diff --git a/l2/app/__init__.py b/l2/app/__init__.py
index add533b..e4938e2 100644
--- a/l2/app/__init__.py
+++ b/l2/app/__init__.py
@@ -4,16 +4,38 @@ Art-DAG L2 Server Application Factory.
 Creates and configures the FastAPI application with all routers and middleware.
 """
 
+import secrets
+import time
 from pathlib import Path
 from contextlib import asynccontextmanager
+from urllib.parse import quote
+
 from fastapi import FastAPI, Request
-from fastapi.responses import JSONResponse, HTMLResponse
+from fastapi.responses import JSONResponse, HTMLResponse, RedirectResponse
 
 from artdag_common import create_jinja_env
 from artdag_common.middleware.auth import get_user_from_cookie
 
 from .config import settings
 
+# Paths that should never trigger a silent auth check
+_SKIP_PREFIXES = ("/auth/", "/.well-known/", "/health",
+                  "/internal/", "/static/", "/inbox")
+_SILENT_CHECK_COOLDOWN = 300  # 5 minutes
+_DEVICE_COOKIE = "artdag_did"
+_DEVICE_COOKIE_MAX_AGE = 30 * 24 * 3600  # 30 days
+
+# Derive external base URL from oauth_redirect_uri (e.g. https://artdag.rose-ash.com)
+_EXTERNAL_BASE = settings.oauth_redirect_uri.rsplit("/auth/callback", 1)[0]
+
+
+def _external_url(request: Request) -> str:
+    """Build external URL from request path + query, using configured base domain."""
+    url = f"{_EXTERNAL_BASE}{request.url.path}"
+    if request.url.query:
+        url += f"?{request.url.query}"
+    return url
+
 
 @asynccontextmanager
 async def lifespan(app: FastAPI):
@@ -38,6 +60,64 @@ def create_app() -> FastAPI:
         lifespan=lifespan,
     )
 
+    # Silent auth check — auto-login via prompt=none OAuth
+    # NOTE: registered BEFORE device_id so device_id is outermost (runs first)
+    @app.middleware("http")
+    async def silent_auth_check(request: Request, call_next):
+        path = request.url.path
+        if (
+            request.method != "GET"
+            or any(path.startswith(p) for p in _SKIP_PREFIXES)
+            or request.headers.get("hx-request")  # skip HTMX
+        ):
+            return await call_next(request)
+
+        # Already logged in — pass through
+        if get_user_from_cookie(request):
+            return await call_next(request)
+
+        # Check cooldown — don't re-check within 5 minutes
+        pnone_at = request.cookies.get("pnone_at")
+        if pnone_at:
+            try:
+                pnone_ts = float(pnone_at)
+                if (time.time() - pnone_ts) < _SILENT_CHECK_COOLDOWN:
+                    return await call_next(request)
+            except (ValueError, TypeError):
+                pass
+
+        # Redirect to silent OAuth check
+        current_url = _external_url(request)
+        return RedirectResponse(
+            url=f"/auth/login?prompt=none&next={quote(current_url, safe='')}",
+            status_code=302,
+        )
+
+    # Device ID middleware — track browser identity across domains
+    # Registered AFTER silent_auth_check so it's outermost (always runs)
+    @app.middleware("http")
+    async def device_id_middleware(request: Request, call_next):
+        did = request.cookies.get(_DEVICE_COOKIE)
+        if did:
+            request.state.device_id = did
+            request.state._new_device_id = False
+        else:
+            request.state.device_id = secrets.token_urlsafe(32)
+            request.state._new_device_id = True
+
+        response = await call_next(request)
+
+        if getattr(request.state, "_new_device_id", False):
+            response.set_cookie(
+                key=_DEVICE_COOKIE,
+                value=request.state.device_id,
+                max_age=_DEVICE_COOKIE_MAX_AGE,
+                httponly=True,
+                samesite="lax",
+                secure=True,
+            )
+        return response
+
     # Coop fragment pre-fetch — inject nav-tree, auth-menu, cart-mini
     _FRAG_SKIP = ("/auth/", "/.well-known/", "/health",
                    "/internal/", "/static/", "/inbox")
diff --git a/l2/app/config.py b/l2/app/config.py
index d88d435..d08b3ed 100644
--- a/l2/app/config.py
+++ b/l2/app/config.py
@@ -33,6 +33,14 @@ class Settings:
     jwt_algorithm: str = "HS256"
     access_token_expire_minutes: int = 60 * 24 * 30  # 30 days
 
+    # OAuth SSO (via account.rose-ash.com)
+    oauth_authorize_url: str = os.environ.get("OAUTH_AUTHORIZE_URL", "https://account.rose-ash.com/auth/oauth/authorize")
+    oauth_token_url: str = os.environ.get("OAUTH_TOKEN_URL", "https://account.rose-ash.com/auth/oauth/token")
+    oauth_client_id: str = os.environ.get("OAUTH_CLIENT_ID", "artdag_l2")
+    oauth_redirect_uri: str = os.environ.get("OAUTH_REDIRECT_URI", "https://artdag.rose-ash.com/auth/callback")
+    oauth_logout_url: str = os.environ.get("OAUTH_LOGOUT_URL", "https://account.rose-ash.com/auth/sso-logout/")
+    secret_key: str = os.environ.get("SECRET_KEY", "change-me-in-production")
+
     def __post_init__(self):
         # Parse L1 servers
         l1_str = os.environ.get("L1_SERVERS", "https://celery-artdag.rose-ash.com")
diff --git a/l2/app/routers/auth.py b/l2/app/routers/auth.py
index 4691caf..98fea5a 100644
--- a/l2/app/routers/auth.py
+++ b/l2/app/routers/auth.py
@@ -1,223 +1,164 @@
 """
-Authentication routes for L2 server.
+Authentication routes — OAuth2 authorization code flow via account.rose-ash.com.
 
-Handles login, registration, logout, and token verification.
+GET  /auth/login    — redirect to account OAuth authorize
+GET  /auth/callback — exchange code for user info, set session cookie
+GET  /auth/logout   — clear cookie, redirect through account SSO logout
 """
 
-import hashlib
-from datetime import datetime, timezone
+import secrets
+import time
 
-from fastapi import APIRouter, Request, Form, HTTPException, Depends
-from fastapi.responses import HTMLResponse, RedirectResponse
-from fastapi.security import HTTPBearer, HTTPAuthorizationCredentials
+import httpx
+from fastapi import APIRouter, Request
+from fastapi.responses import RedirectResponse
+from itsdangerous import URLSafeSerializer
 
-from artdag_common import render
-from artdag_common.middleware import wants_html
+from artdag_common.middleware.auth import UserContext, set_auth_cookie, clear_auth_cookie
 
 from ..config import settings
-from ..dependencies import get_templates, get_user_from_cookie
 
 router = APIRouter()
-security = HTTPBearer(auto_error=False)
+
+_signer = None
 
 
-@router.get("/login", response_class=HTMLResponse)
-async def login_page(request: Request, return_to: str = None):
-    """Login page."""
-    username = get_user_from_cookie(request)
+def _get_signer() -> URLSafeSerializer:
+    global _signer
+    if _signer is None:
+        _signer = URLSafeSerializer(settings.secret_key, salt="oauth-state")
+    return _signer
 
-    if username:
-        templates = get_templates(request)
-        return render(templates, "auth/already_logged_in.html", request,
-            user={"username": username},
-        )
 
-    templates = get_templates(request)
-    return render(templates, "auth/login.html", request,
-        return_to=return_to,
+@router.get("/login")
+async def login(request: Request):
+    """Store state + next in signed cookie, redirect to account OAuth authorize."""
+    next_url = request.query_params.get("next", "/")
+    prompt = request.query_params.get("prompt", "")
+    state = secrets.token_urlsafe(32)
+
+    signer = _get_signer()
+    state_payload = signer.dumps({"state": state, "next": next_url, "prompt": prompt})
+
+    device_id = getattr(request.state, "device_id", "")
+    authorize_url = (
+        f"{settings.oauth_authorize_url}"
+        f"?client_id={settings.oauth_client_id}"
+        f"&redirect_uri={settings.oauth_redirect_uri}"
+        f"&device_id={device_id}"
+        f"&state={state}"
     )
+    if prompt:
+        authorize_url += f"&prompt={prompt}"
 
-
-@router.post("/login", response_class=HTMLResponse)
-async def login_submit(
-    request: Request,
-    username: str = Form(...),
-    password: str = Form(...),
-    return_to: str = Form(None),
-):
-    """Handle login form submission."""
-    from auth import authenticate_user, create_access_token
-
-    if not username or not password:
-        return HTMLResponse(
-            '<div class="text-red-400">Username and password are required</div>'
-        )
-
-    user = await authenticate_user(settings.data_dir, username.strip(), password)
-    if not user:
-        return HTMLResponse(
-            '<div class="text-red-400">Invalid username or password</div>'
-        )
-
-    token = create_access_token(user.username, l2_server=f"https://{settings.domain}")
-
-    # Handle return_to redirect
-    if return_to and return_to.startswith("http"):
-        separator = "&" if "?" in return_to else "?"
-        redirect_url = f"{return_to}{separator}auth_token={token.access_token}"
-        response = HTMLResponse(f'''
-            <div class="text-green-400">Login successful! Redirecting...</div>
-            <script>window.location.href = "{redirect_url}";</script>
-        ''')
-    else:
-        response = HTMLResponse('''
-            <div class="text-green-400">Login successful! Redirecting...</div>
-            <script>window.location.href = "/";</script>
-        ''')
-
+    response = RedirectResponse(url=authorize_url, status_code=302)
     response.set_cookie(
-        key="auth_token",
-        value=token.access_token,
+        key="oauth_state",
+        value=state_payload,
+        max_age=600,  # 10 minutes
         httponly=True,
-        max_age=60 * 60 * 24 * 30,
         samesite="lax",
         secure=True,
     )
     return response
 
 
-@router.get("/register", response_class=HTMLResponse)
-async def register_page(request: Request):
-    """Registration page."""
-    username = get_user_from_cookie(request)
+@router.get("/callback")
+async def callback(request: Request):
+    """Validate state, exchange code via token endpoint, set session cookie."""
+    code = request.query_params.get("code", "")
+    state = request.query_params.get("state", "")
+    error = request.query_params.get("error", "")
+    account_did = request.query_params.get("account_did", "")
 
-    if username:
-        templates = get_templates(request)
-        return render(templates, "auth/already_logged_in.html", request,
-            user={"username": username},
-        )
-
-    templates = get_templates(request)
-    return render(templates, "auth/register.html", request)
-
-
-@router.post("/register", response_class=HTMLResponse)
-async def register_submit(
-    request: Request,
-    username: str = Form(...),
-    password: str = Form(...),
-    password2: str = Form(...),
-    email: str = Form(None),
-):
-    """Handle registration form submission."""
-    from auth import create_user, create_access_token
-
-    if not username or not password:
-        return HTMLResponse('<div class="text-red-400">Username and password are required</div>')
-
-    if password != password2:
-        return HTMLResponse('<div class="text-red-400">Passwords do not match</div>')
-
-    if len(password) < 6:
-        return HTMLResponse('<div class="text-red-400">Password must be at least 6 characters</div>')
+    # Adopt account's device ID as our own (one identity across all apps)
+    if account_did:
+        request.state.device_id = account_did
+        request.state._new_device_id = True  # device_id middleware will set cookie
 
+    # Recover state from signed cookie
+    state_cookie = request.cookies.get("oauth_state", "")
+    signer = _get_signer()
     try:
-        user = await create_user(settings.data_dir, username.strip(), password, email)
-    except ValueError as e:
-        return HTMLResponse(f'<div class="text-red-400">{str(e)}</div>')
+        payload = signer.loads(state_cookie) if state_cookie else {}
+    except Exception:
+        payload = {}
 
-    token = create_access_token(user.username, l2_server=f"https://{settings.domain}")
+    next_url = payload.get("next", "/")
 
-    response = HTMLResponse('''
-        <div class="text-green-400">Registration successful! Redirecting...</div>
-        <script>window.location.href = "/";</script>
-    ''')
-    response.set_cookie(
-        key="auth_token",
-        value=token.access_token,
-        httponly=True,
-        max_age=60 * 60 * 24 * 30,
-        samesite="lax",
-        secure=True,
-    )
+    # Handle prompt=none rejection (user not logged in on account)
+    if error == "login_required":
+        response = RedirectResponse(url=next_url, status_code=302)
+        response.delete_cookie("oauth_state")
+        # Set cooldown cookie — don't re-check for 5 minutes
+        response.set_cookie(
+            key="pnone_at",
+            value=str(time.time()),
+            max_age=300,
+            httponly=True,
+            samesite="lax",
+            secure=True,
+        )
+        # Set device cookie if adopted
+        if account_did:
+            response.set_cookie(
+                key="artdag_did",
+                value=account_did,
+                max_age=30 * 24 * 3600,
+                httponly=True,
+                samesite="lax",
+                secure=True,
+            )
+        return response
+
+    # Normal callback — validate state + code
+    if not state_cookie or not code or not state:
+        return RedirectResponse(url="/", status_code=302)
+
+    if payload.get("state") != state:
+        return RedirectResponse(url="/", status_code=302)
+
+    # Exchange code for user info via account's token endpoint
+    async with httpx.AsyncClient(timeout=10) as client:
+        try:
+            resp = await client.post(
+                settings.oauth_token_url,
+                json={
+                    "code": code,
+                    "client_id": settings.oauth_client_id,
+                    "redirect_uri": settings.oauth_redirect_uri,
+                },
+            )
+        except httpx.HTTPError:
+            return RedirectResponse(url="/", status_code=302)
+
+    if resp.status_code != 200:
+        return RedirectResponse(url="/", status_code=302)
+
+    data = resp.json()
+    if "error" in data:
+        return RedirectResponse(url="/", status_code=302)
+
+    # Map OAuth response to artdag UserContext
+    # Note: account token endpoint returns user.email as "username"
+    username = data.get("username", "")
+    email = username  # OAuth response "username" is the user's email
+    actor_id = f"@{username}"
+
+    user = UserContext(username=username, actor_id=actor_id, email=email)
+
+    response = RedirectResponse(url=next_url, status_code=302)
+    set_auth_cookie(response, user)
+    response.delete_cookie("oauth_state")
+    response.delete_cookie("pnone_at")
     return response
 
 
 @router.get("/logout")
-async def logout(request: Request):
-    """Handle logout."""
-    import db
-    import requests
-    from auth import get_token_claims
-
-    token = request.cookies.get("auth_token")
-    claims = get_token_claims(token) if token else None
-    username = claims.get("sub") if claims else None
-
-    if username and token and claims:
-        # Revoke token in database
-        token_hash = hashlib.sha256(token.encode()).hexdigest()
-        expires_at = datetime.fromtimestamp(claims.get("exp", 0), tz=timezone.utc)
-        await db.revoke_token(token_hash, username, expires_at)
-
-        # Revoke on attached L1 servers
-        attached = await db.get_user_renderers(username)
-        for l1_url in attached:
-            try:
-                requests.post(
-                    f"{l1_url}/auth/revoke-user",
-                    json={"username": username, "l2_server": f"https://{settings.domain}"},
-                    timeout=5,
-                )
-            except Exception:
-                pass
-
-    response = RedirectResponse(url="/", status_code=302)
-    response.delete_cookie("auth_token")
+async def logout():
+    """Clear session cookie, redirect through account SSO logout."""
+    response = RedirectResponse(url=settings.oauth_logout_url, status_code=302)
+    clear_auth_cookie(response)
+    response.delete_cookie("oauth_state")
+    response.delete_cookie("pnone_at")
     return response
-
-
-@router.get("/verify")
-async def verify_token(
-    request: Request,
-    credentials: HTTPAuthorizationCredentials = Depends(security),
-):
-    """
-    Verify a token is valid.
-
-    Called by L1 servers to verify tokens during auth callback.
-    Returns user info if valid, 401 if not.
-    """
-    import db
-    from auth import verify_token as verify_jwt, get_token_claims
-
-    # Get token from Authorization header or query param
-    token = None
-    if credentials:
-        token = credentials.credentials
-    else:
-        # Try Authorization header manually (for clients that don't use Bearer format)
-        auth_header = request.headers.get("Authorization", "")
-        if auth_header.startswith("Bearer "):
-            token = auth_header[7:]
-
-    if not token:
-        raise HTTPException(401, "No token provided")
-
-    # Verify JWT signature and expiry
-    username = verify_jwt(token)
-    if not username:
-        raise HTTPException(401, "Invalid or expired token")
-
-    # Check if token is revoked
-    claims = get_token_claims(token)
-    if claims:
-        token_hash = hashlib.sha256(token.encode()).hexdigest()
-        if await db.is_token_revoked(token_hash):
-            raise HTTPException(401, "Token has been revoked")
-
-    return {
-        "valid": True,
-        "username": username,
-        "claims": claims,
-    }
diff --git a/l2/docker-compose.yml b/l2/docker-compose.yml
index 0f67e81..9c91ea4 100644
--- a/l2/docker-compose.yml
+++ b/l2/docker-compose.yml
@@ -52,7 +52,13 @@ services:
       - INTERNAL_URL_BLOG=http://blog:8000
       - INTERNAL_URL_CART=http://cart:8000
       - INTERNAL_URL_ACCOUNT=http://account:8000
-      # DATABASE_URL, ARTDAG_DOMAIN, ARTDAG_USER, JWT_SECRET from .env file
+      # OAuth SSO
+      - OAUTH_AUTHORIZE_URL=https://account.rose-ash.com/auth/oauth/authorize
+      - OAUTH_TOKEN_URL=https://account.rose-ash.com/auth/oauth/token
+      - OAUTH_CLIENT_ID=artdag_l2
+      - OAUTH_REDIRECT_URI=https://artdag.rose-ash.com/auth/callback
+      - OAUTH_LOGOUT_URL=https://account.rose-ash.com/auth/sso-logout/
+      # DATABASE_URL, ARTDAG_DOMAIN, ARTDAG_USER, JWT_SECRET, SECRET_KEY from .env file
     healthcheck:
       test: ["CMD", "python", "-c", "import urllib.request; urllib.request.urlopen('http://localhost:8200/')"]
       interval: 10s
diff --git a/l2/requirements.txt b/l2/requirements.txt
index 5d228c5..83b95d2 100644
--- a/l2/requirements.txt
+++ b/l2/requirements.txt
@@ -7,6 +7,7 @@ bcrypt>=4.0.0
 python-jose[cryptography]>=3.3.0
 markdown>=3.5.0
 python-multipart>=0.0.6
+itsdangerous>=2.1.0
 asyncpg>=0.29.0
 boto3>=1.34.0
 # common (artdag_common) installed from local dir in Dockerfile