No way to set isError=True for arbitrary tool result content

[ezyang]

There appears to be no way to manually toggle isError=True for non-text content. For example, you can't return an image with isError=True. I noticed this when implementing a proxy using mcp.tool that recursively dispatches to call_tool. Returning content is fine, but there's no good way to make use of isError, unless you have a single string in which case you can raise an Exception and get isError toggled that way.

[Kludex]

I think the best experience here would be to raise a specific exception that has some content inside, and then we could translate to isError internally.

[hanzili]

Hi @Kludex, I implemented your suggested approach in #1824 - adding an optional content parameter to ToolError that gets translated to isError=True internally.

Happy to close if you're already working on this or prefer a different approach!

[blackwell-systems]

I'd like to take this. I see #1824 has been open since January with merge conflicts and no maintainer review, so I'll submit a fresh PR rebased on current main with the same maintainer-endorsed approach (adding optional content to ToolError).

Context: I maintain mcp-assert, an MCP server testing tool. We scanned 54 MCP servers and found this exact isError gap causing real-world issues, so this fix is something I care about getting right.