diff --git a/backend/app/gateway/auth/errors.py b/backend/app/gateway/auth/errors.py
index b5899ebd..1c8065d2 100644
--- a/backend/app/gateway/auth/errors.py
+++ b/backend/app/gateway/auth/errors.py
@@ -21,6 +21,7 @@ class AuthErrorCode(StrEnum):
     PROVIDER_NOT_FOUND = "provider_not_found"
     NOT_AUTHENTICATED = "not_authenticated"
     SYSTEM_ALREADY_INITIALIZED = "system_already_initialized"
+    WORKSPACE_REQUIRED = "workspace_required"
 
 
 class TokenError(StrEnum):
@@ -29,6 +30,7 @@ class TokenError(StrEnum):
     EXPIRED = "expired"
     INVALID_SIGNATURE = "invalid_signature"
     MALFORMED = "malformed"
+    WORKSPACE_MISSING = "workspace_missing"
 
 
 class AuthErrorResponse(BaseModel):
@@ -42,4 +44,6 @@ def token_error_to_code(err: TokenError) -> AuthErrorCode:
     """Map TokenError to AuthErrorCode — single source of truth."""
     if err == TokenError.EXPIRED:
         return AuthErrorCode.TOKEN_EXPIRED
+    if err == TokenError.WORKSPACE_MISSING:
+        return AuthErrorCode.WORKSPACE_REQUIRED
     return AuthErrorCode.TOKEN_INVALID
diff --git a/backend/app/gateway/auth/jwt.py b/backend/app/gateway/auth/jwt.py
index cc004840..52d7e3e8 100644
--- a/backend/app/gateway/auth/jwt.py
+++ b/backend/app/gateway/auth/jwt.py
@@ -68,10 +68,20 @@ def decode_token(token: str) -> TokenPayload | TokenError:
     config = get_auth_config()
     try:
         payload = jwt.decode(token, config.jwt_secret, algorithms=["HS256"])
-        return TokenPayload(**payload)
     except jwt.ExpiredSignatureError:
         return TokenError.EXPIRED
     except jwt.InvalidSignatureError:
         return TokenError.INVALID_SIGNATURE
     except jwt.PyJWTError:
         return TokenError.MALFORMED
+
+    # Reject legacy pre-PR4 tokens that lack the wid claim. Reported as
+    # WORKSPACE_MISSING (not MALFORMED) so middleware can surface a
+    # specific 401 telling the frontend to re-issue via /select-workspace.
+    if "wid" not in payload or payload.get("wid") is None:
+        return TokenError.WORKSPACE_MISSING
+
+    try:
+        return TokenPayload(**payload)
+    except Exception:
+        return TokenError.MALFORMED
diff --git a/backend/tests/test_auth.py b/backend/tests/test_auth.py
index f19c83c7..89fdcacd 100644
--- a/backend/tests/test_auth.py
+++ b/backend/tests/test_auth.py
@@ -101,7 +101,7 @@ def test_create_and_decode_token():
     import os
 
     os.environ["AUTH_JWT_SECRET"] = "test-secret-key-for-jwt-testing-minimum-32-chars"
-    token = create_access_token(user_id)
+    token = create_access_token(user_id, workspace_id="ws-test", role="owner")
     assert isinstance(token, str)
 
     payload = decode_token(token)
@@ -132,7 +132,7 @@ def test_decode_token_invalid():
 def test_create_token_custom_expiry():
     """Custom expiry is respected."""
     user_id = str(uuid4())
-    token = create_access_token(user_id, expires_delta=timedelta(hours=1))
+    token = create_access_token(user_id, expires_delta=timedelta(hours=1), workspace_id="ws-test", role="owner")
     payload = decode_token(token)
     assert payload is not None
     assert payload.sub == user_id
@@ -420,7 +420,7 @@ def test_jwt_encodes_ver():
     from app.gateway.auth.errors import TokenError
 
     os.environ["AUTH_JWT_SECRET"] = "test-secret-key-for-jwt-testing-minimum-32-chars"
-    token = create_access_token(str(uuid4()), token_version=3)
+    token = create_access_token(str(uuid4()), token_version=3, workspace_id="ws-test", role="owner")
     payload = decode_token(token)
     assert not isinstance(payload, TokenError)
     assert payload.ver == 3
@@ -433,7 +433,7 @@ def test_jwt_default_ver_zero():
     from app.gateway.auth.errors import TokenError
 
     os.environ["AUTH_JWT_SECRET"] = "test-secret-key-for-jwt-testing-minimum-32-chars"
-    token = create_access_token(str(uuid4()))
+    token = create_access_token(str(uuid4()), workspace_id="ws-test", role="owner")
     payload = decode_token(token)
     assert not isinstance(payload, TokenError)
     assert payload.ver == 0
@@ -447,7 +447,7 @@ def test_token_version_mismatch_rejects():
     os.environ["AUTH_JWT_SECRET"] = "test-secret-key-for-jwt-testing-minimum-32-chars"
 
     user_id = str(uuid4())
-    token = create_access_token(user_id, token_version=0)
+    token = create_access_token(user_id, token_version=0, workspace_id="ws-test", role="owner")
 
     mock_user = User(id=user_id, email="test@example.com", password_hash="hash", token_version=1)
 
diff --git a/backend/tests/test_auth_errors.py b/backend/tests/test_auth_errors.py
index b3b46c75..dc60f997 100644
--- a/backend/tests/test_auth_errors.py
+++ b/backend/tests/test_auth_errors.py
@@ -69,7 +69,7 @@ def test_decode_token_returns_token_error_on_malformed():
 
 def test_decode_token_returns_payload_on_valid():
     _setup_config()
-    token = create_access_token("user-123")
+    token = create_access_token("user-123", workspace_id="ws-test", role="owner")
     result = decode_token(token)
     assert not isinstance(result, TokenError)
     assert result.sub == "user-123"
diff --git a/backend/tests/test_langgraph_auth.py b/backend/tests/test_langgraph_auth.py
index d2ee8105..f5db4052 100644
--- a/backend/tests/test_langgraph_auth.py
+++ b/backend/tests/test_langgraph_auth.py
@@ -74,7 +74,7 @@ def test_expired_jwt_raises_401():
 
 
 def test_user_not_found_raises_401():
-    token = create_access_token("ghost")
+    token = create_access_token("ghost", workspace_id="ws-test", role="owner")
     with patch("app.gateway.langgraph_auth.get_local_provider", return_value=_mock_provider(None)):
         with pytest.raises(Auth.exceptions.HTTPException) as exc:
             asyncio.run(authenticate(_req({"access_token": token})))
@@ -84,7 +84,7 @@ def test_user_not_found_raises_401():
 
 def test_token_version_mismatch_raises_401():
     user = _user(token_version=2)
-    token = create_access_token(str(user.id), token_version=1)
+    token = create_access_token(str(user.id), token_version=1, workspace_id="ws-test", role="owner")
     with patch("app.gateway.langgraph_auth.get_local_provider", return_value=_mock_provider(user)):
         with pytest.raises(Auth.exceptions.HTTPException) as exc:
             asyncio.run(authenticate(_req({"access_token": token})))
@@ -94,7 +94,7 @@ def test_token_version_mismatch_raises_401():
 
 def test_valid_token_returns_user_id():
     user = _user(token_version=0)
-    token = create_access_token(str(user.id), token_version=0)
+    token = create_access_token(str(user.id), token_version=0, workspace_id="ws-test", role="owner")
     with patch("app.gateway.langgraph_auth.get_local_provider", return_value=_mock_provider(user)):
         result = asyncio.run(authenticate(_req({"access_token": token})))
     assert result == str(user.id)
@@ -102,7 +102,7 @@ def test_valid_token_returns_user_id():
 
 def test_valid_token_matching_version():
     user = _user(token_version=5)
-    token = create_access_token(str(user.id), token_version=5)
+    token = create_access_token(str(user.id), token_version=5, workspace_id="ws-test", role="owner")
     with patch("app.gateway.langgraph_auth.get_local_provider", return_value=_mock_provider(user)):
         result = asyncio.run(authenticate(_req({"access_token": token})))
     assert result == str(user.id)
@@ -113,7 +113,7 @@ def test_valid_token_matching_version():
 
 def test_provider_exception_propagates():
     """Provider raises → should not be swallowed silently."""
-    token = create_access_token("user-1")
+    token = create_access_token("user-1", workspace_id="ws-test", role="owner")
     p = AsyncMock()
     p.get_user = AsyncMock(side_effect=RuntimeError("DB down"))
     with patch("app.gateway.langgraph_auth.get_local_provider", return_value=p):
@@ -126,7 +126,11 @@ def test_jwt_missing_ver_defaults_to_zero():
     import jwt as pyjwt
 
     uid = str(uuid4())
-    raw = pyjwt.encode({"sub": uid, "exp": 9999999999, "iat": 1000000000}, _JWT_SECRET, algorithm="HS256")
+    raw = pyjwt.encode(
+        {"sub": uid, "wid": "ws-test", "role": "owner", "exp": 9999999999, "iat": 1000000000},
+        _JWT_SECRET,
+        algorithm="HS256",
+    )
     user = _user(user_id=uid, token_version=0)
     with patch("app.gateway.langgraph_auth.get_local_provider", return_value=_mock_provider(user)):
         result = asyncio.run(authenticate(_req({"access_token": raw})))
@@ -138,7 +142,11 @@ def test_jwt_missing_ver_rejected_when_user_version_nonzero():
     import jwt as pyjwt
 
     uid = str(uuid4())
-    raw = pyjwt.encode({"sub": uid, "exp": 9999999999, "iat": 1000000000}, _JWT_SECRET, algorithm="HS256")
+    raw = pyjwt.encode(
+        {"sub": uid, "wid": "ws-test", "role": "owner", "exp": 9999999999, "iat": 1000000000},
+        _JWT_SECRET,
+        algorithm="HS256",
+    )
     user = _user(user_id=uid, token_version=1)
     with patch("app.gateway.langgraph_auth.get_local_provider", return_value=_mock_provider(user)):
         with pytest.raises(Auth.exceptions.HTTPException) as exc:
@@ -221,7 +229,7 @@ def test_filter_with_empty_metadata():
 
 
 def test_shared_jwt_secret():
-    token = create_access_token("user-1", token_version=3)
+    token = create_access_token("user-1", token_version=3, workspace_id="ws-test", role="owner")
     payload = decode_token(token)
     from app.gateway.auth.errors import TokenError
 
diff --git a/backend/tests/test_legacy_token_compat.py b/backend/tests/test_legacy_token_compat.py
new file mode 100644
index 00000000..96a8031f
--- /dev/null
+++ b/backend/tests/test_legacy_token_compat.py
@@ -0,0 +1,60 @@
+"""Legacy 4-field JWT compatibility (Stage 0 PR4 T4.6).
+
+Before PR4 every JWT carried only `{sub, exp, iat, ver}`. After PR4 the
+server expects `wid` (workspace_id) on every protected request. Old
+cookies in the wild must NOT collapse into ``TokenError.MALFORMED`` —
+that hides the actual problem (workspace required) and prevents the
+frontend from steering the user to ``/select-workspace``.
+
+The contract: ``decode_token`` returns ``TokenError.WORKSPACE_MISSING``
+specifically when the JWT signature checks out and the payload is
+otherwise well-formed but does NOT carry a ``wid`` claim.
+"""
+
+from __future__ import annotations
+
+from datetime import UTC, datetime, timedelta
+
+import jwt
+import pytest
+
+from app.gateway.auth.config import get_auth_config
+from app.gateway.auth.errors import TokenError
+from app.gateway.auth.jwt import decode_token
+
+
+@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_legacy_token(*, expired: bool = False) -> str:
+    """Encode a pre-PR4 JWT directly (bypassing create_access_token)."""
+    now = datetime.now(UTC)
+    payload = {
+        "sub": "u-legacy",
+        "exp": now + (timedelta(seconds=-1) if expired else timedelta(hours=1)),
+        "iat": now,
+        "ver": 0,
+    }
+    return jwt.encode(payload, get_auth_config().jwt_secret, algorithm="HS256")
+
+
+def test_decode_legacy_token_returns_workspace_missing_error() -> None:
+    """4-field token (no wid) → TokenError.WORKSPACE_MISSING (not MALFORMED)."""
+    token = _make_legacy_token()
+    result = decode_token(token)
+    assert result == TokenError.WORKSPACE_MISSING
+
+
+def test_decode_legacy_token_with_expired_still_reports_expired() -> None:
+    """Expired legacy tokens keep reporting EXPIRED — that signal takes priority.
+
+    Why: an expired token must trigger /auth/refresh logic before we
+    decide it also lacks workspace; reporting WORKSPACE_MISSING on an
+    expired token would steer the user to /select-workspace instead.
+    """
+    token = _make_legacy_token(expired=True)
+    result = decode_token(token)
+    assert result == TokenError.EXPIRED