From 0447ba6a9b7f6d8a7b7e3279dd53289b071e1833 Mon Sep 17 00:00:00 2001
From: gym-cmd <186399764+gym-cmd@users.noreply.github.com>
Date: Thu, 21 May 2026 00:30:45 +0100
Subject: [PATCH] fix(mcp): use resource metadata for passthrough reauth

---
 .../proxy/_experimental/mcp_server/server.py  | 55 ++++++++++++-------
 1 file changed, 36 insertions(+), 19 deletions(-)

diff --git a/litellm/proxy/_experimental/mcp_server/server.py b/litellm/proxy/_experimental/mcp_server/server.py
index 610edb3be9e..800b7d9f0ee 100644
--- a/litellm/proxy/_experimental/mcp_server/server.py
+++ b/litellm/proxy/_experimental/mcp_server/server.py
@@ -2834,6 +2834,30 @@ if MCP_AVAILABLE:
             )
         return user_api_key_auth.model_copy(update={"object_permission": updated_op})
 
+    def _get_passthrough_resource_metadata_url(scope: Scope, server_name: str) -> str:
+        request = StarletteRequest(scope)
+        base_url = get_request_base_url(request)
+        _path = scope.get("_original_path") or scope.get("path", "") or ""
+
+        if _path.startswith(f"/{server_name}/mcp"):
+            return f"{base_url}/.well-known/oauth-protected-resource/{server_name}/mcp"
+        return f"{base_url}/.well-known/oauth-protected-resource/mcp/{server_name}"
+
+    def _get_passthrough_www_authenticate(
+        scope: Scope,
+        server_name: str,
+        invalid_token: bool = False,
+    ) -> str:
+        resource_metadata_url = _get_passthrough_resource_metadata_url(
+            scope=scope,
+            server_name=server_name,
+        )
+        params = []
+        if invalid_token:
+            params.append('error="invalid_token"')
+        params.append(f'resource_metadata="{resource_metadata_url}"')
+        return "Bearer " + ", ".join(params)
+
     async def _raise_preemptive_401_for_unauthenticated_servers(
         scope: Scope,
         mcp_servers: Optional[List[str]],
@@ -2894,18 +2918,10 @@ if MCP_AVAILABLE:
                     server, oauth2_headers, mcp_server_auth_headers
                 )
             ):
-                request = StarletteRequest(scope)
-                base_url = get_request_base_url(request)
-                _path = scope.get("_original_path") or scope.get("path", "") or ""
-
-                # Pick the well-known resource-metadata form that matches the inbound
-                # route pattern so the metadata's `resource` field round-trips to what
-                # the client actually hit (RFC 9728 §3.2).
-                if _path.startswith(f"/{server_name}/mcp"):
-                    resource_metadata_url = f"{base_url}/.well-known/oauth-protected-resource/{server_name}/mcp"
-                else:
-                    resource_metadata_url = f"{base_url}/.well-known/oauth-protected-resource/mcp/{server_name}"
-                www_authenticate = f'Bearer resource_metadata="{resource_metadata_url}"'
+                www_authenticate = _get_passthrough_www_authenticate(
+                    scope=scope,
+                    server_name=server_name,
+                )
                 raise HTTPException(
                     status_code=401,
                     detail="Unauthorized",
@@ -3040,19 +3056,20 @@ if MCP_AVAILABLE:
                 for srv in passthrough_servers
             ]
         )
-        request = StarletteRequest(scope)
-        base_url = get_request_base_url(request)
         for srv, (probe_status, _) in zip(passthrough_servers, probe_results):
             if probe_status == 401:
-                # Token is missing or expired — direct the client to re-authorize.
-                authorization_uri = (
-                    f"Bearer authorization_uri="
-                    f"{base_url}/.well-known/oauth-authorization-server/{srv.name}"
+                # Token is missing or expired: keep pass-through clients on the
+                # protected-resource discovery flow so they re-authorize against
+                # the upstream IdP metadata proxied by LiteLLM.
+                www_authenticate = _get_passthrough_www_authenticate(
+                    scope=scope,
+                    server_name=srv.name,
+                    invalid_token=True,
                 )
                 raise HTTPException(
                     status_code=401,
                     detail="Unauthorized",
-                    headers={"WWW-Authenticate": authorization_uri},
+                    headers={"WWW-Authenticate": www_authenticate},
                 )
             if probe_status == 403:
                 # Token is valid but the caller lacks permission — do not hint