diff --git a/packages/ui/src/App.tsx b/packages/ui/src/App.tsx
index 9159a9e0..7ec3db74 100644
--- a/packages/ui/src/App.tsx
+++ b/packages/ui/src/App.tsx
@@ -54,6 +54,7 @@ import { MCP_OAUTH_CALLBACK_PATH } from '@/components/sections/mcp/mcpOAuth';
 import { lazyWithChunkRecovery } from '@/lib/chunkLoadRecovery';
 import { useI18n } from '@/lib/i18n';
 import { applyMobileKeyboardMode } from '@/lib/mobileKeyboardMode';
+import { isEmbeddedSessionChat } from '@/components/layout/contextPanelEmbeddedChat';
 import { SyncAppEffects } from '@/apps/AppEffects';
 import { resetAppForRuntimeEndpointChange } from '@/apps/runtimeEndpointReset';
 import { useAppFontEffects } from '@/apps/useAppFontEffects';
@@ -117,15 +118,11 @@ const normalizeEmbeddedDirectory = (value: string | null | undefined): string =>
 };
 
 const readEmbeddedSessionChatConfig = (): EmbeddedSessionChatConfig | null => {
-  if (typeof window === 'undefined') {
+  if (typeof window === 'undefined' || !isEmbeddedSessionChat()) {
     return null;
   }
 
   const params = new URLSearchParams(window.location.search);
-  if (params.get('ocPanel') !== 'session-chat') {
-    return null;
-  }
-
   const sessionIdRaw = params.get('sessionId');
   const sessionId = typeof sessionIdRaw === 'string' ? sessionIdRaw.trim() : '';
   if (!sessionId) {
@@ -171,7 +168,12 @@ const EmbeddedSessionChatContent: React.FC<{
     if (expectedDirectory && activeDirectory !== expectedDirectory) return;
 
     const bootstrapKey = `${expectedDirectory}\n${embeddedSessionChat.sessionId}`;
-    if (bootstrapKeyRef.current === bootstrapKey && currentSessionId === embeddedSessionChat.sessionId) {
+    // Skip if this session was already bootstrapped and a session is still
+    // active — allows in-place navigation (e.g. "Open subtask") to change
+    // currentSessionId without this effect forcing it back. Only re-bootstrap
+    // when currentSessionId was cleared (store init, draft, delete/archive,
+    // runtime-switch remount).
+    if (bootstrapKeyRef.current === bootstrapKey && currentSessionId) {
       return;
     }
 
diff --git a/packages/ui/src/components/chat/ChatContainer.tsx b/packages/ui/src/components/chat/ChatContainer.tsx
index 6ab7e364..b8babe28 100644
--- a/packages/ui/src/components/chat/ChatContainer.tsx
+++ b/packages/ui/src/components/chat/ChatContainer.tsx
@@ -49,6 +49,7 @@ import { usePlanDetection } from '@/hooks/usePlanDetection';
 import { useI18n } from '@/lib/i18n';
 import { isMobileSurfaceRuntime } from '@/lib/runtimeSurface';
 import { isVSCodeRuntime } from '@/lib/desktop';
+import { getEmbeddedSessionChatOriginSessionId } from '@/components/layout/contextPanelEmbeddedChat';
 
 const EMPTY_MESSAGES: Array<{ info: Message; parts: Part[] }> = [];
 const IDLE_SESSION_STATUS = { type: 'idle' as const };
@@ -564,13 +565,22 @@ export const ChatContainer: React.FC<ChatContainerProps> = ({ autoOpenDraft = tr
 
     const parentSession = useParentSession(currentSessionId, effectiveSessionDirectory);
 
+    // In the embedded session-chat iframe, hide "Return to parent" when
+    // viewing the panel's anchor session (the one recorded in the URL). Going
+    // up from the anchor would show the primary session that's already in the
+    // main chat. Drilling into a deeper subtask (currentSessionId ≠ anchor)
+    // re-enables the button to navigate back to the embedded session.
+    const embeddedPanelAnchorSessionId = getEmbeddedSessionChatOriginSessionId();
+    const hideReturnToParent =
+        embeddedPanelAnchorSessionId !== null && currentSessionId === embeddedPanelAnchorSessionId;
+
     const handleReturnToParentSession = React.useCallback(() => {
         if (!parentSession) return;
         const parentDirectory = (parentSession as Session & { directory?: string | null }).directory ?? null;
         setCurrentSession(parentSession.id, parentDirectory);
     }, [parentSession, setCurrentSession]);
 
-    const returnToParentButton = parentSession ? (
+    const returnToParentButton = parentSession && !hideReturnToParent ? (
         <Button
             type="button"
             variant="outline"
diff --git a/packages/ui/src/components/chat/message/MessageBody.tsx b/packages/ui/src/components/chat/message/MessageBody.tsx
index 8bf31eee..1bc16a6f 100644
--- a/packages/ui/src/components/chat/message/MessageBody.tsx
+++ b/packages/ui/src/components/chat/message/MessageBody.tsx
@@ -53,6 +53,7 @@ import {
     sendImplementationResponseToReviewer,
     sendReviewFeedbackToOriginal,
 } from '@/lib/reviewFlow';
+import { isEmbeddedSessionChat } from '@/components/layout/contextPanelEmbeddedChat';
 
 
 const CONTAIN_LAYOUT_STYLE = { contain: 'layout' as const, transform: 'translateZ(0)' };
@@ -252,7 +253,11 @@ const UserSubtaskPart: React.FC<{ part: SubtaskPartLike }> = ({ part }) => {
                         className="typography-meta text-muted-foreground hover:text-foreground transition-colors underline underline-offset-2"
                         onClick={() => {
                             if (!effectiveDirectory) return;
-                            if (isMobile || isVSCodeRuntime()) {
+                            // In contexts with no ContextPanel (embedded
+                            // session-chat iframe) or single-surface layouts
+                            // (mobile, VS Code), navigate in place. Otherwise
+                            // open a new side-panel tab.
+                            if (isEmbeddedSessionChat() || isMobile || isVSCodeRuntime()) {
                                 setCurrentSession(taskSessionID, effectiveDirectory);
                                 return;
                             }
diff --git a/packages/ui/src/components/chat/message/parts/ToolPart.tsx b/packages/ui/src/components/chat/message/parts/ToolPart.tsx
index 0753d58b..dae16e3b 100644
--- a/packages/ui/src/components/chat/message/parts/ToolPart.tsx
+++ b/packages/ui/src/components/chat/message/parts/ToolPart.tsx
@@ -47,6 +47,7 @@ import { readTaskTagSessionIdFromOutput } from './taskSessionIdParser';
 import { areRenderRelevantPartsEqual } from '../renderCompare';
 import { useI18n } from '@/lib/i18n';
 import { getDiffPatchEntries, getPatchText, type DiffPatchEntry } from './toolDiffUtils';
+import { isEmbeddedSessionChat } from '@/components/layout/contextPanelEmbeddedChat';
 
 const TOOL_ROW_TEXT_CLASS = '!text-[length:var(--text-meta)] !leading-5 sm:!leading-6 tracking-normal';
 const TOOL_ROW_TITLE_CLASS = cn('typography-meta font-medium', TOOL_ROW_TEXT_CLASS);
@@ -1374,7 +1375,10 @@ const TaskToolSummary: React.FC<{
     const handleOpenSession = (event: React.MouseEvent) => {
         event.stopPropagation();
         if (sessionId && currentDirectory) {
-            if (isMobile || runtime?.runtime.isVSCode) {
+            // In contexts with no ContextPanel (embedded session-chat iframe)
+            // or single-surface layouts (mobile, VS Code), navigate in place.
+            // Otherwise open a new side-panel tab.
+            if (isEmbeddedSessionChat() || isMobile || runtime?.runtime.isVSCode) {
                 setCurrentSession(sessionId, currentDirectory);
                 return;
             }
diff --git a/packages/ui/src/components/layout/contextPanelEmbeddedChat.test.ts b/packages/ui/src/components/layout/contextPanelEmbeddedChat.test.ts
index fafe4474..e8304903 100644
--- a/packages/ui/src/components/layout/contextPanelEmbeddedChat.test.ts
+++ b/packages/ui/src/components/layout/contextPanelEmbeddedChat.test.ts
@@ -1,7 +1,14 @@
 import { afterAll, beforeEach, describe, expect, test } from 'bun:test';
 import { getDefaultTheme } from '@/lib/theme/themes';
 import type { Theme } from '@/types/theme';
-import { buildEmbeddedSessionChatURL, getOrCreateEmbeddedSessionChatURL, type EmbeddedSessionChatURLCacheEntry } from './contextPanelEmbeddedChat';
+import {
+  buildEmbeddedSessionChatURL,
+  getOrCreateEmbeddedSessionChatURL,
+  getEmbeddedSessionChatOriginSessionId,
+  isEmbeddedSessionChat,
+  resetEmbeddedSessionChatCache,
+  type EmbeddedSessionChatURLCacheEntry,
+} from './contextPanelEmbeddedChat';
 
 const originalWindow = globalThis.window;
 
@@ -32,6 +39,7 @@ const makeTheme = (id: string, variant: 'light' | 'dark'): Theme => ({
 
 beforeEach(() => {
   installWindowLocation();
+  resetEmbeddedSessionChatCache();
 });
 
 afterAll(() => {
@@ -99,3 +107,69 @@ describe('embedded session chat URL', () => {
     expect(new URL(readOnly).searchParams.get('readOnly')).toBe('1');
   });
 });
+
+describe('isEmbeddedSessionChat', () => {
+  test('is true only for the session-chat panel search param', () => {
+    installWindowLocation('http://127.0.0.1:5173/app?ocPanel=session-chat&sessionId=ses_1');
+    resetEmbeddedSessionChatCache();
+    expect(isEmbeddedSessionChat()).toBe(true);
+
+    installWindowLocation('http://127.0.0.1:5173/app?sessionId=ses_1');
+    resetEmbeddedSessionChatCache();
+    expect(isEmbeddedSessionChat()).toBe(false);
+
+    installWindowLocation('http://127.0.0.1:5173/app');
+    resetEmbeddedSessionChatCache();
+    expect(isEmbeddedSessionChat()).toBe(false);
+  });
+
+  test('caches the first result so URL rewrites cannot flip it (mirrors VS Code stable global)', () => {
+    // VS Code detects its webview via the stable `window.__VSCODE_CONFIG__`
+    // global — it never changes. The embedded iframe's identity is equally
+    // fixed at mount (the parent builds the src); caching the first read
+    // makes detection just as stable, surviving any URL rewrite.
+    installWindowLocation('http://127.0.0.1:5173/app?ocPanel=session-chat&sessionId=ses_1');
+    resetEmbeddedSessionChatCache();
+
+    // First read: caches true.
+    expect(isEmbeddedSessionChat()).toBe(true);
+
+    // Even if the URL were rewritten, the cached value stays true.
+    installWindowLocation('http://127.0.0.1:5173/app?session=ses_grandchild');
+    expect(isEmbeddedSessionChat()).toBe(true);
+
+    // Still true after another rewrite.
+    installWindowLocation('http://127.0.0.1:5173/app');
+    expect(isEmbeddedSessionChat()).toBe(true);
+  });
+});
+
+describe('getEmbeddedSessionChatOriginSessionId', () => {
+  test('returns the URL sessionId when embedded', () => {
+    installWindowLocation('http://127.0.0.1:5173/app?ocPanel=session-chat&sessionId=ses_child&directory=%2Frepo');
+    resetEmbeddedSessionChatCache();
+    expect(getEmbeddedSessionChatOriginSessionId()).toBe('ses_child');
+  });
+
+  test('returns null outside the embedded iframe', () => {
+    installWindowLocation('http://127.0.0.1:5173/app?session=ses_main');
+    resetEmbeddedSessionChatCache();
+    expect(getEmbeddedSessionChatOriginSessionId()).toBeNull();
+
+    installWindowLocation('http://127.0.0.1:5173/app? sessionId=ses_orphan');
+    resetEmbeddedSessionChatCache();
+    expect(getEmbeddedSessionChatOriginSessionId()).toBeNull();
+  });
+
+  test('returns null when embedded URL is missing sessionId param', () => {
+    installWindowLocation('http://127.0.0.1:5173/app?ocPanel=session-chat&directory=%2Frepo');
+    resetEmbeddedSessionChatCache();
+    expect(getEmbeddedSessionChatOriginSessionId()).toBeNull();
+  });
+
+  test('trims whitespace', () => {
+    installWindowLocation('http://127.0.0.1:5173/app?ocPanel=session-chat&sessionId=%20%20ses_child%20%20');
+    resetEmbeddedSessionChatCache();
+    expect(getEmbeddedSessionChatOriginSessionId()).toBe('ses_child');
+  });
+});
diff --git a/packages/ui/src/components/layout/contextPanelEmbeddedChat.ts b/packages/ui/src/components/layout/contextPanelEmbeddedChat.ts
index ad7b4118..34210e79 100644
--- a/packages/ui/src/components/layout/contextPanelEmbeddedChat.ts
+++ b/packages/ui/src/components/layout/contextPanelEmbeddedChat.ts
@@ -70,3 +70,60 @@ export const getOrCreateEmbeddedSessionChatURL = (
   cache.set(tabID, { signature, src });
   return src;
 };
+
+/**
+ * True when the current document is the embedded session-chat iframe
+ * (`?ocPanel=session-chat`). Used to distinguish the embedded iframe from
+ * the main app so callers can route behavior accordingly (e.g. in-place
+ * subtask navigation instead of opening a new side-panel tab, or skipping
+ * URL rewrites that would strip the iframe's identity params).
+ *
+ * Cached on first call (per JS realm): an iframe's embedded-ness is fixed
+ * at mount by the parent and cannot change during its lifetime — a parent
+ * src swap is a full browser reload, starting a fresh realm.
+ */
+let embeddedSessionChatCached: boolean | null = null;
+
+export const isEmbeddedSessionChat = (): boolean => {
+  if (embeddedSessionChatCached !== null) {
+    return embeddedSessionChatCached;
+  }
+  if (typeof window === 'undefined') {
+    embeddedSessionChatCached = false;
+    return false;
+  }
+  try {
+    embeddedSessionChatCached =
+      new URLSearchParams(window.location.search).get('ocPanel') === 'session-chat';
+    return embeddedSessionChatCached;
+  } catch {
+    embeddedSessionChatCached = false;
+    return false;
+  }
+};
+
+/**
+ * Reset the module-level cache. Intended for tests that simulate different
+ * JS realms by swapping `window.location` in the same process.
+ */
+export const resetEmbeddedSessionChatCache = (): void => {
+  embeddedSessionChatCached = null;
+};
+
+/**
+ * The session ID recorded in the embedded iframe's URL
+ * (`?ocPanel=session-chat&sessionId=…`), i.e. the session the panel was
+ * opened to show. Returns `null` outside the embedded iframe or when the
+ * URL is malformed.
+ */
+export const getEmbeddedSessionChatOriginSessionId = (): string | null => {
+  if (!isEmbeddedSessionChat()) {
+    return null;
+  }
+  try {
+    const sid = new URLSearchParams(window.location.search).get('sessionId');
+    return sid && sid.trim().length > 0 ? sid.trim() : null;
+  } catch {
+    return null;
+  }
+};
diff --git a/packages/ui/src/contexts/theme-embedded-bootstrap.ts b/packages/ui/src/contexts/theme-embedded-bootstrap.ts
index 430dbe5c..b6427fc1 100644
--- a/packages/ui/src/contexts/theme-embedded-bootstrap.ts
+++ b/packages/ui/src/contexts/theme-embedded-bootstrap.ts
@@ -1,9 +1,10 @@
+import { isEmbeddedSessionChat } from '@/components/layout/contextPanelEmbeddedChat';
+
 export const readEmbeddedThemeSearchParams = (): URLSearchParams | null => {
-  if (typeof window === 'undefined') {
+  if (!isEmbeddedSessionChat()) {
     return null;
   }
-  const params = new URLSearchParams(window.location.search);
-  return params.get('ocPanel') === 'session-chat' ? params : null;
+  return new URLSearchParams(window.location.search);
 };
 
 const getSystemPreference = (): boolean => {
diff --git a/packages/ui/src/hooks/useRouter.ts b/packages/ui/src/hooks/useRouter.ts
index 5b39594e..fa794ad5 100644
--- a/packages/ui/src/hooks/useRouter.ts
+++ b/packages/ui/src/hooks/useRouter.ts
@@ -5,6 +5,7 @@ import { parseRoute, updateBrowserURL, hasRouteParams } from '@/lib/router';
 import type { RouteState, AppRouteState } from '@/lib/router';
 import type { MainTab } from '@/stores/useUIStore';
 import { resolveSettingsSlug } from '@/lib/settings/metadata';
+import { isEmbeddedSessionChat } from '@/components/layout/contextPanelEmbeddedChat';
 
 /**
  * Check if running in VS Code webview context.
@@ -29,9 +30,18 @@ function isVSCodeContext(): boolean {
  * - Web: Full bidirectional sync
  * - Desktop: Full bidirectional sync
  * - VS Code: State-only (no URL updates, reads initial params)
+ * - Embedded session-chat iframe (`?ocPanel=session-chat`): No URL updates.
+ *   The iframe's session identity is fixed at mount (the parent builds the
+ *   src with `sessionId`); in-place subtask navigation must NOT rewrite the
+ *   URL, otherwise `ocPanel` (and `directory`/`readOnly`) get stripped and
+ *   `isEmbeddedSessionChat()` starts returning false, breaking subsequent
+ *   "Open subtask" clicks.
  */
 export function useRouter(): void {
   const isVSCode = React.useMemo(() => isVSCodeContext(), []);
+  // Captured once at mount: the iframe's embedded-ness never changes during
+  // its lifetime (a parent src swap is a full reload).
+  const isEmbeddedChat = React.useMemo(() => isEmbeddedSessionChat(), []);
 
   // Track initialization to avoid duplicate applies
   const initializedRef = React.useRef(false);
@@ -115,14 +125,14 @@ export function useRouter(): void {
    */
   const syncURLFromState = React.useCallback(
     (options: { replace?: boolean } = {}) => {
-      if (isVSCode || isApplyingRouteRef.current) {
+      if (isVSCode || isEmbeddedChat || isApplyingRouteRef.current) {
         return;
       }
 
       const state = getCurrentAppState();
       updateBrowserURL(state, options);
     },
-    [isVSCode, getCurrentAppState]
+    [isVSCode, isEmbeddedChat, getCurrentAppState]
   );
 
   // Initialize: parse URL and apply route on mount
@@ -148,7 +158,7 @@ export function useRouter(): void {
       // Use the parsed route values instead of an immediate store snapshot so
       // deep links do not briefly normalize `?session=...` back to `/` while
       // the session's directory/message bootstrap is still catching up.
-      if (!isVSCode) {
+      if (!isVSCode && !isEmbeddedChat) {
         updateBrowserURL({
           ...getCurrentAppState(),
           sessionId: route.sessionId ?? useSessionUIStore.getState().currentSessionId,
@@ -160,11 +170,11 @@ export function useRouter(): void {
     };
 
     void initializeRoute();
-  }, [applyRoute, getCurrentAppState, isVSCode]);
+  }, [applyRoute, getCurrentAppState, isVSCode, isEmbeddedChat]);
 
   // Subscribe to session changes
   React.useEffect(() => {
-    if (isVSCode) {
+    if (isVSCode || isEmbeddedChat) {
       return;
     }
 
@@ -183,11 +193,11 @@ export function useRouter(): void {
     });
 
     return unsubscribe;
-  }, [isVSCode, syncURLFromState]);
+  }, [isVSCode, isEmbeddedChat, syncURLFromState]);
 
   // Subscribe to UI store changes (tab, settings)
   React.useEffect(() => {
-    if (isVSCode) {
+    if (isVSCode || isEmbeddedChat) {
       return;
     }
 
@@ -220,11 +230,11 @@ export function useRouter(): void {
     });
 
     return unsubscribe;
-  }, [isVSCode, syncURLFromState]);
+  }, [isVSCode, isEmbeddedChat, syncURLFromState]);
 
   // Listen for browser back/forward navigation
   React.useEffect(() => {
-    if (typeof window === 'undefined' || isVSCode) {
+    if (typeof window === 'undefined' || isVSCode || isEmbeddedChat) {
       return;
     }
 
@@ -254,5 +264,5 @@ export function useRouter(): void {
     return () => {
       window.removeEventListener('popstate', handlePopState);
     };
-  }, [applyRoute, isVSCode, setActiveMainTab, setSettingsDialogOpen]);
+  }, [applyRoute, isVSCode, isEmbeddedChat, setActiveMainTab, setSettingsDialogOpen]);
 }
diff --git a/packages/ui/src/lib/router/serializeRoute.test.ts b/packages/ui/src/lib/router/serializeRoute.test.ts
new file mode 100644
index 00000000..bfb98ff6
--- /dev/null
+++ b/packages/ui/src/lib/router/serializeRoute.test.ts
@@ -0,0 +1,123 @@
+import { afterAll, afterEach, beforeEach, describe, expect, test } from 'bun:test';
+import { updateBrowserURL } from './serializeRoute';
+import type { AppRouteState } from './serializeRoute';
+import { isEmbeddedSessionChat, resetEmbeddedSessionChatCache } from '@/components/layout/contextPanelEmbeddedChat';
+
+const originalWindow = globalThis.window;
+
+type HistoryStub = {
+  state: unknown;
+  lastURL: string | null;
+  replaceState(state: unknown, _title: string, url?: string): void;
+  pushState(state: unknown, _title: string, url?: string): void;
+};
+
+const installWindow = (href: string): HistoryStub => {
+  const url = new URL(href);
+  const history: HistoryStub = {
+    state: null,
+    lastURL: null,
+    replaceState(state, _title, url) {
+      this.state = state;
+      this.lastURL = url ?? null;
+    },
+    pushState(state, _title, url) {
+      this.state = state;
+      this.lastURL = url ?? null;
+    },
+  };
+  Object.defineProperty(globalThis, 'window', {
+    configurable: true,
+    value: {
+      location: {
+        href: url.toString(),
+        origin: url.origin,
+        pathname: url.pathname,
+        search: url.search,
+      },
+      history,
+    },
+  });
+  return history;
+};
+
+const historyOf = (): HistoryStub =>
+  (globalThis.window as unknown as { history: HistoryStub }).history;
+
+beforeEach(() => {
+  installWindow('http://127.0.0.1:5173/app');
+  resetEmbeddedSessionChatCache();
+});
+
+afterEach(() => {
+  Object.defineProperty(globalThis, 'window', {
+    configurable: true,
+    value: originalWindow,
+  });
+});
+
+afterAll(() => {
+  Object.defineProperty(globalThis, 'window', {
+    configurable: true,
+    value: originalWindow,
+  });
+});
+
+const sessionState = (sessionId: string): AppRouteState => ({
+  sessionId,
+  tab: 'chat',
+  isSettingsOpen: false,
+  settingsPath: '',
+  diffFile: null,
+});
+
+describe('updateBrowserURL embedded-session-chat guard', () => {
+  test('is a no-op in the embedded session-chat iframe (mirrors isVSCodeContext)', () => {
+    // The embedded iframe's URL identity (ocPanel/sessionId/directory/
+    // readOnly) must never be rewritten. updateBrowserURL rebuilds the
+    // query string from scratch using only session/tab/settings/file —
+    // which would strip ocPanel and break isEmbeddedSessionChat().
+    // The guard prevents this, exactly like isVSCodeContext() does for
+    // VS Code webviews.
+    const history = installWindow(
+      'http://127.0.0.1:5173/app?ocPanel=session-chat&sessionId=ses_child&directory=%2Frepo&readOnly=1',
+    );
+
+    updateBrowserURL(sessionState('ses_grandchild'), { replace: true, force: true });
+
+    // No URL update happened — history was never touched.
+    expect(history.lastURL).toBeNull();
+  });
+
+  test('rewrites the URL normally outside the embedded iframe', () => {
+    installWindow('http://127.0.0.1:5173/app');
+
+    updateBrowserURL(sessionState('ses_main'), { replace: true, force: true });
+
+    const writtenURL = historyOf().lastURL ?? '';
+    expect(writtenURL).toContain('session=ses_main');
+  });
+});
+
+describe('isEmbeddedSessionChat caching', () => {
+  test('caches the first result so URL rewrites cannot flip it (mirrors VS Code stable global)', () => {
+    // VS Code detects its webview via the stable `window.__VSCODE_CONFIG__`
+    // global — it never changes. The embedded iframe's identity is equally
+    // fixed at mount (the parent builds the src); caching the first read
+    // makes detection just as stable, surviving any URL rewrite.
+    //
+    // We need a fresh module cache for this test. Since the cache is
+    // module-level, we test the invariant: once true, always true.
+    installWindow(
+      'http://127.0.0.1:5173/app?ocPanel=session-chat&sessionId=ses_child&directory=%2Frepo&readOnly=1',
+    );
+
+    // First read: caches true.
+    expect(isEmbeddedSessionChat()).toBe(true);
+
+    // Even if the URL were rewritten (the guard above prevents this, but
+    // defense in depth), the cached value stays true.
+    installWindow('http://127.0.0.1:5173/app?session=ses_grandchild');
+    expect(isEmbeddedSessionChat()).toBe(true);
+  });
+});
diff --git a/packages/ui/src/lib/router/serializeRoute.ts b/packages/ui/src/lib/router/serializeRoute.ts
index 6c9a9c50..c4c27c11 100644
--- a/packages/ui/src/lib/router/serializeRoute.ts
+++ b/packages/ui/src/lib/router/serializeRoute.ts
@@ -1,4 +1,5 @@
 import type { MainTab } from '@/stores/useUIStore';
+import { isEmbeddedSessionChat } from '@/components/layout/contextPanelEmbeddedChat';
 import { ROUTE_PARAMS } from './types';
 
 /**
@@ -100,7 +101,8 @@ function routeMatchesURL(state: AppRouteState): boolean {
 
 /**
  * Update the browser URL using pushState or replaceState.
- * Does nothing if URL already matches or in VS Code context.
+ * Does nothing if URL already matches, in VS Code context, or in the
+ * embedded session-chat iframe (whose URL identity is fixed at mount).
  */
 export function updateBrowserURL(
   state: AppRouteState,
@@ -110,8 +112,10 @@ export function updateBrowserURL(
     return;
   }
 
-  // Skip URL updates in VS Code webview
-  if (isVSCodeContext()) {
+  // Both VS Code webviews and embedded session-chat iframes carry session
+  // identity outside the route params (`__VSCODE_CONFIG__` / `?ocPanel=…`).
+  // Rebuilding the URL here would strip those params, so skip entirely.
+  if (isVSCodeContext() || isEmbeddedSessionChat()) {
     return;
   }