fix(e2e): refuse a read-back that no replica serves

A read-back over an empty replica mapping satisfied every predicate and
returned as if it had converged, so it would have asserted nothing and
passed. No wiring can produce that today, since the replica list always
falls back to at least one URL, but a helper whose whole job is proving a
write reached every replica should not have a shape that passes vacuously.

litellm/proxy/_experimental/mcp_server/toolset_db.py

@@ -132,9 +132,8 @@ async def update_mcp_toolset(
     data: UpdateMCPToolsetRequest,
     touched_by: str,
 ) -> MCPToolset | None:
-    """A partial update: a field the caller left out keeps its stored value and a
-    field sent as null is cleared, where a null ``tools`` is an empty list and a null
-    ``toolset_name`` is ignored because a toolset always has a name."""
+    """A partial update: absent keeps, null clears, except that a toolset always has a
+    name, so a null ``toolset_name`` is ignored."""
     data_dict: Final = data.model_dump(exclude_unset=True, exclude={"toolset_id"})
     if "tools" in data_dict:
         data_dict["tools"] = json.dumps(data_dict["tools"] or ())

tests/e2e/proxy_client.py

@@ -542,8 +542,11 @@ class ProxyClient:
         """The replicas that serve `path`: every data-plane replica for an LLM route,
         and for a management route the control-plane replicas, which in a split
         deployment is the one service that serves it (the data-plane replicas trim
-        management routes) and in a monolith is every replica."""
-        return self.control_replicas if is_control_plane_path(path) else self.replicas
+        management routes) and in a monolith is every replica. Never empty: a
+        read-back against no replica would assert nothing and pass."""
+        replicas: Final = self.control_replicas if is_control_plane_path(path) else self.replicas
+        assert replicas, f"no replica is configured to serve {path}, so a read-back there would prove nothing"
+        return replicas
 
     def read_back_everywhere[R: BaseModel](
         self, path: str, response_type: type[R], *, settled: Callable[[R], bool]

tests/e2e/test_e2e_http.py

@@ -177,7 +177,7 @@ _JSON: Final[TypeAdapter[object]] = TypeAdapter(object)
 
 @dataclass
 class FakeJsonResponse:
-    """The `_classify` view of a response: a status, the raw body bytes, and the
+    """The `classify` view of a response: a status, the raw body bytes, and the
     parse that would raise on an empty one."""
 
     status_code: int

tests/e2e/test_proxy_client.py

@@ -13,10 +13,9 @@ from __future__ import annotations
 from collections.abc import Iterable, Mapping
 from dataclasses import dataclass
 from itertools import chain, repeat
-from typing import Final
+from typing import Final, cast
 
 import pytest
-
 from e2e_config import parse_replica_urls
 from e2e_http import Success
 from models import ModelListEntry, ModelsListResponse
@@ -25,14 +24,17 @@ from proxy_client import (
     ModelsPoller,
     NeverConvergedOn,
     NotServableOn,
+    ProxyClient,
     ReplicaRead,
     Servable,
     await_everywhere,
     await_servable_everywhere,
     build_proxy_client,
 )
+from transport import Transport
 
 MODEL: Final = "gpt-under-test"
+_NO_TRANSPORTS: Final = cast(Transport, None)
 TIMEOUT: Final = 10.0
 INTERVAL: Final = 2.0
 
@@ -156,3 +158,10 @@ class TestReplicasFor:
             replica_urls=("http://pod-1", "http://pod-2"),
         )
         assert set(client.replicas_for("/v1/mcp/server/abc")) == {"http://pod-1", "http://pod-2"}
+
+    def test_a_route_no_replica_serves_is_refused_rather_than_read_back_vacuously(self) -> None:
+        """A read-back over zero replicas would satisfy every predicate and assert
+        nothing, so asking for one fails instead of passing silently."""
+        client: Final = ProxyClient(transport=_NO_TRANSPORTS, replicas={}, control_replicas={})
+        with pytest.raises(AssertionError, match="no replica is configured"):
+            _ = client.replicas_for("/v1/models")