Should CallToolResult.structuredContent be a JsonArray?

[JoseAlcerreca]

Given that CallToolResult.content is a List<PromptMessageContent>, I'd expect structuredContent to be a JsonArray, but it's a JsonObject.

I can wrap the results in an object with something like

           CallToolResult(
               content = jsonResult.toPromptMessageContent(), // Converts array to list of TextContent
               structuredContent = buildJsonObject { put("results", jsonResult) }
           )

However, when using the MCP Inspector, I get this warning:

⚠ No text block matches structured content

Which makes me think this implementation might be incompatible with some clients. A workaround can be:

            val wrapperResult = buildJsonObject { put("results", jsonResult) }
            CallToolResult(
                content = listOf(TextContent(wrapperResult.toString())),
                structuredContent = wrapperResult
            )

But passing a single item to content doesn't look optimal.

Is this the recommended approach or am I missing something? Thank you!

[devcrocod]

Given that CallToolResult.content is a List, I'd expect structuredContent to be a JsonArray, but it's a JsonObject.

structuredContent JsonObject according to the specification: https://modelcontextprotocol.io/specification/2025-06-18/schema#calltoolresult
Since it represents the result of a tool call, it can only be a JSON object.

As you can see, according to the specification, content is a list. We represented the specification directly, so it needs to be wrapped in a list. I think such cases could be improved by providing a more convenient API