modelcontextprotocol / modelcontextprotocol/python-sdk
No way to set isError=True for arbitrary tool result content
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@Kludex is already working on this.
Since Oct 3, 2025.
enhancement
good first issue
P3
ready for work
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Description
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.
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