MCP integration records tool calls that return isError as successful

[4ktLuffy]

Version

2.71.0 (master @ 6a4eb20)

Steps to Reproduce

Call an MCPServer (mcp 2.3.0) tool that raises ValueError, with MCPIntegration enabled.

Expected Result

The client gets isError: true and the tools/call span is marked as an error.

Actual Result

The client gets isError: true, but the span status stays ok, there is no error.type, and the MCP integration captures no error, so MCP tool error rates read 0%.

The mcp 2.x middleware never reads result["isError"] (mcp.py#L457-L501), and the mcp 1.x wrapper ignores a returned CallToolResult(isError=True) (mcp.py#L357-L395). Affected: raised errors on mcp 2.3.0 and fastmcp 4.0.11, and returned isError results on mcp 1.30.0.

I'd like to open a PR to fix this, with tests, if that's okay with you.

[linear-code]

PY-2934

[alexander-alderman-webb]

Thanks for reporting!
We'll be tackling these gaps in the coming months.

[4ktLuffy]

Some numbers in case they help: with 25% of MCP tool calls returning isError, Sentry's span data shows a 0% failure rate for the Python server and 25% for the JS one. https://4ktluffy.github.io/mcp-incident-lab/

The JS integration already handles this (correlation.js). Happy to port it to Python with tests if you're open to a PR.