diff --git a/src/client/SubagentView.module.css b/src/client/SubagentView.module.css
index 14ddd9b3..de5892e6 100644
--- a/src/client/SubagentView.module.css
+++ b/src/client/SubagentView.module.css
@@ -545,3 +545,30 @@
 .jobsPaneError {
   color: var(--dsw-alias-state-error-primary);
 }
+
+/* "Active only" toolbar row under the main agent card: collapses idle
+   topology branches so busy trees surface just the running subagents. */
+.subagentToolbar {
+  padding: 2px 10px 4px;
+}
+
+.subagentToggle {
+  border: none;
+  border-radius: 999px;
+  background: transparent;
+  color: var(--dsw-alias-label-secondary);
+  cursor: pointer;
+  font: var(--dsw-font-xxxs-11);
+  padding: 2px 10px;
+}
+
+.subagentToggle:hover {
+  background: var(--dsw-alias-interactive-bg-hover);
+  color: var(--dsw-alias-label-primary);
+}
+
+.subagentToggleOn,
+.subagentToggleOn:hover {
+  background: var(--dsw-alias-interactive-bg-active);
+  color: var(--dsw-alias-label-primary);
+}
diff --git a/src/client/SubagentView.tsx b/src/client/SubagentView.tsx
index 5ea2f744..2f681474 100644
--- a/src/client/SubagentView.tsx
+++ b/src/client/SubagentView.tsx
@@ -40,6 +40,7 @@ import {
   countSubagentDescendants,
   isSideThreadSummary,
   rootAncestor,
+  runningVisibilitySet,
 } from './subagent-detect.ts'
 import { type LastActivity } from '../subagent-activity.ts'
 import { SIDE_LABEL_PREFIX } from '../sidechat-core.ts'
@@ -65,6 +66,26 @@ const ARGS_PREVIEW = 60
 const JOB_POLL_MS = 2000
 /** How long the kill button stays armed before it needs re-confirming. */
 const JOB_KILL_ARM_MS = 3000
+/** localStorage key of the "active only" toggle (a pure view preference). */
+const ACTIVE_ONLY_KEY = 'dsh-better-sidebar.subagent.activeOnly'
+
+/** Read the persisted "active only" preference; failures fall back to off. */
+function readActiveOnlyPref(): boolean {
+  try {
+    return globalThis.localStorage?.getItem(ACTIVE_ONLY_KEY) === '1'
+  } catch {
+    return false
+  }
+}
+
+/** Persist the "active only" preference; storage failures are non-fatal. */
+function writeActiveOnlyPref(value: boolean): void {
+  try {
+    globalThis.localStorage?.setItem(ACTIVE_ONLY_KEY, value ? '1' : '0')
+  } catch {
+    // Private mode / quota errors must never break the page.
+  }
+}
 
 /** The direct subagent children of one parent (durable `origin` rows;
  *  Side Chat threads ride the same origin but are tab-strip conversations,
@@ -240,6 +261,13 @@ interface RowsProps {
   currentSessionId: string
   /** The batch live-preview map (child id → latest activity). */
   live: Readonly<Record<string, LastActivity>>
+  /**
+   * Active-only filter (`null` = off): session ids worth showing — running
+   * nodes plus ancestors with running descendants. Entries outside the set
+   * (and diagnostics) are hidden; `entry.activity === 'running'` always
+   * passes so a catalog row never lags its own summary flag.
+   */
+  activeKeepSet: ReadonlySet<string> | null
   openChild: (address: SidebarSubagentAddress) => void
   refresh: (parentSessionId: string) => void
 }
@@ -247,6 +275,7 @@ interface RowsProps {
 /** Render one topology level; branches are always expanded (lazy catalogs). */
 function CatalogRows({
   parentSessionId, catalog, catalogs, byId, level, currentSessionId, live,
+  activeKeepSet,
   openChild, refresh,
 }: RowsProps) {
   const emptyLoading = catalog?.state === 'loading' && catalog.entries.length === 0
@@ -255,6 +284,10 @@ function CatalogRows({
   // strip owns them). Legacy threads created before the descriptor fix still
   // arrive as corrupt diagnostics; they are recognized by summary title.
   const visibleEntries = (catalog?.entries ?? []).filter((entry) => {
+    if (activeKeepSet !== null && !(activeKeepSet.has(entry.id)
+      || (entry.kind === 'child' && entry.activity === 'running'))) {
+      return false
+    }
     if (entry.kind === 'child') return !(entry.label?.startsWith(SIDE_LABEL_PREFIX) ?? false)
     return !(byId[entry.id]?.displayTitle.startsWith(SIDE_LABEL_PREFIX) ?? false)
   })
@@ -361,6 +394,7 @@ function CatalogRows({
                       level={level + 1}
                       currentSessionId={currentSessionId}
                       live={live}
+                      activeKeepSet={activeKeepSet}
                       openChild={openChild}
                       refresh={refresh}
                     />
@@ -664,6 +698,22 @@ export function SubagentView(props: {
   const rootSummary = rootId === undefined ? undefined : byId[rootId]
   const live = useSubagentLive(rootId, active)
 
+  // "Active only" view mode: hide idle topology branches so a busy tree
+  // surfaces just the running subagents (and the ancestors that own them).
+  // A pure view preference — persisted client-side, defaulting to off.
+  const [activeOnly, setActiveOnly] = useState(readActiveOnlyPref)
+  const toggleActiveOnly = useCallback(() => {
+    setActiveOnly(value => {
+      const next = !value
+      writeActiveOnlyPref(next)
+      return next
+    })
+  }, [])
+  const activeKeepSet = useMemo(
+    () => (rootId === undefined || !activeOnly ? null : runningVisibilitySet(byId, rootId)),
+    [byId, rootId, activeOnly],
+  )
+
   /** Catalog owners currently consuming live membership updates. */
   const observedRef = useRef(new Set<string>())
 
@@ -844,6 +894,19 @@ export function SubagentView(props: {
               </span>
             </div>
           )}
+          {rootId !== undefined && totals.count > 0 && (
+            <div className={css.subagentToolbar}>
+              <button
+                type="button"
+                aria-pressed={activeOnly}
+                title={t('subagentActiveOnlyTitle')}
+                className={clsx(css.subagentToggle, activeOnly && css.subagentToggleOn)}
+                onClick={toggleActiveOnly}
+              >
+                {activeOnly ? `✓ ${t('subagentActiveOnly')}` : t('subagentActiveOnly')}
+              </button>
+            </div>
+          )}
           {rootId !== undefined && (
             <div className={css.subagentChildren} role="group" aria-busy={summaryBackedLoading || undefined}>
               {summaryBackedLoading && (
@@ -858,6 +921,7 @@ export function SubagentView(props: {
                   level={1}
                   currentSessionId={sessionId}
                   live={live}
+                  activeKeepSet={activeKeepSet}
                   openChild={openChild}
                   refresh={refresh}
                 />
diff --git a/src/client/locales-ar.ts b/src/client/locales-ar.ts
index 34310111..5c5e919c 100644
--- a/src/client/locales-ar.ts
+++ b/src/client/locales-ar.ts
@@ -277,6 +277,8 @@ export const ar: Record<string, string> = {
   subagent: 'المهام',
   openSubagent: 'المهام',
   subagentMainAgent: 'الوكيل الرئيسي',
+  subagentActiveOnly: "للنشطين فقط",
+  subagentActiveOnlyTitle: "طيّ الوكلاء الفرعيين الخاملين؛ إظهار الوكلاء قيد التشغيل وسلسلة آبائهم فقط",
   subagentEmpty: 'لا وكلاء فرعيون',
   subagentEmptyDesc: 'الوكلاء الفرعيون المُنتَجون تحت الوكيل الرئيسي سيظهرون هنا',
   subagentRunning: 'قيد التشغيل',
diff --git a/src/client/locales-de.ts b/src/client/locales-de.ts
index a73124e6..b66b455a 100644
--- a/src/client/locales-de.ts
+++ b/src/client/locales-de.ts
@@ -262,6 +262,8 @@ export const de: Record<string, string> = {
   subagent: 'Aufgaben',
   openSubagent: 'Aufgaben',
   subagentMainAgent: 'Hauptagent',
+  subagentActiveOnly: "Nur aktive",
+  subagentActiveOnlyTitle: "Untätige Subagenten einklappen; nur laufende und deren Vorfahren anzeigen",
   subagentEmpty: 'Keine Subagenten',
   subagentEmptyDesc: 'Subagenten, die unter dem Hauptagenten erzeugt werden, erscheinen hier',
   subagentRunning: 'Läuft',
diff --git a/src/client/locales-fr.ts b/src/client/locales-fr.ts
index eb645a69..3fc241d4 100644
--- a/src/client/locales-fr.ts
+++ b/src/client/locales-fr.ts
@@ -269,6 +269,8 @@ export const fr: Record<string, string> = {
   subagent: 'Gestion des tâches',
   openSubagent: 'Gestion des tâches',
   subagentMainAgent: 'Agent principal',
+  subagentActiveOnly: "Actifs uniquement",
+  subagentActiveOnlyTitle: "Replier les sous-agents inactifs ; garder les actifs et leurs ancêtres",
   subagentEmpty: 'Aucun sous-agent pour l’instant',
   subagentEmptyDesc: 'Les sous-agents dérivés de l’agent principal actuel s’afficheront ici',
   subagentRunning: 'En cours',
diff --git a/src/client/locales-hi.ts b/src/client/locales-hi.ts
index cf98156a..207d077d 100644
--- a/src/client/locales-hi.ts
+++ b/src/client/locales-hi.ts
@@ -276,6 +276,8 @@ export const hi: Record<string, string> = {
   subagent: 'कार्य',
   openSubagent: 'कार्य',
   subagentMainAgent: 'मुख्य एजेंट',
+  subagentActiveOnly: "केवल सक्रिय",
+  subagentActiveOnlyTitle: "निष्क्रिय सब-एजेंट समेटें; केवल चल रहे और उनके पूर्वज दिखाएँ",
   subagentEmpty: 'कोई सबएजेंट नहीं',
   subagentEmptyDesc: 'मुख्य एजेंट के अंतर्गत बने सबएजेंट यहाँ दिखेंगे',
   subagentRunning: 'चल रहा',
diff --git a/src/client/locales-id.ts b/src/client/locales-id.ts
index e14502f5..16e406ab 100644
--- a/src/client/locales-id.ts
+++ b/src/client/locales-id.ts
@@ -274,6 +274,8 @@ export const id: Record<string, string> = {
   subagent: 'Tasks',
   openSubagent: 'Tasks',
   subagentMainAgent: 'Agen utama',
+  subagentActiveOnly: "Hanya aktif",
+  subagentActiveOnlyTitle: "Ciutkan subagen menganggur; tampilkan yang sedang berjalan dan leluhurnya",
   subagentEmpty: 'Tidak ada subagen',
   subagentEmptyDesc: 'Subagen yang dibangkitkan di bawah agen utama akan muncul di sini',
   subagentRunning: 'Berjalan',
diff --git a/src/client/locales-it.ts b/src/client/locales-it.ts
index dcff13f0..f15729ff 100644
--- a/src/client/locales-it.ts
+++ b/src/client/locales-it.ts
@@ -267,6 +267,8 @@ export const it: Record<string, string> = {
   subagent: 'Attività',
   openSubagent: 'Attività',
   subagentMainAgent: 'Agente principale',
+  subagentActiveOnly: "Solo attivi",
+  subagentActiveOnlyTitle: "Comprimi i sotto-agenti inattivi; mostra solo quelli attivi e i loro antenati",
   subagentEmpty: 'Nessun sottoagente',
   subagentEmptyDesc: 'I sottoagenti generati dall’agente principale compariranno qui',
   subagentRunning: 'In esecuzione',
diff --git a/src/client/locales-ja.ts b/src/client/locales-ja.ts
index f984147f..b9081d03 100644
--- a/src/client/locales-ja.ts
+++ b/src/client/locales-ja.ts
@@ -276,6 +276,8 @@ export const ja: Record<string, string> = {
   subagent: 'タスク管理',
   openSubagent: 'タスク管理',
   subagentMainAgent: 'メインエージェント',
+  subagentActiveOnly: 'アクティブのみ',
+  subagentActiveOnlyTitle: 'アイドル中のサブエージェントを折りたたみ、実行中のものとその親チェーンのみを表示',
   subagentEmpty: 'サブエージェントなし',
   subagentEmptyDesc: 'メインエージェントが派生したサブエージェントはここに表示されます',
   subagentRunning: '実行中',
diff --git a/src/client/locales-ko.ts b/src/client/locales-ko.ts
index 9b8d241f..36751f3a 100644
--- a/src/client/locales-ko.ts
+++ b/src/client/locales-ko.ts
@@ -268,6 +268,8 @@ export const ko: Record<string, string> = {
   subagent: '작업 관리',
   openSubagent: '작업 관리',
   subagentMainAgent: '주 에이전트',
+  subagentActiveOnly: "활성만",
+  subagentActiveOnlyTitle: "유휴 상태의 하위 에이전트는 접고, 실행 중인 것과 그 조상 체인만 표시",
   subagentEmpty: '서브 에이전트 없음',
   subagentEmptyDesc: '현재 주 에이전트에서 파생된 서브 에이전트가 여기에 표시됩니다',
   subagentRunning: '실행 중',
diff --git a/src/client/locales-nl.ts b/src/client/locales-nl.ts
index dd5854fa..14a7f9f1 100644
--- a/src/client/locales-nl.ts
+++ b/src/client/locales-nl.ts
@@ -274,6 +274,8 @@ export const nl: Record<string, string> = {
   subagent: 'Taken',
   openSubagent: 'Taken',
   subagentMainAgent: 'Hoofdagent',
+  subagentActiveOnly: "Alleen actief",
+  subagentActiveOnlyTitle: "Inactieve subagenten inklappen; alleen draaiende en hun voorouders tonen",
   subagentEmpty: 'Geen subagents',
   subagentEmptyDesc: 'Subagents voortgebracht door de hoofdagent verschijnen hier',
   subagentRunning: 'Actief',
diff --git a/src/client/locales-pl.ts b/src/client/locales-pl.ts
index 0b2fe4f8..4b250b1f 100644
--- a/src/client/locales-pl.ts
+++ b/src/client/locales-pl.ts
@@ -278,6 +278,8 @@ export const pl: Record<string, string> = {
   subagent: 'Zadania',
   openSubagent: 'Zadania',
   subagentMainAgent: 'Agent główny',
+  subagentActiveOnly: "Tylko aktywne",
+  subagentActiveOnlyTitle: "Zwiń bezczynne podagenty; pokazuj tylko działające i ich przodków",
   subagentEmpty: 'Brak podagentów',
   subagentEmptyDesc: 'Podagenci powołani przez agenta głównego pojawią się tutaj',
   subagentRunning: 'Działa',
diff --git a/src/client/locales-pt.ts b/src/client/locales-pt.ts
index 88044502..757f4c24 100644
--- a/src/client/locales-pt.ts
+++ b/src/client/locales-pt.ts
@@ -259,6 +259,8 @@ export const pt: Record<string, string> = {
   subagent: 'Tarefas',
   openSubagent: 'Tarefas',
   subagentMainAgent: 'Agente principal',
+  subagentActiveOnly: "Somente ativos",
+  subagentActiveOnlyTitle: "Recolher subagentes ociosos; mostrar apenas os ativos e seus ancestrais",
   subagentEmpty: 'Nenhum subagente',
   subagentEmptyDesc: 'Subagentes gerados sob o agente principal aparecerão aqui',
   subagentRunning: 'Executando',
diff --git a/src/client/locales-ru.ts b/src/client/locales-ru.ts
index 5afceb96..1e45a87a 100644
--- a/src/client/locales-ru.ts
+++ b/src/client/locales-ru.ts
@@ -274,6 +274,8 @@ export const ru: Record<string, string> = {
   subagent: 'Задачи',
   openSubagent: 'Задачи',
   subagentMainAgent: 'Главный агент',
+  subagentActiveOnly: "Только активные",
+  subagentActiveOnlyTitle: "Свернуть простаивающие субагенты; показывать только выполняющиеся и их предков",
   subagentEmpty: 'Нет субагентов',
   subagentEmptyDesc: 'Субагенты, порождённые главным агентом, появятся здесь',
   subagentRunning: 'Выполняется',
diff --git a/src/client/locales-sv.ts b/src/client/locales-sv.ts
index cdb7824b..5f7343e4 100644
--- a/src/client/locales-sv.ts
+++ b/src/client/locales-sv.ts
@@ -259,6 +259,8 @@ export const sv: Record<string, string> = {
   subagent: 'Uppgifter',
   openSubagent: 'Uppgifter',
   subagentMainAgent: 'Huvudagent',
+  subagentActiveOnly: "Endast aktiva",
+  subagentActiveOnlyTitle: "Fäll ihop inaktiva underagenter; visa endast körande och deras förfäder",
   subagentEmpty: 'Inga subagenter',
   subagentEmptyDesc: 'Subagenter skapade under huvudagenten visas här',
   subagentRunning: 'Kör',
diff --git a/src/client/locales-th.ts b/src/client/locales-th.ts
index 937d557d..0d009a19 100644
--- a/src/client/locales-th.ts
+++ b/src/client/locales-th.ts
@@ -276,6 +276,8 @@ export const th: Record<string, string> = {
   subagent: 'งาน',
   openSubagent: 'งาน',
   subagentMainAgent: 'ตัวแทนหลัก',
+  subagentActiveOnly: "เฉพาะที่กำลังทำงาน",
+  subagentActiveOnlyTitle: "ย่อซับเอเจนต์ที่ว่างงาน แสดงเฉพาะตัวที่กำลังทำงานและบรรพบุรุษของมัน",
   subagentEmpty: 'ไม่มีตัวแทนย่อย',
   subagentEmptyDesc: 'ตัวแทนย่อยที่สร้างภายใต้ตัวแทนหลักจะปรากฏที่นี่',
   subagentRunning: 'กำลังทำงาน',
diff --git a/src/client/locales-tr.ts b/src/client/locales-tr.ts
index 45010eb2..c1d9f81b 100644
--- a/src/client/locales-tr.ts
+++ b/src/client/locales-tr.ts
@@ -276,6 +276,8 @@ export const tr: Record<string, string> = {
   subagent: 'Görevler',
   openSubagent: 'Görevler',
   subagentMainAgent: 'Ana aracı',
+  subagentActiveOnly: "Sadece aktif",
+  subagentActiveOnlyTitle: "Boşta bekleyen alt ajanları daralt; yalnızca çalışanlar ve üst zincirlerini göster",
   subagentEmpty: 'Alt aracı yok',
   subagentEmptyDesc: 'Ana aracının altında doğan alt aracılar burada görünür',
   subagentRunning: 'Çalışıyor',
diff --git a/src/client/locales-vi.ts b/src/client/locales-vi.ts
index 94c8e6ab..daff3db0 100644
--- a/src/client/locales-vi.ts
+++ b/src/client/locales-vi.ts
@@ -276,6 +276,8 @@ export const vi: Record<string, string> = {
   subagent: 'Quản lý tác vụ',
   openSubagent: 'Quản lý tác vụ',
   subagentMainAgent: 'Tác nhân chính',
+  subagentActiveOnly: "Chỉ mục đang chạy",
+  subagentActiveOnlyTitle: "Thu gọn subagent nhàn rỗi; chỉ hiện mục đang chạy và chuỗi cha của nó",
   subagentEmpty: 'Không có tác nhân con',
   subagentEmptyDesc: 'Tác nhân con do tác nhân chính tạo ra sẽ hiển thị ở đây',
   subagentRunning: 'Đang chạy',
diff --git a/src/client/locales-zh-HK.ts b/src/client/locales-zh-HK.ts
index 7e8cb941..0094a5ae 100644
--- a/src/client/locales-zh-HK.ts
+++ b/src/client/locales-zh-HK.ts
@@ -291,6 +291,8 @@ export const zhHK: Record<string, string> = {
   subagent: '任務管理',
   openSubagent: '任務管理',
   subagentMainAgent: '主代理',
+  subagentActiveOnly: "只看活躍",
+  subagentActiveOnlyTitle: "收起空閒子代理，僅顯示執行中的子代理及其父鏈",
   subagentEmpty: '暫無子代理',
   subagentEmptyDesc: '目前主代理派生的子代理將顯示在這裡',
   subagentRunning: '執行中',
diff --git a/src/client/locales-zh-MO.ts b/src/client/locales-zh-MO.ts
index e21f71b6..2d9d6484 100644
--- a/src/client/locales-zh-MO.ts
+++ b/src/client/locales-zh-MO.ts
@@ -291,6 +291,8 @@ export const zhMO: Record<string, string> = {
   subagent: '任務管理',
   openSubagent: '任務管理',
   subagentMainAgent: '主代理',
+  subagentActiveOnly: "只看活躍",
+  subagentActiveOnlyTitle: "收起空閒子代理，僅顯示執行中的子代理及其父鏈",
   subagentEmpty: '暫無子代理',
   subagentEmptyDesc: '目前主代理派生的子代理將顯示在這裡',
   subagentRunning: '執行中',
diff --git a/src/client/locales-zh-TW.ts b/src/client/locales-zh-TW.ts
index 2d81e3ff..3a235aca 100644
--- a/src/client/locales-zh-TW.ts
+++ b/src/client/locales-zh-TW.ts
@@ -291,6 +291,8 @@ export const zhTW: Record<string, string> = {
   subagent: '任務管理',
   openSubagent: '任務管理',
   subagentMainAgent: '主代理',
+  subagentActiveOnly: "只看活躍",
+  subagentActiveOnlyTitle: "摺疊閒置子代理，僅顯示執行中的子代理及其父鏈",
   subagentEmpty: '暫無子代理',
   subagentEmptyDesc: '目前主代理派生的子代理將顯示在這裡',
   subagentRunning: '執行中',
diff --git a/src/client/locales.ts b/src/client/locales.ts
index 23ee8f77..0ecf7078 100644
--- a/src/client/locales.ts
+++ b/src/client/locales.ts
@@ -283,6 +283,8 @@ export const zh = {
   subagent: '任务管理',
   openSubagent: '任务管理',
   subagentMainAgent: '主代理',
+  subagentActiveOnly: '只看活跃',
+  subagentActiveOnlyTitle: '收起空闲子代理，仅显示运行中的子代理及其父链',
   subagentEmpty: '暂无子代理',
   subagentEmptyDesc: '当前主代理派生的子代理将显示在这里',
   subagentRunning: '运行中',
@@ -627,6 +629,8 @@ export const en: Record<keyof typeof zh, string> = {
   subagent: 'Tasks',
   openSubagent: 'Tasks',
   subagentMainAgent: 'Main agent',
+  subagentActiveOnly: 'Active only',
+  subagentActiveOnlyTitle: 'Collapse idle subagents; keep running ones and their ancestors',
   subagentEmpty: 'No subagents',
   subagentEmptyDesc: 'Subagents spawned under the main agent will appear here',
   subagentRunning: 'Running',
diff --git a/src/client/subagent-detect.ts b/src/client/subagent-detect.ts
index f45b3ad5..8d6cfb31 100644
--- a/src/client/subagent-detect.ts
+++ b/src/client/subagent-detect.ts
@@ -135,3 +135,44 @@ export function countSubagentDescendants(
   }
   return totals
 }
+
+/**
+ * The session ids worth showing under "active only" mode: every running
+ * subagent plus each ancestor whose subtree contains one (hiding an idle
+ * parent must never orphan its running descendants). The root itself joins
+ * only when its subtree runs — the page renders it separately regardless.
+ * Cycles fail soft (same discipline as {@link countSubagentDescendants}).
+ */
+export function runningVisibilitySet(
+  byId: SidebarSessionList['byId'],
+  rootId: string,
+): Set<string> {
+  const keep = new Set<string>()
+  const childrenOf = new Map<string, string[]>()
+  for (const summary of Object.values(byId)) {
+    if (summary.origin !== 'subagent' || isSideThreadSummary(summary)
+      || summary.parentId === undefined) continue
+    const list = childrenOf.get(summary.parentId)
+    if (list) list.push(summary.id)
+    else childrenOf.set(summary.parentId, [summary.id])
+  }
+  const visit = (id: string): boolean => {
+    const seen = new Set<string>()
+    const walk = (current: string): boolean => {
+      if (seen.has(current)) return false
+      seen.add(current)
+      let subtreeRuns = false
+      for (const childId of childrenOf.get(current) ?? []) {
+        if (walk(childId)) subtreeRuns = true
+      }
+      if (subtreeRuns || byId[current]?.running === true) {
+        keep.add(current)
+        return true
+      }
+      return false
+    }
+    return walk(id)
+  }
+  visit(rootId)
+  return keep
+}
diff --git a/tests/subagent-detect.spec.ts b/tests/subagent-detect.spec.ts
index 2fe4e069..1eadac3d 100644
--- a/tests/subagent-detect.spec.ts
+++ b/tests/subagent-detect.spec.ts
@@ -1,7 +1,7 @@
 import { describe, expect, it } from 'vitest'
 import {
   collectBranchIds, countSubagentDescendants, detectNewDirectSubagent,
-  directSubagentCount, rootAncestor,
+  directSubagentCount, rootAncestor, runningVisibilitySet,
 } from '../src/client/subagent-detect.ts'
 import type { SidebarSessionList, SidebarSubagentCatalog } from '../src/context-types.ts'
 
@@ -159,4 +159,62 @@ describe('subagent detection over the sessions list feed', () => {
     }
     expect(collectBranchIds(cyclic, 'root')).toEqual(['a', 'root'])
   })
+
+  describe('runningVisibilitySet (the "active only" view filter)', () => {
+    it('returns an empty set when every subagent is idle', () => {
+      const byId = list('p1', ['c1', 'c2']).byId
+      const keep = runningVisibilitySet(byId, 'p1')
+      // The root row is rendered separately (never through this filter), so
+      // an all-idle tree yields an empty set: every catalog entry hides.
+      expect(keep.size).toBe(0)
+      expect(keep.has('c1')).toBe(false)
+      expect(keep.has('c2')).toBe(false)
+    })
+
+    it('keeps a running child and the ancestors that own it', () => {
+      const byId = list('p1', ['c1', 'c2', 'c3'], ['c2']).byId
+      const keep = runningVisibilitySet(byId, 'p1')
+      expect(keep.has('p1')).toBe(true) // ancestor of the running child
+      expect(keep.has('c2')).toBe(true) // running
+      expect(keep.has('c1')).toBe(false) // idle sibling
+      expect(keep.has('c3')).toBe(false)
+    })
+
+    it('keeps an idle parent only when a deeper descendant runs', () => {
+      const byId: SidebarSessionList['byId'] = {
+        p1: { id: 'p1', displayTitle: 'P1' },
+        c1: { id: 'c1', displayTitle: 'C1', origin: 'subagent', parentId: 'p1' },
+        g1: { id: 'g1', displayTitle: 'G1', origin: 'subagent', parentId: 'c1', running: true },
+        c2: { id: 'c2', displayTitle: 'C2', origin: 'subagent', parentId: 'p1' },
+      }
+      const keep = runningVisibilitySet(byId, 'p1')
+      expect(keep.has('c1')).toBe(true) // idle itself, owns the running g1
+      expect(keep.has('g1')).toBe(true)
+      expect(keep.has('p1')).toBe(true)
+      expect(keep.has('c2')).toBe(false)
+    })
+
+    it('ignores Side Chat threads when deciding what stays visible', () => {
+      const byId: SidebarSessionList['byId'] = {
+        p1: { id: 'p1', displayTitle: 'P1' },
+        c1: { id: 'c1', displayTitle: 'C1', origin: 'subagent', parentId: 'p1' },
+        s1: {
+          id: 's1', displayTitle: 'Side: refactor plan', origin: 'subagent',
+          parentId: 'p1', running: true,
+        },
+      }
+      const keep = runningVisibilitySet(byId, 'p1')
+      expect(keep.has('s1')).toBe(false)
+      expect(keep.has('c1')).toBe(false)
+    })
+
+    it('terminates on lineage cycles (fail soft)', () => {
+      const cyclic: SidebarSessionList['byId'] = {
+        a: { id: 'a', displayTitle: 'A', origin: 'subagent', parentId: 'b', running: true },
+        b: { id: 'b', displayTitle: 'B', origin: 'subagent', parentId: 'a' },
+      }
+      expect(() => runningVisibilitySet(cyclic, 'a')).not.toThrow()
+      expect(runningVisibilitySet(cyclic, 'a').has('a')).toBe(true)
+    })
+  })
 })