diff --git a/packages/web-shell/client/components/ChatPane.module.css b/packages/web-shell/client/components/ChatPane.module.css
index 15713b2a09..8cea48ea5d 100644
--- a/packages/web-shell/client/components/ChatPane.module.css
+++ b/packages/web-shell/client/components/ChatPane.module.css
@@ -48,7 +48,8 @@
   color: var(--foreground);
 }
 
-.closeButton {
+.closeButton,
+.maximizeButton {
   flex: 0 0 auto;
   display: inline-flex;
   align-items: center;
@@ -62,11 +63,19 @@
   cursor: pointer;
 }
 
-.closeButton:hover {
+.closeButton:hover,
+.maximizeButton:hover {
   color: var(--foreground);
   background: color-mix(in srgb, var(--foreground) 8%, transparent);
 }
 
+/* The maximized pane keeps its toggle visually "active" so it's clear which
+   control returns to the tiled layout. */
+.maximizeButton[aria-pressed='true'] {
+  color: var(--foreground);
+  background: color-mix(in srgb, var(--foreground) 10%, transparent);
+}
+
 /* The transcript takes the remaining height and scrolls independently per pane. */
 .body {
   flex: 1 1 auto;
diff --git a/packages/web-shell/client/components/ChatPane.test.tsx b/packages/web-shell/client/components/ChatPane.test.tsx
index 7b62ad3d67..4c79fa1c82 100644
--- a/packages/web-shell/client/components/ChatPane.test.tsx
+++ b/packages/web-shell/client/components/ChatPane.test.tsx
@@ -534,6 +534,36 @@ describe('ChatPane', () => {
     expect(onClose).toHaveBeenCalledTimes(1);
   });
 
+  it('renders no maximize toggle without onToggleMaximize', () => {
+    render({ onClose: () => {} });
+    expect(container!.querySelector('[aria-label="Maximize pane"]')).toBeNull();
+    expect(container!.querySelector('[aria-label="Restore pane"]')).toBeNull();
+  });
+
+  it('invokes onToggleMaximize from the header maximize button', () => {
+    const onToggleMaximize = vi.fn();
+    render({ onToggleMaximize });
+    const maximizeBtn = container!.querySelector(
+      '[aria-label="Maximize pane"]',
+    );
+    expect(maximizeBtn).not.toBeNull();
+    // A toggle button always exposes its pressed state; not maximized here.
+    expect(maximizeBtn!.getAttribute('aria-pressed')).toBe('false');
+    act(() =>
+      maximizeBtn!.dispatchEvent(new MouseEvent('click', { bubbles: true })),
+    );
+    expect(onToggleMaximize).toHaveBeenCalledTimes(1);
+  });
+
+  it('shows the restore affordance while maximized', () => {
+    render({ onToggleMaximize: () => {}, isMaximized: true });
+    const restoreBtn = container!.querySelector('[aria-label="Restore pane"]');
+    expect(restoreBtn).not.toBeNull();
+    expect(restoreBtn!.getAttribute('aria-pressed')).toBe('true');
+    // The label flips to "restore" — no stale "maximize" affordance remains.
+    expect(container!.querySelector('[aria-label="Maximize pane"]')).toBeNull();
+  });
+
   it('cancels the active turn via the composer cancel action', () => {
     render();
     act(() =>
diff --git a/packages/web-shell/client/components/ChatPane.tsx b/packages/web-shell/client/components/ChatPane.tsx
index 8d202720c3..6e6142f792 100644
--- a/packages/web-shell/client/components/ChatPane.tsx
+++ b/packages/web-shell/client/components/ChatPane.tsx
@@ -5,6 +5,7 @@
  */
 
 import { useCallback, useEffect, useMemo, useRef } from 'react';
+import { Maximize2Icon, Minimize2Icon } from 'lucide-react';
 import {
   useActions,
   useConnection,
@@ -77,6 +78,14 @@ export interface ChatPaneProps {
    */
   workspaceCwd?: string;
   onClose?: () => void;
+  /**
+   * Toggle this pane between maximized (solo, filling the whole split) and the
+   * tiled layout. Omitted when only one pane is open — there's nothing to
+   * maximize against.
+   */
+  onToggleMaximize?: () => void;
+  /** Whether this pane is currently the maximized (solo) one. */
+  isMaximized?: boolean;
   onError?: (error: unknown, fallback: string) => void;
   onRightPanelOpen?: (request: TurnOutputOpenRequest) => void;
   onPaneArtifactsChange?: (
@@ -98,6 +107,8 @@ export function ChatPane({
   title,
   workspaceCwd,
   onClose,
+  onToggleMaximize,
+  isMaximized = false,
   onError,
   onRightPanelOpen,
   onPaneArtifactsChange,
@@ -397,6 +408,27 @@ export function ChatPane({
         <span className={styles.title} title={headerLabel}>
           {headerLabel}
         </span>
+        {onToggleMaximize && (
+          <button
+            type="button"
+            className={styles.maximizeButton}
+            onClick={onToggleMaximize}
+            aria-pressed={isMaximized}
+            aria-label={t(
+              isMaximized ? 'splitView.restorePane' : 'splitView.maximizePane',
+            )}
+            title={t(
+              isMaximized ? 'splitView.restorePane' : 'splitView.maximizePane',
+            )}
+          >
+            {/* Same icon vocabulary as the dialog fullscreen toggle. */}
+            {isMaximized ? (
+              <Minimize2Icon size={16} aria-hidden />
+            ) : (
+              <Maximize2Icon size={16} aria-hidden />
+            )}
+          </button>
+        )}
         {onClose && (
           <button
             type="button"
diff --git a/packages/web-shell/client/components/SplitView.module.css b/packages/web-shell/client/components/SplitView.module.css
index 913971d7eb..901cb03b47 100644
--- a/packages/web-shell/client/components/SplitView.module.css
+++ b/packages/web-shell/client/components/SplitView.module.css
@@ -144,6 +144,12 @@
   min-height: 0;
 }
 
+/* While one pane is maximized, its siblings are hidden (but stay mounted, so
+   their sessions keep streaming). The lone visible slot then fills the row. */
+.paneSlot[data-pane-hidden] {
+  display: none;
+}
+
 .empty {
   flex: 1 1 auto;
   display: flex;
diff --git a/packages/web-shell/client/components/SplitView.test.tsx b/packages/web-shell/client/components/SplitView.test.tsx
index d7429947f8..89e016ffd0 100644
--- a/packages/web-shell/client/components/SplitView.test.tsx
+++ b/packages/web-shell/client/components/SplitView.test.tsx
@@ -66,8 +66,17 @@ vi.mock('./ChatPane', () => ({
     // Let a test force a render crash to exercise the per-pane ErrorBoundary.
     if (props.title === 'BOOM') throw new Error('pane exploded');
     return (
-      <div data-testid="chat-pane" data-pane-workspace={props.workspaceCwd}>
+      <div
+        data-testid="chat-pane"
+        data-pane-workspace={props.workspaceCwd}
+        data-maximized={props.isMaximized ? 'true' : 'false'}
+      >
         <span data-testid="pane-title">{props.title}</span>
+        {props.onToggleMaximize && (
+          <button data-testid="pane-maximize" onClick={props.onToggleMaximize}>
+            max
+          </button>
+        )}
         {props.onClose && (
           <button data-testid="pane-close" onClick={props.onClose}>
             x
@@ -372,6 +381,258 @@ describe('SplitView', () => {
     expect(titles()).toEqual(['Two']);
   });
 
+  function maximizeButtons(): HTMLElement[] {
+    return Array.from(
+      container!.querySelectorAll('[data-testid="pane-maximize"]'),
+    );
+  }
+  function hiddenSlots(): HTMLElement[] {
+    return Array.from(container!.querySelectorAll('[data-pane-hidden]'));
+  }
+
+  it('offers a maximize toggle only when more than one pane is open', () => {
+    // A lone pane already fills the split — nothing to maximize against.
+    render();
+    expect(maximizeButtons()).toHaveLength(0);
+    // Adding a second pane makes the toggle available on both.
+    openPicker();
+    const options = container!.querySelectorAll('[role="option"] button');
+    act(() =>
+      options[0].dispatchEvent(new MouseEvent('click', { bubbles: true })),
+    );
+    expect(maximizeButtons()).toHaveLength(2);
+  });
+
+  it('maximizing a pane hides the others but keeps them all mounted', () => {
+    render({ sessionIds: ['s1', 's2', 's3'] });
+    expect(panes()).toHaveLength(3);
+    expect(hiddenSlots()).toHaveLength(0);
+
+    act(() =>
+      maximizeButtons()[0].dispatchEvent(
+        new MouseEvent('click', { bubbles: true }),
+      ),
+    );
+    // All three panes stay mounted (their sessions keep streaming)…
+    expect(panes()).toHaveLength(3);
+    // …but the two non-maximized slots are hidden, leaving one visible.
+    expect(hiddenSlots()).toHaveLength(2);
+    // The maximized pane reflects its state down to ChatPane.
+    const maximized = container!.querySelector('[data-maximized="true"]');
+    expect(
+      maximized?.querySelector('[data-testid="pane-title"]')?.textContent,
+    ).toBe('One');
+  });
+
+  it('toggles back to the tiled layout when the maximized pane’s button is clicked again', () => {
+    render({ sessionIds: ['s1', 's2'] });
+    act(() =>
+      maximizeButtons()[0].dispatchEvent(
+        new MouseEvent('click', { bubbles: true }),
+      ),
+    );
+    expect(hiddenSlots()).toHaveLength(1);
+    // The still-mounted maximized pane's own toggle restores the split.
+    act(() =>
+      maximizeButtons()[0].dispatchEvent(
+        new MouseEvent('click', { bubbles: true }),
+      ),
+    );
+    expect(hiddenSlots()).toHaveLength(0);
+    expect(container!.querySelector('[data-maximized="true"]')).toBeNull();
+  });
+
+  it('restores the tiled layout on Escape', () => {
+    render({ sessionIds: ['s1', 's2'] });
+    act(() =>
+      maximizeButtons()[0].dispatchEvent(
+        new MouseEvent('click', { bubbles: true }),
+      ),
+    );
+    expect(hiddenSlots()).toHaveLength(1);
+    // A plain Escape (not aimed at the composer or picker) restores all panes.
+    act(() =>
+      document.dispatchEvent(
+        new KeyboardEvent('keydown', { key: 'Escape', bubbles: true }),
+      ),
+    );
+    expect(hiddenSlots()).toHaveLength(0);
+  });
+
+  it('keeps a pane maximized when Escape originates from an editable field', () => {
+    render({ sessionIds: ['s1', 's2'] });
+    act(() =>
+      maximizeButtons()[0].dispatchEvent(
+        new MouseEvent('click', { bubbles: true }),
+      ),
+    );
+    expect(hiddenSlots()).toHaveLength(1);
+    // Escape from the composer cancels the turn / closes its menus — it must not
+    // also un-maximize. An <input> stands in for the CodeMirror editor here.
+    const input = document.createElement('input');
+    container!.appendChild(input);
+    act(() =>
+      input.dispatchEvent(
+        new KeyboardEvent('keydown', { key: 'Escape', bubbles: true }),
+      ),
+    );
+    expect(hiddenSlots()).toHaveLength(1);
+    input.remove();
+  });
+
+  it('moves maximize to another pane when its toggle is clicked', () => {
+    render({ sessionIds: ['s1', 's2', 's3'] });
+    // Maximize s1.
+    act(() =>
+      maximizeButtons()[0].dispatchEvent(
+        new MouseEvent('click', { bubbles: true }),
+      ),
+    );
+    let maximized = container!.querySelector('[data-maximized="true"]');
+    expect(
+      maximized?.querySelector('[data-testid="pane-title"]')?.textContent,
+    ).toBe('One');
+    // Click s2's toggle while s1 is maximized — maximize MOVES to s2 (it does
+    // not restore to tiled). Guards the toggle's switch branch, which a
+    // "clear on any second click" regression would break.
+    act(() =>
+      maximizeButtons()[1].dispatchEvent(
+        new MouseEvent('click', { bubbles: true }),
+      ),
+    );
+    maximized = container!.querySelector('[data-maximized="true"]');
+    expect(
+      maximized?.querySelector('[data-testid="pane-title"]')?.textContent,
+    ).toBe('Two');
+    // Exactly one pane maximized, the other two hidden.
+    expect(container!.querySelectorAll('[data-maximized="true"]')).toHaveLength(
+      1,
+    );
+    expect(hiddenSlots()).toHaveLength(2);
+  });
+
+  it('closes the picker on Escape without un-maximizing', () => {
+    render({ sessionIds: ['s1', 's2'] });
+    act(() =>
+      maximizeButtons()[0].dispatchEvent(
+        new MouseEvent('click', { bubbles: true }),
+      ),
+    );
+    expect(hiddenSlots()).toHaveLength(1);
+    // Open the add-session picker, then press Escape: it closes the picker but
+    // must NOT also un-maximize — Escape defers to the open picker first.
+    openPicker();
+    expect(container!.querySelector('[role="listbox"]')).not.toBeNull();
+    act(() =>
+      document.dispatchEvent(
+        new KeyboardEvent('keydown', { key: 'Escape', bubbles: true }),
+      ),
+    );
+    expect(container!.querySelector('[role="listbox"]')).toBeNull();
+    expect(hiddenSlots()).toHaveLength(1);
+  });
+
+  it('drops maximize when the maximized pane is closed', () => {
+    // Uncontrolled so the close button removes the pane locally (a controlled
+    // split only reports removals up via onPanesChange).
+    render();
+    openPicker();
+    const options = container!.querySelectorAll('[role="option"] button');
+    act(() =>
+      options[0].dispatchEvent(new MouseEvent('click', { bubbles: true })),
+    );
+    expect(panes()).toHaveLength(2); // seed 'Three' + added 'One'
+    // Maximize the seed pane, then close it via its (visible) close button.
+    act(() =>
+      maximizeButtons()[0].dispatchEvent(
+        new MouseEvent('click', { bubbles: true }),
+      ),
+    );
+    expect(hiddenSlots()).toHaveLength(1);
+    const closes = container!.querySelectorAll('[data-testid="pane-close"]');
+    act(() =>
+      closes[0].dispatchEvent(new MouseEvent('click', { bubbles: true })),
+    );
+    // The lone survivor tiles normally — maximize was dropped with its pane.
+    expect(titles()).toEqual(['One']);
+    expect(hiddenSlots()).toHaveLength(0);
+  });
+
+  it('drops maximize when a controlled sync removes the maximized pane', () => {
+    render({ sessionIds: ['s1', 's2'] });
+    act(() =>
+      maximizeButtons()[0].dispatchEvent(
+        new MouseEvent('click', { bubbles: true }),
+      ),
+    );
+    expect(hiddenSlots()).toHaveLength(1);
+    // The parent drops the maximized session (s1) from the split…
+    act(() =>
+      root!.render(
+        <I18nProvider language="en">
+          <SplitView onExit={() => {}} sessionIds={['s2']} />
+        </I18nProvider>,
+      ),
+    );
+    // …so nothing stays maximized — the lone survivor tiles normally.
+    expect(titles()).toEqual(['Two']);
+    expect(hiddenSlots()).toHaveLength(0);
+  });
+
+  it('clears a stale maximize when a controlled split shrinks to one pane then regrows', () => {
+    const rerender = (ids: string[]) =>
+      act(() =>
+        root!.render(
+          <I18nProvider language="en">
+            <SplitView onExit={() => {}} sessionIds={ids} />
+          </I18nProvider>,
+        ),
+      );
+    render({ sessionIds: ['s1', 's2'] });
+    act(() =>
+      maximizeButtons()[0].dispatchEvent(
+        new MouseEvent('click', { bubbles: true }),
+      ),
+    );
+    expect(hiddenSlots()).toHaveLength(1); // s1 maximized, s2 hidden
+    // The parent drops the *other* pane, leaving the maximized one alone — the
+    // stale maximize must clear (it can't hold against a single pane)…
+    rerender(['s1']);
+    expect(titles()).toEqual(['One']);
+    expect(hiddenSlots()).toHaveLength(0);
+    // …so re-adding a pane shows a clean tiled split, not a silently re-hidden
+    // one from the earlier maximize.
+    rerender(['s1', 's2']);
+    expect(titles()).toEqual(['One', 'Two']);
+    expect(hiddenSlots()).toHaveLength(0);
+  });
+
+  it('reveals a newly added pane by exiting maximize', () => {
+    // Uncontrolled so the picker mounts the new pane locally.
+    render();
+    openPicker();
+    let options = container!.querySelectorAll('[role="option"] button');
+    act(() =>
+      options[0].dispatchEvent(new MouseEvent('click', { bubbles: true })),
+    );
+    expect(panes()).toHaveLength(2);
+    act(() =>
+      maximizeButtons()[0].dispatchEvent(
+        new MouseEvent('click', { bubbles: true }),
+      ),
+    );
+    expect(hiddenSlots()).toHaveLength(1);
+    // Adding another session drops the maximize so the new pane isn't hidden
+    // behind a still-maximized one.
+    openPicker();
+    options = container!.querySelectorAll('[role="option"] button');
+    act(() =>
+      options[0].dispatchEvent(new MouseEvent('click', { bubbles: true })),
+    );
+    expect(panes()).toHaveLength(3);
+    expect(hiddenSlots()).toHaveLength(0);
+  });
+
   it('reloads the session list when the picker opens (never a stale list)', () => {
     render({ sessionIds: ['s1'] });
     // `useSessions` only fetches on mount; nothing reloads until the user acts.
diff --git a/packages/web-shell/client/components/SplitView.tsx b/packages/web-shell/client/components/SplitView.tsx
index 3d697bfb67..486eef08b9 100644
--- a/packages/web-shell/client/components/SplitView.tsx
+++ b/packages/web-shell/client/components/SplitView.tsx
@@ -31,6 +31,7 @@ import {
   mergeSessionsById,
   workspaceBasename,
 } from '../utils/workspace';
+import { isEditableTarget } from '../utils/dom';
 import styles from './SplitView.module.css';
 
 const MAX_PANES = MAX_SPLIT_PANES;
@@ -133,6 +134,10 @@ export function SplitView({
     return currentSessionId ? [currentSessionId] : [];
   });
   const [pickerOpen, setPickerOpen] = useState(false);
+  // Which pane, if any, is maximized to fill the whole split. Purely visual and
+  // ephemeral (not deep-linked via `?split=`, like the dialog fullscreen toggle
+  // it mirrors): the other panes stay mounted and streaming, just hidden.
+  const [maximizedPaneId, setMaximizedPaneId] = useState<string | null>(null);
   const addWrapRef = useRef<HTMLDivElement | null>(null);
   // A per-tab/per-mount nonce: two browser tabs opening the same split must not
   // register the same daemon client id, or suppressOwnUserEcho would treat one
@@ -249,6 +254,9 @@ export function SplitView({
         return;
       }
       const next = [...currentPaneIds, sessionId];
+      // Reveal the freshly added pane rather than leaving it hidden behind a
+      // still-maximized one.
+      setMaximizedPaneId(null);
       if (sessionIdsControlled) {
         onPanesChange?.(next);
       } else {
@@ -292,11 +300,48 @@ export function SplitView({
     [onPanesChange, sessionIdsControlled],
   );
 
+  const toggleMaximize = useCallback((sessionId: string) => {
+    setMaximizedPaneId((current) => (current === sessionId ? null : sessionId));
+  }, []);
+
+  // Maximize only makes sense against another pane, so drop it whenever it no
+  // longer can hold: the maximized pane left the set (closed here, or removed by
+  // a controlled-mode sync), or the split shrank to a lone pane. Without the
+  // length guard a surviving maximized pane would keep a stale `maximizedPaneId`
+  // that silently re-hides the next pane a controlled parent adds back.
+  useEffect(() => {
+    if (
+      maximizedPaneId &&
+      (paneIds.length < 2 || !paneIds.includes(maximizedPaneId))
+    ) {
+      setMaximizedPaneId(null);
+    }
+  }, [paneIds, maximizedPaneId]);
+
+  // Escape restores the tiled layout, but only when the key is otherwise unused:
+  // defer to an open picker (its own Escape closes it first), and never steal
+  // Escape from the composer — it cancels the in-flight turn / closes its menus —
+  // or from an open dialog. `isEditableTarget` covers `.cm-editor` and dialog
+  // keyboard scopes, so a maximized pane's composer keeps its Escape.
+  useEffect(() => {
+    if (!maximizedPaneId) return;
+    const onKeyDown = (event: KeyboardEvent) => {
+      if (event.key !== 'Escape' || event.defaultPrevented) return;
+      if (pickerOpen || isEditableTarget(event.target)) return;
+      setMaximizedPaneId(null);
+    };
+    document.addEventListener('keydown', onKeyDown);
+    return () => document.removeEventListener('keydown', onKeyDown);
+  }, [maximizedPaneId, pickerOpen]);
+
   const available = useMemo(
     () => allSessions.filter((session) => !paneIds.includes(session.sessionId)),
     [allSessions, paneIds],
   );
   const canAdd = paneIds.length < MAX_PANES && available.length > 0;
+  // Only offer per-pane maximize once there's another pane to maximize against —
+  // a lone pane already fills the split.
+  const canMaximize = paneIds.length > 1;
 
   return (
     <div className={styles.split} data-testid="split-view">
@@ -374,9 +419,14 @@ export function SplitView({
         ) : (
           paneIds.map((sessionId) => {
             const paneWorkspaceCwd = workspaceCwdById.get(sessionId);
+            const isMaximized = maximizedPaneId === sessionId;
+            // When one pane is maximized, the rest stay mounted (their sessions
+            // keep streaming) but are hidden via CSS — a purely visual solo.
+            const isHidden = maximizedPaneId !== null && !isMaximized;
             return (
               <div
                 className={styles.paneSlot}
+                data-pane-hidden={isHidden ? '' : undefined}
                 // Include the resolved workspace in the key on a multi-workspace
                 // daemon so a pane whose workspace resolves only after mount (e.g.
                 // a `?split=` deep link) remounts under the right workspace rather
@@ -433,6 +483,12 @@ export function SplitView({
                       title={titleById.get(sessionId)}
                       workspaceCwd={paneWorkspaceCwd}
                       onClose={() => removePane(sessionId)}
+                      onToggleMaximize={
+                        canMaximize
+                          ? () => toggleMaximize(sessionId)
+                          : undefined
+                      }
+                      isMaximized={isMaximized}
                       onError={onError}
                       onRightPanelOpen={onRightPanelOpen}
                       onPaneArtifactsChange={onPaneArtifactsChange}
diff --git a/packages/web-shell/client/i18n.tsx b/packages/web-shell/client/i18n.tsx
index 64e9902d8e..17bbac730b 100644
--- a/packages/web-shell/client/i18n.tsx
+++ b/packages/web-shell/client/i18n.tsx
@@ -1876,6 +1876,8 @@ const EN: Messages = {
   'splitView.count': (v) => `${v?.count ?? 0} panes`,
   'splitView.addPane': 'Add session',
   'splitView.closePane': 'Close pane',
+  'splitView.maximizePane': 'Maximize pane',
+  'splitView.restorePane': 'Restore pane',
   'splitView.paneError': 'This session pane hit an error',
   'splitView.paneConnectionError': 'Connection lost',
   'splitView.outerApprovalPending':
@@ -3746,6 +3748,8 @@ const ZH: Messages = {
   'splitView.count': (v) => `${v?.count ?? 0} 个窗格`,
   'splitView.addPane': '添加会话',
   'splitView.closePane': '关闭窗格',
+  'splitView.maximizePane': '最大化窗格',
+  'splitView.restorePane': '还原窗格',
   'splitView.paneError': '此会话窗格出错',
   'splitView.paneConnectionError': '连接已断开',
   'splitView.outerApprovalPending': '主会话正在等待审批。',