[v2] structuredContent with string wrapped in result: mismatch with specification

[tobiasBora]

What happened?

I'm confused, I created a server like

import type { DiagramConfClass } from '$lib/contexts/context.svelte';
import { McpServer } from '@modelcontextprotocol/server';
import * as z from 'zod/v4';

export function createServer(diagramConfClass: DiagramConfClass): McpServer {
  const server = new McpServer({ name: 'proofdiag', version: '0.0.1' });

  server.registerTool(
    'ping',
    {
      description: 'Returns pong',
      outputSchema: z.string()
    },
    async () => {
      const str = `Pong at ${new Date()}`
      // content seems to be automatically added by the SDK (needed for backward compatibility)
      return { structuredContent: str };
    }
  );

  return server;
}

The specs specifies that when calling such a tool, we should get back the structured content directly

Structured content is returned as a JSON value in the structuredContent field of a result. This can be any JSON value (object, array, string, number, boolean, or null) that conforms to the tool’s outputSchema if one is defined.

and they provide this valid return:

{
  "jsonrpc": "2.0",
  "id": 5,
  "result": {
    "resultType": "complete",
    "content": [
      {
        "type": "text",
        "text": "{\"temperature\": 22.5, \"conditions\": \"Partly cloudy\", \"humidity\": 65}"
      }
    ],
    "structuredContent": {
      "temperature": 22.5,
      "conditions": "Partly cloudy",
      "humidity": 65
    }
  }
}

However, when used with strings (and possibly other non-object types), not only typescript complains, but this library nests the structuredContent inside a {result: str}, so I need to access the final content via myjsonrpc.result.structuredContent.result instead of myjsonrpc.result.structuredContent. When considering objects, it works fine.

Screenshot:

What did you expect?

StructuredContent with strings should not be nested inside a result.

Code to reproduce

import type { DiagramConfClass } from '$lib/contexts/context.svelte';
import { McpServer } from '@modelcontextprotocol/server';
import * as z from 'zod/v4';

export function createServer(diagramConfClass: DiagramConfClass): McpServer {
  const server = new McpServer({ name: 'proofdiag', version: '0.0.1' });

  server.registerTool(
    'ping',
    {
      description: 'Returns pong',
      outputSchema: z.string()
    },
    async () => {
      const str = `Pong at ${new Date()}`
      // content seems to be automatically added by the SDK (needed for backward compatibility)
      return { structuredContent: str };
    }
  );

  return server;
}

SDK version

@modelcontextprotocol/server@2.0.0-beta.4

Area

Server

[tobiasBora]

Ok, edited the issue as it occurs only when returning non-object (at least for string). May relate to #1308 and #1149 even if the problem is not identical (no crash…).