fix(mcp): use global fallback before raising missing per-user env var 412

A variable declared with both global and user scope is covered by the
global value (globals win in the merge), so the tool-call path must
resolve it from the global rather than raising a 412 when the user has
not filled in the per-user value. Restrict the missing-var check to
referenced user vars that lack a global fallback.

litellm/proxy/_experimental/mcp_server/mcp_server_manager.py

@@ -1645,12 +1645,18 @@ class MCPServerManager:
         if not global_values and not user_specs:
             return static_headers
 
-        # Figure out which user-scoped vars are actually referenced.
+        # Figure out which user-scoped vars are actually referenced. A var that
+        # also carries a global value is always covered by that global (globals
+        # win in the merge below), so it can never be genuinely "missing" even if
+        # the user hasn't filled it in -- only vars without a global fallback do.
         referenced = collect_env_var_references(strings=(static_headers or {}).values())
         referenced_user_vars = referenced & user_var_names
+        required_user_vars = {
+            name for name in referenced_user_vars if name not in global_values
+        }
 
         user_values: Dict[str, str] = {}
-        if referenced_user_vars:
+        if required_user_vars:
             try:
                 user_values = await self._load_user_env_vars(server, user_api_key_auth)
             except Exception as exc:
@@ -1669,7 +1675,7 @@ class MCPServerManager:
 
             if raise_on_missing:
                 missing = sorted(
-                    name for name in referenced_user_vars if not user_values.get(name)
+                    name for name in required_user_vars if not user_values.get(name)
                 )
                 if missing:
                     # A cached negative must never produce a 412: cache
@@ -1681,9 +1687,7 @@ class MCPServerManager:
                         server, user_api_key_auth, force_refresh=True
                     )
                     missing = sorted(
-                        name
-                        for name in referenced_user_vars
-                        if not user_values.get(name)
+                        name for name in required_user_vars if not user_values.get(name)
                     )
                 if missing:
                     raise MCPMissingUserEnvVarsError(

tests/test_litellm/proxy/_experimental/mcp_server/test_mcp_env_vars.py

@@ -446,6 +446,50 @@ async def test_resolve_static_headers_stale_user_value_cannot_override_global(
     assert headers == {"X-DB-URL": "admin-db/alice"}
 
 
+@pytest.mark.asyncio
+async def test_resolve_static_headers_dual_scope_var_uses_global_without_412(
+    monkeypatch,
+):
+    """A var declared with both ``global`` and ``user`` scope is covered by the
+    global value (globals win in the merge), so the tool-call path must resolve
+    it from the global instead of raising a 412 when the user hasn't filled it
+    in. This happens during a global-to-user (or user-to-global) migration."""
+    from litellm.proxy._experimental.mcp_server.mcp_server_manager import (
+        MCPServerManager,
+    )
+    from litellm.types.mcp_server.mcp_server_manager import MCPServer
+
+    manager = MCPServerManager()
+    server = MCPServer(
+        server_id="srv-5",
+        name="srv5",
+        transport="http",
+        url="https://example.com",
+        static_headers={"Authorization": "Bearer ${SHARED_TOKEN}"},
+        env_vars=[
+            {"name": "SHARED_TOKEN", "value": "global-secret", "scope": "global"},
+            {"name": "SHARED_TOKEN", "value": "", "scope": "user"},
+        ],
+    )
+
+    load_calls = []
+
+    async def fake_load_user_env_vars(
+        server, user_api_key_auth, *, force_refresh=False
+    ):
+        load_calls.append(force_refresh)
+        return {}
+
+    monkeypatch.setattr(manager, "_load_user_env_vars", fake_load_user_env_vars)
+
+    headers = await manager._resolve_static_headers_with_env_vars(
+        server, user_api_key_auth=object()
+    )
+    assert headers == {"Authorization": "Bearer global-secret"}
+    # The global fully covers the reference, so no per-user lookup is needed.
+    assert load_calls == []
+
+
 # ── health-check skip for per-user-env-var-backed headers ──────────────────