diff --git a/packages/ui/src/App.tsx b/packages/ui/src/App.tsx
index ba0feeda..cb9c3efa 100644
--- a/packages/ui/src/App.tsx
+++ b/packages/ui/src/App.tsx
@@ -12,7 +12,7 @@ import { useMenuActions } from '@/hooks/useMenuActions';
 import { useMessageSync } from '@/hooks/useMessageSync';
 import { useSessionStatusBootstrap } from '@/hooks/useSessionStatusBootstrap';
 import { useSessionAutoCleanup } from '@/hooks/useSessionAutoCleanup';
-import { useSessionDeepLink } from '@/hooks/useSessionDeepLink';
+import { useRouter } from '@/hooks/useRouter';
 import { usePushVisibilityBeacon } from '@/hooks/usePushVisibilityBeacon';
 import { GitPollingProvider } from '@/hooks/useGitPolling';
 import { useConfigStore } from '@/stores/useConfigStore';
@@ -160,7 +160,7 @@ function App({ apis }: AppProps) {
 
   usePushVisibilityBeacon();
 
-  useSessionDeepLink();
+  useRouter();
 
   useKeyboardShortcuts();
 
diff --git a/packages/ui/src/components/views/SettingsView.tsx b/packages/ui/src/components/views/SettingsView.tsx
index 1d75b226..6f953a38 100644
--- a/packages/ui/src/components/views/SettingsView.tsx
+++ b/packages/ui/src/components/views/SettingsView.tsx
@@ -2,6 +2,7 @@ import React from 'react';
 import { cn, getModifierLabel } from '@/lib/utils';
 import { SIDEBAR_SECTIONS } from '@/constants/sidebar';
 import type { SidebarSection } from '@/constants/sidebar';
+import { useUIStore } from '@/stores/useUIStore';
 import { RiArrowDownSLine, RiArrowLeftSLine, RiCloseLine, RiFolderLine } from '@remixicon/react';
 import {
   DropdownMenu,
@@ -60,7 +61,25 @@ interface SettingsViewProps {
 export const SettingsView: React.FC<SettingsViewProps> = ({ onClose, forceMobile }) => {
   const deviceInfo = useDeviceInfo();
   const isMobile = forceMobile ?? deviceInfo.isMobile;
-  const [activeTab, setActiveTab] = React.useState<SidebarSection>('settings');
+
+  // Sync activeTab with store's sidebarSection for routing support
+  const storeSidebarSection = useUIStore((state) => state.sidebarSection);
+  const setSidebarSection = useUIStore((state) => state.setSidebarSection);
+
+  // Use store's sidebarSection as the source of truth, but filter to valid settings sections
+  const activeTab = React.useMemo<SidebarSection>(() => {
+    // If store has a valid settings section (not 'sessions'), use it
+    if (storeSidebarSection !== 'sessions') {
+      return storeSidebarSection;
+    }
+    // Default to 'settings' if store has 'sessions'
+    return 'settings';
+  }, [storeSidebarSection]);
+
+  // Update store when tab changes
+  const setActiveTab = React.useCallback((tab: SidebarSection) => {
+    setSidebarSection(tab);
+  }, [setSidebarSection]);
   const [selectedOpenChamberSection, setSelectedOpenChamberSection] = React.useState<OpenChamberSection>('visual');
   // Mobile drill-down state: show page content instead of sidebar
   const [showMobilePageContent, setShowMobilePageContent] = React.useState(false);
@@ -257,7 +276,7 @@ export const SettingsView: React.FC<SettingsViewProps> = ({ onClose, forceMobile
     setActiveTab(tab);
     // Reset mobile drill-down state when changing tabs
     setShowMobilePageContent(false);
-  }, []);
+  }, [setActiveTab]);
 
   // Handle mobile sidebar item selection (drill-down to page)
   const handleMobileSidebarClick = React.useCallback(() => {
diff --git a/packages/ui/src/hooks/useRouter.ts b/packages/ui/src/hooks/useRouter.ts
new file mode 100644
index 00000000..33a77422
--- /dev/null
+++ b/packages/ui/src/hooks/useRouter.ts
@@ -0,0 +1,348 @@
+import React from 'react';
+import { useSessionStore } from '@/stores/useSessionStore';
+import { useUIStore } from '@/stores/useUIStore';
+import { parseRoute, updateBrowserURL, hasRouteParams } from '@/lib/router';
+import type { RouteState, AppRouteState } from '@/lib/router';
+import type { SidebarSection } from '@/constants/sidebar';
+import type { MainTab } from '@/stores/useUIStore';
+
+/**
+ * Check if running in VS Code webview context.
+ */
+function isVSCodeContext(): boolean {
+  if (typeof window === 'undefined') {
+    return false;
+  }
+  const win = window as { __VSCODE_CONFIG__?: unknown };
+  return win.__VSCODE_CONFIG__ !== undefined;
+}
+
+/**
+ * Hook that provides bidirectional URL routing for OpenChamber.
+ *
+ * On mount:
+ * - Parses URL parameters and applies them to app state
+ * - Sets up subscriptions to sync state changes back to URL
+ * - Listens for browser back/forward navigation
+ *
+ * Works in:
+ * - Web: Full bidirectional sync
+ * - Desktop (Tauri): Full bidirectional sync
+ * - VS Code: State-only (no URL updates, reads initial params)
+ */
+export function useRouter(): void {
+  const isVSCode = React.useMemo(() => isVSCodeContext(), []);
+
+  // Track initialization to avoid duplicate applies
+  const initializedRef = React.useRef(false);
+  const isApplyingRouteRef = React.useRef(false);
+
+  // Get store actions (stable references)
+  const setCurrentSession = useSessionStore((state) => state.setCurrentSession);
+  const setActiveMainTab = useUIStore((state) => state.setActiveMainTab);
+  const setSettingsDialogOpen = useUIStore((state) => state.setSettingsDialogOpen);
+  const setSidebarSection = useUIStore((state) => state.setSidebarSection);
+  const navigateToDiff = useUIStore((state) => state.navigateToDiff);
+
+  /**
+   * Apply a parsed route state to the application stores.
+   */
+  const applyRoute = React.useCallback(
+    async (route: RouteState) => {
+      if (isApplyingRouteRef.current) {
+        return;
+      }
+
+      isApplyingRouteRef.current = true;
+
+      try {
+        // 1. Apply session first (may trigger async operations)
+        if (route.sessionId) {
+          const currentSessionId = useSessionStore.getState().currentSessionId;
+          if (route.sessionId !== currentSessionId) {
+            await setCurrentSession(route.sessionId);
+          }
+        }
+
+        // 2. Handle settings (takes precedence over tabs - it's a full-screen overlay)
+        if (route.settingsSection) {
+          setSidebarSection(route.settingsSection);
+          setSettingsDialogOpen(true);
+          // Don't process tab when settings is open
+          return;
+        }
+
+        // Close settings if URL has no settings section
+        if (useUIStore.getState().isSettingsDialogOpen) {
+          setSettingsDialogOpen(false);
+        }
+
+        // 3. Apply tab
+        if (route.tab) {
+          setActiveMainTab(route.tab);
+        }
+
+        // 4. Apply diff file (only if going to diff tab)
+        if (route.diffFile && (route.tab === 'diff' || !route.tab)) {
+          navigateToDiff(route.diffFile);
+        }
+      } finally {
+        isApplyingRouteRef.current = false;
+      }
+    },
+    [setCurrentSession, setActiveMainTab, setSettingsDialogOpen, setSidebarSection, navigateToDiff]
+  );
+
+  /**
+   * Get current app state for URL serialization.
+   */
+  const getCurrentAppState = React.useCallback((): AppRouteState => {
+    const sessionState = useSessionStore.getState();
+    const uiState = useUIStore.getState();
+
+    return {
+      sessionId: sessionState.currentSessionId,
+      tab: uiState.activeMainTab,
+      isSettingsOpen: uiState.isSettingsDialogOpen,
+      settingsSection: uiState.sidebarSection,
+      diffFile: uiState.pendingDiffFile,
+    };
+  }, []);
+
+  /**
+   * Sync current app state to URL.
+   */
+  const syncURLFromState = React.useCallback(
+    (options: { replace?: boolean } = {}) => {
+      if (isVSCode || isApplyingRouteRef.current) {
+        return;
+      }
+
+      const state = getCurrentAppState();
+      updateBrowserURL(state, options);
+    },
+    [isVSCode, getCurrentAppState]
+  );
+
+  // Initialize: parse URL and apply route on mount
+  React.useEffect(() => {
+    if (initializedRef.current) {
+      return;
+    }
+    initializedRef.current = true;
+
+    // Only process if URL has route params
+    if (!hasRouteParams()) {
+      // No route params - just set up sync (URL will update when user navigates)
+      return;
+    }
+
+    const route = parseRoute();
+
+    // Apply the initial route
+    const initializeRoute = async () => {
+      await applyRoute(route);
+
+      // After applying, update URL to normalized form (use replaceState)
+      if (!isVSCode) {
+        syncURLFromState({ replace: true });
+      }
+    };
+
+    void initializeRoute();
+  }, [applyRoute, isVSCode, syncURLFromState]);
+
+  // Subscribe to session changes
+  React.useEffect(() => {
+    if (isVSCode) {
+      return;
+    }
+
+    let prevSessionId: string | null = useSessionStore.getState().currentSessionId;
+
+    const unsubscribe = useSessionStore.subscribe((state) => {
+      const sessionId = state.currentSessionId;
+
+      // Skip if no change or if we're currently applying a route
+      if (sessionId === prevSessionId || isApplyingRouteRef.current) {
+        return;
+      }
+
+      prevSessionId = sessionId;
+      syncURLFromState();
+    });
+
+    return unsubscribe;
+  }, [isVSCode, syncURLFromState]);
+
+  // Subscribe to UI store changes (tab, settings)
+  React.useEffect(() => {
+    if (isVSCode) {
+      return;
+    }
+
+    let prevTab: MainTab = useUIStore.getState().activeMainTab;
+    let prevSettingsOpen: boolean = useUIStore.getState().isSettingsDialogOpen;
+    let prevSettingsSection: SidebarSection = useUIStore.getState().sidebarSection;
+    let prevDiffFile: string | null = useUIStore.getState().pendingDiffFile;
+
+    const unsubscribe = useUIStore.subscribe((state) => {
+      // Skip if we're currently applying a route
+      if (isApplyingRouteRef.current) {
+        return;
+      }
+
+      const tabChanged = state.activeMainTab !== prevTab;
+      const settingsOpenChanged = state.isSettingsDialogOpen !== prevSettingsOpen;
+      const settingsSectionChanged = state.sidebarSection !== prevSettingsSection;
+      const diffFileChanged = state.pendingDiffFile !== prevDiffFile && state.activeMainTab === 'diff';
+
+      // Update tracking vars
+      prevTab = state.activeMainTab;
+      prevSettingsOpen = state.isSettingsDialogOpen;
+      prevSettingsSection = state.sidebarSection;
+      prevDiffFile = state.pendingDiffFile;
+
+      // Only sync if something relevant changed
+      if (tabChanged || settingsOpenChanged || settingsSectionChanged || diffFileChanged) {
+        syncURLFromState();
+      }
+    });
+
+    return unsubscribe;
+  }, [isVSCode, syncURLFromState]);
+
+  // Listen for browser back/forward navigation
+  React.useEffect(() => {
+    if (typeof window === 'undefined' || isVSCode) {
+      return;
+    }
+
+    const handlePopState = () => {
+      // Parse the new URL and apply it
+      const route = parseRoute();
+
+      // Check if this is a route with any params, or if we should restore defaults
+      if (hasRouteParams()) {
+        void applyRoute(route);
+      } else {
+        // URL has no route params - this might be a "back to home" navigation
+        // Close settings if open, keep current session
+        const uiState = useUIStore.getState();
+        if (uiState.isSettingsDialogOpen) {
+          setSettingsDialogOpen(false);
+        }
+        // Reset to chat tab if not already there
+        if (uiState.activeMainTab !== 'chat') {
+          setActiveMainTab('chat');
+        }
+      }
+    };
+
+    window.addEventListener('popstate', handlePopState);
+
+    return () => {
+      window.removeEventListener('popstate', handlePopState);
+    };
+  }, [applyRoute, isVSCode, setActiveMainTab, setSettingsDialogOpen]);
+}
+
+/**
+ * Programmatically navigate to a route.
+ * Can be used from outside React components.
+ */
+export function navigateToRoute(route: Partial<RouteState>): void {
+  if (typeof window === 'undefined') {
+    return;
+  }
+
+  // Check VS Code context
+  const win = window as { __VSCODE_CONFIG__?: unknown };
+  if (win.__VSCODE_CONFIG__ !== undefined) {
+    // In VS Code, just apply state changes directly
+    if (route.sessionId) {
+      void useSessionStore.getState().setCurrentSession(route.sessionId);
+    }
+    if (route.settingsSection) {
+      useUIStore.getState().setSidebarSection(route.settingsSection);
+      useUIStore.getState().setSettingsDialogOpen(true);
+    } else if (route.tab) {
+      useUIStore.getState().setActiveMainTab(route.tab);
+    }
+    if (route.diffFile) {
+      useUIStore.getState().navigateToDiff(route.diffFile);
+    }
+    return;
+  }
+
+  // Build URL and navigate
+  const params = new URLSearchParams();
+
+  if (route.sessionId) {
+    params.set('session', route.sessionId);
+  }
+  if (route.settingsSection) {
+    params.set('settings', route.settingsSection);
+  } else if (route.tab && route.tab !== 'chat') {
+    if (useUIStore.getState().isSettingsDialogOpen) {
+      useUIStore.getState().setSettingsDialogOpen(false);
+    }
+    params.set('tab', route.tab);
+  }
+  if (route.diffFile) {
+    params.set('file', route.diffFile);
+  }
+
+  const search = params.toString();
+  const url = search ? `${window.location.pathname}?${search}` : window.location.pathname;
+
+  window.history.pushState({ route }, '', url);
+
+  // Also apply to state
+  if (route.sessionId) {
+    void useSessionStore.getState().setCurrentSession(route.sessionId);
+  }
+  if (route.settingsSection) {
+    useUIStore.getState().setSidebarSection(route.settingsSection);
+    useUIStore.getState().setSettingsDialogOpen(true);
+  } else if (route.tab) {
+    useUIStore.getState().setActiveMainTab(route.tab);
+  }
+  if (route.diffFile) {
+    useUIStore.getState().navigateToDiff(route.diffFile);
+  }
+}
+
+/**
+ * Get a shareable URL for the current state.
+ */
+export function getShareableURL(): string {
+  if (typeof window === 'undefined') {
+    return '/';
+  }
+
+  const sessionState = useSessionStore.getState();
+  const uiState = useUIStore.getState();
+
+  const params = new URLSearchParams();
+
+  if (sessionState.currentSessionId) {
+    params.set('session', sessionState.currentSessionId);
+  }
+
+  if (uiState.isSettingsDialogOpen) {
+    const settingsSection = uiState.sidebarSection === 'sessions' ? 'settings' : uiState.sidebarSection;
+    params.set('settings', settingsSection);
+  } else if (uiState.activeMainTab !== 'chat') {
+    params.set('tab', uiState.activeMainTab);
+  }
+
+  if (uiState.activeMainTab === 'diff' && uiState.pendingDiffFile) {
+    params.set('file', uiState.pendingDiffFile);
+  }
+
+  const search = params.toString();
+  const base = `${window.location.origin}${window.location.pathname}`;
+
+  return search ? `${base}?${search}` : base;
+}
diff --git a/packages/ui/src/hooks/useSessionDeepLink.ts b/packages/ui/src/hooks/useSessionDeepLink.ts
deleted file mode 100644
index 6c9dca1a..00000000
--- a/packages/ui/src/hooks/useSessionDeepLink.ts
+++ /dev/null
@@ -1,42 +0,0 @@
-import React from 'react';
-import { useSessionStore } from '@/stores/useSessionStore';
-import { useUIStore } from '@/stores/useUIStore';
-
-export const useSessionDeepLink = () => {
-  const setCurrentSession = useSessionStore((state) => state.setCurrentSession);
-
-  React.useEffect(() => {
-    if (typeof window === 'undefined') {
-      return;
-    }
-
-    let sessionId: string | null = null;
-    try {
-      const params = new URLSearchParams(window.location.search);
-      sessionId = params.get('session');
-    } catch {
-      return;
-    }
-
-    if (!sessionId || sessionId.trim().length === 0) {
-      return;
-    }
-
-    const run = async () => {
-      try {
-        useUIStore.getState().setActiveMainTab('chat');
-        await setCurrentSession(sessionId as string);
-      } finally {
-        try {
-          const url = new URL(window.location.href);
-          url.searchParams.delete('session');
-          window.history.replaceState({}, '', url.toString());
-        } catch {
-          // ignore
-        }
-      }
-    };
-
-    void run();
-  }, [setCurrentSession]);
-};
diff --git a/packages/ui/src/lib/router/index.ts b/packages/ui/src/lib/router/index.ts
new file mode 100644
index 00000000..40571d88
--- /dev/null
+++ b/packages/ui/src/lib/router/index.ts
@@ -0,0 +1,31 @@
+/**
+ * Router module for URL-based navigation in OpenChamber.
+ *
+ * Provides bidirectional sync between URL query parameters and application state.
+ * Works across web, desktop (Tauri), and VS Code (state-only mode).
+ *
+ * URL Schema:
+ * - `?session=<id>` - Navigate to specific session
+ * - `?tab=<chat|git|diff|terminal|files>` - Active main tab
+ * - `?settings=<section>` - Open settings to specific section
+ * - `?file=<path>` - Diff view with file selected
+ *
+ * Examples:
+ * - `/?session=abc123` - Open session abc123
+ * - `/?tab=git` - Open git tab
+ * - `/?settings=providers` - Open settings to providers section
+ * - `/?tab=diff&file=src/main.ts` - Open diff view with file
+ */
+
+export type { RouteState, RouterContext } from './types';
+export { VALID_TABS, VALID_SETTINGS_SECTIONS, ROUTE_PARAMS } from './types';
+
+export { parseRoute, hasRouteParams } from './parseRoute';
+
+export type { AppRouteState } from './serializeRoute';
+export {
+  serializeRoute,
+  buildURL,
+  routeMatchesURL,
+  updateBrowserURL,
+} from './serializeRoute';
diff --git a/packages/ui/src/lib/router/parseRoute.ts b/packages/ui/src/lib/router/parseRoute.ts
new file mode 100644
index 00000000..544af095
--- /dev/null
+++ b/packages/ui/src/lib/router/parseRoute.ts
@@ -0,0 +1,133 @@
+import type { SidebarSection } from '@/constants/sidebar';
+import type { MainTab } from '@/stores/useUIStore';
+import {
+  type RouteState,
+  VALID_TABS,
+  VALID_SETTINGS_SECTIONS,
+  ROUTE_PARAMS,
+} from './types';
+
+/**
+ * Parse the current URL search parameters into a RouteState.
+ * Returns null values for any parameter that is missing or invalid.
+ */
+export function parseRoute(searchParams?: URLSearchParams): RouteState {
+  const params = searchParams ?? getSearchParams();
+
+  return {
+    sessionId: parseSessionId(params),
+    tab: parseTab(params),
+    settingsSection: parseSettingsSection(params),
+    diffFile: parseDiffFile(params),
+  };
+}
+
+/**
+ * Safely get URLSearchParams from the current location.
+ */
+function getSearchParams(): URLSearchParams {
+  if (typeof window === 'undefined') {
+    return new URLSearchParams();
+  }
+
+  try {
+    return new URLSearchParams(window.location.search);
+  } catch {
+    return new URLSearchParams();
+  }
+}
+
+/**
+ * Parse session ID from URL parameters.
+ * Returns null if missing or empty.
+ */
+function parseSessionId(params: URLSearchParams): string | null {
+  const value = params.get(ROUTE_PARAMS.SESSION);
+  if (!value || value.trim().length === 0) {
+    return null;
+  }
+  return value.trim();
+}
+
+/**
+ * Parse main tab from URL parameters.
+ * Returns null if missing or invalid.
+ */
+function parseTab(params: URLSearchParams): MainTab | null {
+  const value = params.get(ROUTE_PARAMS.TAB);
+  if (!value) {
+    return null;
+  }
+
+  const normalized = value.toLowerCase().trim() as MainTab;
+  if (VALID_TABS.includes(normalized)) {
+    return normalized;
+  }
+
+  return null;
+}
+
+/**
+ * Parse settings section from URL parameters.
+ * Returns null if missing or invalid.
+ */
+function parseSettingsSection(params: URLSearchParams): SidebarSection | null {
+  const value = params.get(ROUTE_PARAMS.SETTINGS);
+  if (!value) {
+    return null;
+  }
+
+  const normalized = value.toLowerCase().trim();
+
+  // Check if it's a valid section
+  if ((VALID_SETTINGS_SECTIONS as readonly string[]).includes(normalized)) {
+    return normalized as SidebarSection;
+  }
+
+  // Handle common aliases
+  if (normalized === 'openchamber' || normalized === 'general' || normalized === 'preferences') {
+    return 'settings';
+  }
+
+  return null;
+}
+
+/**
+ * Parse diff file path from URL parameters.
+ * Returns null if missing or empty.
+ */
+function parseDiffFile(params: URLSearchParams): string | null {
+  const value = params.get(ROUTE_PARAMS.FILE);
+  if (!value || value.trim().length === 0) {
+    return null;
+  }
+
+  // URL decode the file path
+  try {
+    return decodeURIComponent(value.trim());
+  } catch {
+    // If decoding fails, return the raw value
+    return value.trim();
+  }
+}
+
+/**
+ * Check if the current URL has any route parameters.
+ */
+export function hasRouteParams(): boolean {
+  if (typeof window === 'undefined') {
+    return false;
+  }
+
+  try {
+    const params = new URLSearchParams(window.location.search);
+    return (
+      params.has(ROUTE_PARAMS.SESSION) ||
+      params.has(ROUTE_PARAMS.TAB) ||
+      params.has(ROUTE_PARAMS.SETTINGS) ||
+      params.has(ROUTE_PARAMS.FILE)
+    );
+  } catch {
+    return false;
+  }
+}
diff --git a/packages/ui/src/lib/router/serializeRoute.ts b/packages/ui/src/lib/router/serializeRoute.ts
new file mode 100644
index 00000000..eee2a09f
--- /dev/null
+++ b/packages/ui/src/lib/router/serializeRoute.ts
@@ -0,0 +1,149 @@
+import type { SidebarSection } from '@/constants/sidebar';
+import type { MainTab } from '@/stores/useUIStore';
+import { ROUTE_PARAMS } from './types';
+
+/**
+ * Application state relevant for URL serialization.
+ */
+export interface AppRouteState {
+  sessionId: string | null;
+  tab: MainTab;
+  isSettingsOpen: boolean;
+  settingsSection: SidebarSection;
+  diffFile: string | null;
+}
+
+/**
+ * Default tab when none is specified.
+ */
+const DEFAULT_TAB: MainTab = 'chat';
+
+/**
+ * Serialize application state to URL search parameters.
+ * Only includes parameters that differ from defaults to keep URLs clean.
+ */
+export function serializeRoute(state: AppRouteState): URLSearchParams {
+  const params = new URLSearchParams();
+
+  // Session ID - always include if present
+  if (state.sessionId && state.sessionId.trim().length > 0) {
+    params.set(ROUTE_PARAMS.SESSION, state.sessionId);
+  }
+
+  // Settings takes precedence - if open, include settings section
+  if (state.isSettingsOpen) {
+    const settingsSection = state.settingsSection === 'sessions' ? 'settings' : state.settingsSection;
+    params.set(ROUTE_PARAMS.SETTINGS, settingsSection);
+    // Don't include tab when settings is open (it's a full-screen overlay)
+    return params;
+  }
+
+  // Tab - only include if not the default
+  if (state.tab !== DEFAULT_TAB) {
+    params.set(ROUTE_PARAMS.TAB, state.tab);
+  }
+
+  // Diff file - only include when on diff tab
+  if (state.tab === 'diff' && state.diffFile && state.diffFile.trim().length > 0) {
+    params.set(ROUTE_PARAMS.FILE, state.diffFile);
+  }
+
+  return params;
+}
+
+/**
+ * Convert URLSearchParams to a URL string.
+ * Returns just the pathname if no params, otherwise pathname + search string.
+ */
+export function buildURL(params: URLSearchParams, pathname?: string): string {
+  const path = pathname ?? (typeof window !== 'undefined' ? window.location.pathname : '/');
+  const search = params.toString();
+
+  if (!search) {
+    return path;
+  }
+
+  return `${path}?${search}`;
+}
+
+/**
+ * Check if the current URL matches the given route state.
+ * Used to avoid unnecessary URL updates.
+ */
+export function routeMatchesURL(state: AppRouteState): boolean {
+  if (typeof window === 'undefined') {
+    return true;
+  }
+
+  try {
+    const currentParams = new URLSearchParams(window.location.search);
+    const newParams = serializeRoute(state);
+
+    // Compare sorted param strings for equality
+    const currentSorted = [...currentParams.entries()].sort((a, b) => a[0].localeCompare(b[0]));
+    const newSorted = [...newParams.entries()].sort((a, b) => a[0].localeCompare(b[0]));
+
+    if (currentSorted.length !== newSorted.length) {
+      return false;
+    }
+
+    for (let i = 0; i < currentSorted.length; i++) {
+      if (currentSorted[i][0] !== newSorted[i][0] || currentSorted[i][1] !== newSorted[i][1]) {
+        return false;
+      }
+    }
+
+    return true;
+  } catch {
+    return true;
+  }
+}
+
+/**
+ * Update the browser URL using pushState or replaceState.
+ * Does nothing if URL already matches or in VS Code context.
+ */
+export function updateBrowserURL(
+  state: AppRouteState,
+  options: { replace?: boolean; force?: boolean } = {}
+): void {
+  if (typeof window === 'undefined') {
+    return;
+  }
+
+  // Skip URL updates in VS Code webview
+  if (isVSCodeContext()) {
+    return;
+  }
+
+  // Skip if URL already matches (unless forced)
+  if (!options.force && routeMatchesURL(state)) {
+    return;
+  }
+
+  try {
+    const params = serializeRoute(state);
+    const url = buildURL(params);
+
+    if (options.replace) {
+      window.history.replaceState({ ...window.history.state, route: state }, '', url);
+    } else {
+      window.history.pushState({ route: state }, '', url);
+    }
+  } catch {
+    // Silently fail - URL updates are non-critical
+  }
+}
+
+/**
+ * Check if running in VS Code webview context.
+ */
+function isVSCodeContext(): boolean {
+  if (typeof window === 'undefined') {
+    return false;
+  }
+
+  // Check for VS Code config object
+  const win = window as { __VSCODE_CONFIG__?: unknown };
+  return win.__VSCODE_CONFIG__ !== undefined;
+}
diff --git a/packages/ui/src/lib/router/types.ts b/packages/ui/src/lib/router/types.ts
new file mode 100644
index 00000000..a0f212b7
--- /dev/null
+++ b/packages/ui/src/lib/router/types.ts
@@ -0,0 +1,54 @@
+import type { SidebarSection } from '@/constants/sidebar';
+import type { MainTab } from '@/stores/useUIStore';
+
+/**
+ * Represents the current route state derived from URL parameters.
+ * All fields are nullable - null means "not specified in URL" (use app defaults).
+ */
+export interface RouteState {
+  /** Session ID to navigate to */
+  sessionId: string | null;
+  /** Main tab to display (chat, git, diff, terminal, files) */
+  tab: MainTab | null;
+  /** Settings section - when non-null, settings dialog should be open */
+  settingsSection: SidebarSection | null;
+  /** File path for diff view */
+  diffFile: string | null;
+}
+
+/**
+ * Context for router operations - determines what capabilities are available.
+ */
+export interface RouterContext {
+  /** Whether running in VS Code webview (limited URL capabilities) */
+  isVSCode: boolean;
+  /** Whether URL can be updated (false in VS Code, true elsewhere) */
+  canUpdateURL: boolean;
+}
+
+/**
+ * Valid main tab values for URL routing.
+ */
+export const VALID_TABS: readonly MainTab[] = ['chat', 'git', 'diff', 'terminal', 'files'] as const;
+
+/**
+ * Valid settings section values for URL routing.
+ */
+export const VALID_SETTINGS_SECTIONS: readonly SidebarSection[] = [
+  'settings',
+  'agents',
+  'commands',
+  'skills',
+  'providers',
+  'git-identities',
+] as const;
+
+/**
+ * URL parameter names used for routing.
+ */
+export const ROUTE_PARAMS = {
+  SESSION: 'session',
+  TAB: 'tab',
+  SETTINGS: 'settings',
+  FILE: 'file',
+} as const;