Add defer tool loading to enable Anthropic's "Tool Search" pattern

[wolfeidau]

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

Anthropic has introduced a beta feature where tools can be marked with defer_loading: true. For more information see Anthropic Advanced Tool Use Documentation.

Also a good summary here https://x.com/trq212/status/2011523109871108570

Describe the solution you'd like

This would involve adding a property to the Tool structure.

	// Support for deferred loading
	DeferLoading bool `json:"defer_loading"`

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.

I have this working with mark3labs/mcp-go#644 using the environment variable ENABLE_TOOL_SEARCH=true with claude code, you can see mcp loaded on demand, and no MCP tools in context in the screenshot below.

Interested in thoughts on this, happy to make a PR.

[maciej-kisiel]

Hi @wolfeidau, could you explain why would you need this on the SDK level?

In general, the SDK's goal is MCP specification conformance and supporting a beta functionality from one of the AI companies does not really align with it. Couldn't this be achieved on the client application side by decorating the results of ListTools?