McpServerResource not working.

[BenLampson]

Describe the bug
I just defined two resources to test,

[McpServerResource(MimeType = "text/json",UriTemplate ="KBInfo://summary"), Description("Get this mcp's Knowledge summary info")]
public static string KBSummary()
{
    var allReadme = Directory.GetFiles("./KB", "x.md", SearchOption.AllDirectories);
    var res = new List<KBItemResponse>();
    foreach (var item in allReadme)
    {
        res.Add(new()
        {
            FilePath = $"{Path.GetRelativePath(".", item)}",
            SummaryInfo = File.ReadAllText(item)
        });
    }
    return System.Text.Json.JsonSerializer.Serialize(res);
}

[McpServerResource(MimeType = "text/markdown", UriTemplate = "KBInfo://{path}"), Description("Get some knowledge's detailinfo")]
public static string GetKnowledgeDetailInfo(string path)
{

    return string.Empty;
}

To Reproduce
Use AI VS code like the Roo Code etc.
Ask some question wants to use this resource.

Expected behavior
Error accessing MCP resource:
MCP error -32602: Unknown resource URI: 'KBInfo://KB/docs2/topic-auth.md'

Logs

ModelContextProtocol.McpException: Unknown resource URI: 'KBInfo://KB/docs2/topic-auth.md'
         at ModelContextProtocol.Server.McpServerImpl.<>c.<<ConfigureResources>b__48_2>d.MoveNext()
      --- End of stack trace from previous location ---
         at ModelContextProtocol.Server.McpServerImpl.<>c__DisplayClass48_1.<<ConfigureResources>b__8>d.MoveNext()
      --- End of stack trace from previous location ---
         at ModelContextProtocol.AspNetCore.AuthorizationFilterSetup.<>c__DisplayClass14_0.<<CheckReadResourceFilter>b__1>d.MoveNext()
      --- End of stack trace from previous location ---
         at ModelContextProtocol.Server.McpServerImpl.<>c__DisplayClass48_2.<<ConfigureResources>b__9>d.MoveNext()
      --- End of stack trace from previous location ---
         at ModelContextProtocol.Server.McpServerImpl.<InvokeHandlerAsync>g__InvokeScopedAsync|52_0[TParams,TResult](McpRequestHandler`2 handler, TParams args, JsonRpcRequest jsonRpcRequest, CancellationToken cancellationToken)
         at ModelContextProtocol.Server.McpServerImpl.<InvokeHandlerAsync>g__InvokeScopedAsync|52_0[TParams,TResult](McpRequestHandler`2 handler, TParams args, JsonRpcRequest jsonRpcRequest, CancellationToken cancellationToken)
         at ModelContextProtocol.RequestHandlers.<>c__DisplayClass0_0`2.<<Set>b__0>d.MoveNext()
      --- End of stack trace from previous location ---
         at ModelContextProtocol.McpSessionHandler.HandleRequest(JsonRpcRequest request, CancellationToken cancellationToken)
         at ModelContextProtocol.McpSessionHandler.HandleMessageAsync(JsonRpcMessage message, CancellationToken cancellationToken)
         at ModelContextProtocol.McpSessionHandler.<>c__DisplayClass26_1.<<ProcessMessagesCoreAsync>g__ProcessMessageAsync|1>d.MoveNext()

Additional context
None

[stephentoub]

Can you share a full repro? How are you registering these resources with your server?

[PederHP]

I am not sure "KBInfo://KB/docs2/topic-auth.md" is a valid expansion of "KBInfo://{path}"

This is a level 2 template and should be "KBInfo://{+path}"

As per:
https://www.rfc-editor.org/rfc/rfc6570.html

I am not sure the SDK supports level 2 and 3 templates. The MCP spec isn't clear on what level of templates should be supported, but I think most SDKs only support level 1.

[stephentoub]

The MCP spec isn't clear on what level of templates should be supported, but I think most SDKs only support level 1.

It should support through level 4...at least that was the intent.

[PederHP]

Then editing to:

[McpServerResource(MimeType = "text/markdown", UriTemplate = "KBInfo://{+path}"), Description("Get some knowledge's detailinfo")]
public static string GetKnowledgeDetailInfo(string path)
{

    return string.Empty;
}

Should be sufficient.

[StevieJski]

I get same error when defining 2 templated resources. Something like
[McpServerResource(UriTemplate = "myresource://schemas/entity/{typeName}", Name = "Schemas Entity")]
and
[McpServerResource(UriTemplate = "myresource://entities/get/{type}{?where,maxResults,firstResultIndex}", Name = "Entities Get")]

I believe this comes down to primitive matching in this section of McpServerImpl.ConfigureResources()

                        // Fall back to an O(N) lookup, trying to match against each URI template.
                        // The number of templates is controlled by the server developer, and the number is expected to be
                        // not terribly large. If that changes, this can be tweaked to enable a more efficient lookup.
                        foreach (var resourceTemplate in resources)
                        {
                            // Check if this template would handle the request by testing if ReadAsync would succeed
                            if (resourceTemplate.IsTemplated)
                            {
                                // This is a simplified check - a more robust implementation would match the URI pattern
                                // For now, we'll let the actual handler attempt the match
                                request.MatchedPrimitive = resourceTemplate;
                                break;
                            }
                        }

since it always matches on the first resource where IsTemplated == true. This means in my case that resource reads work for url
myresource://schemas/entity/Strategy
but get the "Unknown resource URI" for e.g.
myresource://entities/get/Strategy?maxResults=100
as when it calls matchedResource.ReadAsync() the url won't parse and so result is null, because its using the wrong template

if (request.MatchedPrimitive is McpServerResource matchedResource)
{
    if (await matchedResource.ReadAsync(request, cancellationToken).ConfigureAwait(false) is { } result)
    {
        return result;
    }
}

I've locally 'fixed' this by doing what I think the intention was from the existing code comments, i.e. call ReadAsync() to see if the template matches

                        // Fall back to an O(N) lookup, trying to match against each URI template.
                        // The number of templates is controlled by the server developer, and the number is expected to be
                        // not terribly large. If that changes, this can be tweaked to enable a more efficient lookup.
                        foreach (var resourceTemplate in resources)
                        {
                            // Check if this template would handle the request by testing if ReadAsync would succeed
                            if (resourceTemplate.IsTemplated )
                            {
                                var match = await resourceTemplate.ReadAsync(request, cancellationToken);
                                if (match != null)
                                {
                                  request.MatchedPrimitive = resourceTemplate;
                                  break;
                                }
                            }
                        }

[stephentoub]

@halter73, I think this broke in #733. This loop:

                    // Fall back to an O(N) lookup, trying to match against each URI template.
                    // The number of templates is controlled by the server developer, and the number is expected to be
                    // not terribly large. If that changes, this can be tweaked to enable a more efficient lookup.
                    foreach (var resourceTemplate in resources)
                    {
                        if (await resourceTemplate.ReadAsync(request, cancellationToken).ConfigureAwait(false) is { } result)
                        {
                            return result;
                        }
                    }

got changed to this:

                        // Fall back to an O(N) lookup, trying to match against each URI template.
                        // The number of templates is controlled by the server developer, and the number is expected to be
                        // not terribly large. If that changes, this can be tweaked to enable a more efficient lookup.
                        foreach (var resourceTemplate in resources)
                        {
                            // Check if this template would handle the request by testing if ReadAsync would succeed
                            if (resourceTemplate.IsTemplated)
                            {
                                // This is a simplified check - a more robust implementation would match the URI pattern
                                // For now, we'll let the actual handler attempt the match
                                request.MatchedPrimitive = resourceTemplate;
                                break;
                            }
                        }

Can you take a look?