Support object type tool result

[logan272]

Is your feature request related to a problem? Please describe.
Currently, the Tool Result can only return Text Content, Image Content, or Embedded Resources. However, there are many instances where I need the tool call to return a JSON object.

Describe the solution you'd like
I propose adding an object content type to the tool result, allowing for structured data to be returned directly. For example:

return {
   content: [
      {
           type: 'object',
           data: { ... }
      }
   ]
}

Describe alternatives you've considered
Currently, when I want to return structured data from a tool call, I have to use the following workaround, which is not ideal:

return {
   content: [
      {
           type: 'text',
           text: JSON.stringify(data)
      }
   ]
}

[pgayvallet]

Most of the library's zod schemas are using passthrough (for unclear reasons, but anyway), and CallToolResultSchema is no exception, so the way I worked around that limitation is to simply use another property than content to return my objects.

E.g an example of response from one of our tools:

{
  content: [], // still need to define an empty list to pass validation
  objects: myObjects // but then you're free using another prop to pass any serializable data 
}

The suggestion of having an object content makes total sense though, and would be the correct way to do that.

[yesennes]

The spec already defines this: https://modelcontextprotocol.io/specification/2025-06-18/server/tools#structured-content. Definitely should be implemented.

[felixweinberger]

Structured content was implemented and should meet this usecase? #454