diff --git a/src/web/public/app.js b/src/web/public/app.js
index 8acdae7e..bcb2cf99 100644
--- a/src/web/public/app.js
+++ b/src/web/public/app.js
@@ -7587,7 +7587,9 @@ class CodemanApp {
     }
   }
 
-  _cleanupPreviousSession(newSessionId) {
+  // `skipSnapshot`: the tile grid opening on the session it parks
+  // (tile-grid.js openTileGrid), whose snapshot closing the grid discards.
+  _cleanupPreviousSession(newSessionId, { skipSnapshot = false } = {}) {
     // Snapshot the OUTGOING session's xterm rendered state (viewport + scrollback +
     // colors/attrs) before the terminal gets cleared/reset. Lets us restore the
     // exact view on switch-back rather than replaying codex's byte stream, which
@@ -7598,6 +7600,7 @@ class CodemanApp {
     // quota for them. Unknown/undefined mode still snapshots, matching restore.
     const outgoingSession = this.activeSessionId ? this.sessions?.get?.(this.activeSessionId) : null;
     if (
+      !skipSnapshot &&
       this.activeSessionId &&
       outgoingSession?.mode !== 'shell' &&
       this._serializeAddon &&
diff --git a/src/web/public/tile-grid.js b/src/web/public/tile-grid.js
index c8c04f85..84bfc7c8 100644
--- a/src/web/public/tile-grid.js
+++ b/src/web/public/tile-grid.js
@@ -202,7 +202,11 @@ Object.assign(CodemanApp.prototype, {
       return true;
     }
 
-    this._cleanupPreviousSession(focus);
+    // No snapshot of a session that becomes a tile: closing the grid drops the
+    // main terminal's snapshot of every tiled id (stale by then), so taking one
+    // here (a 1000-line serialize and up to 256 KB of localStorage for a
+    // non-shell session) only ever produced a copy that was thrown away.
+    this._cleanupPreviousSession(focus, { skipSnapshot: wanted.includes(this.activeSessionId) });
     grid.open = true;
     grid.ids = [];
     grid.focusedId = focus;
diff --git a/test/tile-grid-park-guards.test.ts b/test/tile-grid-park-guards.test.ts
index d7a21b65..f9565a7f 100644
--- a/test/tile-grid-park-guards.test.ts
+++ b/test/tile-grid-park-guards.test.ts
@@ -82,6 +82,35 @@ beforeEach(() => {
 });
 
 describe('parking the main terminal', () => {
+  it('opening takes no snapshot of a session that becomes a tile, and keeps it for one that does not', () => {
+    // Closing the grid drops the snapshot of every tiled id, so one taken of a
+    // tiled session at opening was always thrown away.
+    const tiled = makeApp();
+    tiled.openTileGrid(IDS);
+    expect(tiled._cleanupPreviousSession).toHaveBeenCalledWith('s-a', { skipSnapshot: true });
+
+    resetGridHarness();
+    const elsewhere = makeApp(); // s-a is active, the grid opens on the others
+    elsewhere.openTileGrid(['s-b', 's-c']);
+    expect(elsewhere._cleanupPreviousSession).toHaveBeenCalledWith('s-b', { skipSnapshot: false });
+  });
+
+  it('_cleanupPreviousSession skips the snapshot only when asked', () => {
+    const app = makeApp();
+    delete app._cleanupPreviousSession; // the real one
+    app._disconnectWs = vi.fn();
+    const serialize = vi.fn(() => 'snapshot of s-a\r\n'.repeat(4));
+    app._serializeAddon = { serialize };
+    app._isUsableXtermSnapshot = () => true;
+    app._persistXtermSnapshot = vi.fn();
+    app._cleanupPreviousSession('s-b', { skipSnapshot: true });
+    expect(serialize).not.toHaveBeenCalled();
+    expect(app._xtermSnapshots.has('s-a')).toBe(false);
+    app._cleanupPreviousSession('s-b');
+    expect(serialize).toHaveBeenCalledTimes(1);
+    expect(app._xtermSnapshots.has('s-a')).toBe(true);
+  });
+
   it('opening parks it ONCE and puts a tile per session in its place, focused tile active', () => {
     const app = makeApp();
     expect(app.openTileGrid(IDS, { focusedId: 's-b' })).toBe(true);