diff --git a/backend/app/gateway/auth_middleware.py b/backend/app/gateway/auth_middleware.py
index 578e8241..7dcaf90c 100644
--- a/backend/app/gateway/auth_middleware.py
+++ b/backend/app/gateway/auth_middleware.py
@@ -9,6 +9,7 @@ owner filtering works automatically via the sentinel pattern.
 Fine-grained permission checks remain in authz.py decorators.
 """
 
+import logging
 from collections.abc import Callable
 
 from fastapi import HTTPException, Request, Response
@@ -24,6 +25,8 @@ from app.gateway.internal_auth import INTERNAL_AUTH_HEADER_NAME, get_internal_us
 from deerflow.runtime.user_context import reset_current_user, set_current_user
 from deerflow.runtime.workspace_context import reset_current_workspace, set_current_workspace
 
+logger = logging.getLogger(__name__)
+
 # Paths that never require authentication.
 _PUBLIC_PATH_PREFIXES: tuple[str, ...] = (
     "/health",
@@ -87,7 +90,11 @@ class AuthMiddleware(BaseHTTPMiddleware):
         if auth_header.startswith("Bearer dfk_"):
             token = auth_header[len("Bearer ") :]
             backend = build_api_key_backend()
-            result = await backend.authenticate(token) if backend is not None else None
+            try:
+                result = await backend.authenticate(token) if backend is not None else None
+            except Exception:
+                logger.exception("API key authentication failed unexpectedly")
+                return JSONResponse(status_code=503, content={"detail": "Authentication service unavailable"})
             if result is None:
                 return JSONResponse(
                     status_code=401,
diff --git a/backend/app/gateway/authz.py b/backend/app/gateway/authz.py
index 0579f72e..1830f90c 100644
--- a/backend/app/gateway/authz.py
+++ b/backend/app/gateway/authz.py
@@ -38,6 +38,7 @@ from typing import TYPE_CHECKING, Any, ParamSpec, TypeVar
 from fastapi import HTTPException, Request
 
 if TYPE_CHECKING:
+    from app.gateway.auth.api_key_backend import ServicePrincipal
     from app.gateway.auth.models import User
 
 P = ParamSpec("P")
@@ -65,13 +66,14 @@ class AuthContext:
     Stored in request.state.auth after require_auth decoration.
 
     Attributes:
-        user: The authenticated user, or None if anonymous
+        user: The authenticated principal (human ``User`` or
+            ``ServicePrincipal`` for API-key requests), or None if anonymous
         permissions: List of permission strings (e.g., "threads:read")
     """
 
     __slots__ = ("user", "permissions")
 
-    def __init__(self, user: User | None = None, permissions: list[str] | None = None):
+    def __init__(self, user: User | ServicePrincipal | None = None, permissions: list[str] | None = None):
         self.user = user
         self.permissions = permissions or []
 
@@ -93,8 +95,11 @@ class AuthContext:
         permission = f"{resource}:{action}"
         return permission in self.permissions
 
-    def require_user(self) -> User:
-        """Get user or raise 401.
+    def require_user(self) -> User | ServicePrincipal:
+        """Get the authenticated principal or raise 401.
+
+        Returns the human ``User`` or the ``ServicePrincipal`` backing an
+        API key, depending on how the request authenticated.
 
         Raises:
             HTTPException 401 if not authenticated
diff --git a/backend/tests/test_auth_middleware_api_key.py b/backend/tests/test_auth_middleware_api_key.py
index ec56d42b..24206676 100644
--- a/backend/tests/test_auth_middleware_api_key.py
+++ b/backend/tests/test_auth_middleware_api_key.py
@@ -122,3 +122,13 @@ async def test_non_dfk_bearer_falls_through_to_cookie_path(tmp_path):
         assert r.json()["detail"]["code"] == "not_authenticated"
     finally:
         await _cleanup()
+
+
+async def test_bare_prefix_bearer_returns_401(tmp_path):
+    await _seed_key(tmp_path)
+    try:
+        client = TestClient(_make_app())
+        r = client.get("/api/probe", headers={"Authorization": "Bearer dfk_"})
+        assert r.status_code == 401
+    finally:
+        await _cleanup()