C# MCP Server Testing

Workflow

Step 1: Create the test project

dotnet new xunit -n <ServerName>.Tests
cd <ServerName>.Tests
dotnet add reference ../<ServerName>/<ServerName>.csproj
dotnet add package ModelContextProtocol
dotnet add package Moq
dotnet add package FluentAssertions

Step 2: Write unit tests for tool methods

Test tool methods directly — fastest and most isolated:

public class MyToolTests
{
    [Fact]
    public void Echo_ReturnsFormattedMessage()
    {
        var result = MyTools.Echo("Hello");
        result.Should().Be("Echo: Hello");
    }

    [Theory]
    [InlineData("")]
    [InlineData("   ")]
    public void Echo_HandlesEdgeCases(string input)
    {
        var result = MyTools.Echo(input);
        result.Should().StartWith("Echo:");
    }
}

For tools with DI dependencies, mock the dependency:

public class ApiToolTests
{
    [Fact]
    public async Task FetchData_ReturnsApiResponse()
    {
        var handler = new MockHttpMessageHandler("""{"id": 1}""");
        var httpClient = new HttpClient(handler);

        var result = await ApiTools.FetchData(httpClient, "resource-1");
        result.Should().Contain("id");
    }
}

Step 3: Write integration tests with MCP client

Test the full MCP protocol using a client-server connection:

using ModelContextProtocol.Client;

public class ServerIntegrationTests : IAsyncLifetime
{
    private McpClient _client = null!;

    public async Task InitializeAsync()
    {
        var transport = new StdioClientTransport(new StdioClientTransportOptions
        {
            Name = "TestClient",
            Command = "dotnet",
            Arguments = ["run", "--project", "../<ServerName>/<ServerName>.csproj"]
        });
        _client = await McpClient.CreateAsync(transport);
    }

    public async Task DisposeAsync() => await _client.DisposeAsync();

    [Fact]
    public async Task Server_ListsExpectedTools()
    {
        var tools = await _client.ListToolsAsync();
        tools.Should().Contain(t => t.Name == "echo");
    }

    [Fact]
    public async Task Tool_ReturnsExpectedResult()
    {
        var result = await _client.CallToolAsync("echo",
            new Dictionary<string, object?> { ["message"] = "Test" });
        var text = result.Content.OfType<TextContentBlock>().First().Text;
        text.Should().Contain("Test");
    }
}

For the SDK's `ClientServerTestBase` (in-memory testing) and HTTP testing with `WebApplicationFactory`, see references/test-patterns.md.

Step 4: Run tests

# Run all tests
dotnet test

# Run a specific test class
dotnet test --filter "FullyQualifiedName~MyToolTests"

# Run with coverage
dotnet test --collect:"XPlat Code Coverage"

Step 5: Write evaluations

Evaluations measure how well an LLM uses your tools. Good evaluation questions should be:

• Read-only and non-destructive — never modify data as a side effect
• Deterministic — have a single verifiable correct answer
• Multi-step — require the LLM to call multiple tools or reason across results

For the evaluation format, example questions, and detailed guidance, see references/evaluations.md.

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