Fix #177: Returning multiple tool results #222
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The resource/read message in MCP allows of multiple resources to be returned. However, in the SDK we do not allow this. This change is such that we allow returning multiple resource in the lowlevel API if needed. However in FastMCP we stick to one, since a FastMCP resource defines the mime_type in the decorator and hence a resource cannot dynamically return different mime_typed resources. It also is just the better default to only return one resource. However in the lowlevel API we will allow this. Strictly speaking this is not a BC break since the new return value is additive, but if people subclassed server, it will break them.
jerome3o-anthropic
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Feb 20, 2025
dsp-ant
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* feat: allow lowlevel servers to return a list of resources The resource/read message in MCP allows of multiple resources to be returned. However, in the SDK we do not allow this. This change is such that we allow returning multiple resource in the lowlevel API if needed. However in FastMCP we stick to one, since a FastMCP resource defines the mime_type in the decorator and hence a resource cannot dynamically return different mime_typed resources. It also is just the better default to only return one resource. However in the lowlevel API we will allow this. Strictly speaking this is not a BC break since the new return value is additive, but if people subclassed server, it will break them. * feat: lower the type requriements for call_tool to Iterable
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This was an attempt to fix #177. However I noted that in FastMCP a resource is bound to mime type in the decorator. Hence return a list of multiple resources, as the spec allows, is nonsensical without overcomplicating the interface. I hence only changed it in the lowlevel SDK. The assumption here is that needing to return multiple resources is rare enough to justify dropping to the lowlevel API.
I can see us in the longrun expanding resource() interface in FastMCP to allow for this, but i am honestly not yet convinced this is in the spirit of FastMCP.
Notable tools can already return multiple values in FastMCP.