fix(ssh): gate paired-viewer pane recovery on the narrowed session-gone predicate

isSshSessionGoneError landed on the IPC transport, which never calls
terminal.recoverPane. The one caller that does — recoverExpiredHostPane in the
paired-viewer transport — still triggered on a bare SSH_SESSION_EXPIRED
substring, so the identity-mismatch reply (the relay found a LIVE PTY under that
id owned by another pane, which is evidence of presence) still asked the HUB to
replace the pane, putting a second agent on one transcript. Main already refuses
the respawn on that same reply; this makes the two agree.

A pane whose shell genuinely died is unaffected: plain SSH_SESSION_EXPIRED still
matches. The mismatch reply now surfaces as an error instead of a respawn.

src/renderer/src/components/terminal-pane/remote-runtime-pty-transport-expired-pane-recovery.test.ts

@@ -132,6 +132,45 @@ describe('createRemoteRuntimePtyTransport', () => {
     expect(onError).not.toHaveBeenCalled()
   })
 
+  it('does not recover a pane whose relay reply was an identity mismatch', async () => {
+    // The mismatch suffix means the relay found a LIVE PTY under that id owned by ANOTHER pane, so
+    // it is evidence of presence, not absence. This transport is the only caller of
+    // terminal.recoverPane, so a bare SSH_SESSION_EXPIRED substring test here put a second agent on
+    // one transcript even though main already refuses the respawn on the same reply.
+    const onError = vi.fn()
+    resolvedPaneHandle = 'terminal-mismatch'
+    const { createRemoteRuntimePtyTransport } = await import('./remote-runtime-pty-transport')
+    const transport = createRemoteRuntimePtyTransport('hub-env', {
+      worktreeId: 'wt-1',
+      tabId: 'web-terminal-host-tab-1',
+      leafId: 'pane:1'
+    })
+    transport.attach({
+      existingPtyId: 'remote:hub-env@@terminal-mismatch',
+      callbacks: { onError }
+    })
+    await vi.waitFor(() => expect(subscriptionSendBinary).toHaveBeenCalled())
+    runtimeCall.mockClear()
+
+    subscriptionCallbacks?.onResponse({
+      ok: true,
+      result: {
+        type: 'error',
+        streamId: latestSubscribePayload().streamId,
+        message: 'SSH_SESSION_EXPIRED: pty-1 SSH_PTY_IDENTITY_MISMATCH'
+      }
+    })
+
+    await vi.waitFor(() => expect(onError).toHaveBeenCalled())
+    expect(runtimeCall).not.toHaveBeenCalledWith(
+      expect.objectContaining({ method: 'terminal.recoverPane' })
+    )
+    expect(runtimeCall).not.toHaveBeenCalledWith(
+      expect.objectContaining({ method: 'terminal.create' })
+    )
+    expect(transport.getPtyId()).toBe('remote:hub-env@@terminal-mismatch')
+  })
+
   it('fails closed when an older HUB cannot recover an expired SSH pane', async () => {
     const onError = vi.fn()
     resolvedPaneHandle = 'terminal-expired'

src/renderer/src/components/terminal-pane/remote-runtime-pty-transport.ts

@@ -32,6 +32,7 @@ import type {
   PtyTransportRecoveryState
 } from './pty-transport-types'
 import { createPtyOutputProcessor } from './pty-transport'
+import { isSshSessionGoneError } from './pty-connection/pty-connect-limits'
 import { RuntimeRpcCallError, unwrapRuntimeRpcResult } from '../../runtime/runtime-rpc-client'
 import {
   getRemoteRuntimePtyEnvironmentId,
@@ -116,7 +117,6 @@ type RemoteAgentSessionLaunchResult =
   | RuntimeEnsureAgentSessionResult
   | RuntimeCreateAgentSessionResult
   | { terminal: RuntimeTerminalCreate; disposition?: undefined }
-const SSH_SESSION_EXPIRED_ERROR = 'SSH_SESSION_EXPIRED'
 
 function isRemoteTerminalStaleMessage(message: string): boolean {
   return message.includes('terminal_handle_stale')
@@ -1600,8 +1600,12 @@ export function createRemoteRuntimePtyTransport(
       retireRemoteTerminalId()
       return
     }
-    if (message.includes(SSH_SESSION_EXPIRED_ERROR)) {
-      // Why: only the HUB may replace its expired SSH pane; a paired viewer must never fall back to client-local SSH.
+    if (isSshSessionGoneError(message)) {
+      // Why: only the HUB may replace its expired SSH pane; a paired viewer must never fall back to
+      // client-local SSH. The identity-mismatch suffix is excluded because it means the opposite —
+      // the relay found a LIVE PTY under that id owned by another pane — and this is the one
+      // transport that actually calls terminal.recoverPane, so a bare substring test here spawned
+      // a second agent onto one transcript.
       recoverExpiredHostPane()
       return
     }