The CallToolResult#structuredContent type is insufficient to represent list or array of objects.

[tzolov]

Bug description
CallToolResult#structuredContent is typed as Map<String, Object>, preventing structured responses like List<DomainObject> or DomainObject[].

Environment

• Affects: 0.12.x, 0.13.x
• Breaking Change: Yes

Expected behavior
Support all valid JSON response types including arrays and lists.

[tzolov]

Apparently this an issue inherited from the MCP Spec: This MCP issue is addressed here: modelcontextprotocol/modelcontextprotocol#834

Currently the MCP CallToolResult schema reference defines structuredContent)?: { [key: string]: unknown };. Later is not capable to represent JSON array types as pointed in #834

[Kehrlann]

As discussed today, the MCP spec type examples seem written in Typescript, so you can experiment here. e.g.

interface CallToolResult {
  _meta?: { [key: string]: unknown };
  //content: ContentBlock[];
  isError?: boolean;
  structuredContent?: { [key: string]: unknown };
  [key: string]: unknown;
}

// e.g. this does not compile
const x: CallToolResult = {
    structuredContent: [{"x": 42}]
}