Support MCP tool call response `structuredContent`

[phemmer]

Description

According to the MCP spec, a tool call response that provides structuredContent is not required to also provide content. It is only recommended. Thus content may be empty, which causes pydantic-ai to see the tool call response as empty.

MCP Specification section 5.2.6 Structured Content:

For backwards compatibility, a tool that returns structured content SHOULD also return the serialized JSON in a TextContent block.

While it is a valid argument that the content field should be provided by the server, this appears to be controlled on a per-tool basis, and not by the SDK in use, which makes it harder to enforce. Since the MCP spec clearly indicates that content is not required, and the language even implies that it's leagacy, I figure pydantic-ai should be supporting and preferring structuredContent when present.

Specifically the issue is here, where only content is supported:

pydantic-ai/pydantic_ai_slim/pydantic_ai/mcp.py

Line 225 in 62f4ddc

content = [await self._map_tool_result_part(part) for part in result.content]

References

No response

[DouweM]

@phemmer Thanks for the report, makes sense to me. Would you be up for filing a PR with a fix?