diff --git a/backend/app/gateway/auth/repositories/sqlite.py b/backend/app/gateway/auth/repositories/sqlite.py
index 3ee3978e..767a2e21 100644
--- a/backend/app/gateway/auth/repositories/sqlite.py
+++ b/backend/app/gateway/auth/repositories/sqlite.py
@@ -46,6 +46,7 @@ class SQLiteUserRepository(UserRepository):
             oauth_id=row.oauth_id,
             needs_setup=row.needs_setup,
             token_version=row.token_version,
+            default_workspace_id=row.default_workspace_id,
         )
 
     @staticmethod
@@ -60,6 +61,7 @@ class SQLiteUserRepository(UserRepository):
             oauth_id=user.oauth_id,
             needs_setup=user.needs_setup,
             token_version=user.token_version,
+            default_workspace_id=user.default_workspace_id,
         )
 
     # ── CRUD ──────────────────────────────────────────────────────────
@@ -106,6 +108,7 @@ class SQLiteUserRepository(UserRepository):
             row.oauth_id = user.oauth_id
             row.needs_setup = user.needs_setup
             row.token_version = user.token_version
+            row.default_workspace_id = user.default_workspace_id
             await session.commit()
         return user
 
diff --git a/backend/app/gateway/routers/auth.py b/backend/app/gateway/routers/auth.py
index 3a41e13e..c2c36612 100644
--- a/backend/app/gateway/routers/auth.py
+++ b/backend/app/gateway/routers/auth.py
@@ -15,11 +15,59 @@ 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.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
 
 logger = logging.getLogger(__name__)
 
+
+async def _ensure_default_workspace(user) -> str:
+    """Create the user's personal workspace + owner membership, set default_workspace_id.
+
+    Returns the new workspace id. Idempotent for users who already
+    have a default_workspace_id (used by both the registration flow
+    and the lifespan backfill in app.py).
+    """
+    from deerflow.persistence.engine import get_session_factory
+    from deerflow.persistence.workspace import WorkspaceRepository
+    from deerflow.persistence.workspace.sql import SLUG_BLACKLIST
+    from deerflow.persistence.workspace_membership import WorkspaceMembershipRepository
+
+    if user.default_workspace_id:
+        return user.default_workspace_id
+
+    sf = get_session_factory()
+    ws_repo = WorkspaceRepository(sf)
+    m_repo = WorkspaceMembershipRepository(sf)
+
+    base_slug = auto_slug_from_email(user.email)
+
+    async def slug_exists(s: str) -> bool:
+        # Treat blacklisted slugs as "taken" so the walker skips them
+        # instead of letting WorkspaceRepository.create raise after a
+        # successful slug computation (the user picked a reserved name
+        # like "admin@example.com" → base slug "admin").
+        if s in SLUG_BLACKLIST:
+            return True
+        return (await ws_repo.get_by_slug(s)) is not None
+
+    unique_slug = await next_available_slug(base_slug, exists_check=slug_exists)
+
+    display_local = user.email.split("@", 1)[0]
+    workspace = await ws_repo.create(
+        name=f"{display_local}'s Workspace"[:64],
+        slug=unique_slug,
+        owner_id=str(user.id),
+    )
+    await m_repo.add(workspace_id=workspace["id"], user_id=str(user.id), role="owner")
+
+    user.default_workspace_id = workspace["id"]
+    await get_local_provider().update_user(user)
+
+    return workspace["id"]
+
+
 router = APIRouter(prefix="/api/v1/auth", tags=["auth"])
 
 
@@ -452,7 +500,14 @@ async def initialize_admin(request: Request, response: Response, body: Initializ
             detail=AuthErrorResponse(code=AuthErrorCode.SYSTEM_ALREADY_INITIALIZED, message="System already initialized").model_dump(),
         )
 
-    token = create_access_token(str(user.id), token_version=user.token_version)
+    workspace_id = await _ensure_default_workspace(user)
+
+    token = create_access_token(
+        str(user.id),
+        token_version=user.token_version,
+        workspace_id=workspace_id,
+        role="owner",
+    )
     _set_session_cookie(response, token, request)
 
     return UserResponse(id=str(user.id), email=user.email, system_role=user.system_role)
diff --git a/backend/packages/harness/deerflow/persistence/workspace/sql.py b/backend/packages/harness/deerflow/persistence/workspace/sql.py
index 5a213633..6aad5108 100644
--- a/backend/packages/harness/deerflow/persistence/workspace/sql.py
+++ b/backend/packages/harness/deerflow/persistence/workspace/sql.py
@@ -32,7 +32,7 @@ _SLUG_MAX_LEN = 32
 
 # slug 黑名单（应用层校验，不写 DB constraint）。包含 ADR-007 §4 保留 slug
 # + 路径 + Next.js 保留 + 业务保留词。
-_SLUG_BLACKLIST = frozenset(
+SLUG_BLACKLIST = frozenset(
     {
         "admin",
         "api",
@@ -77,7 +77,7 @@ def _validate_slug(slug: str) -> None:
         raise WorkspaceValidationError(f"slug length must be between {_SLUG_MIN_LEN} and {_SLUG_MAX_LEN}, got {len(slug)}")
     if not _SLUG_PATTERN.fullmatch(slug):
         raise WorkspaceValidationError(f"slug {slug!r} does not match required pattern ^[a-z0-9](-?[a-z0-9])*$")
-    if slug in _SLUG_BLACKLIST:
+    if slug in SLUG_BLACKLIST:
         raise WorkspaceValidationError(f"slug {slug!r} is reserved")
 
 
diff --git a/backend/tests/test_register_creates_workspace.py b/backend/tests/test_register_creates_workspace.py
new file mode 100644
index 00000000..51718d25
--- /dev/null
+++ b/backend/tests/test_register_creates_workspace.py
@@ -0,0 +1,172 @@
+"""Registration endpoints (POST /initialize, POST /register) auto-create a workspace.
+
+Stage 0 PR4 T4.8 + T4.9. Every newly registered user must end up with:
+
+- a single ``workspaces`` row (their personal workspace),
+- a single ``workspace_memberships`` row with ``role='owner'``,
+- ``users.default_workspace_id`` pointing at that workspace,
+- a session cookie whose JWT carries the workspace as the ``wid`` claim.
+
+Tests run against a per-test SQLite engine bootstrapped by the
+fixture; the registration router is exercised through the real
+TestClient so the full handler + DB transaction path is covered.
+"""
+
+from __future__ import annotations
+
+import asyncio
+import os
+
+import jwt
+import pytest
+from fastapi.testclient import TestClient
+
+os.environ.setdefault("AUTH_JWT_SECRET", "test-secret-key-register-workspace-32+")
+
+from app.gateway.auth.config import AuthConfig, set_auth_config
+
+_TEST_SECRET = "test-secret-key-register-workspace-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}/register_ws.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))
+    app = create_app()
+    yield TestClient(app)
+
+
+def _init_payload(**extra):
+    return {"email": "admin@example.com", "password": "Str0ng!Pass99", **extra}
+
+
+def _register_payload(email: str = "alice@example.com", **extra):
+    return {"email": email, "password": "Tr0ub4dor3a-strong!", **extra}
+
+
+def _decode(token: str) -> dict:
+    """Decode a JWT (signature-checked) and return the raw payload."""
+    return jwt.decode(token, _TEST_SECRET, algorithms=["HS256"])
+
+
+async def _read_workspace_state(user_id: str) -> dict:
+    """Inspect the per-user workspace state after a registration call.
+
+    Returns a dict with the workspace row, the owner membership row and
+    the user's default_workspace_id, so each test can pick what it
+    cares about.
+    """
+    from sqlalchemy import select
+
+    from deerflow.persistence.engine import get_session_factory
+    from deerflow.persistence.user.model import UserRow
+    from deerflow.persistence.workspace.model import WorkspaceRow
+    from deerflow.persistence.workspace_membership.model import WorkspaceMembershipRow
+
+    sf = get_session_factory()
+    async with sf() as session:
+        user = await session.get(UserRow, user_id)
+        memberships = (await session.execute(select(WorkspaceMembershipRow).where(WorkspaceMembershipRow.user_id == user_id))).scalars().all()
+        workspaces = []
+        if memberships:
+            workspaces = (await session.execute(select(WorkspaceRow).where(WorkspaceRow.id.in_([m.workspace_id for m in memberships])))).scalars().all()
+        return {
+            "user_default_workspace_id": getattr(user, "default_workspace_id", None) if user else None,
+            "memberships": [(m.workspace_id, m.user_id, m.role) for m in memberships],
+            "workspaces": [(w.id, w.slug, w.owner_id) for w in workspaces],
+        }
+
+
+# ---------- T4.8 — /initialize ---------------------------------------------
+
+
+def test_initialize_creates_admin_with_default_workspace(client):
+    """POST /initialize → admin + workspace + owner membership + wid cookie."""
+    resp = client.post("/api/v1/auth/initialize", json=_init_payload())
+    assert resp.status_code == 201, resp.text
+    user_id = resp.json()["id"]
+
+    state = asyncio.run(_read_workspace_state(user_id))
+
+    assert len(state["workspaces"]) == 1, state
+    ws_id, ws_slug, ws_owner = state["workspaces"][0]
+    assert ws_owner == user_id
+    # base slug "admin" is reserved, walker bumps to first free suffix
+    assert ws_slug == "admin-2"
+
+    assert state["memberships"] == [(ws_id, user_id, "owner")]
+    assert state["user_default_workspace_id"] == ws_id
+
+    token = resp.cookies["access_token"]
+    claims = _decode(token)
+    assert claims["wid"] == ws_id
+    assert claims["role"] == "owner"
+
+
+# ---------- T4.9 — /register -----------------------------------------------
+
+
+def test_register_creates_user_with_default_workspace(client):
+    """POST /register → user + workspace + owner membership + wid cookie."""
+    # Initialize an admin first so the system is past first-boot.
+    client.post("/api/v1/auth/initialize", json=_init_payload())
+
+    resp = client.post("/api/v1/auth/register", json=_register_payload())
+    assert resp.status_code == 201, resp.text
+    user_id = resp.json()["id"]
+
+    state = asyncio.run(_read_workspace_state(user_id))
+
+    assert len(state["workspaces"]) == 1, state
+    ws_id, ws_slug, ws_owner = state["workspaces"][0]
+    assert ws_owner == user_id
+    assert ws_slug == "alice"
+
+    assert state["memberships"] == [(ws_id, user_id, "owner")]
+    assert state["user_default_workspace_id"] == ws_id
+
+    token = resp.cookies["access_token"]
+    claims = _decode(token)
+    assert claims["wid"] == ws_id
+    assert claims["role"] == "owner"
+
+
+def test_two_registrations_isolate_workspaces_and_avoid_slug_collision(client):
+    """Two users with colliding email local-parts → distinct workspaces, slug suffix bump."""
+    client.post("/api/v1/auth/initialize", json=_init_payload())
+
+    r1 = client.post("/api/v1/auth/register", json=_register_payload(email="alice@example.com"))
+    r2 = client.post("/api/v1/auth/register", json=_register_payload(email="alice@somewhere.else"))
+    assert r1.status_code == 201, r1.text
+    assert r2.status_code == 201, r2.text
+
+    s1 = asyncio.run(_read_workspace_state(r1.json()["id"]))
+    s2 = asyncio.run(_read_workspace_state(r2.json()["id"]))
+
+    ws1 = s1["workspaces"][0]
+    ws2 = s2["workspaces"][0]
+    assert ws1[0] != ws2[0], "workspaces must be distinct"
+    assert ws1[1] == "alice"
+    assert ws2[1] == "alice-2", "slug collision walker should land on -2"