@@ -7,4 +7,4 @@ An object timeline with a `groupByField` draws one group per value, headed by th
 - **A lookup groups by the referenced record's name.** The timeline's own query expands every declared relation, so a lookup value arrives as the related record, and the group header used to read "[object Object]" for every row: a timeline grouped by an owner or an account drew one group. The group key now reads the field's display value through the same resolver the entry title and description use. A select field still groups by its option's label. An empty value groups under "Unassigned", and when no row has a display value the timeline keeps its date buckets. A lookup that arrives unexpanded groups under its id, which is what the title shows for it too.
 - **A key's rows are gathered into one group.** Entries are sorted by start date, and a group used to end wherever the next entry had another value, so a field whose values alternate in date order (a channel, say) drew the same header many times. Each value now heads exactly one group. Groups appear in the order of their earliest entry, and entries stay in date order inside each group, so a timeline whose values were already contiguous renders as before.
 
-Unchanged: a timeline with no `groupByField` (its date buckets), and the horizontal timeline, which draws no groups and keeps its date order. Nothing is added to the package entry: no export, prop or language-pack key.
+Unchanged: a timeline with no `groupByField` (its date buckets), the horizontal timeline, which draws no groups and keeps its date order, and the group key of a multi-valued field (a multiselect, tags, or a lookup with `multiple`), which reads as before. Nothing is added to the package entry: no export, prop or language-pack key.
@@ -816,16 +816,23 @@ export const ObjectTimeline: React.FC<ObjectTimelineProps> = ({
       // unexpanded foreign key groups under the id itself, which is what the
       // title slot shows for it too (AGENTS.md #0.1).
       //
+      // An ARRAY (a multi-valued field: `multiselect`, `tags`, a lookup with
+      // `multiple`) keeps the key it had, its stringified value. The resolver
+      // reads no display value off an array, so routing one through it would
+      // move every such row under "Unassigned", and how a multi-valued row
+      // should group (one combined group, or one group per value) is a
+      // decision this change does not make.
+      //
       // One group per key is drawn by the renderer's vertical branch, which
       // buckets by this key in the order of each key's earliest entry. The
       // order is NOT changed here: the horizontal branch draws the same array,
       // draws no groups, and must keep its date order.
       const groupSelectOptions = optionMap(groupByField);
       const groupKeys = mapped.map((m): string | undefined => {
         const opt = m.group != null && m.group !== '' ? groupSelectOptions[String(m.group)] : undefined;
-        return opt?.label
-          ? fieldOptionLabel(objectName, groupByField, String(m.group), opt.label)
-          : recordDisplayValueAt(m._data, groupByField);
+        if (opt?.label) return fieldOptionLabel(objectName, groupByField, String(m.group), opt.label);
+        if (Array.isArray(m.group)) return String(m.group);
+        return recordDisplayValueAt(m._data, groupByField);
       });
       if (groupKeys.some((key) => key !== undefined)) {
         return mapped.map((m, i) => ({ ...m, group: groupKeys[i] ?? t('timeline.bucket.unassigned') }));
 
@@ -36,8 +36,8 @@
  * ── The controls ────────────────────────────────────────────────────────────
  * The date buckets (no `groupByField`), an unexpanded lookup id (the
  * resolver's own answer: the id itself, no name is invented for it, AGENTS.md
- * #0.1), and the horizontal rail, which draws no groups and keeps its date
- * order. Each was right before the fix and must stay right after it. The
+ * #0.1), a multi-valued field (its key is left as it was), and the horizontal
+ * rail, which draws no groups and keeps its date order. Each was right before the fix and must stay right after it. The
  * horizontal arm is also why the bucketing lives in the renderer's vertical
  * branch and not in the composer: both branches draw the same item array.
  *
@@ -243,6 +243,19 @@ describe('an explicit groupByField draws one group per display value (objectui#1
     expect(feed()).toEqual(['u1: First, Second', 'u2: Third']);
   });
 
+  // An array has no display value under the resolver, so a multi-valued field
+  // keeps the key it had (its stringified value) rather than moving every row
+  // under "Unassigned". How a multi-valued row should group is not decided here.
+  it('CONTROL: a multi-valued field keeps the group key it had', () => {
+    mountInline(block({ groupByField: 'tags' }), [
+      { id: 't1', subject: 'First', due: '2026-03-01', tags: ['urgent', 'vip'] },
+      { id: 't2', subject: 'Second', due: '2026-03-02', tags: ['urgent', 'vip'] },
+      { id: 't3', subject: 'Third', due: '2026-03-03', tags: ['vip'] },
+    ]);
+
+    expect(feed()).toEqual(['urgent,vip: First, Second', 'vip: Third']);
+  });
+
   it('CONTROL: a timeline with no groupByField draws its date buckets, in date order, exactly as before', () => {
     vi.useFakeTimers({ toFake: ['Date'] });
     // Tuesday 2026-10-06, noon, local time; every value is date-only, which