[Yuxin-Lou]

fix #4652

Summary

• contain exceptions raised while rendering malformed MCP tool results
• mark MCP timeouts, internal cancellations, retry failures, and execution exceptions as structured tool errors. This is the main difference from PR fix: contain malformed mcp tool results #4660 .

Changes

• wrap MCP result rendering in a local exception boundary
• convert malformed responses to ToolResult.error
• convert timeout, server-side cancellation, transient retry failure, and generic MCP exceptions to ToolResult.error
• continue propagating genuine external CancelledError signals
• add regression coverage for malformed empty content and registry-level correction hints

[hamb1y]

yours is way better :)

[chengyongru]

Maintainer edit pushed: transient MCP stream failures such as closed streams now use the existing reconnect handler before retrying, so a stale MCP session can be refreshed without requiring a process restart. I also added regression coverage for reconnect success and retry failure returning a structured tool error.

[chengyongru]

Follow-up pushed: aligned the existing transient retry regression test with the reconnect-first behavior, and kept retry-exhaustion covered as a structured ToolResult.error. GitHub Actions are now green on the latest head.

[Re-bin]

LGTM. This fixes the MCP tool-result failure path cleanly: malformed MCP content, timeouts, cancellations, and retry exhaustion now surface as structured tool errors instead of successful-looking text. The reconnect follow-up also keeps transient closed-stream failures on the existing MCP recovery path.

Validated locally:

• python -m pytest tests/tools/test_mcp_tool.py tests/agent/test_mcp_transient_retry.py -q
• python -m ruff check nanobot/agent/tools/mcp.py tests/agent/test_mcp_transient_retry.py tests/tools/test_mcp_tool.py

GitHub Actions is green across the matrix.