@@ -0,0 +1,8 @@
+---
+"@object-ui/plugin-calendar": patch
+---
+
+Clause-②: no
+Metadata-protocol: none
+
+Fix calendar events reading `[object Object]` when `calendar.titleField` names a lookup field. `ObjectCalendar` now passes the author's `titleField` to the unified record display-name resolver instead of stringifying the value itself, so an expanded lookup shows the referenced record's display name, a raw id string still shows as it did, and an expanded reference with no usable name falls through to the object-level title ladder.
@@ -1013,20 +1013,21 @@ export const ObjectCalendar: React.FC<ObjectCalendarComponentProps> = ({
     const resolveColorFieldValue = createFieldColorResolver(
       (objectSchema?.fields as Record<string, any> | undefined)?.[colorField ?? ''],
     );
-    const resolveTitle = (record: Record<string, any>): string => {
-      // 1. Explicit titleField wins when present on the record.
-      if (titleField) {
-        const v = record[titleField];
-        const s = typeof v === 'string' ? v.trim() : v;
-        if (s) return String(s);
-      }
-      // 2-4. Unified object-level resolver (ADR-0079): objectSchema.titleFormat
-      //   → objectSchema.displayNameField → type-aware field derivation →
-      //   `Record #<id>` floor. Replaces the old per-view chain (template render
-      //   → NAME_FIELD_KEY → hard-coded name list → "Untitled") so an event
-      //   object whose name lives in e.g. `activity_name` shows the real name.
-      return getRecordDisplayName(objectSchema, record);
-    };
+    // The author-chosen `titleField` goes to the unified resolver as its
+    // explicit rung rather than being stringified here: a lookup arrives
+    // expanded to an object (`{ id, name, … }`) and `String(object)` is the
+    // literal `[object Object]`. The resolver reduces an expanded reference to
+    // its display name, treats a whitespace-only value as empty, and leaves a
+    // raw id string as it is.
+    //
+    // Precedence: explicit titleField (when it yields a value) →
+    //   objectSchema.nameField / displayNameField → objectSchema.titleFormat →
+    //   type-aware field derivation → `Record #<id>` floor (ADR-0079). That
+    //   replaces the old per-view chain (template render → NAME_FIELD_KEY →
+    //   hard-coded name list → "Untitled") so an event object whose name lives
+    //   in e.g. `activity_name` shows the real name.
+    const resolveTitle = (record: Record<string, any>): string =>
+      getRecordDisplayName(objectSchema, record, { titleField });
 
     const scheduled: CalendarViewEvent[] = [];
     const unscheduled: UnscheduledRecord[] = [];
 
@@ -0,0 +1,115 @@
+/**
+ * ObjectUI
+ * Copyright (c) 2024-present ObjectStack Inc.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+/**
+ * objectui#13471 — a calendar whose `titleField` names a LOOKUP.
+ *
+ * The record query asks for the lookup expanded, so the value arrives as an
+ * object (`{ _id, name }`). `ObjectCalendar` used to stringify the author's
+ * `titleField` itself, so every event read the literal `[object Object]`. It now
+ * hands the field to the unified record display-name resolver as the explicit
+ * rung, which reduces an expanded reference to its display name.
+ *
+ * Events are read off the props `CalendarView` receives, the same seam the
+ * `colorField` ladder test uses.
+ */
+
+import React from 'react';
+import { render, waitFor, screen } from '@testing-library/react';
+import { describe, it, expect, vi } from 'vitest';
+import { ObjectCalendar } from '../ObjectCalendar';
+
+vi.mock('@object-ui/plugin-detail', async (importOriginal) => ({
+  ...(await importOriginal<typeof import('@object-ui/plugin-detail')>()),
+  RecordDetailDrawer: () => null,
+  deriveRecordPageHref: () => null,
+}));
+
+let lastEvents: any[] = [];
+
+vi.mock('../CalendarView', async (importOriginal) => {
+  const actual = await importOriginal<any>();
+  return {
+    ...actual,
+    CalendarView: ({ events }: any) => {
+      lastEvents = events;
+      return <div data-testid="calendar-view" data-event-count={String(events.length)} />;
+    },
+  };
+});
+
+const OBJECT_SCHEMA = {
+  name: 'srm_supplier_audit',
+  nameField: 'audit_no',
+  fields: {
+    audit_no: { name: 'audit_no', type: 'text' },
+    supplier: { name: 'supplier', type: 'lookup', reference_to: 'srm_supplier' },
+    planned_date: { name: 'planned_date', type: 'datetime' },
+  },
+};
+
+function makeDataSource(rows: any[]) {
+  return {
+    find: vi.fn(async () => ({ data: rows, total: rows.length })),
+    findOne: vi.fn(),
+    create: vi.fn(),
+    update: vi.fn(),
+    delete: vi.fn(),
+    getObjectSchema: vi.fn(async () => OBJECT_SCHEMA),
+  } as any;
+}
+
+async function titlesFor(rows: any[]): Promise<string[]> {
+  lastEvents = [];
+  const schema: any = {
+    type: 'object-calendar',
+    objectName: 'srm_supplier_audit',
+    calendar: { startDateField: 'planned_date', titleField: 'supplier' },
+  };
+  const { unmount } = render(<ObjectCalendar schema={schema} dataSource={makeDataSource(rows)} />);
+  try {
+    let settled: any[] = [];
+    await waitFor(() => {
+      const events = lastEvents;
+      expect(events).toHaveLength(rows.length);
+      settled = events;
+    });
+    return settled.map((e) => e.title);
+  } finally {
+    unmount();
+    expect(screen.queryAllByTestId('calendar-view')).toHaveLength(0);
+  }
+}
+
+describe('objectui#13471 — calendar titleField naming a lookup', () => {
+  it('an expanded lookup object shows the referenced record name', async () => {
+    const titles = await titlesFor([
+      { id: 'a1', audit_no: 'AUD-1', planned_date: '2026-10-05T09:00:00Z', supplier: { _id: 's1', name: 'Acme Metals' } },
+      { id: 'a2', audit_no: 'AUD-2', planned_date: '2026-10-06T09:00:00Z', supplier: { id: 's2', name: 'Borealis Plastics' } },
+    ]);
+    expect(titles).toEqual(['Acme Metals', 'Borealis Plastics']);
+  });
+
+  it('a raw id string still shows as it did', async () => {
+    const titles = await titlesFor([
+      { id: 'a1', audit_no: 'AUD-1', planned_date: '2026-10-05T09:00:00Z', supplier: 'sup-001' },
+    ]);
+    expect(titles).toEqual(['sup-001']);
+  });
+
+  it('an expanded object with no usable name never prints [object Object]', async () => {
+    const titles = await titlesFor([
+      { id: 'a1', audit_no: 'AUD-1', planned_date: '2026-10-05T09:00:00Z', supplier: { _id: 's1' } },
+      { id: 'a2', audit_no: 'AUD-2', planned_date: '2026-10-06T09:00:00Z', supplier: null },
+    ]);
+    expect(titles.some((t) => t.includes('[object Object]'))).toBe(false);
+    // Falls through to the object-level resolver (`nameField`), the same ladder
+    // an empty titleField always took.
+    expect(titles).toEqual(['AUD-1', 'AUD-2']);
+  });
+});