diff --git a/backend/app/gateway/routers/auth.py b/backend/app/gateway/routers/auth.py
index ff4a8e72..dd8a7ff5 100644
--- a/backend/app/gateway/routers/auth.py
+++ b/backend/app/gateway/routers/auth.py
@@ -340,7 +340,15 @@ async def login_local(
         )
 
     _record_login_success(client_ip)
-    token = create_access_token(str(user.id), token_version=user.token_version)
+    # Ensure the user has a workspace (covers pre-PR4 users still in DB
+    # whose default_workspace_id was never backfilled by the lifespan hook).
+    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 LoginResponse(
@@ -423,8 +431,17 @@ async def change_password(request: Request, response: Response, body: ChangePass
 
     await provider.update_user(user)
 
-    # Re-issue cookie with new token_version
-    token = create_access_token(str(user.id), token_version=user.token_version)
+    # Re-issue cookie with new token_version. wid + role must be carried
+    # forward so the re-signed JWT still passes the AuthMiddleware
+    # workspace gate; _ensure_default_workspace fills in for the (rare)
+    # case where the user predates PR4 and has not been backfilled.
+    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 MessageResponse(message="Password changed successfully")
diff --git a/backend/tests/test_change_password_keeps_wid.py b/backend/tests/test_change_password_keeps_wid.py
new file mode 100644
index 00000000..9dcaacd4
--- /dev/null
+++ b/backend/tests/test_change_password_keeps_wid.py
@@ -0,0 +1,108 @@
+"""``change_password`` and ``login_local`` re-issue JWTs carrying wid + role.
+
+Stage 0 PR4 T4.12. The contract:
+
+- After ``POST /auth/change-password`` the new session cookie's JWT must
+  still encode the user's workspace under ``wid`` (and ``role='owner'``),
+  with ``ver`` bumped. Dropping ``wid`` here would lock the user out of
+  every protected endpoint immediately after a password change.
+- Same for ``POST /auth/login/local`` — it issues a fresh JWT and must
+  also encode ``wid``.
+"""
+
+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-change-password-wid-32x")
+
+from app.gateway.auth.config import AuthConfig, set_auth_config
+
+_TEST_SECRET = "test-secret-key-change-password-wid-32x"
+
+
+@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}/change_pwd.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 _decode(token: str) -> dict:
+    return jwt.decode(token, _TEST_SECRET, algorithms=["HS256"])
+
+
+def _bootstrap_user(client) -> tuple[str, dict]:
+    """Register a user and return (user_id, initial claims)."""
+    client.post("/api/v1/auth/initialize", json={"email": "admin@example.com", "password": "Str0ng!Pass99"})
+    resp = client.post("/api/v1/auth/register", json={"email": "alice@example.com", "password": "Tr0ub4dor3a-strong!"})
+    assert resp.status_code == 201, resp.text
+    return resp.json()["id"], _decode(resp.cookies["access_token"])
+
+
+def test_change_password_keeps_wid_and_bumps_ver(client):
+    """change_password re-signs the JWT with wid + role; ver moves forward."""
+    user_id, initial_claims = _bootstrap_user(client)
+
+    # /register set the csrf cookie; the matching header is required on
+    # the change-password POST (Double Submit Cookie pattern).
+    csrf = client.cookies.get("csrf_token")
+    assert csrf, "register must have set csrf_token cookie"
+
+    resp = client.post(
+        "/api/v1/auth/change-password",
+        json={"current_password": "Tr0ub4dor3a-strong!", "new_password": "Tr0ub4dor3a-strong2!"},
+        headers={"X-CSRF-Token": csrf},
+    )
+    assert resp.status_code == 200, resp.text
+
+    new_claims = _decode(resp.cookies["access_token"])
+    assert new_claims["sub"] == user_id
+    assert new_claims["wid"] == initial_claims["wid"], "wid must survive a password change"
+    assert new_claims["role"] == "owner"
+    assert new_claims["ver"] == initial_claims["ver"] + 1, "token_version must advance"
+
+
+def test_login_issues_wid_carrying_jwt(client):
+    """/auth/login/local issues a JWT that the workspace middleware will accept."""
+    user_id, _ = _bootstrap_user(client)
+
+    # Clear the session cookie set by /register so the login response is observed in isolation.
+    client.cookies.clear()
+    resp = client.post(
+        "/api/v1/auth/login/local",
+        data={"username": "alice@example.com", "password": "Tr0ub4dor3a-strong!"},
+    )
+    assert resp.status_code == 200, resp.text
+
+    claims = _decode(resp.cookies["access_token"])
+    assert claims["sub"] == user_id
+    assert claims.get("wid"), "login must include wid"
+    assert claims["role"] == "owner"