From 877bbdebf8d6000b85a6a2673005d8d5a78171d9 Mon Sep 17 00:00:00 2001
From: Shahar Mor <shaharmor@microsoft.com>
Date: Fri, 24 Jul 2026 09:42:35 +0300
Subject: [PATCH] fix(agents): stop forking a duplicate Pi tab for a live
 background session (#9729)

---
 .../pi-live-session-no-duplicate-tab.test.ts  | 82 +++++++++++++++++++
 .../src/lib/sleeping-agent-pane-ownership.ts  | 36 ++++++++
 2 files changed, 118 insertions(+)
 create mode 100644 src/renderer/src/lib/pi-live-session-no-duplicate-tab.test.ts

diff --git a/src/renderer/src/lib/pi-live-session-no-duplicate-tab.test.ts b/src/renderer/src/lib/pi-live-session-no-duplicate-tab.test.ts
new file mode 100644
index 000000000..8d369760c
--- /dev/null
+++ b/src/renderer/src/lib/pi-live-session-no-duplicate-tab.test.ts
@@ -0,0 +1,82 @@
+import { afterEach, describe, expect, it } from 'vitest'
+import type { SleepingAgentSessionRecord } from '../../../shared/agent-session-resume'
+import { useAppStore } from '@/store'
+import { resumeSleepingAgentSessionsForWorktree } from './resume-sleeping-agent-session'
+
+const initialAppStoreState = useAppStore.getState()
+const LEAF_ID = '11111111-1111-1111-8111-111111111111'
+const PANE_KEY = `pi-tab:${LEAF_ID}`
+
+afterEach(() => {
+  useAppStore.setState(initialAppStoreState, true)
+})
+
+describe('Pi live session does not spawn a duplicate resume tab', () => {
+  it('keeps a done-but-alive Pi pane instead of forking a new tab', () => {
+    const providerSession = {
+      key: 'session_id' as const,
+      id: 'pi-1',
+      transcriptPath: '/tmp/pi-session-1.jsonl'
+    }
+    const record: SleepingAgentSessionRecord = {
+      paneKey: PANE_KEY,
+      tabId: 'pi-tab',
+      worktreeId: 'wt-1',
+      agent: 'pi',
+      providerSession,
+      prompt: '',
+      state: 'working',
+      capturedAt: 1,
+      updatedAt: 1,
+      origin: 'live'
+    }
+    useAppStore.setState({
+      activeWorktreeId: 'wt-1',
+      activeTabType: 'editor',
+      activeTabId: null,
+      tabsByWorktree: {
+        'wt-1': [
+          {
+            id: 'pi-tab',
+            ptyId: 'pty-1',
+            worktreeId: 'wt-1',
+            title: 'pi',
+            customTitle: null,
+            color: null,
+            sortOrder: 0,
+            createdAt: 1
+          }
+        ]
+      },
+      ptyIdsByTabId: { 'pi-tab': ['pty-1'] },
+      terminalLayoutsByTabId: {
+        'pi-tab': {
+          root: { type: 'leaf', leafId: LEAF_ID },
+          activeLeafId: LEAF_ID,
+          expandedLeafId: null,
+          ptyIdsByLeafId: { [LEAF_ID]: 'pty-1' }
+        }
+      },
+      agentStatusByPaneKey: {
+        [PANE_KEY]: {
+          state: 'done',
+          prompt: '',
+          updatedAt: 10,
+          stateStartedAt: 10,
+          agentType: 'pi',
+          paneKey: PANE_KEY,
+          worktreeId: 'wt-1',
+          tabId: 'pi-tab',
+          providerSession
+        }
+      },
+      sleepingAgentSessionsByPaneKey: { [PANE_KEY]: record }
+    } as never)
+
+    const launched = resumeSleepingAgentSessionsForWorktree('wt-1')
+
+    expect(launched).toBe(0)
+    expect(useAppStore.getState().tabsByWorktree['wt-1']).toHaveLength(1)
+    expect(useAppStore.getState().sleepingAgentSessionsByPaneKey[PANE_KEY]).toBe(record)
+  })
+})
diff --git a/src/renderer/src/lib/sleeping-agent-pane-ownership.ts b/src/renderer/src/lib/sleeping-agent-pane-ownership.ts
index 90b154cb7..a1e0f7b74 100644
--- a/src/renderer/src/lib/sleeping-agent-pane-ownership.ts
+++ b/src/renderer/src/lib/sleeping-agent-pane-ownership.ts
@@ -78,6 +78,30 @@ function hasRestorableStablePanePty(
   )
 }
 
+// Why: a pane whose PTY is live *right now* already owns its running session
+// — e.g. a Pi TUI that finished a turn but stays alive in a background tab.
+// Resume must never fork such a pane into a duplicate tab, even when it isn't
+// the pane that reconnects on activation. Liveness comes from the runtime
+// live-PTY map (ptyIdsByTabId), not the layout's ptyIdsByLeafId snapshot, which
+// persists stale across sleep/restart.
+function stablePaneHasLivePty(
+  tabId: string,
+  leafId: string,
+  ptyIdsByTabId: Record<string, string[]>,
+  layout: TerminalLayoutSnapshot | undefined
+): boolean {
+  const livePtyIds = ptyIdsByTabId[tabId] ?? []
+  if (livePtyIds.length === 0) {
+    return false
+  }
+  const leafPtyId = layout?.ptyIdsByLeafId?.[leafId]
+  if (leafPtyId) {
+    return livePtyIds.includes(leafPtyId)
+  }
+  // Single-leaf tabs have no per-leaf binding; the tab's live PTY is this leaf's.
+  return layout?.root?.type === 'leaf' && layout.root.leafId === leafId
+}
+
 function paneWillConnectOnActivation(
   worktreeId: string,
   tabId: string,
@@ -119,6 +143,18 @@ export function recordPaneIsOwnedByPreservedPane(
     if (isPassiveCompletedHibernationEvidence(record)) {
       return true
     }
+    // Why: a pane with a live PTY owns its running session regardless of which
+    // pane reconnects on activation; forking it would duplicate the session.
+    if (
+      stablePaneHasLivePty(
+        tabId,
+        stable.leafId,
+        state.ptyIdsByTabId,
+        state.terminalLayoutsByTabId[tabId]
+      )
+    ) {
+      return true
+    }
     // Why: active sessions rely on pane-level cold restore. A preserved leaf
     // without a PTY/session id can repaint scrollback but cannot resume.
     return (