Support tool structured content and output schema

[ptitjes]

Is your feature request related to a problem? Please describe.

The 2025-06-18 version of the protocol introduces the Tool's "Structured Content" and "Output Schema" support which allows servers to optionally specify a tool output schema and a tool result structured content.

cf. https://modelcontextprotocol.io/specification/2025-06-18/server/tools#structured-content
and https://modelcontextprotocol.io/specification/2025-06-18/server/tools#output-schema

Describe the solution you'd like

• Add an optional outputSchema field to Tool
• Add an optional structuredContent field to CallToolResult
  • How should this articulate with the existing CompatibilityCallToolResult?

Open question:

The specification says:

If an output schema is provided:

• Servers MUST provide structured results that conform to this schema.
• Clients SHOULD validate structured results against this schema.

Should we enforce that in the SDK or leave it to the responsibility of the host?

Describe alternatives you've considered
N/A

Additional context

• Tools – Structured Content
• Tools – Output Schema

[ptitjes]

I would be happy to contribute a PR, provided you guys advise me on the two questions above (what to do about CompatibilityCallToolResult and structured results validation against schema).

[devcrocod]

That would be great if you could take on this issue

How should this articulate with the existing CompatibilityCallToolResult?

As I see it, should add structuredContent to CallToolResult. It will then appear in CompatibilityCallToolResult, but remain empty

Should we enforce that in the SDK or leave it to the responsibility of the host?

If I understand the question correctly, it would be better to leave this to the host. For now, we can just provide the relevant fields and that's enough