fix(prebuilt): handle plain ToolException in default tool error handler

[Balaji Seshadri (gitbalaji)]

Summary

Fixes #6449.

_default_handle_tool_errors only caught ToolInvocationError (a subclass of ToolException). A plain ToolException — as raised by MCP adapters (langchain_mcp_adapters) and other tool wrappers — was re-raised instead of being returned as an error ToolMessage, crashing the graph.

Before:

def _default_handle_tool_errors(e: Exception) -> str:
    if isinstance(e, ToolInvocationError):
        return e.message
    raise e  # ← ToolException from MCP falls through here

After:

def _default_handle_tool_errors(e: Exception) -> str:
    if isinstance(e, ToolException):  # catches both ToolException and ToolInvocationError
        return str(e)
    raise e

ToolInvocationError calls super().__init__(self.message), so str(e) is identical to e.message — no behaviour change for the existing path.

Test plan

• Added test_tool_node_default_handler_catches_tool_exception — verifies a plain ToolException from an async tool is returned as an error ToolMessage with the default handler
• Existing test_tool_node_error_handling and test_tool_node_handle_tool_errors_false still pass
• make format — clean
• make lint — clean (Success: no issues found in 5 source files)

🤖 Generated with Claude Code