CONVERSIONS.find((c) => c.id === SUBJECT);
+    expect(subject?.toMajor).toBe(18);
+    expect(subject?.retiredFromLoadPath).toBe(true);
+    expect(compareVersions('18.0.0', subject!.retiredAfter!), 'floor 18.0.0 is above the stamp').toBe(1);
+    expect(issuePaths(builtDuringPrerelease('^18'))).toEqual(['objects.0.indexes.0.unique']);
+    expect(issuePaths(builtDuringPrerelease('^18', 'global'))).toEqual([]);
+  });
+
+  // Done, first half: built before the retirement, it boots, the entry named.
+  for (const label of AT_OR_BELOW_THE_FLOOR) {
+    it(`runtime labelled ${label}: the subject replays, its notice names it, and the strict parse passes`, () => {
+      const def = builtDuringPrerelease('^18');
+      const result = applyArtifactForwardConversions(def, { runtimeSpecVersion: label });
+
+      expect(result.verdict).toBe('converted-major-step');
+      expect(result.authoredFloor).toBe('18.0.0');
+      expect(result.notices.map((n) => [n.conversionId, n.path, n.from, n.to])).toEqual([
+        [SUBJECT, 'objects[0].indexes[0].unique', 'true', 'global'],
+      ]);
+      expect((result.definition as ReturnType<typeof builtDuringPrerelease>).objects[0]!.indexes[0]!.unique).toBe('global');
+      expect(issuePaths(result.definition)).toEqual([]);
+      // The per-entry half's own list: only entries stamped at or above the
+      // floor, never the step it opened on the floor's reason.
+      expect(new Map(result.replayedRetirements.map((r) => [r.conversionId, r.retiredAfter]))).toEqual(openedByRule('18.0.0'));
+    });
+  }
+
+  it('from 18.0.1 on, the same artifact converts through the label half — the rule reduces to the old one', () => {
+    const result = applyArtifactForwardConversions(builtDuringPrerelease('^18'), { runtimeSpecVersion: '18.0.1' });
+    expect(result.verdict).toBe('converted-forward');
+    expect(result.replayedRetirements).toEqual([]);
+    expect(result.notices.map((n) => n.conversionId)).toEqual([SUBJECT]);
+    expect(issuePaths(result.definition)).toEqual([]);
+  });
+
+  // Done, second half: built after the retirement, it is unchanged.
+  it('a ^18 artifact built after the retirement is unchanged: the window is open, nothing fires, the same reference comes back', () => {
+    for (const label of AT_OR_BELOW_THE_FLOOR) {
+      const def = builtDuringPrerelease('^18', 'global');
+      const bytes = JSON.stringify(def);
+      const result = applyArtifactForwardConversions(def, { runtimeSpecVersion: label });
+      // ⭐ ANTI-VACUITY: the major-step half really is open on this input.
+      expect(result.verdict, label).toBe('converted-major-step');
+      expect(result.notices, label).toEqual([]);
+      expect(result.definition, label).toBe(def);
+      expect(JSON.stringify(result.definition), label).toBe(bytes);
+    }
+  });
+
+  it('the half opens only for a first release: a ^18.0.1 floor on a prerelease runtime still meets the tombstone', () => {
+    const def = builtDuringPrerelease('^18.0.1');
+    const result = applyArtifactForwardConversions(def, { runtimeSpecVersion: '17.7.0' });
+    expect(result.verdict).toBe(openedByRule('18.0.1').size > 0 ? 'converted-retired-after' : 'authored-current');
+    expect(result.notices.map((n) => n.conversionId)).not.toContain(SUBJECT);
+    expect(issuePaths(result.definition)).toEqual(['objects.0.indexes.0.unique']);
+  });
+
+  // CONTROL: a 17.x floor keeps #20390's per-entry verdict on this tree's own label.
+  it('CONTROL — a ^17.7.0 floor on a 17.7.0 label keeps the per-entry verdict, and an older step-18 retirement still meets its tombstone', () => {
+    const def = legacyPermissionDefinition('^17.7.0');
+    const result = applyArtifactForwardConversions(def, { runtimeSpecVersion: '17.7.0' });
+    const expected = openedByRule('17.7.0');
+    expect(result.verdict).toBe(expected.size > 0 ? 'converted-retired-after' : 'authored-current');
+    expect(new Map(result.replayedRetirements.map((r) => [r.conversionId, r.retiredAfter]))).toEqual(expected);
+    // `permission-allow-restore-purge-removed` is in step 18 too, retired long
+    // before 17.7.0: a 17.7.0 floor postdates it, so it is NOT replayed.
+    expect(result.notices).toEqual([]);
+    expect(result.definition).toBe(def);
+    expect(def.permissions[0]!.objects.crm_ticket).toHaveProperty('allowRestore');
+  });
+
+  // CONTROL: the protocol-18 handshake, untouched — it refuses ^17 before any conversion could matter.
+  it('CONTROL — the protocol-18 handshake still refuses a ^17 artifact, prescribing the migration, and admits ^18', () => {
+    const refused = checkProtocolCompat({ id: 'com.example.prerelease-probe', engines: { protocol: '^17' } }, '18.0.0');
+    expect(refused.status).toBe('incompatible');
+    if (refused.status !== 'incompatible') return;
+    expect(refused.diagnostic.code).toBe('OS_PROTOCOL_INCOMPATIBLE');
+    expect(refused.diagnostic.migrateCommand).toBe('objectstack migrate meta --from 17');
+    expect(checkProtocolCompat({ id: 'com.example.prerelease-probe', engines: { protocol: '^18' } }, '18.0.0').status).toBe('ok');
+  });
+
+  /**
+   * Reading (ii), both directions, over EVERY retired step-18 entry rather than
+   * a sample — each entry's own fixture pair, carried in a `^18` manifest (no
+   * conversion reads a manifest's `engines`), on a runtime labelled 17.7.0.
+   */
+  describe('reading (ii) over the whole step — every retired step-18 entry, both directions', () => {
+    const STEP_18 = ALL_CONVERSIONS.filter((c) => c.toMajor === 18 && c.retiredFromLoadPath === true);
+
+    const inCaretEighteen = (stack: Record<string, unknown>): Record<string, unknown> => {
+      const manifest = (stack.manifest ?? {}) as Record<string, unknown>;
+      const engines = (manifest.engines ?? {}) as Record<string, unknown>;
+      return { ...stack, manifest: { ...manifest, engines: { ...engines, protocol: '^18' } } };
+    };
+
+    it('the step carries the subject, and the door refuses its default flips by id', () => {
+      expect(STEP_18.map((c) => c.id)).toContain(SUBJECT);
+      expect(STEP_18.filter((c) => DOOR_DEFAULT_FLIPS.includes(c.id)).map((c) => c.id)).toEqual([
+        'flow-decision-mode-inclusive-explicit',
+      ]);
+    });
+
+    // Built BEFORE: each entry's old shape replays to exactly its canonical shape.
+    for (const entry of STEP_18) {
+      const flip = DOOR_DEFAULT_FLIPS.includes(entry.id);
+      it(`${entry.id}: ${flip ? 'a default flip — never replayed, even with the step open' : 'built before it, a ^18 artifact converts to its canonical fixture'}`, () => {
+        const before = inCaretEighteen(structuredClone(entry.fixture.before));
+        const result = applyArtifactForwardConversions(before, { runtimeSpecVersion: '17.7.0' });
+        expect(result.verdict).toBe('converted-major-step');
+        if (flip) {
+          expect(result.notices.map((n) => n.conversionId)).not.toContain(entry.id);
+          expect(result.definition).toBe(before);
+          return;
+        }
+        expect(result.definition).toEqual(inCaretEighteen(structuredClone(entry.fixture.after)));
+        expect(result.notices).toHaveLength(entry.fixture.expectedNotices);
+        for (const n of result.notices) expect(n.conversionId).toBe(entry.id);
+      });
+    }
+
+    // Built AFTER: over every entry's canonical fixture, the whole replayed step changes nothing.
+    for (const entry of STEP_18) {
+      it(`${entry.id}: built after it, a ^18 artifact comes back by reference with the whole step replayed over it`, () => {
+        const after = inCaretEighteen(structuredClone(entry.fixture.after));
+        const result = applyArtifactForwardConversions(after, { runtimeSpecVersion: '17.7.0' });
+        expect(result.verdict).toBe('converted-major-step');
+        expect(result.notices.map((n) => `${n.conversionId} at ${n.path}`)).toEqual([]);
+        expect(result.definition).toBe(after);
+      });
+    }
+  });
+});
+
 /**
  * #15429 — the second member of the DEFAULT-FLIP class this door refuses.
  *
@@ -756,6 +948,24 @@ describe('the artifact door never writes `mode: inclusive` onto an authored excl
     expect(result.notices.map((n) => n.conversionId)).not.toContain(ID);
   });
 
+  /**
+   * [#23001] Nor does the major-step half. The entry belongs to step 18
+   * (`toMajor: 18`), so a `^18` floor on a runtime labelled below 18.0.0 opens
+   * every OTHER retired entry of that step — and the refusal list is read
+   * before `toMajor` exactly as it is before `retiredAfter`.
+   */
+  it('stays refused inside the major-step half too — the refusal list is read before toMajor', () => {
+    expect(ALL_CONVERSIONS.find((c) => c.id === ID)?.toMajor, 'the entry is in step 18').toBe(18);
+    const def = twoBranchDecisionDefinition('^18');
+    const result = applyArtifactForwardConversions(def, { runtimeSpecVersion: '17.7.0' });
+
+    // ⭐ ANTI-VACUITY: the major-step half really is open on this input.
+    expect(result.verdict).toBe('converted-major-step');
+    expect(verdictNodeOf(result.definition).config).toBeUndefined();
+    expect(result.notices.map((n) => n.conversionId)).not.toContain(ID);
+    expect(result.definition).toBe(def);
+  });
+
   it('floor ^99.0.0 — the window is shut and nothing is replayed at all', () => {
     const def = twoBranchDecisionDefinition('^99.0.0');
     const result = applyArtifactForwardConversions(def, { runtimeSpecVersion: '17.4.0' });