fix: Gracefully handle errors when listing MCP tools

When listing (and lazily indexing) tools, it's possible for an error
to get thrown by individual toolgroups if for example an MCP toolgroup
is unable to connect to its `mcp_endpoint`.

This logs a warning in the server when that happens, logs a full stack
trace of the error if debug logging is enabled, and just returns the
list of tools from all working toolgroups instead of throwing an error
to the client when a single toolgroup is temporarily or permanently
misbehaving.

Fixes #2540

A new unit test was added to test this exception handling, which is
run as part of our regular test suite but also manually run to
specifically verify this fix via:

```
uv run pytest -sv --asyncio-mode=auto \
tests/unit/distribution/routers/test_routing_tables.py
```

To verify the additional debug logging is printing properly:

```
LLAMA_STACK_LOGGING=core=debug \
uv run pytest -sv --asyncio-mode=auto \
tests/unit/distribution/routers/test_routing_tables.py
```

Signed-off-by: Ben Browning <bbrownin@redhat.com>

llama_stack/distribution/routing_tables/toolgroups.py

@@ -52,9 +52,14 @@ class ToolGroupsRoutingTable(CommonRoutingTableImpl, ToolGroups):
 
         all_tools = []
         for toolgroup in toolgroups:
-            if toolgroup.identifier not in self.toolgroups_to_tools:
-                await self._index_tools(toolgroup)
-            all_tools.extend(self.toolgroups_to_tools[toolgroup.identifier])
+            try:
+                if toolgroup.identifier not in self.toolgroups_to_tools:
+                    await self._index_tools(toolgroup)
+                all_tools.extend(self.toolgroups_to_tools[toolgroup.identifier])
+            except Exception as e:
+                logger.warning(f"Error listing tools for toolgroup {toolgroup.identifier}: {e}")
+                logger.debug(e, exc_info=True)
+                continue
 
         return ListToolsResponse(data=all_tools)
 

tests/unit/distribution/routers/test_routing_tables.py

@@ -10,6 +10,7 @@ from unittest.mock import AsyncMock
 
 import pytest
 
+from llama_stack.apis.common.content_types import URL
 from llama_stack.apis.common.type_system import NumberType
 from llama_stack.apis.datasets.datasets import Dataset, DatasetPurpose, URIDataSource
 from llama_stack.apis.datatypes import Api
@@ -296,3 +297,26 @@ async def test_tool_groups_routing_table(cached_disk_dist_registry):
     await table.unregister_toolgroup(toolgroup_id="test-toolgroup")
     tool_groups = await table.list_tool_groups()
     assert len(tool_groups.data) == 0
+
+
+@pytest.mark.asyncio
+async def test_tool_groups_routing_table_exception_handling(cached_disk_dist_registry):
+    """Test that the tool group routing table handles exceptions when listing tools, like if an MCP server is unreachable."""
+
+    exception_throwing_tool_groups_impl = ToolGroupsImpl()
+    exception_throwing_tool_groups_impl.list_runtime_tools = AsyncMock(side_effect=Exception("Test exception"))
+
+    table = ToolGroupsRoutingTable(
+        {"test_provider": exception_throwing_tool_groups_impl}, cached_disk_dist_registry, {}
+    )
+    await table.initialize()
+
+    await table.register_tool_group(
+        toolgroup_id="test-toolgroup-exceptions",
+        provider_id="test_provider",
+        mcp_endpoint=URL(uri="http://localhost:8479/foo/bar"),
+    )
+
+    tools = await table.list_tools(toolgroup_id="test-toolgroup-exceptions")
+
+    assert len(tools.data) == 0