diff --git a/apps/web/tests/snapshots/subagent-conversation/ui.expected.md b/apps/web/tests/snapshots/subagent-conversation/ui.expected.md
index 3a9b03fffe..14b397d9b4 100644
--- a/apps/web/tests/snapshots/subagent-conversation/ui.expected.md
+++ b/apps/web/tests/snapshots/subagent-conversation/ui.expected.md
@@ -3,8 +3,8 @@
     - button "Ask a research subagent to"
     - text: /
     - button "event-sourcing researcher" [disabled]
-  - button "1 subagents":
-    - text: 1 subagents
+  - button "1 subagent":
+    - text: 1 subagent
     - img
   - tablist:
     - tab "Chat" [selected]
diff --git a/apps/web/tests/subagent-conversation.e2e.ts b/apps/web/tests/subagent-conversation.e2e.ts
index 83ce78f3e4..904fcfcd09 100644
--- a/apps/web/tests/subagent-conversation.e2e.ts
+++ b/apps/web/tests/subagent-conversation.e2e.ts
@@ -331,7 +331,7 @@ describe('web e2e: persisted subagent conversation and human continuation', () =
 
   it('opens an unavailable persisted grandchild after recording the available child', async () => {
     onTestFailed(() => saveFailureShot(page, 'web-e2e-subagent-grandchild'))
-    await page.getByRole('button', { name: '1 subagents' }).click()
+    await page.getByRole('button', { name: '1 subagent' }).click()
     await page.getByRole('treeitem', { name: new RegExp(NESTED_LABEL) }).click()
     await page.getByText('The parent session is offline; reopen it to continue sending messages.').waitFor()
     const hierarchy = page.getByRole('navigation', { name: 'Session hierarchy' })
diff --git a/packages/client/ui-subagent/src/client/SubagentCatalogAction.tsx b/packages/client/ui-subagent/src/client/SubagentCatalogAction.tsx
index 359827780f..14f8169d82 100644
--- a/packages/client/ui-subagent/src/client/SubagentCatalogAction.tsx
+++ b/packages/client/ui-subagent/src/client/SubagentCatalogAction.tsx
@@ -324,6 +324,8 @@ export function SubagentCatalogAction({
   // The catalog can arrive before the session-list baseline; never undercount
   // the already-visible direct rows during that short bootstrap window.
   const descendantCount = Math.max(healthy.length, descendants.count)
+  const totalCountKey = descendantCount === 1 ? 'count.total.one' : 'count.total.other'
+  const runningCountKey = descendantCount === 1 ? 'count.running.one' : 'count.running.other'
 
   const observeCatalog = (parentSessionId: SessionId, next: boolean): void => {
     if (next) observedCatalogs.current.add(parentSessionId)
@@ -432,7 +434,7 @@ export function SubagentCatalogAction({
         className={css.trigger}
         aria-haspopup="tree"
         aria-expanded={open}
-        aria-label={t(descendants.running ? 'count.running' : 'count.total', { count: descendantCount })}
+        aria-label={t(descendants.running ? runningCountKey : totalCountKey, { count: descendantCount })}
         onClick={() => { changeOpen(!open) }}
         onKeyDown={(event) => {
           if (event.key !== 'ArrowDown') return
@@ -444,7 +446,7 @@ export function SubagentCatalogAction({
         <span className={css.activitySlot}>
           {descendants.running && <StateDot state="ongoing" />}
         </span>
-        <span className={css.count}>{t('count.total', { count: descendantCount })}</span>
+        <span className={css.count}>{t(totalCountKey, { count: descendantCount })}</span>
         <IconChevronDownOutline14 className={open ? css.triggerOpen : undefined} />
       </button>
       {open && (
diff --git a/packages/client/ui-subagent/src/client/locales.ts b/packages/client/ui-subagent/src/client/locales.ts
index 2ecf1be4f5..86562534b0 100644
--- a/packages/client/ui-subagent/src/client/locales.ts
+++ b/packages/client/ui-subagent/src/client/locales.ts
@@ -24,8 +24,10 @@ export const zh = {
   'activity.inactive': '当前未运行',
   'branch.collapse': '收起 {label} 的下级子代理',
   'branch.expand': '展开 {label} 的下级子代理',
-  'count.total': '{count} 个子代理',
-  'count.running': '{count} 个子代理，正在运行',
+  'count.total.one': '{count} 个子代理',
+  'count.total.other': '{count} 个子代理',
+  'count.running.one': '{count} 个子代理，正在运行',
+  'count.running.other': '{count} 个子代理，正在运行',
   'tree.aria': '子代理会话',
   'readonly.oneShot.title': '一次性子代理记录',
   'readonly.title': '此子代理暂时只读',
@@ -54,8 +56,10 @@ export const en: Record<SubagentKey, string> = {
   'activity.inactive': 'not running',
   'branch.collapse': 'Collapse {label} descendants',
   'branch.expand': 'Expand {label} descendants',
-  'count.total': '{count} subagents',
-  'count.running': '{count} subagents running',
+  'count.total.one': '{count} subagent',
+  'count.total.other': '{count} subagents',
+  'count.running.one': '{count} subagent running',
+  'count.running.other': '{count} subagents running',
   'tree.aria': 'Subagent sessions',
   'readonly.oneShot.title': 'One-shot subagent record',
   'readonly.title': 'This subagent is read-only for now',
diff --git a/packages/client/ui-subagent/tests/conversation-ui.spec.tsx b/packages/client/ui-subagent/tests/conversation-ui.spec.tsx
index 1687b1ffc1..5649b2b257 100644
--- a/packages/client/ui-subagent/tests/conversation-ui.spec.tsx
+++ b/packages/client/ui-subagent/tests/conversation-ui.spec.tsx
@@ -167,6 +167,24 @@ describe('SubagentCatalogAction', () => {
     expect(input.setCatalogOpen).toHaveBeenLastCalledWith(PARENT, false)
   })
 
+  it('selects singular count keys for one descendant', () => {
+    const base = props(catalog({
+      entries: [{
+        kind: 'child', id: CHILD, mode: 'continuable', label: 'worker',
+        activity: 'running', hasChildren: false,
+      }],
+    }), {}, {
+      [CHILD]: {
+        ...summary(CHILD, Date.now()), parentId: PARENT, origin: 'subagent', running: true,
+      },
+    })
+    const translate = vi.fn(base.t)
+    render(<SubagentCatalogAction {...base} t={translate} />)
+
+    expect(translate).toHaveBeenCalledWith('count.running.one', { count: 1 })
+    expect(translate).toHaveBeenCalledWith('count.total.one', { count: 1 })
+  })
+
   it('supports trigger/menu keyboard traversal, Escape focus restore, and outside close', async () => {
     const input = props(catalog())
     render(<SubagentCatalogAction {...input} />)