Add `ResourceTemplate` and related attribute(s) for implementing resource template handlers

[Redth]

Is your feature request related to a problem? Please describe.
Just like McpToolType and McpTool works today, it would be great to have a way to use attributes to define resource templates.

Describe the solution you'd like
Would be nice to annotate a type and method(s) to specify resource templates.

Resource URI Templates per the spec should conform to RFC6570. I'm not sure if this is what ASP.NET uses, but perhaps there is some way to leverage the route parsing logic within the SDK here?

[ResourceTemplateType]
public static class SampleResourceTemplates {

  // RFC6570 URI Template
  [ResourceTemplate("nuget://{packageId}/{version}")]
  public static byte[] NuGetPackage(string packageId, string version) {
    // Implement Handler logic to fetch and return
  }

}

// in Program.cs
builder.Services
  .AddMcpServer()
  .WithStdioServerTransport()
  .WithTools()
  .WithResourceTemplates();

Additional context
This probably should be considered with #72 so that methods could return actual types marked as ResourceType and have that handled automatically.

[Redth]

For template parsing and wiring up to method calls, we could potentially use something like https://github.com/corvus-dotnet/Corvus.UriTemplates which already implements the RFC and has some patterns for programmatically extracting a dictionary of parameters given a URI template and URI.

[PederHP]

I really like this approach. I think it could work fine for Resource as well. Isn't a Resource effectively just a ResourceTemplate with no parameters?

[IvanMurzak]

Thanks for this idea! I was expected for the same approach as with Tools, it was a surprise to see the existed implementation which doesn't have Attributes.

@PederHP do you have any plans for this feature?

[stephentoub]

We just haven't got to it yet.

[johnkors]

We just haven't got to it yet.

Is this why my MCP server logs go: No handler found for request resources/list for server Server?

My server:

        builder.Services.AddMcpServer()
            .WithStdioServerTransport()
            .WithPromptsFromAssembly()
            .WithToolsFromAssembly();

I'm using my MCP-server together with Claude and it looks like it asks for resources every 5 seconds or so. Shutting down Claude, and the polling for resources/lists stops.

Logs:

[06:17:22] WRN ModelContextProtocol.Server.McpServer bf23e145-b3c9-4c52-94ff-6e060dd62278 No handler found for request resources/list for server Server (Loftbot.App 1.0.0.0)
[06:17:22] ERR ModelContextProtocol.Server.McpServer bf23e145-b3c9-4c52-94ff-6e060dd62278 Request handler error for Server (Loftbot.App 1.0.0.0) with method resources/list
[06:17:22] INF ModelContextProtocol.Server.McpServer bf23e145-b3c9-4c52-94ff-6e060dd62278 Request handler called for Server (Loftbot.App 1.0.0.0) with method prompts/list
[06:17:22] INF ModelContextProtocol.Server.McpServer bf23e145-b3c9-4c52-94ff-6e060dd62278 Request handler completed for Server (Loftbot.App 1.0.0.0) with method prompts/list
[06:17:27] WRN ModelContextProtocol.Server.McpServer bf23e145-b3c9-4c52-94ff-6e060dd62278 No handler found for request resources/list for server Server (Loftbot.App 1.0.0.0)
[06:17:27] ERR ModelContextProtocol.Server.McpServer bf23e145-b3c9-4c52-94ff-6e060dd62278 Request handler error for Server (Loftbot.App 1.0.0.0) with method resources/list
[06:17:27] INF ModelContextProtocol.Server.McpServer bf23e145-b3c9-4c52-94ff-6e060dd62278 Request handler called for Server (Loftbot.App 1.0.0.0) with method prompts/list
[06:17:27] INF ModelContextProtocol.Server.McpServer bf23e145-b3c9-4c52-94ff-6e060dd62278 Request handler completed for Server (Loftbot.App 1.0.0.0) with method prompts/list
[06:17:32] WRN ModelContextProtocol.Server.McpServer bf23e145-b3c9-4c52-94ff-6e060dd62278 No handler found for request resources/list for server Server (Loftbot.App 1.0.0.0)
[06:17:32] ERR ModelContextProtocol.Server.McpServer bf23e145-b3c9-4c52-94ff-6e060dd62278 Request handler error for Server (Loftbot.App 1.0.0.0) with method resources/list
[06:17:32] INF ModelContextProtocol.Server.McpServer bf23e145-b3c9-4c52-94ff-6e060dd62278 Request handler called for Server (Loftbot.App 1.0.0.0) with method prompts/list
[06:17:32] INF ModelContextProtocol.Server.McpServer bf23e145-b3c9-4c52-94ff-6e060dd62278 Request handler completed for Server (Loftbot.App 1.0.0.0) with method prompts/list
[06:17:37] WRN ModelContextProtocol.Server.McpServer bf23e145-b3c9-4c52-94ff-6e060dd62278 No handler found for request resources/list for server Server (Loftbot.App 1.0.0.0)
[06:17:37] ERR ModelContextProtocol.Server.McpServer bf23e145-b3c9-4c52-94ff-6e060dd62278 Request handler error for Server (Loftbot.App 1.0.0.0) with method resources/list
[06:17:37] INF ModelContextProtocol.Server.McpServer bf23e145-b3c9-4c52-94ff-6e060dd62278 Request handler called for Server (Loftbot.App 1.0.0.0) with method prompts/list
[06:17:37] INF ModelContextProtocol.Server.McpServer bf23e145-b3c9-4c52-94ff-6e060dd62278 Request handler completed for Server (Loftbot.App 1.0.0.0) with method prompts/list

[IvanMurzak]

@johnkors I don't think so. You just need to set the handler. I made 3 static functions to make it pretty.

builder.Services
    .AddMcpServer()
    .WithStdioServerTransport()
    .WithPromptsFromAssembly()
    .WithToolsFromAssembly()
    .WithListResourceTemplatesHandler(ResourceRouter.ListResourceTemplates)
    .WithListResourcesHandler(ResourceRouter.ListResources)
    .WithReadResourceHandler(ResourceRouter.ReadResource);

[stephentoub]

Is this why my MCP server logs go: No handler found for request resources/list for server Server?

That looks like an issue with Claude. If the server doesn't advertise a resources capability, the client shouldn't be issuing requests to list resources.

[johnkors]

You just need to set the handler.

I don't have resources, so adding a resource handler would only avoid the error messages. Wouldn't it make more sense for the server to return an empty list if none defined instead of erroring out?

That looks like an issue with Claude. If the server doesn't advertise a resources capability, the client shouldn't be issuing requests to list resources.

Yeah, Copilot in VS Code does not poll - so probably an issue with Claude. Didn't they invent the spec, though? :)

[stephentoub]

Wouldn't it make more sense for the server to return an empty list if none defined instead of erroring out?

It could, but the spec actually states it should be an error:

"Error Handling
Servers SHOULD return standard JSON-RPC errors for common failure cases:
Resource not found: -32002"

[PederHP]

Yeah, Copilot in VS Code does not poll - so probably an issue with Claude. Didn't they invent the spec, though? :)

They did, but the Python and TypeScript SDKs have a number of open issues, and there have been a few odd implementation choices (the initial prompt implementation in the app comes to mind) so the Claude desktop app should not be considered a reference client. The mcp-inspector is probably thing that exists to a reference client.

[johnkors]

It could, but the spec actually states it should be an error:

I understand, but would it be too magic if there always were a default no-op implementation, so I don't need to do this to avoid error logs?

.WithListResourcesHandler((ctx, ct) => new NoOpListResourceHandler()) // returns empty lists

Or just lower the log level. Some client asking for resources when there aren't any, WRN instead of ERR?

[stephentoub]

Or just lower the log level.

That's reasonable