fix: tolerate empty MCP list results

litellm/experimental_mcp_client/client.py

@@ -34,14 +34,21 @@ except ImportError:
 from mcp.types import CallToolRequestParams as MCPCallToolRequestParams
 from mcp.types import CallToolResult as MCPCallToolResult
 from mcp.types import (
+    ClientRequest,
     GetPromptRequestParams,
     GetPromptResult,
+    ListPromptsRequest,
+    ListPromptsResult,
+    ListResourceTemplatesRequest,
+    ListResourceTemplatesResult,
+    ListResourcesRequest,
+    ListResourcesResult,
     Prompt,
     ResourceTemplate,
     TextContent,
 )
 from mcp.types import Tool as MCPTool
-from pydantic import AnyUrl
+from pydantic import AnyUrl, Field
 from litellm._logging import verbose_logger
 from litellm.constants import MCP_CLIENT_TIMEOUT, MCP_NPM_CACHE_DIR
 from litellm.llms.custom_httpx.http_handler import get_ssl_configuration
@@ -84,6 +91,18 @@ def _first_non_cancelled_cause(exc: BaseException) -> Optional[BaseException]:
 TSessionResult = TypeVar("TSessionResult")
 
 
+class _LenientListPromptsResult(ListPromptsResult):
+    prompts: List[Prompt] = Field(default_factory=list)
+
+
+class _LenientListResourcesResult(ListResourcesResult):
+    resources: List[Resource] = Field(default_factory=list)
+
+
+class _LenientListResourceTemplatesResult(ListResourceTemplatesResult):
+    resourceTemplates: List[ResourceTemplate] = Field(default_factory=list)
+
+
 class MCPSigV4Auth(httpx.Auth):
     """
     httpx Auth class that signs each request with AWS SigV4.
@@ -628,7 +647,10 @@ class MCPClient:
         )
 
         async def _list_prompts_operation(session: ClientSession):
-            return await session.list_prompts()
+            return await session.send_request(
+                ClientRequest(ListPromptsRequest()),
+                _LenientListPromptsResult,
+            )
 
         try:
             result = await self.run_with_session(_list_prompts_operation)
@@ -713,7 +735,10 @@ class MCPClient:
         )
 
         async def _list_resources_operation(session: ClientSession):
-            return await session.list_resources()
+            return await session.send_request(
+                ClientRequest(ListResourcesRequest()),
+                _LenientListResourcesResult,
+            )
 
         try:
             result = await self.run_with_session(_list_resources_operation)
@@ -751,7 +776,10 @@ class MCPClient:
         )
 
         async def _list_resource_templates_operation(session: ClientSession):
-            return await session.list_resource_templates()
+            return await session.send_request(
+                ClientRequest(ListResourceTemplatesRequest()),
+                _LenientListResourceTemplatesResult,
+            )
 
         try:
             result = await self.run_with_session(_list_resource_templates_operation)

tests/test_litellm/experimental_mcp_client/test_mcp_client.py

@@ -1,6 +1,5 @@
 import asyncio
 import os
-import ssl
 import sys
 from unittest.mock import AsyncMock, MagicMock, patch
 
@@ -15,9 +14,32 @@ from litellm.experimental_mcp_client.client import (
     MCPClient,
     _first_non_cancelled_cause,
 )
+from mcp.types import (
+    ListPromptsRequest,
+    ListResourceTemplatesRequest,
+    ListResourcesRequest,
+)
 from litellm.types.mcp import MCPAuth, MCPStdioConfig, MCPTransport
 
 
+class _MissingListResultSession:
+    def __init__(self):
+        self.requests = []
+
+    async def send_request(self, request, result_type):
+        self.requests.append((request, result_type))
+        return result_type.model_validate({})
+
+    async def list_prompts(self):
+        raise AssertionError("strict list_prompts should not be used")
+
+    async def list_resources(self):
+        raise AssertionError("strict list_resources should not be used")
+
+    async def list_resource_templates(self):
+        raise AssertionError("strict list_resource_templates should not be used")
+
+
 class _FakeExceptionGroup(Exception):
     """Duck-typed stand-in for an anyio/builtin ExceptionGroup.
 
@@ -48,6 +70,32 @@ class TestMCPClient:
         assert client.stdio_config.get("command") == "python"
         assert client.stdio_config.get("args") == ["-m", "my_mcp_server"]
 
+    @pytest.mark.asyncio
+    @pytest.mark.parametrize(
+        ("client_method", "request_type"),
+        [
+            ("list_prompts", ListPromptsRequest),
+            ("list_resources", ListResourcesRequest),
+            ("list_resource_templates", ListResourceTemplatesRequest),
+        ],
+    )
+    async def test_list_methods_tolerate_missing_result_collections(
+        self, client_method, request_type
+    ):
+        client = MCPClient(server_url="http://example.com/mcp", transport_type="http")
+        session = _MissingListResultSession()
+
+        async def _run_operation(operation):
+            return await operation(session)
+
+        client.run_with_session = AsyncMock(side_effect=_run_operation)
+
+        result = await getattr(client, client_method)()
+
+        assert result == []
+        assert len(session.requests) == 1
+        assert isinstance(session.requests[0][0].root, request_type)
+
     @pytest.mark.asyncio
     async def test_mcp_client_stdio_connect_error(self):
         """Test MCP client stdio connection error handling"""