diff --git a/src/main/native-chat/agent-session-journal/journal-submission-positions.test.ts b/src/main/native-chat/agent-session-journal/journal-submission-positions.test.ts
index d8aa5b4870c..2ae99a42b45 100644
--- a/src/main/native-chat/agent-session-journal/journal-submission-positions.test.ts
+++ b/src/main/native-chat/agent-session-journal/journal-submission-positions.test.ts
@@ -7,6 +7,7 @@ import { mkdtemp, rm } from 'node:fs/promises'
 import { tmpdir } from 'node:os'
 import { join } from 'node:path'
 import { afterEach, describe, expect, it } from 'vitest'
+import { codexProviderHandle } from '../../../shared/agent-session-provider-handle-encoding'
 import { agentSessionFailureFact } from '../../../shared/agent-session-failure'
 import { agentSessionFailureWords } from '../../../shared/agent-session-failure-words'
 import {
@@ -26,7 +27,7 @@ const IDENTITY: AgentSessionJournalIdentity = {
   workspaceId: 'workspace-1',
   hostId: 'host-1',
   agent: 'codex',
-  providerHandle: { kind: 'codex', threadId: 'thread-1' }
+  providerHandle: codexProviderHandle('thread-1')
 }
 const BODY: AgentJournalMessageItem = {
   kind: 'message',
diff --git a/src/main/persistence-ui-state.test.ts b/src/main/persistence-ui-state.test.ts
index 10bea531533..4324e61cdf5 100644
--- a/src/main/persistence-ui-state.test.ts
+++ b/src/main/persistence-ui-state.test.ts
@@ -92,6 +92,19 @@ describe('Store', () => {
     expect(ui.dismissedUpdateVersion).toBeNull()
   })
 
+  it.each([false, true])(
+    'restores sidebarOpen=%s from disk without changing the right sidebar',
+    async (sidebarOpen) => {
+      const store = await createStore()
+      store.updateUI({ sidebarOpen, rightSidebarOpen: false })
+      store.flush()
+
+      const reloaded = await createStore()
+      expect(reloaded.getUI().sidebarOpen).toBe(sidebarOpen)
+      expect(reloaded.getUI().rightSidebarOpen).toBe(false)
+    }
+  )
+
   it('round-trips and normalizes the host-qualified manual repo order', async () => {
     const store = await createStore()
     store.updateUI({
diff --git a/src/main/runtime/rpc/methods/client-ui-pairing-local-fields.test.ts b/src/main/runtime/rpc/methods/client-ui-pairing-local-fields.test.ts
index 4e3beb3a771..9bb125d6065 100644
--- a/src/main/runtime/rpc/methods/client-ui-pairing-local-fields.test.ts
+++ b/src/main/runtime/rpc/methods/client-ui-pairing-local-fields.test.ts
@@ -46,6 +46,7 @@ describe('client UI RPC pairing-local field seams', () => {
       { hostId: 'runtime:web-11111111-2222-3333-4444-555555555555', repoId: 'repo-a' }
     ],
     workspaceHostOrder: ['runtime:web-11111111-2222-3333-4444-555555555555', 'local'],
+    sidebarOpen: false,
     agentsVisibleHostIds: ['runtime:web-11111111-2222-3333-4444-555555555555'],
     agentsFilterRepoIds: ['repo-a'],
     agentsHideWorkspacesFromOtherDevices: true,
diff --git a/src/renderer/src/app-shell/use-persisted-ui-writer.ts b/src/renderer/src/app-shell/use-persisted-ui-writer.ts
index 168fb87f302..3b0556a161e 100644
--- a/src/renderer/src/app-shell/use-persisted-ui-writer.ts
+++ b/src/renderer/src/app-shell/use-persisted-ui-writer.ts
@@ -145,6 +145,7 @@ export function usePersistedUIWriter(): void {
   const ui = useAppStore(
     useShallow((s): PersistedUIWriteBaseline => ({
       sidebarWidth: s.sidebarWidth,
+      sidebarOpen: s.sidebarOpen,
       rightSidebarOpen: s.rightSidebarOpen,
       rightSidebarTab: s.rightSidebarTab,
       rightSidebarExplorerView: s.rightSidebarExplorerView,
diff --git a/src/renderer/src/app-shell/workspace-view-cross-client-sync.test.tsx b/src/renderer/src/app-shell/workspace-view-cross-client-sync.test.tsx
index f5d404b4e10..be236272bf4 100644
--- a/src/renderer/src/app-shell/workspace-view-cross-client-sync.test.tsx
+++ b/src/renderer/src/app-shell/workspace-view-cross-client-sync.test.tsx
@@ -270,6 +270,40 @@ describe('workspace view preferences: cross-client persistence (STA-5781)', () =
     expect(after.hideCliCreatedWorkspaces).toBe(before.hideCliCreatedWorkspaces)
   })
 
+  it('persists a left sidebar close across an unrelated sync and restores it on startup', async () => {
+    act(() => {
+      store.getState().toggleSidebar()
+      authority.set({ sidebarWidth: 320 })
+    })
+    deliverBroadcasts()
+    expect(store.getState().sidebarOpen).toBe(false)
+    await flushDesktopDebounce()
+    expect(authority.get().sidebarOpen).toBe(false)
+
+    const restarted = createUIStore()
+    restarted.getState().hydratePersistedUI(authority.get(), 'startup')
+    expect(restarted.getState().sidebarOpen).toBe(false)
+    expect(restarted.getState().sidebarWidth).toBe(320)
+  })
+
+  it('persists a left sidebar reopen while the close acknowledgement is still pending', async () => {
+    holdAcks = true
+    act(() => store.getState().toggleSidebar())
+    await flushDesktopDebounce()
+    expect(authority.get().sidebarOpen).toBe(false)
+
+    act(() => store.getState().toggleSidebar())
+    deliverBroadcasts()
+    expect(store.getState().sidebarOpen).toBe(true)
+    await resolveAcks()
+    await flushDesktopDebounce()
+    await resolveAcks()
+    deliverBroadcasts()
+    expect(authority.get().sidebarOpen).toBe(true)
+    expect(store.getState().sidebarOpen).toBe(true)
+    expect(store.getState().persistedUIWriteInFlightCounts).toEqual({})
+  })
+
   it('a mobile tap must not revert a desktop change the mobile mirror has not seen', async () => {
     const mobile = createMobileClient(authority)
     mobile.sync()
diff --git a/src/renderer/src/store/slices/persisted-ui-write-baseline.test.ts b/src/renderer/src/store/slices/persisted-ui-write-baseline.test.ts
index f39ef11b03f..0efae37ca0c 100644
--- a/src/renderer/src/store/slices/persisted-ui-write-baseline.test.ts
+++ b/src/renderer/src/store/slices/persisted-ui-write-baseline.test.ts
@@ -10,6 +10,7 @@ import {
 function makeBaseline(overrides: Partial<PersistedUIWriteBaseline> = {}): PersistedUIWriteBaseline {
   return {
     sidebarWidth: 280,
+    sidebarOpen: true,
     rightSidebarOpen: true,
     rightSidebarTab: 'explorer',
     rightSidebarExplorerView: 'files',
@@ -145,3 +146,10 @@ describe('persistedUIWriteFieldsToWireUpdate', () => {
     expect(update).toEqual({ hideDefaultBranchWorkspace: true, groupBy: 'none' })
   })
 })
+
+describe('sidebarOpen write round-trip', () => {
+  it('sends a left sidebar close to main under its persisted key', () => {
+    const changed = diffPersistedUIWriteFields(makeBaseline({ sidebarOpen: false }), makeBaseline())
+    expect(persistedUIWriteFieldsToWireUpdate(changed)).toEqual({ sidebarOpen: false })
+  })
+})
diff --git a/src/renderer/src/store/slices/persisted-ui-write-baseline.ts b/src/renderer/src/store/slices/persisted-ui-write-baseline.ts
index 158e1b31bf2..47cd8a46b3f 100644
--- a/src/renderer/src/store/slices/persisted-ui-write-baseline.ts
+++ b/src/renderer/src/store/slices/persisted-ui-write-baseline.ts
@@ -10,6 +10,7 @@ import type { PersistedUIState } from '../../../../shared/persisted-ui-state-typ
  */
 export type PersistedUIWriteBaseline = {
   sidebarWidth: number
+  sidebarOpen: boolean
   rightSidebarOpen: boolean
   rightSidebarTab: PersistedUIState['rightSidebarTab']
   rightSidebarExplorerView: PersistedUIState['rightSidebarExplorerView']
@@ -40,6 +41,7 @@ export type PersistedUIWriteBaseline = {
 // this module exists to close (see ui-state-schema-parity.ts for the same lesson).
 const PERSISTED_UI_WRITE_BASELINE_FIELD_SET = {
   sidebarWidth: true,
+  sidebarOpen: true,
   rightSidebarOpen: true,
   rightSidebarTab: true,
   rightSidebarExplorerView: true,
diff --git a/src/renderer/src/store/slices/ui-contextual-tours.test.ts b/src/renderer/src/store/slices/ui-contextual-tours.test.ts
index 7a9adc8af7b..3db02f6e587 100644
--- a/src/renderer/src/store/slices/ui-contextual-tours.test.ts
+++ b/src/renderer/src/store/slices/ui-contextual-tours.test.ts
@@ -384,8 +384,7 @@ describe('createUISlice contextual tours', () => {
     stubContextualTourTargets([
       '[data-contextual-tour-target="terminal-pane-split-target"], [data-contextual-tour-target="workspace-agent-terminal-tip"]'
     ])
-    store.setState({ sidebarOpen: false })
-    store.getState().hydratePersistedUI(makeAutoTourEligibleUI())
+    store.getState().hydratePersistedUI(makeAutoTourEligibleUI({ sidebarOpen: false }))
     store
       .getState()
       .requestContextualTour('workspace-agent-sessions', 'setup_guide_parallel_work', false, {
diff --git a/src/renderer/src/store/slices/ui-hydration-view-layout.test.ts b/src/renderer/src/store/slices/ui-hydration-view-layout.test.ts
index fd01ee2d2a7..7173826d473 100644
--- a/src/renderer/src/store/slices/ui-hydration-view-layout.test.ts
+++ b/src/renderer/src/store/slices/ui-hydration-view-layout.test.ts
@@ -225,6 +225,43 @@ describe('createUISlice hydratePersistedUI', () => {
     expect(store.getState().rightSidebarWidth).toBe(360)
   })
 
+  it('hydrates a persisted closed left sidebar preference', () => {
+    const store = createUIStore()
+
+    store.getState().hydratePersistedUI(makePersistedUI({ sidebarOpen: false }))
+
+    expect(store.getState().sidebarOpen).toBe(false)
+  })
+
+  it('hydrates a persisted open left sidebar preference', () => {
+    const store = createUIStore()
+
+    store.getState().hydratePersistedUI(makePersistedUI({ sidebarOpen: true }))
+
+    expect(store.getState().sidebarOpen).toBe(true)
+  })
+
+  it('hydrates a missing left sidebar preference as open', () => {
+    const store = createUIStore()
+
+    store.setState({ sidebarOpen: false })
+    store.getState().hydratePersistedUI({ ...makePersistedUI(), sidebarOpen: undefined })
+
+    expect(store.getState().sidebarOpen).toBe(true)
+  })
+
+  it('keeps an unsaved left sidebar close when a sync omits the left sidebar preference', () => {
+    const store = createUIStore()
+    store.getState().hydratePersistedUI(makePersistedUI(), 'startup')
+
+    store.getState().setSidebarOpen(false)
+    store.getState().hydratePersistedUI({ ...makePersistedUI(), sidebarOpen: undefined }, 'sync')
+
+    // Why: the baseline must stay open or the writer sees no diff and the close never persists.
+    expect(store.getState().sidebarOpen).toBe(false)
+    expect(store.getState().persistedUIWriteBaseline?.sidebarOpen).toBe(true)
+  })
+
   it('hydrates a persisted closed right sidebar preference', () => {
     const store = createUIStore()
 
diff --git a/src/renderer/src/store/slices/ui/ui-slice-hydration-actions.ts b/src/renderer/src/store/slices/ui/ui-slice-hydration-actions.ts
index c8da9a29dcc..9df5868597d 100644
--- a/src/renderer/src/store/slices/ui/ui-slice-hydration-actions.ts
+++ b/src/renderer/src/store/slices/ui/ui-slice-hydration-actions.ts
@@ -88,10 +88,6 @@ export function createUiHydrationActions(set: UISliceSet, _get: UISliceGet): Par
         // Migration: one-shot old-'recent'→'smart' runs in main (_sortBySmartMigrated), not here, so a deliberate 'recent' choice survives restart.
         const sortBy = ui.sortBy
         const statusBarItemsWithGrok = hydrateStatusBarItems(ui)
-        const rightSidebarRoute = normalizeRightSidebarRoute(
-          ui.rightSidebarTab,
-          ui.rightSidebarExplorerView
-        )
         const hydrated = {
           // Why: persisted widths may be stale/corrupt/hand-edited; clamp during hydration so invalid values can't break layout.
           sidebarWidth: sanitizePersistedSidebarWidth(
@@ -114,9 +110,9 @@ export function createUiHydrationActions(set: UISliceSet, _get: UISliceGet): Par
             undefined,
             s.combinedDiffFileTreeWidth
           ),
+          sidebarOpen: typeof ui.sidebarOpen === 'boolean' ? ui.sidebarOpen : true,
           rightSidebarOpen: typeof ui.rightSidebarOpen === 'boolean' ? ui.rightSidebarOpen : true,
-          rightSidebarTab: rightSidebarRoute.rightSidebarTab,
-          rightSidebarExplorerView: rightSidebarRoute.rightSidebarExplorerView,
+          ...normalizeRightSidebarRoute(ui.rightSidebarTab, ui.rightSidebarExplorerView),
           groupBy: (ui.groupBy as UISlice['groupBy'] | 'parent') === 'parent' ? 'repo' : ui.groupBy,
           sortBy,
           // Why: main-process getUI() already normalized this (defaulting to 'manual'); read it through without migrating.
diff --git a/src/renderer/src/web/preload-api/web-preference-normalization.ts b/src/renderer/src/web/preload-api/web-preference-normalization.ts
index 37b57d0e401..1b805b3c223 100644
--- a/src/renderer/src/web/preload-api/web-preference-normalization.ts
+++ b/src/renderer/src/web/preload-api/web-preference-normalization.ts
@@ -69,6 +69,7 @@ export function mergeHostWebUIState(
     hideWorkspacesFromOtherDevices: local.hideWorkspacesFromOtherDevices === true,
     manualRepoOrder: local.manualRepoOrder,
     workspaceHostOrder: local.workspaceHostOrder,
+    sidebarOpen: local.sidebarOpen,
     agentsVisibleHostIds: local.agentsVisibleHostIds,
     agentsFilterRepoIds: local.agentsFilterRepoIds,
     agentsHideWorkspacesFromOtherDevices: local.agentsHideWorkspacesFromOtherDevices === true,
diff --git a/src/renderer/src/web/web-preload-api-ui-pairing-local-fields.test.ts b/src/renderer/src/web/web-preload-api-ui-pairing-local-fields.test.ts
index e1f9ede3818..7a1cffeaedf 100644
--- a/src/renderer/src/web/web-preload-api-ui-pairing-local-fields.test.ts
+++ b/src/renderer/src/web/web-preload-api-ui-pairing-local-fields.test.ts
@@ -21,14 +21,13 @@ describe('web UI preload API pairing-local fields', () => {
     vi.doUnmock('./web-runtime-client')
   })
 
-  // Census-driven, matching the host-side seam tests: a field added to PAIRING_LOCAL_UI_FIELDS
-  // without wiring the web read seam fails here rather than shipping. The host sample differs from
-  // the browser's for every field, so only the pin makes this pass.
+  // Different host samples expose any missing browser-local pin.
   const browserLocalUiSamples: Record<PairingLocalUiField, unknown> = {
     automationHostFilter: { kind: 'host', hostKey: 'browser-local-host-key' },
     hideWorkspacesFromOtherDevices: true,
     manualRepoOrder: [{ hostId: 'runtime:web-env-1', repoId: 'repo-b' }],
     workspaceHostOrder: ['runtime:web-env-1', 'local'],
+    sidebarOpen: false,
     agentsVisibleHostIds: ['runtime:web-env-1'],
     agentsFilterRepoIds: ['repo-b'],
     agentsHideWorkspacesFromOtherDevices: true,
@@ -47,6 +46,7 @@ describe('web UI preload API pairing-local fields', () => {
     hideWorkspacesFromOtherDevices: false,
     manualRepoOrder: [{ hostId: 'local', repoId: 'repo-a' }],
     workspaceHostOrder: ['local', 'ssh:box'],
+    sidebarOpen: true,
     agentsVisibleHostIds: ['local'],
     agentsFilterRepoIds: ['repo-a'],
     agentsHideWorkspacesFromOtherDevices: false,
@@ -61,9 +61,13 @@ describe('web UI preload API pairing-local fields', () => {
     manuallyUnreadTurnsByPaneKey: { 'tab-2:leaf-2': 654 }
   }
 
-  it.each(PAIRING_LOCAL_UI_FIELDS.map((field) => [field] as const))(
-    'keeps the browser-local %s and never sends it to the host',
-    async (field) => {
+  it.each(
+    PAIRING_LOCAL_UI_FIELDS.flatMap((field) =>
+      (['set', 'setWithAck'] as const).map((method) => [field, method] as const)
+    )
+  )(
+    'keeps the browser-local %s and never sends it to the host through %s',
+    async (field, method) => {
       const runtimeCalls: { method: string; params: unknown }[] = []
       vi.doMock('./web-runtime-client', () => ({
         WebRuntimeClient: class {
@@ -81,14 +85,18 @@ describe('web UI preload API pairing-local fields', () => {
         }
       }))
 
-      const browserLocal = { [field]: browserLocalUiSamples[field] } as Partial<PersistedUIState>
+      const browserLocal: Partial<PersistedUIState> = { [field]: browserLocalUiSamples[field] }
       const globals = installBrowserGlobals('Linux')
       writeStoredRuntimeEnvironment(globals.storage)
-      globals.storage.setItem('orca.web.ui.v1', JSON.stringify(browserLocal))
       const { installWebPreloadApi } = await import('./web-preload-api')
       installWebPreloadApi()
 
-      await globals.window.api.ui.set({ ...browserLocal, sidebarWidth: 280 })
+      const write = globals.window.api.ui[method]
+      expect(write).toBeTypeOf('function')
+      if (!write) {
+        throw new Error('Missing UI write method')
+      }
+      await write({ ...browserLocal, sidebarWidth: 280 })
 
       expect(runtimeCalls[0]).toEqual({ method: 'ui.set', params: { sidebarWidth: 280 } })
       await expect(globals.window.api.ui.get()).resolves.toMatchObject(browserLocal)
diff --git a/src/shared/pairing-local-ui-fields.test.ts b/src/shared/pairing-local-ui-fields.test.ts
index 780b471a1ca..24042ffaa50 100644
--- a/src/shared/pairing-local-ui-fields.test.ts
+++ b/src/shared/pairing-local-ui-fields.test.ts
@@ -10,6 +10,7 @@ describe('pairing-local UI fields', () => {
       'hideWorkspacesFromOtherDevices',
       'manualRepoOrder',
       'workspaceHostOrder',
+      'sidebarOpen',
       'agentsVisibleHostIds',
       'agentsFilterRepoIds',
       'agentsHideWorkspacesFromOtherDevices',
diff --git a/src/shared/pairing-local-ui-fields.ts b/src/shared/pairing-local-ui-fields.ts
index 65e036699ac..b514db0a3b9 100644
--- a/src/shared/pairing-local-ui-fields.ts
+++ b/src/shared/pairing-local-ui-fields.ts
@@ -12,6 +12,8 @@ export const PAIRING_LOCAL_UI_FIELDS = [
   'hideWorkspacesFromOtherDevices',
   'manualRepoOrder',
   'workspaceHostOrder',
+  // A phone-width browser and a desktop window each choose their own left sidebar, and old hosts reject the unknown key.
+  'sidebarOpen',
   // Agent View filters and presentation belong to each client's host catalog and viewport.
   'agentsVisibleHostIds',
   'agentsFilterRepoIds',
diff --git a/src/shared/persisted-ui-state-types.ts b/src/shared/persisted-ui-state-types.ts
index f7708d50206..ecd51429533 100644
--- a/src/shared/persisted-ui-state-types.ts
+++ b/src/shared/persisted-ui-state-types.ts
@@ -32,6 +32,7 @@ export type PersistedUIState = {
   /** Active top-level view at save time, restored on relaunch; sanitized to 'terminal' if unknown or now-gated. */
   activeView: TopLevelView
   sidebarWidth: number
+  sidebarOpen?: boolean
   rightSidebarOpen: boolean
   rightSidebarTab: RightSidebarTab
   rightSidebarExplorerView: RightSidebarExplorerView
diff --git a/src/shared/rpc-contract/client-ui-params.ts b/src/shared/rpc-contract/client-ui-params.ts
index 08113208e23..9f41e92674e 100644
--- a/src/shared/rpc-contract/client-ui-params.ts
+++ b/src/shared/rpc-contract/client-ui-params.ts
@@ -127,6 +127,7 @@ export const UiUpdateFields = z
     // Why: sync hydration ignores this persisted startup view, so paired windows stay put.
     activeView: TopLevelViewSchema.optional(),
     sidebarWidth: z.number().finite().optional(),
+    sidebarOpen: z.boolean().optional(),
     rightSidebarOpen: z.boolean().optional(),
     rightSidebarTab: RightSidebarTabParam.optional(),
     rightSidebarExplorerView: z.enum(['files', 'search']).optional(),