diff --git a/backend/app/gateway/auth/models.py b/backend/app/gateway/auth/models.py
index 1e9f91a7..4a904fec 100644
--- a/backend/app/gateway/auth/models.py
+++ b/backend/app/gateway/auth/models.py
@@ -47,6 +47,26 @@ class UserResponse(BaseModel):
     needs_setup: bool = False
 
 
+class UserMeWorkspace(BaseModel):
+    """One workspace entry in ``GET /auth/me`` (Stage 0 PR4)."""
+
+    id: str
+    name: str
+    slug: str
+    role: str
+
+
+class UserMeResponse(BaseModel):
+    """Response model for ``GET /auth/me`` — extends UserResponse with workspaces."""
+
+    id: str
+    email: str
+    system_role: Literal["admin", "user"]
+    needs_setup: bool = False
+    default_workspace_id: str | None = None
+    workspaces: list[UserMeWorkspace] = []
+
+
 class ActiveWorkspace(BaseModel):
     """Lightweight workspace proxy injected into the request-scoped contextvar.
 
diff --git a/backend/app/gateway/routers/auth.py b/backend/app/gateway/routers/auth.py
index dd8a7ff5..5635bcdd 100644
--- a/backend/app/gateway/routers/auth.py
+++ b/backend/app/gateway/routers/auth.py
@@ -15,6 +15,7 @@ from app.gateway.auth import (
 )
 from app.gateway.auth.config import get_auth_config
 from app.gateway.auth.errors import AuthErrorCode, AuthErrorResponse
+from app.gateway.auth.models import UserMeResponse, UserMeWorkspace
 from app.gateway.auth.workspace_slug import auto_slug_from_email, next_available_slug
 from app.gateway.csrf_middleware import is_secure_request
 from app.gateway.deps import get_current_user_from_request, get_local_provider
@@ -447,11 +448,41 @@ async def change_password(request: Request, response: Response, body: ChangePass
     return MessageResponse(message="Password changed successfully")
 
 
-@router.get("/me", response_model=UserResponse)
+@router.get("/me", response_model=UserMeResponse)
 async def get_me(request: Request):
-    """Get current authenticated user info."""
+    """Get current authenticated user info, including the workspaces they belong to."""
+    from deerflow.persistence.engine import get_session_factory
+    from deerflow.persistence.workspace import WorkspaceRepository
+    from deerflow.persistence.workspace_membership import WorkspaceMembershipRepository
+
     user = await get_current_user_from_request(request)
-    return UserResponse(id=str(user.id), email=user.email, system_role=user.system_role, needs_setup=user.needs_setup)
+
+    sf = get_session_factory()
+    workspaces: list[UserMeWorkspace] = []
+    if sf is not None:
+        ws_repo = WorkspaceRepository(sf)
+        m_repo = WorkspaceMembershipRepository(sf)
+        ws_rows = await ws_repo.list_by_user(user_id=str(user.id))
+        memberships = await m_repo.list_by_user(user_id=str(user.id))
+        role_by_ws = {m["workspace_id"]: m["role"] for m in memberships}
+        workspaces = [
+            UserMeWorkspace(
+                id=w["id"],
+                name=w["name"],
+                slug=w["slug"],
+                role=role_by_ws.get(w["id"], "member"),
+            )
+            for w in ws_rows
+        ]
+
+    return UserMeResponse(
+        id=str(user.id),
+        email=user.email,
+        system_role=user.system_role,
+        needs_setup=user.needs_setup,
+        default_workspace_id=user.default_workspace_id,
+        workspaces=workspaces,
+    )
 
 
 _SETUP_STATUS_COOLDOWN: dict[str, float] = {}
diff --git a/backend/tests/test_auth_me_returns_workspaces.py b/backend/tests/test_auth_me_returns_workspaces.py
new file mode 100644
index 00000000..1a853958
--- /dev/null
+++ b/backend/tests/test_auth_me_returns_workspaces.py
@@ -0,0 +1,90 @@
+"""``GET /auth/me`` returns the user's workspace memberships.
+
+Stage 0 PR4 T4.11. After PR4 the frontend needs to discover which
+workspaces the current user belongs to (eventually for a picker UI).
+Each membership entry surfaces ``id``, ``name``, ``slug``, ``role``
+so the picker can render the list without a second roundtrip.
+"""
+
+from __future__ import annotations
+
+import asyncio
+import os
+
+import pytest
+from fastapi.testclient import TestClient
+
+os.environ.setdefault("AUTH_JWT_SECRET", "test-secret-key-auth-me-workspaces-32+")
+
+from app.gateway.auth.config import AuthConfig, set_auth_config
+
+_TEST_SECRET = "test-secret-key-auth-me-workspaces-32+"
+
+
+@pytest.fixture(autouse=True)
+def _setup_auth(tmp_path):
+    from app.gateway import deps
+    from app.gateway.routers.auth import _SETUP_STATUS_COOLDOWN
+    from deerflow.persistence.engine import close_engine, init_engine
+
+    set_auth_config(AuthConfig(jwt_secret=_TEST_SECRET))
+    url = f"sqlite+aiosqlite:///{tmp_path}/auth_me.db"
+    asyncio.run(init_engine("sqlite", url=url, sqlite_dir=str(tmp_path)))
+    deps._cached_local_provider = None
+    deps._cached_repo = None
+    _SETUP_STATUS_COOLDOWN.clear()
+    try:
+        yield
+    finally:
+        deps._cached_local_provider = None
+        deps._cached_repo = None
+        _SETUP_STATUS_COOLDOWN.clear()
+        asyncio.run(close_engine())
+
+
+@pytest.fixture()
+def client(_setup_auth):
+    from app.gateway.app import create_app
+
+    set_auth_config(AuthConfig(jwt_secret=_TEST_SECRET))
+    yield TestClient(create_app())
+
+
+def test_auth_me_returns_workspaces_list(client):
+    """After registration, /auth/me lists the user's single personal workspace."""
+    client.post("/api/v1/auth/initialize", json={"email": "admin@example.com", "password": "Str0ng!Pass99"})
+    reg = client.post("/api/v1/auth/register", json={"email": "alice@example.com", "password": "Tr0ub4dor3a-strong!"})
+    user_id = reg.json()["id"]
+
+    resp = client.get("/api/v1/auth/me")
+    assert resp.status_code == 200, resp.text
+
+    body = resp.json()
+    assert body["id"] == user_id
+    assert body["email"] == "alice@example.com"
+    assert body["default_workspace_id"], "user should land with a default workspace"
+
+    assert len(body["workspaces"]) == 1, body
+    ws = body["workspaces"][0]
+    assert ws["id"] == body["default_workspace_id"]
+    assert ws["slug"] == "alice"
+    assert ws["role"] == "owner"
+    assert ws["name"]  # whatever the helper picks — just sanity check it's non-empty
+
+
+def test_auth_me_workspaces_isolated_per_user(client):
+    """Two users see only their own workspaces in /auth/me."""
+    client.post("/api/v1/auth/initialize", json={"email": "admin@example.com", "password": "Str0ng!Pass99"})
+    client.post("/api/v1/auth/register", json={"email": "alice@example.com", "password": "Tr0ub4dor3a-strong!"})
+    a_id = client.get("/api/v1/auth/me").json()["id"]
+    a_workspaces = client.get("/api/v1/auth/me").json()["workspaces"]
+
+    client.cookies.clear()
+    client.post("/api/v1/auth/register", json={"email": "bob@example.com", "password": "Tr0ub4dor3a-strong!"})
+    b_id = client.get("/api/v1/auth/me").json()["id"]
+    b_workspaces = client.get("/api/v1/auth/me").json()["workspaces"]
+
+    assert a_id != b_id
+    assert {w["id"] for w in a_workspaces}.isdisjoint({w["id"] for w in b_workspaces})
+    assert {w["slug"] for w in a_workspaces} == {"alice"}
+    assert {w["slug"] for w in b_workspaces} == {"bob"}