diff --git a/apps/web/src/App.tsx b/apps/web/src/App.tsx
index e488e28..af68dea 100644
--- a/apps/web/src/App.tsx
+++ b/apps/web/src/App.tsx
@@ -8,6 +8,8 @@ import { Project } from '@/pages/Project';
 import { Session } from '@/pages/Session';
 import { Toaster } from '@/components/ui/sonner';
 import { useUserEvents } from '@/hooks/useUserEvents';
+import { SidebarDrawerProvider, useSidebarDrawer } from '@/hooks/useSidebarDrawer';
+import { useViewport } from '@/hooks/useViewport';
 
 function SessionRightRail() {
   const { id } = useParams<{ id: string }>();
@@ -27,11 +29,25 @@ function RightRailForSession({ sessionId }: { sessionId: string }) {
   return <RightRail projectId={projectId} />;
 }
 
+function MobileBackdrop() {
+  const { open, setOpen } = useSidebarDrawer();
+  const { isMobile } = useViewport();
+  if (!isMobile || !open) return null;
+  return (
+    <div
+      className="fixed inset-0 z-30 bg-black/40 md:hidden"
+      onClick={() => setOpen(false)}
+      aria-hidden="true"
+    />
+  );
+}
+
 function AppShell() {
   useUserEvents();
   return (
     <div className="dark h-screen flex bg-background text-foreground">
       <ProjectSidebar />
+      <MobileBackdrop />
       <main className="flex-1 flex flex-col min-w-0">
         <Routes>
           <Route path="/" element={<Home />} />
@@ -50,7 +66,9 @@ function AppShell() {
 export default function App() {
   return (
     <BrowserRouter>
-      <AppShell />
+      <SidebarDrawerProvider>
+        <AppShell />
+      </SidebarDrawerProvider>
     </BrowserRouter>
   );
 }
diff --git a/apps/web/src/components/ProjectSidebar.tsx b/apps/web/src/components/ProjectSidebar.tsx
index 8c9159f..9d26f2c 100644
--- a/apps/web/src/components/ProjectSidebar.tsx
+++ b/apps/web/src/components/ProjectSidebar.tsx
@@ -20,6 +20,8 @@ import {
 import { AddProjectModal } from './AddProjectModal';
 import { api } from '@/api/client';
 import { useSidebar } from '@/hooks/useSidebar';
+import { useSidebarDrawer } from '@/hooks/useSidebarDrawer';
+import { useViewport } from '@/hooks/useViewport';
 import type { SidebarProject } from '@/api/types';
 import { giteaUrlFor } from '@/lib/projectUrls';
 import { cn } from '@/lib/utils';
@@ -195,8 +197,23 @@ export function ProjectSidebar() {
   const rowCls = (active: boolean) =>
     active ? 'bg-sidebar-accent text-sidebar-accent-foreground' : 'hover:bg-sidebar-accent/60';
 
+  const { open: drawerOpen } = useSidebarDrawer();
+  const { isMobile } = useViewport();
+
+  // On mobile the sidebar is a slide-in drawer (fixed, z-40, off-screen by
+  // default). On desktop it sits inline as a normal flex column. The
+  // backdrop is rendered by AppShell; drawer-open state lives in
+  // SidebarDrawerProvider.
+  const asideCls = isMobile
+    ? cn(
+        'fixed inset-y-0 left-0 z-40 w-60 border-r bg-sidebar text-sidebar-foreground flex flex-col',
+        'transition-transform duration-200 ease-out',
+        drawerOpen ? 'translate-x-0' : '-translate-x-full',
+      )
+    : 'w-60 shrink-0 border-r bg-sidebar text-sidebar-foreground flex flex-col h-screen';
+
   return (
-    <aside className="w-60 shrink-0 border-r bg-sidebar text-sidebar-foreground flex flex-col h-screen">
+    <aside className={asideCls}>
       <div className="px-4 py-3 border-b flex items-center justify-between">
         <NavLink to="/" className="font-semibold tracking-tight text-base">
           BooCode
diff --git a/apps/web/src/hooks/useSidebarDrawer.tsx b/apps/web/src/hooks/useSidebarDrawer.tsx
new file mode 100644
index 0000000..ecb532f
--- /dev/null
+++ b/apps/web/src/hooks/useSidebarDrawer.tsx
@@ -0,0 +1,36 @@
+import { createContext, useCallback, useContext, useEffect, useState } from 'react';
+import type { ReactNode } from 'react';
+import { useLocation } from 'react-router-dom';
+
+interface SidebarDrawerState {
+  open: boolean;
+  setOpen: (open: boolean) => void;
+  toggle: () => void;
+}
+
+const Ctx = createContext<SidebarDrawerState | null>(null);
+
+export function SidebarDrawerProvider({ children }: { children: ReactNode }) {
+  const [open, setOpen] = useState(false);
+  const location = useLocation();
+
+  // Auto-close on navigation. Effect fires once on mount too (open default
+  // is false, so no observable effect) and on every pathname change after.
+  useEffect(() => {
+    setOpen(false);
+  }, [location.pathname]);
+
+  const toggle = useCallback(() => setOpen((v) => !v), []);
+
+  return <Ctx.Provider value={{ open, setOpen, toggle }}>{children}</Ctx.Provider>;
+}
+
+export function useSidebarDrawer(): SidebarDrawerState {
+  const ctx = useContext(Ctx);
+  if (!ctx) {
+    // Soft fallback so consumers don't crash if rendered outside a provider.
+    // In practice all top-level routes are inside the provider.
+    return { open: false, setOpen: () => {}, toggle: () => {} };
+  }
+  return ctx;
+}
diff --git a/apps/web/src/hooks/useViewport.ts b/apps/web/src/hooks/useViewport.ts
new file mode 100644
index 0000000..618dc93
--- /dev/null
+++ b/apps/web/src/hooks/useViewport.ts
@@ -0,0 +1,45 @@
+import { useEffect, useState } from 'react';
+
+// Breakpoints (px): mobile <768, tablet 768-1023, desktop >=1024.
+const MOBILE_MAX = 767;
+const TABLET_MAX = 1023;
+
+export interface ViewportSnapshot {
+  isMobile: boolean;
+  isTablet: boolean;
+  width: number;
+}
+
+function snapshot(): ViewportSnapshot {
+  if (typeof window === 'undefined') {
+    return { isMobile: false, isTablet: false, width: 1280 };
+  }
+  const width = window.innerWidth;
+  return {
+    isMobile: width <= MOBILE_MAX,
+    isTablet: width > MOBILE_MAX && width <= TABLET_MAX,
+    width,
+  };
+}
+
+// matchMedia-based, no resize polling. We listen to two breakpoint queries
+// and recompute the snapshot on any change.
+export function useViewport(): ViewportSnapshot {
+  const [state, setState] = useState<ViewportSnapshot>(snapshot);
+
+  useEffect(() => {
+    if (typeof window === 'undefined') return;
+    const mobileMq = window.matchMedia(`(max-width: ${MOBILE_MAX}px)`);
+    const tabletMq = window.matchMedia(`(min-width: ${MOBILE_MAX + 1}px) and (max-width: ${TABLET_MAX}px)`);
+    const update = () => setState(snapshot());
+    mobileMq.addEventListener('change', update);
+    tabletMq.addEventListener('change', update);
+    update();
+    return () => {
+      mobileMq.removeEventListener('change', update);
+      tabletMq.removeEventListener('change', update);
+    };
+  }, []);
+
+  return state;
+}
diff --git a/apps/web/src/pages/Project.tsx b/apps/web/src/pages/Project.tsx
index 579692d..bed2216 100644
--- a/apps/web/src/pages/Project.tsx
+++ b/apps/web/src/pages/Project.tsx
@@ -1,12 +1,14 @@
 import { useEffect, useState } from 'react';
 import { Link, useNavigate, useParams } from 'react-router-dom';
-import { Plus, MessageSquare, Trash2, ChevronDown, ChevronRight, RotateCcw } from 'lucide-react';
+import { Plus, MessageSquare, Trash2, ChevronDown, ChevronRight, RotateCcw, Menu } from 'lucide-react';
 import { toast } from 'sonner';
 import { api } from '@/api/client';
 import type { Project as ProjectType, Session } from '@/api/types';
 import { Button } from '@/components/ui/button';
 import { sessionEvents } from '@/hooks/sessionEvents';
 import { useSessions } from '@/hooks/useSessions';
+import { useSidebarDrawer } from '@/hooks/useSidebarDrawer';
+import { useViewport } from '@/hooks/useViewport';
 
 export function Project() {
   const { id } = useParams<{ id: string }>();
@@ -16,6 +18,8 @@ export function Project() {
   const [creating, setCreating] = useState(false);
   const [archivedSessions, setArchivedSessions] = useState<Session[] | null>(null);
   const [showArchived, setShowArchived] = useState(false);
+  const { setOpen: setDrawerOpen } = useSidebarDrawer();
+  const { isMobile } = useViewport();
 
   useEffect(() => {
     if (!id) return;
@@ -76,16 +80,31 @@ export function Project() {
 
   return (
     <div className="flex-1 flex flex-col">
-      <header className="border-b px-6 py-3 flex items-center justify-between">
-        <div>
-          <h1 className="text-lg font-semibold tracking-tight">
-            {project?.name ?? '…'}
-          </h1>
-          <div className="text-xs text-muted-foreground font-mono">
-            {project?.path}
+      <header
+        className="border-b px-6 py-3 flex items-center justify-between gap-2"
+        style={{ paddingTop: 'max(0.75rem, env(safe-area-inset-top))' }}
+      >
+        <div className="flex items-center gap-2 min-w-0">
+          {isMobile && (
+            <button
+              type="button"
+              onClick={() => setDrawerOpen(true)}
+              className="inline-flex items-center justify-center -ml-2 min-w-[44px] min-h-[44px] rounded text-muted-foreground hover:bg-muted hover:text-foreground shrink-0"
+              aria-label="Open sidebar"
+            >
+              <Menu className="size-5" />
+            </button>
+          )}
+          <div className="min-w-0">
+            <h1 className="text-lg font-semibold tracking-tight truncate">
+              {project?.name ?? '…'}
+            </h1>
+            <div className="text-xs text-muted-foreground font-mono truncate">
+              {project?.path}
+            </div>
           </div>
         </div>
-        <Button onClick={handleNew} disabled={creating}>
+        <Button onClick={handleNew} disabled={creating} className="shrink-0">
           <Plus />
           New session
         </Button>
diff --git a/apps/web/src/pages/Session.tsx b/apps/web/src/pages/Session.tsx
index 5d4f82b..fcb1ddd 100644
--- a/apps/web/src/pages/Session.tsx
+++ b/apps/web/src/pages/Session.tsx
@@ -1,10 +1,12 @@
 import { useEffect, useState } from 'react';
 import { Link, useNavigate, useParams } from 'react-router-dom';
-import { ChevronRight } from 'lucide-react';
+import { ChevronRight, Menu } from 'lucide-react';
 import { api } from '@/api/client';
 import type { Project, Session as SessionType } from '@/api/types';
 import { sessionEvents } from '@/hooks/sessionEvents';
 import { useActivePane } from '@/hooks/useActivePane';
+import { useSidebarDrawer } from '@/hooks/useSidebarDrawer';
+import { useViewport } from '@/hooks/useViewport';
 import { Workspace } from '@/components/Workspace';
 import { ModelPicker } from '@/components/ModelPicker';
 
@@ -16,6 +18,8 @@ export function Session() {
   const [name, setName] = useState('');
   const [editingName, setEditingName] = useState(false);
   const active = useActivePane();
+  const { setOpen: setDrawerOpen } = useSidebarDrawer();
+  const { isMobile } = useViewport();
 
   useEffect(() => {
     if (!id) return;
@@ -83,7 +87,17 @@ export function Session() {
 
   return (
     <div className="flex-1 flex flex-col min-h-0">
-      <header className="border-b px-4 py-2 flex items-center gap-1.5 shrink-0 text-sm">
+      <header className="border-b px-4 py-2 flex items-center gap-1.5 shrink-0 text-sm" style={{ paddingTop: 'max(0.5rem, env(safe-area-inset-top))' }}>
+        {isMobile && (
+          <button
+            type="button"
+            onClick={() => setDrawerOpen(true)}
+            className="inline-flex items-center justify-center -ml-1 mr-1 min-w-[44px] min-h-[44px] rounded text-muted-foreground hover:bg-muted hover:text-foreground"
+            aria-label="Open sidebar"
+          >
+            <Menu className="size-5" />
+          </button>
+        )}
         <Link to="/" className="text-muted-foreground hover:text-foreground">
           Projects
         </Link>