From 222cb293ca61a57d6771bbaca43307f2e15f11fd Mon Sep 17 00:00:00 2001
From: mateo-berri <277851410+mateo-berri@users.noreply.github.com>
Date: Tue, 1 Sep 2026 22:14:18 -0700
Subject: [PATCH] fix(mcp): name the access group that resolved when an agent
 vetoes a mixed scope

---
 .../proxy/_experimental/mcp_server/server.py  | 12 ++++---
 .../mcp_server/test_mcp_server.py             | 31 +++++++++++++++++--
 2 files changed, 36 insertions(+), 7 deletions(-)

diff --git a/litellm/proxy/_experimental/mcp_server/server.py b/litellm/proxy/_experimental/mcp_server/server.py
index 9f530ddf862..b3b574673db 100644
--- a/litellm/proxy/_experimental/mcp_server/server.py
+++ b/litellm/proxy/_experimental/mcp_server/server.py
@@ -1489,10 +1489,14 @@ if MCP_AVAILABLE:
                     )
                 }
                 raise HTTPException(status_code=403, detail=agent_denial)
-            vetoed_group: Final = (
-                next((name for name in requested_names if _registered_server_id(name) is None), None)
-                if resolved_ids
-                else None
+            vetoed_group: Final = next(
+                (
+                    name
+                    for name in requested_names
+                    if _registered_server_id(name) is None
+                    and any(name in (server.access_groups or ()) for server in resolved_without_agent)
+                ),
+                None,
             )
             if vetoed_group is not None:
                 group_denial: Final[_McpDeniedDetail] = {
diff --git a/tests/test_litellm/proxy/_experimental/mcp_server/test_mcp_server.py b/tests/test_litellm/proxy/_experimental/mcp_server/test_mcp_server.py
index fb69ad3b1c4..ea64fdf2a5d 100644
--- a/tests/test_litellm/proxy/_experimental/mcp_server/test_mcp_server.py
+++ b/tests/test_litellm/proxy/_experimental/mcp_server/test_mcp_server.py
@@ -1336,11 +1336,13 @@ def _denied_scope_manager(known_server_names_to_ids: dict[str, str]) -> MagicMoc
     return manager
 
 
-def _scope_resolver(resolved_without_agent: list[str]) -> AsyncMock:
+def _scope_resolver(resolved_without_agent: list[str], access_groups: tuple[str, ...] = ()) -> AsyncMock:
     async def resolve(user_api_key_auth, mcp_servers, client_ip=None):
         if user_api_key_auth is not None and user_api_key_auth.agent_id:
             return []
-        return [MagicMock(server_id=server_id) for server_id in resolved_without_agent]
+        return [
+            MagicMock(server_id=server_id, access_groups=list(access_groups)) for server_id in resolved_without_agent
+        ]
 
     return AsyncMock(side_effect=resolve)
 
@@ -1450,7 +1452,7 @@ async def test_scoped_list_access_group_vetoed_by_agent_names_agent_and_group():
         user_api_key_auth,
         ["prod-group"],
         _denied_scope_manager({}),
-        _scope_resolver(resolved_without_agent=["srv-github"]),
+        _scope_resolver(resolved_without_agent=["srv-github"], access_groups=("prod-group",)),
     )
 
     assert denial.status_code == 403
@@ -1460,6 +1462,29 @@ async def test_scoped_list_access_group_vetoed_by_agent_names_agent_and_group():
     assert "mcp_access_groups" in message
 
 
+@pytest.mark.asyncio
+async def test_scoped_list_mixed_unknown_and_vetoed_group_names_the_group_that_resolved():
+    """With an unknown name ahead of the agent-vetoed group in the scope, the 403 must name the group
+    whose servers the key can reach, never the unknown name, or the admin is told to grant a group
+    that does not exist."""
+    pytest.importorskip("litellm.proxy._experimental.mcp_server.server")
+
+    user_api_key_auth = UserAPIKeyAuth(api_key="test_key", user_id="test_user", agent_id="agent-123")
+
+    denial = await _denied_scoped_list(
+        user_api_key_auth,
+        ["no-such-group", "prod-group"],
+        _denied_scope_manager({}),
+        _scope_resolver(resolved_without_agent=["srv-github"], access_groups=("prod-group",)),
+    )
+
+    assert denial.status_code == 403
+    message = denial.detail["error"]
+    assert "access group 'prod-group'" in message
+    assert "no-such-group" not in message
+    assert "agent 'agent-123'" in message
+
+
 @pytest.mark.asyncio
 async def test_scoped_list_agent_key_denied_by_key_grants_raises_generic_403():
     """When the agent-stripped rerun still resolves nothing, the 403 stays generic instead of