Support for OpenAI Apps SDK

[aaazzam]

Enhancement

OpenAI announced the Apps SDK, an extension of mcp-ui. Let’s support this as a first class FastMCP extension.

[BrandonShar]

Hey @aaazzam , I'm going to be spiking on adding this to a FastMCP server this week. I'd be happy to collaborate on upstreaming it if you're interested!

[BrandonShar]

Tough part is the example from OpenAI isn't functional yet, so it's hard to tell whether meta only needs to be added to tool descriptions (which FastMCP already supports) or if it will also need to be added to CallToolResult payloads (which neither FastMCP or the underlying SDK support). The example repo does both, but the docs seem to suggest that the meta in the payload is just an enhancement, not a requirement.

[aaazzam]

@BrandonShar I'd love that -- thanks for your help on this. Let's both try to get something to work and then try to figure out if this is is a thing that needs documentation, sdk support, or both.

[BrandonShar]

Really exciting start here! I got everything working with the OpenAI example app's frontend using existing FastMCP tools and resources.

I'd like to do a couple more tests (planning for tomorrow) with understanding how widget controlled tool calls work, but fingers crossed this may end up mainly being documentation and possibly some syntactic sugar.

The one part we're definitely missing is the ability to return _meta as part of a ToolResult. This is a part of JSON-RPC, not MCP (meta is in the protocol, just not in the sdk!), so it may be tough to add. What _meta in the response controls is the ability to send data from the tool to a frontend component without the model having access to it (both content and structured_content are exposed to the model). From a quick claude code explanation reading of the code it looks like FastMCP delegates ToolResults down to the anthropic sdk and that sdk doesn't have a clear way to add _meta either. The dataclass supports it, but not the handling code.

[BrandonShar]

I created a PR to allow _meta to be returned from tool calls (via allowing CallToolResult to be returned directly) in the underlying sdk here: modelcontextprotocol/python-sdk#1459

[BrandonShar]

The _meta piece I PR'd above should be the only thing we need functionality-wise and then we can create a FastMCP 2.0 version of OpenAI's example server. (and it will look so much nicer 😄 )

Hopefully that PR lands soon. If/when it does, I'll create a corresponding PR here to use it.

[BrandonShar]

Ok, PR just got merged! I'll keep an eye on when it's tagged for release and then open up a PR to upstream it here!