Bug: Marshaling Content omits medadata

[IAmSurajBobade]

Describe the bug Content interface has wireContent method - which marshals bytes to struct implementing Content interface.

Meta property in wireContent, has JSON tag _meta.

type wireContent struct {
        ....
	Meta        Meta              `json:"_meta,omitempty"`
}

however, structs do not have json tag defined, so it expects meta, instead of _meta from byte array.

type ResourceLink struct {
	...
	Meta        Meta
}

We should add json tag for Meta in all structs implementing Content interface. So ResourceLink would become

type ResourceLink struct {
	...
	Meta        Meta              `json:"_meta,omitempty"`
}

[IAmSurajBobade]

Looks like @findleyr has already added relevant TODO comment https://github.com/modelcontextprotocol/go-sdk/blob/main/mcp/content.go#L5.

[findleyr]

Sorry, @IAmSurajBobade, I don't understand. The content types have custom MarshalJSON methods, which appear to preserve metadata. Is this not working?

[IAmSurajBobade]

@findleyr, updated description of the issue. This is the issue, due to which https://github.com/modelcontextprotocol/go-sdk/pull/570#discussion_r2515018632 is not working.

[findleyr]

@IAmSurajBobade this is not a bug: the way this works is that each struct that has a Content field has a custom UnmarshalJSON method, which unmarshals using contentsFromWire.

It is not expected that marshaling directly into content types works as expected. The custom marshalling at the top level is required for selecting the correct content type.