fix(tools): handle toolset errors gracefully in canonical_tools

When an MCP server (or any toolset) is unavailable or raises an
exception during get_tools(), the error previously propagated uncaught
through _convert_tool_union_to_tools() and canonical_tools(), crashing
the entire agent silently with no log output.

This change wraps the toolset.get_tools_with_prefix() call in a
try-except that catches exceptions, logs a warning with the toolset
class name and error details, and returns an empty tool list. This
allows the agent to continue operating with tools from other working
toolsets.

The fix preserves the existing retry behavior — McpToolset's
@retry_on_errors decorator still attempts reconnection before the
error reaches this handler. On subsequent agent invocations,
get_tools() is called again, naturally retrying the connection.

Fixes: #3341

Co-authored-by: Xiang (Sean) Zhou <seanzhougoogle@google.com>
PiperOrigin-RevId: 890512146

Author: seanzhougoogle

src/google/adk/agents/llm_agent.py

@@ -181,7 +181,15 @@ async def _convert_tool_union_to_tools(
     return [FunctionTool(func=tool_union)]
 
   # At this point, tool_union must be a BaseToolset
-  return await tool_union.get_tools_with_prefix(ctx)
+  try:
+    return await tool_union.get_tools_with_prefix(ctx)
+  except Exception as e:
+    logger.warning(
+        'Failed to get tools from toolset %s: %s',
+        type(tool_union).__name__,
+        e,
+    )
+    return []
 
 
 class LlmAgent(BaseAgent):

tests/unittests/agents/test_llm_agent_fields.py

@@ -472,6 +472,40 @@ def _tool_2():
     assert tools[0].name == '_tool_1'
     assert tools[1].name == '_tool_2'
 
+  async def test_canonical_tools_graceful_degradation_on_toolset_error(self):
+    """Test that canonical_tools returns tools from working toolsets when one fails."""
+    from google.adk.tools.base_tool import BaseTool
+    from google.adk.tools.base_toolset import BaseToolset
+
+    class FailingToolset(BaseToolset):
+
+      async def get_tools(self, readonly_context=None):
+        raise ConnectionError('MCP server unavailable')
+
+    class WorkingToolset(BaseToolset):
+
+      async def get_tools(self, readonly_context=None):
+        tool = mock.MagicMock(spec=BaseTool)
+        tool.name = 'working_tool'
+        tool._get_declaration = mock.MagicMock(return_value=None)
+        return [tool]
+
+    def _regular_tool():
+      pass
+
+    agent = LlmAgent(
+        name='test_agent',
+        model='gemini-pro',
+        tools=[_regular_tool, FailingToolset(), WorkingToolset()],
+    )
+    ctx = await _create_readonly_context(agent)
+    tools = await agent.canonical_tools(ctx)
+
+    # Should have the regular tool + working toolset tool, but not crash
+    assert len(tools) == 2
+    assert tools[0].name == '_regular_tool'
+    assert tools[1].name == 'working_tool'
+
 
 # Tests for multi-provider model support via string model names
 @pytest.mark.parametrize(