Fix: Synthesize handlers for empty tool/prompt/resource collections (#865)

* Initial plan

* Allow handlers for empty tool/prompt/resource collections

Changed condition from `IsEmpty: false` to `not null` check in McpServerImpl.cs for resources (line 298), prompts (line 459), and tools (line 547). This allows handlers to be synthesized even when collections are empty, enabling dynamic addition of items after initialization.

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: Stephen Toub <stoub@microsoft.com>

src/ModelContextProtocol.Core/Server/McpServerImpl.cs

@@ -292,7 +292,7 @@ internal sealed partial class McpServerImpl : McpServer
         var subscribe = resourcesCapability?.Subscribe;
 
         // Handle resources provided via DI.
-        if (resources is { IsEmpty: false })
+        if (resources is not null)
         {
             var originalListResourcesHandler = listResourcesHandler;
             listResourcesHandler = async (request, cancellationToken) =>
@@ -453,7 +453,7 @@ internal sealed partial class McpServerImpl : McpServer
         var listChanged = promptsCapability?.ListChanged;
 
         // Handle tools provided via DI by augmenting the handlers to incorporate them.
-        if (prompts is { IsEmpty: false })
+        if (prompts is not null)
         {
             var originalListPromptsHandler = listPromptsHandler;
             listPromptsHandler = async (request, cancellationToken) =>
@@ -541,7 +541,7 @@ internal sealed partial class McpServerImpl : McpServer
         var listChanged = toolsCapability?.ListChanged;
 
         // Handle tools provided via DI by augmenting the handlers to incorporate them.
-        if (tools is { IsEmpty: false })
+        if (tools is not null)
         {
             var originalListToolsHandler = listToolsHandler;
             listToolsHandler = async (request, cancellationToken) =>

tests/Common/Utils/MockHttpHandler.cs

@@ -1,6 +1,4 @@
-using System.Net.Http;
+namespace ModelContextProtocol.Tests.Utils;
-
-namespace ModelContextProtocol.Tests.Utils;
 
 public class MockHttpHandler : HttpMessageHandler
 {

tests/ModelContextProtocol.AspNetCore.Tests/ModelContextProtocol.AspNetCore.Tests.csproj

@@ -44,7 +44,6 @@
     <PackageReference Include="Moq" />
     <PackageReference Include="OpenTelemetry" />
     <PackageReference Include="OpenTelemetry.Exporter.InMemory" />
-    <PackageReference Include="System.Linq.AsyncEnumerable" />
     <PackageReference Include="xunit.v3" />
     <PackageReference Include="xunit.runner.visualstudio">
       <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
@@ -52,6 +51,10 @@
     </PackageReference>
   </ItemGroup>
 
+  <ItemGroup Condition="!$([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', 'net10.0'))">
+    <PackageReference Include="System.Linq.AsyncEnumerable" />
+  </ItemGroup>
+
   <ItemGroup>
     <ProjectReference Include="..\..\samples\TestServerWithHosting\TestServerWithHosting.csproj" />
     <ProjectReference Include="..\..\src\ModelContextProtocol.Core\ModelContextProtocol.Core.csproj" />

tests/ModelContextProtocol.TestSseServer/ModelContextProtocol.TestSseServer.csproj

@@ -9,8 +9,6 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="Microsoft.Extensions.Logging" />
-    <PackageReference Include="Microsoft.Extensions.Logging.Console" />
     <PackageReference Include="Serilog.Extensions.Logging" />
     <PackageReference Include="Serilog.Sinks.File" />
   </ItemGroup>

tests/ModelContextProtocol.Tests/Configuration/McpServerBuilderExtensionsResourcesTests.cs

@@ -7,7 +7,6 @@ using ModelContextProtocol.Server;
 using Moq;
 using System.Collections;
 using System.ComponentModel;
-using System.Text.Json;
 using System.Threading.Channels;
 using static ModelContextProtocol.Tests.Configuration.McpServerBuilderExtensionsPromptsTests;
 

tests/ModelContextProtocol.Tests/ModelContextProtocol.Tests.csproj

@@ -52,7 +52,6 @@
     <PackageReference Include="OpenTelemetry" />
     <PackageReference Include="OpenTelemetry.Exporter.InMemory" />
     <PackageReference Include="Serilog" />
-    <PackageReference Include="System.Linq.AsyncEnumerable" />
     <PackageReference Include="JsonSchema.Net" />
     <PackageReference Include="xunit.v3" />
     <PackageReference Include="xunit.runner.visualstudio">
@@ -61,6 +60,10 @@
     </PackageReference>
   </ItemGroup>
 
+  <ItemGroup Condition="!$([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', 'net10.0'))">
+    <PackageReference Include="System.Linq.AsyncEnumerable" />
+  </ItemGroup>
+
   <ItemGroup>
     <ProjectReference Include="..\..\src\ModelContextProtocol.Core\ModelContextProtocol.Core.csproj" />
     <ProjectReference Include="..\ModelContextProtocol.TestServer\ModelContextProtocol.TestServer.csproj" />

tests/ModelContextProtocol.Tests/Protocol/CancellationTests.cs

@@ -1,5 +1,4 @@
 using Microsoft.Extensions.DependencyInjection;
-using ModelContextProtocol.Client;
 using ModelContextProtocol.Protocol;
 using ModelContextProtocol.Server;
 

tests/ModelContextProtocol.Tests/Protocol/ElicitationTests.cs

@@ -1,7 +1,6 @@
 using Microsoft.Extensions.DependencyInjection;
 using ModelContextProtocol.Client;
 using ModelContextProtocol.Protocol;
-using ModelContextProtocol.Server;
 using System.Text.Json;
 
 namespace ModelContextProtocol.Tests.Configuration;

tests/ModelContextProtocol.Tests/Server/EmptyCollectionTests.cs

@@ -0,0 +1,165 @@
+using Microsoft.Extensions.AI;
+using Microsoft.Extensions.DependencyInjection;
+using ModelContextProtocol.Protocol;
+using ModelContextProtocol.Server;
+
+namespace ModelContextProtocol.Tests.Server;
+
+/// <summary>
+/// Tests to verify that handlers are synthesized for empty collections that can be populated dynamically.
+/// This addresses the issue where handlers were only created when collections had items.
+/// </summary>
+public class EmptyCollectionTests : ClientServerTestBase
+{
+    public EmptyCollectionTests(ITestOutputHelper testOutputHelper) : base(testOutputHelper) { }
+
+    private McpServerResourceCollection _resourceCollection = [];
+    private McpServerPrimitiveCollection<McpServerTool> _toolCollection = [];
+    private McpServerPrimitiveCollection<McpServerPrompt> _promptCollection = [];
+
+    protected override void ConfigureServices(ServiceCollection services, IMcpServerBuilder mcpServerBuilder) =>
+        mcpServerBuilder.Services.Configure<McpServerOptions>(options =>
+        {
+            options.ResourceCollection = _resourceCollection;
+            options.ToolCollection = _toolCollection;
+            options.PromptCollection = _promptCollection;
+        });
+
+    [Fact]
+    public async Task EmptyResourceCollection_CanAddResourcesDynamically()
+    {
+        var client = await CreateMcpClientForServer();
+
+        // Initially, the resource collection is empty
+        var initialResources = await client.ListResourcesAsync(TestContext.Current.CancellationToken);
+        Assert.Empty(initialResources);
+
+        // Add a resource dynamically
+        _resourceCollection.Add(McpServerResource.Create(
+            () => "test content",
+            new() { UriTemplate = "test://resource/1" }));
+
+        // The resource should now be listed
+        var updatedResources = await client.ListResourcesAsync(TestContext.Current.CancellationToken);
+        Assert.Single(updatedResources);
+        Assert.Equal("test://resource/1", updatedResources[0].Uri);
+    }
+
+    [Fact]
+    public async Task EmptyToolCollection_CanAddToolsDynamically()
+    {
+        var client = await CreateMcpClientForServer();
+
+        // Initially, the tool collection is empty
+        var initialTools = await client.ListToolsAsync(cancellationToken: TestContext.Current.CancellationToken);
+        Assert.Empty(initialTools);
+
+        // Add a tool dynamically
+        _toolCollection.Add(McpServerTool.Create(
+            () => "test result",
+            new() { Name = "test_tool", Description = "A test tool" }));
+
+        // The tool should now be listed
+        var updatedTools = await client.ListToolsAsync(cancellationToken: TestContext.Current.CancellationToken);
+        Assert.Single(updatedTools);
+        Assert.Equal("test_tool", updatedTools[0].Name);
+    }
+
+    [Fact]
+    public async Task EmptyPromptCollection_CanAddPromptsDynamically()
+    {
+        var client = await CreateMcpClientForServer();
+
+        // Initially, the prompt collection is empty
+        var initialPrompts = await client.ListPromptsAsync(TestContext.Current.CancellationToken);
+        Assert.Empty(initialPrompts);
+
+        // Add a prompt dynamically
+        _promptCollection.Add(McpServerPrompt.Create(
+            () => new ChatMessage(ChatRole.User, "test prompt"),
+            new() { Name = "test_prompt", Description = "A test prompt" }));
+
+        // The prompt should now be listed
+        var updatedPrompts = await client.ListPromptsAsync(TestContext.Current.CancellationToken);
+        Assert.Single(updatedPrompts);
+        Assert.Equal("test_prompt", updatedPrompts[0].Name);
+    }
+
+    [Fact]
+    public async Task EmptyResourceCollection_CanCallReadResourceAfterAddingDynamically()
+    {
+        var client = await CreateMcpClientForServer();
+
+        // Add a resource dynamically
+        _resourceCollection.Add(McpServerResource.Create(
+            () => "dynamic content",
+            new() { UriTemplate = "test://resource/dynamic" }));
+
+        // Read the resource
+        var result = await client.ReadResourceAsync("test://resource/dynamic", TestContext.Current.CancellationToken);
+        Assert.NotNull(result);
+        Assert.Single(result.Contents);
+        Assert.IsType<TextResourceContents>(result.Contents[0]);
+        Assert.Equal("dynamic content", ((TextResourceContents)result.Contents[0]).Text);
+    }
+
+    [Fact]
+    public async Task EmptyToolCollection_CanCallToolAfterAddingDynamically()
+    {
+        var client = await CreateMcpClientForServer();
+
+        // Add a tool dynamically
+        _toolCollection.Add(McpServerTool.Create(
+            () => "dynamic result",
+            new() { Name = "dynamic_tool", Description = "A dynamic tool" }));
+
+        // Call the tool
+        var result = await client.CallToolAsync("dynamic_tool", cancellationToken: TestContext.Current.CancellationToken);
+        Assert.NotNull(result);
+        Assert.Single(result.Content);
+        Assert.IsType<TextContentBlock>(result.Content[0]);
+        Assert.Equal("dynamic result", ((TextContentBlock)result.Content[0]).Text);
+    }
+
+    [Fact]
+    public async Task EmptyPromptCollection_CanGetPromptAfterAddingDynamically()
+    {
+        var client = await CreateMcpClientForServer();
+
+        // Add a prompt dynamically
+        _promptCollection.Add(McpServerPrompt.Create(
+            () => new ChatMessage(ChatRole.User, "dynamic prompt content"),
+            new() { Name = "dynamic_prompt", Description = "A dynamic prompt" }));
+
+        // Get the prompt
+        var result = await client.GetPromptAsync("dynamic_prompt", cancellationToken: TestContext.Current.CancellationToken);
+        Assert.NotNull(result);
+        Assert.Single(result.Messages);
+        Assert.Equal(Role.User, result.Messages[0].Role);
+        Assert.IsType<TextContentBlock>(result.Messages[0].Content);
+        Assert.Equal("dynamic prompt content", ((TextContentBlock)result.Messages[0].Content).Text);
+    }
+}
+
+/// <summary>
+/// Tests to verify that handlers are NOT synthesized when collections are null.
+/// This ensures we don't unnecessarily create capabilities when nothing is configured.
+/// </summary>
+public class NullCollectionTests : ClientServerTestBase
+{
+    public NullCollectionTests(ITestOutputHelper testOutputHelper) : base(testOutputHelper) { }
+
+    [Fact]
+    public async Task ListFails()
+    {
+        Assert.Null(Server.ServerOptions.Capabilities?.Resources);
+        Assert.Null(Server.ServerOptions.Capabilities?.Tools);
+        Assert.Null(Server.ServerOptions.Capabilities?.Prompts);
+
+        var client = await CreateMcpClientForServer();
+
+        await Assert.ThrowsAsync<McpProtocolException>(async () => await client.ListToolsAsync(cancellationToken: TestContext.Current.CancellationToken));
+        await Assert.ThrowsAsync<McpProtocolException>(async () => await client.ListPromptsAsync(TestContext.Current.CancellationToken));
+        await Assert.ThrowsAsync<McpProtocolException>(async () => await client.ListResourcesAsync(TestContext.Current.CancellationToken));
+    }
+}