diff --git a/backend/app/gateway/auth/models.py b/backend/app/gateway/auth/models.py
index d8f9b954..1e9f91a7 100644
--- a/backend/app/gateway/auth/models.py
+++ b/backend/app/gateway/auth/models.py
@@ -31,6 +31,12 @@ class User(BaseModel):
     needs_setup: bool = Field(default=False, description="True for auto-created admin until setup completes")
     token_version: int = Field(default=0, description="Incremented on password change to invalidate old JWTs")
 
+    # Workspace linkage (Stage 0 PR4)
+    default_workspace_id: str | None = Field(
+        default=None,
+        description="The workspace the user lands in by default after login. NULL → /select-workspace.",
+    )
+
 
 class UserResponse(BaseModel):
     """Response model for user info endpoint."""
@@ -39,3 +45,18 @@ class UserResponse(BaseModel):
     email: str
     system_role: Literal["admin", "user"]
     needs_setup: bool = False
+
+
+class ActiveWorkspace(BaseModel):
+    """Lightweight workspace proxy injected into the request-scoped contextvar.
+
+    Implements the structural ``CurrentWorkspace`` protocol expected by
+    ``deerflow.runtime.workspace_context``: only ``.id`` (str) and
+    ``.role`` (str) are required. We intentionally do *not* embed the
+    full ``WorkspaceRow`` here — the middleware needs to set the
+    contextvar on every request and an extra DB lookup just to populate
+    a name/slug we don't use yet would be wasted work.
+    """
+
+    id: str
+    role: str
diff --git a/backend/app/gateway/auth_middleware.py b/backend/app/gateway/auth_middleware.py
index 6b645226..7895efa7 100644
--- a/backend/app/gateway/auth_middleware.py
+++ b/backend/app/gateway/auth_middleware.py
@@ -17,9 +17,11 @@ from starlette.responses import JSONResponse
 from starlette.types import ASGIApp
 
 from app.gateway.auth.errors import AuthErrorCode, AuthErrorResponse
+from app.gateway.auth.models import ActiveWorkspace
 from app.gateway.authz import _ALL_PERMISSIONS, AuthContext
 from app.gateway.internal_auth import INTERNAL_AUTH_HEADER_NAME, get_internal_user, is_valid_internal_auth_token
 from deerflow.runtime.user_context import reset_current_user, set_current_user
+from deerflow.runtime.workspace_context import reset_current_workspace, set_current_workspace
 
 # Paths that never require authentication.
 _PUBLIC_PATH_PREFIXES: tuple[str, ...] = (
@@ -119,8 +121,22 @@ class AuthMiddleware(BaseHTTPMiddleware):
         # JWT-decode + DB-lookup pipeline a second time per request).
         request.state.user = user
         request.state.auth = AuthContext(user=user, permissions=_ALL_PERMISSIONS)
-        token = set_current_user(user)
+        user_token = set_current_user(user)
+
+        # Inject workspace contextvar from the JWT's wid/role claims.
+        # decode_token has already rejected legacy no-wid tokens upstream,
+        # so by the time we get here payload.wid is guaranteed non-None
+        # for cookie-authenticated requests. Internal-auth requests skip
+        # the workspace contextvar (they don't have a workspace scope —
+        # the internal user is a system actor).
+        ws_token = None
+        payload = getattr(request.state, "auth_payload", None)
+        if payload is not None and payload.wid is not None:
+            ws_token = set_current_workspace(ActiveWorkspace(id=payload.wid, role=payload.role or "owner"))
+
         try:
             return await call_next(request)
         finally:
-            reset_current_user(token)
+            if ws_token is not None:
+                reset_current_workspace(ws_token)
+            reset_current_user(user_token)
diff --git a/backend/app/gateway/deps.py b/backend/app/gateway/deps.py
index 96ea7c5e..96ddecd1 100644
--- a/backend/app/gateway/deps.py
+++ b/backend/app/gateway/deps.py
@@ -220,6 +220,10 @@ async def get_current_user_from_request(request: Request):
             detail=AuthErrorResponse(code=AuthErrorCode.TOKEN_INVALID, message="Token revoked (password changed)").model_dump(),
         )
 
+    # Stash decoded payload on request.state so AuthMiddleware can read
+    # wid/role for the workspace contextvar without a second decode.
+    request.state.auth_payload = payload
+
     return user
 
 
diff --git a/backend/tests/test_auth_middleware_workspace.py b/backend/tests/test_auth_middleware_workspace.py
new file mode 100644
index 00000000..9b208537
--- /dev/null
+++ b/backend/tests/test_auth_middleware_workspace.py
@@ -0,0 +1,130 @@
+"""AuthMiddleware injects the workspace ContextVar (Stage 0 PR4 T4.7).
+
+After PR4 every authenticated request has a workspace bound on
+``deerflow.runtime.workspace_context._current_workspace``. The middleware
+populates it from the JWT's ``wid`` / ``role`` claims, mirrors what it
+already does for ``user_context``, and tears both down in a single
+``try/finally`` so leaks don't cross requests.
+
+Legacy 4-field tokens (no ``wid``) are rejected upstream by
+``decode_token`` (T4.6) — those should never reach the workspace
+injection branch; this file pins that the 401 they trigger carries
+``AuthErrorCode.WORKSPACE_REQUIRED``.
+"""
+
+from __future__ import annotations
+
+from datetime import UTC, datetime, timedelta
+from unittest.mock import AsyncMock, patch
+from uuid import uuid4
+
+import jwt
+import pytest
+from fastapi import FastAPI
+from starlette.testclient import TestClient
+
+from app.gateway.auth import create_access_token
+from app.gateway.auth.config import get_auth_config
+from app.gateway.auth.models import User
+from app.gateway.auth_middleware import AuthMiddleware
+from deerflow.runtime.workspace_context import get_current_workspace
+
+
+@pytest.fixture(autouse=True)
+def _stable_jwt_secret(monkeypatch):
+    monkeypatch.setenv("AUTH_JWT_SECRET", "test-secret-key-for-jwt-testing-minimum-32-chars")
+    yield
+
+
+def _make_app() -> FastAPI:
+    """App with AuthMiddleware + an inspect route that surfaces the contextvar."""
+    app = FastAPI()
+    app.add_middleware(AuthMiddleware)
+
+    @app.get("/api/v1/auth/setup-status")  # public — never gates on wid
+    async def setup_status():
+        return {"needs_setup": False}
+
+    @app.get("/api/models")  # protected — exercises wid injection
+    async def inspect_workspace():
+        ws = get_current_workspace()
+        if ws is None:
+            return {"workspace": None}
+        return {"workspace": {"id": ws.id, "role": ws.role}}
+
+    return app
+
+
+def _make_user(uid: str) -> User:
+    return User(id=uid, email="t@example.com", password_hash="hash", token_version=0)
+
+
+def _make_legacy_token() -> str:
+    """Encode a pre-PR4 JWT (no wid/role) directly."""
+    now = datetime.now(UTC)
+    payload = {
+        "sub": str(uuid4()),
+        "exp": now + timedelta(hours=1),
+        "iat": now,
+        "ver": 0,
+    }
+    return jwt.encode(payload, get_auth_config().jwt_secret, algorithm="HS256")
+
+
+def test_new_jwt_injects_workspace_into_contextvar() -> None:
+    """Cookie with wid+role → route observes the workspace via the contextvar."""
+    uid = str(uuid4())
+    token = create_access_token(uid, workspace_id="ws-abc", role="owner")
+
+    with patch("app.gateway.deps.get_local_provider") as fn:
+        fn.return_value.get_user = AsyncMock(return_value=_make_user(uid))
+        client = TestClient(_make_app())
+        res = client.get("/api/models", cookies={"access_token": token})
+
+    assert res.status_code == 200, res.text
+    assert res.json() == {"workspace": {"id": "ws-abc", "role": "owner"}}
+
+
+def test_legacy_jwt_rejected_with_workspace_required() -> None:
+    """No-wid tokens get 401 with AuthErrorCode.WORKSPACE_REQUIRED, not generic token_invalid."""
+    client = TestClient(_make_app())
+    res = client.get("/api/models", cookies={"access_token": _make_legacy_token()})
+
+    assert res.status_code == 401
+    assert res.json()["detail"]["code"] == "workspace_required"
+
+
+def test_public_path_skips_workspace_check() -> None:
+    """Public whitelist (e.g. /api/v1/auth/setup-status) does not require wid."""
+    client = TestClient(_make_app())
+    res = client.get("/api/v1/auth/setup-status")  # no cookie at all
+    assert res.status_code == 200
+
+
+def test_workspace_contextvar_resets_between_requests() -> None:
+    """After dispatch returns the contextvar must be clear (no leak across requests).
+
+    Why we test this: if the try/finally is wired only for user_context but
+    not workspace_context, two back-to-back requests can see each other's
+    workspace under asyncio task switching.
+    """
+    uid = str(uuid4())
+    token = create_access_token(uid, workspace_id="ws-first", role="owner")
+
+    # First request resolves to ws-first
+    with patch("app.gateway.deps.get_local_provider") as fn:
+        fn.return_value.get_user = AsyncMock(return_value=_make_user(uid))
+        client = TestClient(_make_app())
+        res1 = client.get("/api/models", cookies={"access_token": token})
+    assert res1.json() == {"workspace": {"id": "ws-first", "role": "owner"}}
+
+    # Outside the request scope the contextvar must be empty again.
+    assert get_current_workspace() is None
+
+    # Second request with a different workspace must not see ws-first.
+    token2 = create_access_token(uid, workspace_id="ws-second", role="owner")
+    with patch("app.gateway.deps.get_local_provider") as fn:
+        fn.return_value.get_user = AsyncMock(return_value=_make_user(uid))
+        client = TestClient(_make_app())
+        res2 = client.get("/api/models", cookies={"access_token": token2})
+    assert res2.json() == {"workspace": {"id": "ws-second", "role": "owner"}}