Exceptions from MCP server tool calls aren't serialized properly

[peterj]

What happened?

Describe the bug

The exception that comes from the _format_errors in the MCP tools _base .py file is ok; however, that exception is then caught in the StaticWorkbench (

autogen/python/packages/autogen-core/src/autogen_core/tools/_static_workbench.py

Line 60 in eba05ae

except Exception as e:

) and is cast to a string (e.g. unhandled errors in a TaskGroup)

The return_value_as_string gets called and tries to iterate through the items in the above string and serialize them (return_value_as_string in McpToolAdapter class); since items are not TextContent or EmbeddedResource (they are chars), it returns '{}' for each character in the string, so the actual error that gets returned to the user looks like this:

[{}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}]

To Reproduce

Call an MCP server tool that throws an exception.

Expected behavior
An actual exception should be displayed, but we get:

[{}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}]

Which packages was the bug in?

Python Extensions (autogen-ext)

AutoGen library version.

Python dev (main branch)

Other library version.

No response

Model used

No response

Model provider

None

Other model provider

No response

Python version

None

.NET version

None

Operating system

None

[peterj]

@victordibia I submitted a PR for this issue here: #6482

[victordibia]

Thanks @peterj , will take a look later today!