OpenAPI/Swagger specification to MCP Tools

[gunpal5]

OpenAPI specifications are standard for communicating with a REST API service.

MCP servers can work as a proxy to REST services. Which would bridge the gap between a REST service and AI Models. In turn speed up the development process and migration to AI.

If we can write an adapter or a converter which takes the OpenAPI specs and selectively convert part or full APIs into MCP Tools then it would awesome.

I did a quick search for this idea, there are couple of existing repositories which does similar thing, but those are in different languages.

Let me know your views on this.

[stephentoub]

Seems like a reasonable MCP server implementation you could expose on top of the C# SDK. I wouldn't build it into the C# SDK unless it became a standard feature of the specification. If you hit any issues implementing it on top of the C# SDK, that's something we'd like to know about and fix in the SDK, to ensure things like that are possible and easy.

[eiriktsarpalis]

Sounds like another use case for us making dynamically populated tools easier to implement?

[stephentoub]

Sounds like another use case for us making dynamically populated tools easier to implement?

Is it hard right now? e.g.

foreach (var restTool in ...)
{
    services.AddSingleton(McpServerTool.Create(...));
}

or going a level down

McpServerPrimitiveCollection<McpServerTool> tools = [];
foreach (var restTool in ...)
{
    tools.Add(McpServerTool.Create(...));
}

McpServerOptions options = new()
{
    Capabilities = new()
    {
        Tools = new()
        {
            ToolCollection = tools,
        }
    }
};

...

Are you referring to cleaning up the options, or something deeper?

[PederHP]

My take is that this is an MCP server concern and not an SDK concern. There are already several MCP servers out there which can proxy REST APIs based on OAS files. You can implement one with the SDK as it is, right now, by mapping the OAS to the Tool spec.

It's not a hugely complicated task - you could probably get GitHub Copilot in Edit/Agentic mode or Claude Code to do a reasonable implementation, if they have a good enough understanding of the SDK. It's straight mapping and marshaling exercising.

It's a great use case, but it sounds very narrow for the actual SDK - especially as there are servers which do this right now and none of the other SDKs have anything like this.

[eiriktsarpalis]

Are you referring to cleaning up the options, or something deeper?

It is more about supporting changes to the list of tools within the lifetime of the server and issuing notifications to the client when the list changes per the spec. My understanding is that this can only be implemented manually today.

[stephentoub]

It is more about supporting changes to the list of tools within the lifetime of the server and issuing notifications to the client when the list changes per the spec. My understanding is that this can only be implemented manually today.

We actually already do that. If in my example above you later add/remove a tool from that tools collection, it'll both send a notification to registered clients and start serving that updated too list.

csharp-sdk/tests/ModelContextProtocol.Tests/Configuration/McpServerBuilderExtensionsToolsTests.cs

Lines 264 to 269 in 8fcdf95

	var newTool = McpServerTool.Create([McpServerTool(Name = "NewTool")] () => "42");
	serverTools.Add(newTool);
	await notificationRead;
	tools = await client.ListToolsAsync(cancellationToken: TestContext.Current.CancellationToken);
	Assert.Equal(17, tools.Count);

I'm sure before we're done we can make it even more ergonomic. But I think that's mostly about how one supplies the options initially.

[gunpal5]

@stephentoub @PederHP @eiriktsarpalis Thank you for your inputs. I'll implement this feature as a separate project.

[stephentoub]

Thanks, @gunpal5. Please let us know if you hit any issues along the way, or otherwise have any suggestions based on your experiences.

[gunpal5]

Hi Team @stephentoub @eiriktsarpalis @PederHP

I just managed to create a first draft of the project. It supports multiple configuration options to build the MCP servers from OAS schema. Please take a look into this:

https://github.com/gunpal5/QuickMCP

I was wondering if it would be possible for you to add a link to this repository in ReadMe.md? I believe it would complement the official SDK and save developer's time.