@@ -0,0 +1,185 @@
+// Copyright (c) 2026 ObjectStack. Licensed under the Apache-2.0 license.
+
+/**
+ * [#26167] The summary code's composer, and the read-side overlay, against a
+ * REAL locale: `createMemoryI18n` (what the kernel registers as the `i18n`
+ * fallback) loaded with this plugin's shipped bundle plus an app's labels, so
+ * the key shapes asked are the bundles' own. The real read path is pinned in
+ * `activity-summary-reader-locale.integration.test.ts`.
+ */
+
+import { describe, it, expect } from 'vitest';
+import { createMemoryI18n } from '@objectstack/core';
+
+import { AuditTranslations } from './translations/index.js';
+import {
+  ACTIVITY_SUMMARY_MESSAGE_KEY,
+  composeActivitySummary,
+  composeStoredSummary,
+  localizeActivityRow,
+  readActivitySummaryMessage,
+  readerSummaryLocale,
+  type ActivitySummaryMessage,
+} from './activity-summary-message.js';
+
+const DEFS: Record<string, any> = {
+  biz_deal: {
+    name: 'biz_deal',
+    label: 'Deal',
+    fields: {
+      stage: { label: 'Stage', options: [{ value: 'won', label: 'Won' }] },
+      amount: { label: 'Amount' },
+      owner: { label: 'Owner' },
+    },
+  },
+};
+const getObjectDef = (name: string) => DEFS[name] ?? null;
+
+function makeI18n() {
+  const i18n = createMemoryI18n();
+  for (const [locale, data] of Object.entries(AuditTranslations)) {
+    i18n.loadTranslations(locale, data as Record<string, any>);
+  }
+  i18n.loadTranslations('zh-CN', {
+    objects: {
+      biz_deal: {
+        label: '商机',
+        fields: { stage: { label: '阶段', options: { won: '赢单' } }, amount: { label: '金额' } },
+      },
+    },
+  });
+  return i18n;
+}
+
+const zh = () => readerSummaryLocale(makeI18n(), 'zh-CN', getObjectDef)!;
+const ja = () => readerSummaryLocale(makeI18n(), 'ja-JP', getObjectDef)!;
+
+const CREATED: ActivitySummaryMessage = { key: 'activityCreated', object: 'biz_deal', label: 'Acme' };
+const NAMED: ActivitySummaryMessage = {
+  key: 'activityUpdatedFields',
+  object: 'biz_deal',
+  label: 'Acme',
+  fields: ['amount', 'owner'],
+  more: 2,
+};
+const CHANGES: ActivitySummaryMessage = {
+  key: 'fieldChanges',
+  object: 'biz_deal',
+  changes: [
+    { field: 'stage', from: { text: '∅' }, to: { text: 'Won', option: 'won' } },
+    { field: 'owner', from: { text: '张伟' }, to: { text: 'Ana' } },
+  ],
+};
+
+describe('composeActivitySummary — one code, each reader\'s language (#26167)', () => {
+  it('composes every template in the reader locale, labels resolved per reader', () => {
+    expect(composeActivitySummary(CREATED, zh())).toBe('创建了 商机 "Acme"');
+    expect(composeActivitySummary({ ...CREATED, key: 'activityDeleted' }, zh())).toBe('删除了 商机 "Acme"');
+    expect(composeActivitySummary({ ...CREATED, key: 'activityUpdated' }, zh())).toBe('更新了 商机 "Acme"');
+    // `owner` has no zh-CN label: the authored one answers; the list is zh-CN's.
+    expect(composeActivitySummary(NAMED, zh())).toBe('更新了 商机 "Acme"：金额、Owner和另外 2 个字段');
+    // An option value is the reader's option label; a title is the record's own datum.
+    expect(composeActivitySummary(CHANGES, zh())).toBe('阶段: ∅ → 赢单; Owner: 张伟 → Ana');
+  });
+
+  it('falls back to the authored labels where the reader\'s pack has none', () => {
+    expect(composeActivitySummary(CREATED, ja())).toBe('Deal「Acme」を作成しました');
+    expect(composeActivitySummary(CHANGES, ja())).toBe('Stage: ∅ → Won; Owner: 张伟 → Ana');
+  });
+
+  it('answers undefined for a locale with no template, and the field-less frame where only that exists', () => {
+    const fr = createMemoryI18n();
+    const none = readerSummaryLocale(fr, 'fr-FR', getObjectDef)!;
+    expect(composeActivitySummary(CREATED, none)).toBeUndefined();
+    fr.loadTranslations('fr-FR', { messages: { activityUpdated: 'A mis à jour {{object}} « {{label}} »' } });
+    const older = readerSummaryLocale(fr, 'fr-FR', getObjectDef)!;
+    expect(composeActivitySummary(NAMED, older)).toBe('A mis à jour Deal « Acme »');
+  });
+
+  it('the stored sentence is the write locale\'s, else the English catalog\'s — never a second spelling', () => {
+    expect(composeStoredSummary(NAMED, zh())).toBe('更新了 商机 "Acme"：金额、Owner和另外 2 个字段');
+    const none = readerSummaryLocale(createMemoryI18n(), 'fr-FR', getObjectDef)!;
+    expect(composeStoredSummary(CREATED, none)).toBe('Created Deal "Acme"');
+    expect(composeStoredSummary(NAMED, none)).toBe('Updated Deal "Acme": Amount, Owner, and 2 more');
+  });
+});
+
+describe('readerSummaryLocale — the reader\'s negotiated locale', () => {
+  it('negotiates against the held packs: a bare `zh` reads the zh-CN pack', () => {
+    expect(composeActivitySummary(CREATED, readerSummaryLocale(makeI18n(), 'zh', getObjectDef)!)).toBe(
+      '创建了 商机 "Acme"',
+    );
+  });
+
+  it('composes nothing without a locale or a `t()`', () => {
+    expect(readerSummaryLocale(makeI18n(), undefined, getObjectDef)).toBeUndefined();
+    expect(readerSummaryLocale(makeI18n(), '', getObjectDef)).toBeUndefined();
+    expect(readerSummaryLocale(undefined, 'zh-CN', getObjectDef)).toBeUndefined();
+  });
+
+  it('a throwing service is a miss, never a failed read', () => {
+    const throwing = readerSummaryLocale(
+      {
+        t: () => {
+          throw new Error('boom');
+        },
+      },
+      'zh-CN',
+      getObjectDef,
+    )!;
+    expect(composeActivitySummary(CREATED, throwing)).toBeUndefined();
+  });
+});
+
+describe('readActivitySummaryMessage — only a code this version can compose', () => {
+  it('accepts each stored shape and refuses anything else', () => {
+    for (const m of [CREATED, NAMED, CHANGES]) expect(readActivitySummaryMessage(m)).toEqual(m);
+    expect(readActivitySummaryMessage({ key: 'activityArchived', object: 'biz_deal', label: 'x' })).toBeNull();
+    expect(readActivitySummaryMessage({ key: 'activityCreated', object: 'biz_deal' })).toBeNull();
+    expect(readActivitySummaryMessage({ ...NAMED, fields: [1] })).toBeNull();
+    expect(readActivitySummaryMessage({ key: 'fieldChanges', object: 'biz_deal', changes: [{ field: 'x' }] })).toBeNull();
+    expect(readActivitySummaryMessage('activityCreated')).toBeNull();
+  });
+});
+
+describe('localizeActivityRow — the read-side overlay', () => {
+  const row = (metadata: unknown, summary = 'Created Deal "Acme"') => ({ summary, metadata });
+  const stored = (message: unknown) => JSON.stringify({ old: null, new: { name: 'Acme' }, [ACTIVITY_SUMMARY_MESSAGE_KEY]: message });
+
+  it('overlays the summary and drops the code from what a caller is served', () => {
+    const r: Record<string, unknown> = row(stored(CREATED));
+    localizeActivityRow(r, zh(), true);
+    expect(r.summary).toBe('创建了 商机 "Acme"');
+    expect(JSON.parse(r.metadata as string)).toEqual({ old: null, new: { name: 'Acme' } });
+  });
+
+  it('keeps the code for the system, and works on a parsed `metadata` object too', () => {
+    const r: Record<string, unknown> = row({ old: null, new: {}, [ACTIVITY_SUMMARY_MESSAGE_KEY]: CREATED });
+    localizeActivityRow(r, zh(), false);
+    expect(r.summary).toBe('创建了 商机 "Acme"');
+    expect((r.metadata as Record<string, unknown>)[ACTIVITY_SUMMARY_MESSAGE_KEY]).toEqual(CREATED);
+  });
+
+  it('keeps the stored sentence with no locale, an unknown code, or a missing template — and still drops the code', () => {
+    for (const [message, locale] of [
+      [CREATED, undefined],
+      [{ key: 'activityArchived', object: 'biz_deal', label: 'Acme' }, zh()],
+      [CREATED, readerSummaryLocale(createMemoryI18n(), 'fr-FR', getObjectDef)],
+    ] as const) {
+      const r: Record<string, unknown> = row(stored(message));
+      localizeActivityRow(r, locale, true);
+      expect(r.summary).toBe('Created Deal "Acme"');
+      expect(JSON.parse(r.metadata as string)).not.toHaveProperty(ACTIVITY_SUMMARY_MESSAGE_KEY);
+    }
+  });
+
+  it('a row written before the change (no code) and a row whose summary was not projected are left alone', () => {
+    const legacy: Record<string, unknown> = row(JSON.stringify({ old: null, new: { name: 'Acme' } }));
+    localizeActivityRow(legacy, zh(), true);
+    expect(legacy).toEqual(row(JSON.stringify({ old: null, new: { name: 'Acme' } })));
+
+    const unprojected: Record<string, unknown> = { metadata: stored(CREATED) };
+    localizeActivityRow(unprojected, zh(), true);
+    expect(unprojected).not.toHaveProperty('summary');
+  });
+});