Fix regression matching templated McpServerResources

src/ModelContextProtocol.Core/Server/AIFunctionMcpServerResource.cs

@@ -314,7 +314,28 @@ private AIFunctionMcpServerResource(AIFunction function, ResourceTemplate resour
     public override IReadOnlyList<object> Metadata => _metadata;
 
     /// <inheritdoc />
-    public override async ValueTask<ReadResourceResult?> ReadAsync(
+    public override bool CanReadUri(string uri)
+    {
+        Throw.IfNull(uri);
+        return TryMatch(uri, out _);
+    }
+
+    private bool TryMatch(string uri, out Match? match)
+    {
+        // For templates, use the Regex to parse. For static resources, we can just compare the URIs.
+        if (_uriParser is null)
+        {
+            // This resource is not templated.
+            match = null;
+            return UriTemplate.UriTemplateComparer.Instance.Equals(uri, ProtocolResourceTemplate.UriTemplate);
+        }
+
+        match = _uriParser.Match(uri);
+        return match.Success;
+    }
+
+    /// <inheritdoc />
+    public override async ValueTask<ReadResourceResult> ReadAsync(
         RequestContext<ReadResourceRequestParams> request, CancellationToken cancellationToken = default)
     {
         Throw.IfNull(request);
@@ -323,20 +344,9 @@ private AIFunctionMcpServerResource(AIFunction function, ResourceTemplate resour
 
         cancellationToken.ThrowIfCancellationRequested();
 
-        // Check to see if this URI template matches the request URI. If it doesn't, return null.
-        // For templates, use the Regex to parse. For static resources, we can just compare the URIs.
-        Match? match = null;
-        if (_uriParser is not null)
-        {
-            match = _uriParser.Match(request.Params.Uri);
-            if (!match.Success)
-            {
-                return null;
-            }
-        }
-        else if (!UriTemplate.UriTemplateComparer.Instance.Equals(request.Params.Uri, ProtocolResource!.Uri))
+        if (!TryMatch(request.Params.Uri, out Match? match))
         {
-            return null;
+            throw new InvalidOperationException($"Resource '{ProtocolResourceTemplate.UriTemplate}' does not match the provided URI '{request.Params.Uri}'.");
         }
 
         // Build up the arguments for the AIFunction call, including all of the name/value pairs from the URI.

src/ModelContextProtocol.Core/Server/DelegatingMcpServerResource.cs

@@ -26,7 +26,10 @@ protected DelegatingMcpServerResource(McpServerResource innerResource)
     public override ResourceTemplate ProtocolResourceTemplate => _innerResource.ProtocolResourceTemplate;
 
     /// <inheritdoc />
-    public override ValueTask<ReadResourceResult?> ReadAsync(RequestContext<ReadResourceRequestParams> request, CancellationToken cancellationToken = default) => 
+    public override bool CanReadUri(string uri) => _innerResource.CanReadUri(uri);
+
+    /// <inheritdoc />
+    public override ValueTask<ReadResourceResult> ReadAsync(RequestContext<ReadResourceRequestParams> request, CancellationToken cancellationToken = default) =>
         _innerResource.ReadAsync(request, cancellationToken);
 
     /// <inheritdoc />

src/ModelContextProtocol.Core/Server/McpServerImpl.cs

@@ -342,10 +342,7 @@ await originalListResourceTemplatesHandler(request, cancellationToken).Configure
             {
                 if (request.MatchedPrimitive is McpServerResource matchedResource)
                 {
-                    if (await matchedResource.ReadAsync(request, cancellationToken).ConfigureAwait(false) is { } result)
-                    {
-                        return result;
-                    }
+                    return await matchedResource.ReadAsync(request, cancellationToken).ConfigureAwait(false);
                 }
 
                 return await originalReadResourceHandler(request, cancellationToken).ConfigureAwait(false);
@@ -366,22 +363,17 @@ await originalListResourceTemplatesHandler(request, cancellationToken).Configure
                 if (request.Params?.Uri is { } uri && resources is not null)
                 {
                     // First try an O(1) lookup by exact match.
-                    if (resources.TryGetPrimitive(uri, out var resource))
+                    if (resources.TryGetPrimitive(uri, out var resource) && !resource.IsTemplated)
                     {
                         request.MatchedPrimitive = resource;
                     }
                     else
                     {
                         // 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)
+                            if (resourceTemplate.CanReadUri(uri))
                             {
-                                // 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;
                             }

src/ModelContextProtocol.Core/Server/McpServerResource.cs

@@ -162,6 +162,14 @@ protected McpServerResource()
     /// </remarks>
     public abstract IReadOnlyList<object> Metadata { get; }
 
+    /// <summary>
+    /// Evaluates whether the <paramref name="uri"/> matches the <see cref="ProtocolResourceTemplate"/>
+    /// and can be used as the <see cref="ReadResourceRequestParams.Uri"/> passed to <see cref="ReadAsync"/>.
+    /// </summary>
+    /// <param name="uri">The URI being evaluated for this resource.</param>
+    /// <returns><see langword="true"/> if the <paramref name="uri"/> matches the <see cref="ProtocolResourceTemplate"/>; otherwise, <see langword="false"/>.</returns>
+    public abstract bool CanReadUri(string uri);
+
     /// <summary>
     /// Gets the resource, rendering it with the provided request parameters and returning the resource result.
     /// </summary>
@@ -174,12 +182,14 @@ protected McpServerResource()
     /// </param>
     /// <returns>
     /// A <see cref="ValueTask{ReadResourceResult}"/> representing the asynchronous operation, containing a <see cref="ReadResourceResult"/> with
-    /// the resource content and messages. If and only if this <see cref="McpServerResource"/> doesn't match the <see cref="ReadResourceRequestParams.Uri"/>,
-    /// the method returns <see langword="null"/>.
+    /// the resource content and messages.
     /// </returns>
     /// <exception cref="ArgumentNullException"><paramref name="request"/> is <see langword="null"/>.</exception>
-    /// <exception cref="InvalidOperationException">The resource implementation returned <see langword="null"/> or an unsupported result type.</exception>
-    public abstract ValueTask<ReadResourceResult?> ReadAsync(
+    /// <exception cref="InvalidOperationException">
+    /// The <see cref="ReadResourceRequestParams.Uri"/> did not match the <see cref="ProtocolResourceTemplate"/> for this resource,
+    /// the resource implementation returned <see langword="null"/>, or the resource implementation returned an unsupported result type.
+    /// </exception>
+    public abstract ValueTask<ReadResourceResult> ReadAsync(
         RequestContext<ReadResourceRequestParams> request,
         CancellationToken cancellationToken = default);
 

tests/ModelContextProtocol.Tests/Configuration/McpServerResourceRoutingTests.cs

@@ -0,0 +1,39 @@
+using Microsoft.Extensions.DependencyInjection;
+using ModelContextProtocol.Client;
+using ModelContextProtocol.Protocol;
+using ModelContextProtocol.Server;
+
+namespace ModelContextProtocol.Tests.Configuration;
+
+public sealed class McpServerResourceRoutingTests(ITestOutputHelper testOutputHelper) : ClientServerTestBase(testOutputHelper)
+{
+    protected override void ConfigureServices(ServiceCollection services, IMcpServerBuilder mcpServerBuilder)
+    {
+        mcpServerBuilder.WithResources([
+            McpServerResource.Create(options: new() { UriTemplate = "test://resource/non-templated" } , method: () => "static"),
+            McpServerResource.Create(options: new() { UriTemplate = "test://resource/{id}" }, method: (string id) => $"template: {id}"),
+            McpServerResource.Create(options: new() { UriTemplate = "test://params{?a1,a2,a3}" }, method: (string a1, string a2, string a3) => $"params: {a1}, {a2}, {a3}"),
+        ]);
+    }
+
+    [Fact]
+    public async Task MultipleTemplatedResources_MatchesCorrectResource()
+    {
+        // Verify that when multiple templated resources exist, the correct one is matched based on the URI pattern, not just the first one.
+        // Regression test for https://github.com/modelcontextprotocol/csharp-sdk/issues/821.
+        await using McpClient client = await CreateMcpClientForServer();
+
+        var nonTemplatedResult = await client.ReadResourceAsync("test://resource/non-templated", TestContext.Current.CancellationToken);
+        Assert.Equal("static", ((TextResourceContents)nonTemplatedResult.Contents[0]).Text);
+
+        var templatedResult = await client.ReadResourceAsync("test://resource/12345", TestContext.Current.CancellationToken);
+        Assert.Equal("template: 12345", ((TextResourceContents)templatedResult.Contents[0]).Text);
+
+        var paramsResult = await client.ReadResourceAsync("test://params?a1=a&a2=b&a3=c", TestContext.Current.CancellationToken);
+        Assert.Equal("params: a, b, c", ((TextResourceContents)paramsResult.Contents[0]).Text);
+
+        var mcpEx = await Assert.ThrowsAsync<McpProtocolException>(async () => await client.ReadResourceAsync("test://params{?a1,a2,a3}", TestContext.Current.CancellationToken));
+        Assert.Equal(McpErrorCode.InvalidParams, mcpEx.ErrorCode);
+        Assert.Equal("Request failed (remote): Unknown resource URI: 'test://params{?a1,a2,a3}'", mcpEx.Message);
+    }
+}

tests/ModelContextProtocol.Tests/Server/McpServerResourceTests.cs

@@ -142,7 +142,7 @@ public async Task UriTemplate_CreatedFromParameters_LotsOfTypesSupported()
         const string Name = "Hello";
 
         McpServerResource t;
-        ReadResourceResult? result;
+        ReadResourceResult result;
         McpServer server = new Mock<McpServer>().Object;
 
         t = McpServerResource.Create(() => "42", new() { Name = Name });
@@ -282,11 +282,12 @@ public async Task UriTemplate_CreatedFromParameters_LotsOfTypesSupported()
     [InlineData("resource://mcp/Hello?arg1=42&arg2=84")]
     [InlineData("resource://mcp/Hello?arg1=42&arg2=84&arg3=123")]
     [InlineData("resource://mcp/Hello#fragment")]
-    public async Task UriTemplate_NonMatchingUri_ReturnsNull(string uri)
+    public async Task UriTemplate_NonMatchingUri_DoesNotMatch(string uri)
     {
         McpServerResource t = McpServerResource.Create((string arg1) => arg1, new() { Name = "Hello" });
         Assert.Equal("resource://mcp/Hello{?arg1}", t.ProtocolResourceTemplate.UriTemplate);
-        Assert.Null(await t.ReadAsync(
+        Assert.False(t.CanReadUri(uri));
+        await Assert.ThrowsAsync<InvalidOperationException>(async () => await t.ReadAsync(
             new RequestContext<ReadResourceRequestParams>(new Mock<McpServer>().Object, CreateTestJsonRpcRequest()) { Params = new() { Uri = uri } },
             TestContext.Current.CancellationToken));
     }
@@ -337,7 +338,7 @@ public async Task UriTemplate_MissingOptionalParameter_Succeeds()
         McpServerResource t = McpServerResource.Create((string? arg1 = null, int? arg2 = null) => arg1 + arg2, new() { Name = "Hello" });
         Assert.Equal("resource://mcp/Hello{?arg1,arg2}", t.ProtocolResourceTemplate.UriTemplate);
 
-        ReadResourceResult? result;
+        ReadResourceResult result;
 
         result = await t.ReadAsync(
             new RequestContext<ReadResourceRequestParams>(new Mock<McpServer>().Object, CreateTestJsonRpcRequest()) { Params = new() { Uri = "resource://mcp/Hello" } },