@@ -0,0 +1,10 @@
+---
+'@objectstack/metadata-protocol': patch
+---
+
+fix(metadata-protocol): the layered read on a base's address answers an email template locale added in the base's `.custom` pair with the pair's layer (#28830)
+
+Clause-②: yes (widening)
+Metadata-protocol: none
+
+`GET /meta/email_template/<name>/layers?package=<base>&member=<locale>` (and the same read naming no package) answered `404` for a locale the base does not ship and its `.custom` pair adds, while the by-name read, the base's lists and `sendTemplate` serve that locale since #28802. It now answers the pair's row as the `overlay` and `effective` layers (`code: null`, the base ships no such locale), from the one read the by-name read uses. Another package's added locale stays `404` there and answers under the read naming that package; the base's own locales, the pair's declared overrides and every type whose identity is its name alone are unchanged.
@@ -11630,12 +11630,7 @@ export class ObjectStackProtocolImplementation implements
             // added member is the address's, served from the pair's row under
             // the pair's binding (a dependent's member never is).
             if (!record && readState === 'active' && wantedMember !== undefined && typeof contentPackageId === 'string') {
-                const pairOfAddition = this.pairOfAddedMember(request.type, request.name, contentPackageId, wantedMember);
-                if (pairOfAddition !== undefined) {
-                    record = (await this.findServedOverlayRow({
-                        type: request.type, name: request.name, state: readState, packageId: pairOfAddition, ...memberOption(member),
-                    }, activeRows))?.row;
-                }
+                record = await this.findPairAddedMemberRow(request.type, request.name, contentPackageId, member, wantedMember, activeRows);
             }
             // [#23715] Two or more packages and no package-unbound draft: the
             // best-effort first match ADR-0048 §3.3 states, as before, served
@@ -12335,14 +12330,21 @@ export class ObjectStackProtocolImplementation implements
             // then the `.custom` item, `code` the base's document and
             // `effective` the override (the base's document stays reachable
             // here, ADR-0146 "Decided here" item 5).
-            const served = await this.findServedOverlayRow({
+            let served: { row: any; overrideOf?: string } | undefined = await this.findServedOverlayRow({
                 type: request.type,
                 name: request.name,
                 state: 'active',
                 ...(contentPackageId !== undefined ? { packageId: contentPackageId } : {}),
                 ...memberOption(member),
                 servedOverride: true,
             });
+            // [#28830, #28390 ruling A] The pair's own added member is the
+            // overlay on the base's address, as the by-name read serves it
+            // ({@link findPairAddedMemberRow}); another package's never is.
+            if (!served && wantedMember !== undefined && typeof contentPackageId === 'string') {
+                const row = await this.findPairAddedMemberRow(request.type, request.name, contentPackageId, member, wantedMember);
+                if (row) served = { row };
+            }
             overlayLockLayer = await this.overlayLockLayerAt({
                 type: request.type,
                 name: request.name,
@@ -17525,6 +17527,24 @@ export class ObjectStackProtocolImplementation implements
         return pair.ok ? pair.value : undefined;
     }
 
+    /**
+     * [#28802, #28830] The pair's own stored row at the bundle member `member`
+     * (`wantedMember` its discriminator value) of the name `name`, when only
+     * the pair can add that member on `base`'s address
+     * ({@link pairOfAddedMember}); else `undefined`. The one read of it the
+     * by-name read and the layered read share.
+     */
+    private async findPairAddedMemberRow(
+        type: string, name: string, base: string, member: string | undefined, wantedMember: string,
+        activeRows?: readonly StoredOverlayRow[],
+    ): Promise<any> {
+        const pairOfAddition = this.pairOfAddedMember(type, name, base, wantedMember);
+        if (pairOfAddition === undefined) return undefined;
+        return (await this.findServedOverlayRow({
+            type, name, state: 'active', packageId: pairOfAddition, ...memberOption(member),
+        }, activeRows))?.row;
+    }
+
     /**
      * [#28802] The registry's answer `item` for a bundle member read on the
      * address of `base`, when that address serves it
 
@@ -345,3 +345,72 @@ describe('[#28802] a bundle member on the base\'s address, at member grain', ()
         expect(await executing(protocol, NAME)).toEqual([['en-US', BASE, 'Override EN'], PAIR_FR]);
     });
 });
+
+/** The layered read: `{ code, overlay, effective }` subjects (or view columns), or `404` when no layer answers. */
+const layersOf = async (protocol: ObjectStackProtocolImplementation, type: string, name: string, member?: string, packageId?: string) => {
+    try {
+        const res: any = await (protocol as any).getMetaItemLayered({
+            type, name, ...(member ? { member } : {}), ...(packageId ? { packageId } : {}),
+        });
+        const pick = (item: any) => (item === null || item === undefined
+            ? null
+            : type === 'view'
+                ? (item?.config?.columns ?? item?.columns ?? []).map((c: any) => c?.field ?? c)
+                : [item.subject, item.locale]);
+        // The protocol answers no layer at all as three nulls; the `/layers` door turns that into 404.
+        if (res.code == null && res.overlay == null && res.effective == null) return 404;
+        return { code: pick(res.code), overlay: pick(res.overlay), effective: pick(res.effective) };
+    } catch (error: any) {
+        if (error?.status === 404) return 404;
+        throw error;
+    }
+};
+
+describe('[#28830] the layered read on the base\'s address, at member grain', () => {
+    it('the pair\'s own added member answers with the pair\'s layer, naming the base and naming no package', async () => {
+        const { protocol } = await boot();
+        await install(protocol, PAIR);
+        await publishInto(protocol, PAIR, NAME, 'fr-FR', 'Pair FR');
+
+        const PAIR_LAYERS = { code: null, overlay: ['Pair FR', 'fr-FR'], effective: ['Pair FR', 'fr-FR'] };
+        expect(await layersOf(protocol, TYPE, NAME, 'fr-FR', BASE)).toEqual(PAIR_LAYERS);
+        expect(await layersOf(protocol, TYPE, NAME, 'fr-FR')).toEqual(PAIR_LAYERS);
+        // Unchanged: the pair's own address, and the base's member.
+        expect(await layersOf(protocol, TYPE, NAME, 'fr-FR', PAIR)).toEqual(PAIR_LAYERS);
+        expect(await layersOf(protocol, TYPE, NAME, 'en-US', BASE))
+            .toEqual({ code: ['Base EN', 'en-US'], overlay: null, effective: ['Base EN', 'en-US'] });
+    });
+
+    it('a dependent\'s added member stays 404 on the base\'s layered read and answers under the dependent', async () => {
+        const { protocol } = await boot();
+        await install(protocol, DEPENDENT);
+        await publishInto(protocol, DEPENDENT, NAME, 'fr-FR', 'Dependent FR');
+
+        expect(await layersOf(protocol, TYPE, NAME, 'fr-FR', BASE)).toBe(404);
+        expect(await layersOf(protocol, TYPE, NAME, 'fr-FR')).toBe(404);
+        expect(await layersOf(protocol, TYPE, NAME, 'fr-FR', DEPENDENT))
+            .toEqual({ code: null, overlay: ['Dependent FR', 'fr-FR'], effective: ['Dependent FR', 'fr-FR'] });
+    });
+
+    it('with a dependent\'s member at the same locale, the base\'s layered read answers the pair\'s', async () => {
+        const { protocol } = await boot();
+        await install(protocol, PAIR);
+        await install(protocol, DEPENDENT);
+        await publishInto(protocol, PAIR, NAME, 'fr-FR', 'Pair FR');
+        await publishInto(protocol, DEPENDENT, NAME, 'fr-FR', 'Dependent FR');
+
+        expect(await layersOf(protocol, TYPE, NAME, 'fr-FR', BASE))
+            .toEqual({ code: null, overlay: ['Pair FR', 'fr-FR'], effective: ['Pair FR', 'fr-FR'] });
+    });
+
+    it('control: a declared override of the base\'s member, and a non-bundle type, answer as before', async () => {
+        const { protocol } = await boot();
+        await install(protocol, PAIR);
+        await publishInto(protocol, PAIR, NAME, 'en-US', 'Override EN', { overrides: { package: BASE, basedOn: '1.0.0' } });
+
+        expect(await layersOf(protocol, TYPE, NAME, 'en-US', BASE))
+            .toEqual({ code: ['Base EN', 'en-US'], overlay: ['Override EN', 'en-US'], effective: ['Override EN', 'en-US'] });
+        expect(await layersOf(protocol, 'view', VIEW, undefined, BASE))
+            .toEqual({ code: BASE_COLUMNS, overlay: null, effective: BASE_COLUMNS });
+    });
+});
@@ -75,6 +75,14 @@ function showcaseRig() {
         const item = (doc?.item ?? doc) as Record<string, unknown>;
         return { status: read.status, subject: item.subject, locale: item.locale, _packageId: item._packageId };
     };
+    /** `GET /meta/email_template/:name/layers` at one member — the subjects of `code`, `overlay` and `effective`, or the status. */
+    const layers = async (member: string, pkg?: string) => {
+        const read = await call('GET', `/meta/email_template/${NAME}/layers?member=${member}${pkg ? `&package=${pkg}` : ''}`);
+        if (read.status !== 200) return { status: read.status };
+        const doc = read.json?.data ?? read.json;
+        const subject = (layer: any) => (layer === null || layer === undefined ? null : layer.subject);
+        return { status: 200, code: subject(doc?.code), overlay: subject(doc?.overlay), effective: subject(doc?.effective) };
+    };
     const rowsOf = (items: any[]) => items
         .filter((i) => i?.name === NAME)
         .map((i) => [i.locale, i._packageId, i.subject])
@@ -133,7 +141,7 @@ function showcaseRig() {
         const doc = read.json?.data ?? read.json?.item ?? read.json;
         rig.shipped = doc?.item ?? doc;
     };
-    return { rig, boot, byName, listed, executing, rendered, body, saveInto, publishInto, createDependent, ensurePair };
+    return { rig, boot, byName, layers, listed, executing, rendered, body, saveInto, publishInto, createDependent, ensurePair };
 }
 
 const BASE_EN = { status: 200, subject: SHIPPED_SUBJECT, locale: 'en-US', _packageId: BASE };
@@ -179,6 +187,16 @@ describe('[#28802] a dependent\'s email template locale the base does not ship s
         expect(await r.byName('en-US')).toEqual(BASE_EN);
     });
 
+    it('[#28830] the layered read naming the base or no package stays 404 for the dependent\'s `fr-FR`; naming the dependent it answers', async () => {
+        expect(await r.layers('fr-FR', BASE)).toEqual({ status: 404 });
+        expect(await r.layers('fr-FR')).toEqual({ status: 404 });
+        expect(await r.layers('fr-FR', DEPENDENT))
+            .toEqual({ status: 200, code: null, overlay: DEPENDENT_SUBJECT, effective: DEPENDENT_SUBJECT });
+        // Control: the base's own member.
+        expect(await r.layers('en-US', BASE))
+            .toEqual({ status: 200, code: SHIPPED_SUBJECT, overlay: null, effective: SHIPPED_SUBJECT });
+    });
+
     it('the env-wide, base-scoped and executing lists leave the dependent\'s `fr-FR` out, so a `fr-FR` send renders the base\'s `en-US`', async () => {
         expect(await r.listed()).toEqual([BASE_EN_ROW]);
         expect(await r.listed(BASE)).toEqual([BASE_EN_ROW]);
@@ -195,6 +213,9 @@ describe('[#28802] a dependent\'s email template locale the base does not ship s
         expect(await r.byName('en-US')).toEqual(OVERRIDE_EN);
         expect(await r.listed()).toEqual([['en-US', BASE, OVERRIDE_SUBJECT]]);
         expect(await r.rendered('en-US')).toBe('Override: T');
+        expect(await r.layers('en-US', BASE))
+            .toEqual({ status: 200, code: SHIPPED_SUBJECT, overlay: OVERRIDE_SUBJECT, effective: OVERRIDE_SUBJECT });
+        expect(await r.layers('fr-FR', BASE)).toEqual({ status: 404 });
 
         expect(await r.byName('fr-FR', BASE)).toEqual({ status: 404 });
         expect(await r.byName('fr-FR')).toEqual({ status: 404 });
@@ -223,6 +244,16 @@ describe('[#28802] the pair\'s own email template locale the base does not ship
         expect(await r.byName('en-US')).toEqual(BASE_EN);
     });
 
+    it('[#28830] the layered read naming the base or no package answers the pair\'s `fr-FR` layer, as the other reads serve it', async () => {
+        const PAIR_LAYERS = { status: 200, code: null, overlay: PAIR_SUBJECT, effective: PAIR_SUBJECT };
+        expect(await r.layers('fr-FR', BASE)).toEqual(PAIR_LAYERS);
+        expect(await r.layers('fr-FR')).toEqual(PAIR_LAYERS);
+        // Unchanged: the pair's own address, and the base's member.
+        expect(await r.layers('fr-FR', PAIR)).toEqual(PAIR_LAYERS);
+        expect(await r.layers('en-US', BASE))
+            .toEqual({ status: 200, code: SHIPPED_SUBJECT, overlay: null, effective: SHIPPED_SUBJECT });
+    });
+
     it('the env-wide, base-scoped and executing lists carry the pair\'s `fr-FR` beside the base\'s `en-US`, so a `fr-FR` send renders it', async () => {
         expect(await r.listed()).toEqual([BASE_EN_ROW, PAIR_FR_ROW]);
         expect(await r.listed(BASE)).toEqual([BASE_EN_ROW, PAIR_FR_ROW]);
@@ -240,6 +271,8 @@ describe('[#28802] the pair\'s own email template locale the base does not ship
         expect(await r.byName('fr-FR', DEPENDENT)).toEqual(DEPENDENT_FR);
         expect(await r.byName('fr-FR', BASE)).toEqual(PAIR_FR);
         expect(await r.byName('fr-FR')).toEqual(PAIR_FR);
+        expect(await r.layers('fr-FR', BASE))
+            .toEqual({ status: 200, code: null, overlay: PAIR_SUBJECT, effective: PAIR_SUBJECT });
         expect(await r.listed()).toEqual([BASE_EN_ROW, PAIR_FR_ROW]);
         expect(await r.listed(BASE)).toEqual([BASE_EN_ROW, PAIR_FR_ROW]);
         expect(await r.executing()).toEqual([BASE_EN_ROW, PAIR_FR_ROW]);