fix: support capability tokens in middleware for MCP server file access

backend/core/middleware.py

@@ -2,12 +2,13 @@
 
 import logging
 
-from fastapi import Request
+from fastapi import HTTPException, Request
 from fastapi.responses import RedirectResponse
 from starlette.middleware.base import BaseHTTPMiddleware
 from starlette.responses import Response
 
 from core.auth import get_user_from_header
+from core.capabilities import verify_file_token
 from infrastructure.app_factory import app_factory
 
 logger = logging.getLogger(__name__)
@@ -23,12 +24,33 @@ def __init__(self, app, debug_mode: bool = False):
     async def dispatch(self, request: Request, call_next) -> Response:
         # Log request
         logger.info(f"Request: {request.method} {request.url.path}")
-        
+
         # Skip auth for static files and auth endpoint
         if request.url.path.startswith('/static') or request.url.path == '/auth':
             return await call_next(request)
-
-        # Check authentication
+
+        # Check for capability token in download URLs (allows MCP servers to access files)
+        if request.url.path.startswith('/api/files/download/'):
+            token = request.query_params.get('token')
+            if token:
+                claims = verify_file_token(token)
+                if claims:
+                    # Valid capability token - extract user from token and allow request
+                    # Note: We only validate token authenticity here (authentication).
+                    # The route handler validates that token's file key matches the requested
+                    # file (authorization). This separation of concerns keeps middleware focused
+                    # on authentication while route handlers handle resource-specific authorization.
+                    user_email = claims.get('u')
+                    if user_email:
+                        logger.info(f"Authenticated via capability token for user: {user_email}")
+                        request.state.user_email = user_email
+                        return await call_next(request)
+                    else:
+                        logger.warning("Valid token but missing user email claim")
+                else:
+                    logger.warning("Invalid capability token provided")
+
+        # Check authentication via X-User-Email header
         user_email = None
         if self.debug_mode:
             # In debug mode, honor X-User-Email header if provided, otherwise use config test user
@@ -43,13 +65,18 @@ async def dispatch(self, request: Request, call_next) -> Response:
         else:
             x_email_header = request.headers.get('X-User-Email')
             user_email = get_user_from_header(x_email_header)
-            
+
             if not user_email:
-                logger.warning("Missing X-User-Email, redirecting to auth")
-                return RedirectResponse(url="/auth", status_code=302)
-
+                # Distinguish between API endpoints (return 401) and browser endpoints (redirect)
+                if request.url.path.startswith('/api/'):
+                    logger.warning(f"Missing authentication for API endpoint: {request.url.path}")
+                    raise HTTPException(status_code=401, detail="Unauthorized")
+                else:
+                    logger.warning("Missing X-User-Email, redirecting to auth")
+                    return RedirectResponse(url="/auth", status_code=302)
+
         # Add user to request state
         request.state.user_email = user_email
-        
+
         response = await call_next(request)
         return response