chore(mcp): address mypy + Greptile feedback on PR 27575

- Cast client_ip to str at the bottom of _is_server_accessible_from_ip so
  mypy sees the narrowing the early returns already perform. The runtime
  shape is unchanged — INTERNAL_REQUEST and None are both handled in the
  early branches above.
- Distinguish "client_ip_unknown" from "ip_filtering" in the
  rest_endpoints 403 response. When IP extraction fails on a public
  server the previous error told the operator to set
  available_on_public_internet=True, which was already set; the new
  message points them at use_x_forwarded_for / mcp_trusted_proxy_ranges.
- Preserve the explicit port in the Host header when
  litellm.user_url_validation is False. The opt-out path previously
  dropped the port, sending a Host that didn't match the connection
  target for non-default-port URLs.

litellm/proxy/_experimental/mcp_server/discoverable_endpoints.py

@@ -39,7 +39,10 @@ def _validate_mcp_oauth_outbound_url(
     operator who points an MCP server at an internal IdP doesn't unintentionally
     expose it as an SSRF probe via the proxy."""
     if not getattr(litellm, "user_url_validation", True):
-        return url, urlparse(url).hostname or ""
+        parsed = urlparse(url)
+        host = parsed.hostname or ""
+        host_header = f"{host}:{parsed.port}" if parsed.port else host
+        return url, host_header
     try:
         return validate_url(url)
     except SSRFError as exc:

litellm/proxy/_experimental/mcp_server/mcp_server_manager.py

@@ -3109,12 +3109,14 @@ class MCPServerManager:
         public_ids = set(litellm.public_mcp_servers or [])
         if server.server_id in public_ids:
             return True
-        # Non-public server: only accessible from internal IPs
+        # Non-public server: only accessible from internal IPs. The two
+        # early returns above narrow client_ip to ``str`` for the type
+        # checker; cast keeps mypy happy without a runtime assert.
         general_settings = self._get_general_settings()
         internal_networks = IPAddressUtils.parse_internal_networks(
             general_settings.get("mcp_internal_ip_ranges")
         )
-        return IPAddressUtils.is_internal_ip(client_ip, internal_networks)
+        return IPAddressUtils.is_internal_ip(cast(str, client_ip), internal_networks)
 
     def get_mcp_server_by_id(self, server_id: str) -> Optional[MCPServer]:
         """

litellm/proxy/_experimental/mcp_server/rest_endpoints.py

@@ -397,6 +397,19 @@ if MCP_AVAILABLE:
                     _server, rest_client_ip
                 )
             ):
+                if rest_client_ip is None:
+                    raise HTTPException(
+                        status_code=403,
+                        detail={
+                            "error": "client_ip_unknown",
+                            "message": (
+                                "Cannot determine client IP for IP-based access "
+                                "control. If the proxy is behind a load balancer "
+                                "or reverse proxy, configure use_x_forwarded_for "
+                                "and mcp_trusted_proxy_ranges in general_settings."
+                            ),
+                        },
+                    )
                 raise HTTPException(
                     status_code=403,
                     detail={
@@ -486,6 +499,19 @@ if MCP_AVAILABLE:
                     _server, rest_client_ip
                 )
             ):
+                if rest_client_ip is None:
+                    raise HTTPException(
+                        status_code=403,
+                        detail={
+                            "error": "client_ip_unknown",
+                            "message": (
+                                "Cannot determine client IP for IP-based access "
+                                "control. If the proxy is behind a load balancer "
+                                "or reverse proxy, configure use_x_forwarded_for "
+                                "and mcp_trusted_proxy_ranges in general_settings."
+                            ),
+                        },
+                    )
                 raise HTTPException(
                     status_code=403,
                     detail={

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

@@ -2530,7 +2530,14 @@ class TestOutboundOAuthURLValidation:
                 "http://10.0.0.1:5000/token", role="token"
             )
             assert url == "http://10.0.0.1:5000/token"
-            assert host == "10.0.0.1"
+            # Host header includes the explicit port from the URL.
+            assert host == "10.0.0.1:5000"
+
+            # When the URL omits the port, the Host header omits it too.
+            url2, host2 = _validate_mcp_oauth_outbound_url(
+                "http://10.0.0.1/token", role="token"
+            )
+            assert host2 == "10.0.0.1"
         finally:
             litellm.user_url_validation = original