C# MCP Server Testing

Test MCP servers at two levels: unit tests for individual tool methods, and integration tests that exercise the full MCP protocol in-memory.

When to Use

• Adding automated tests to an MCP server
• Testing individual tool methods with mocked dependencies
• Writing integration tests that validate tool listing and invocation via MCP protocol
• Setting up CI test pipelines for MCP servers

Stop Signals

• No server yet? → Use mcp-csharp-create first
• Server not running? → Use mcp-csharp-debug
• Just need manual/interactive testing? → Use mcp-csharp-debug for MCP Inspector

Inputs

Input	Required	Description
MCP server project path	Yes	Path to the server .csproj being tested
Test framework	Recommended	Default: xUnit. Also supports NUnit or MSTest
Transport type	Recommended	Determines integration test approach (stdio vs HTTP)

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.

Validation

• Unit tests cover all tool methods, including edge cases
• Integration tests verify tool listing via ListToolsAsync()
• Integration tests verify tool invocation via CallToolAsync()
• All tests pass: dotnet test
• Tests run in CI without manual setup

Common Pitfalls

Pitfall	Solution
Integration test hangs on CreateAsync	Server fails to start. Verify dotnet build succeeds first. For stdio, ensure no stdout logging
StdioClientTransport not finding project	Use the correct relative path to .csproj from the test project directory
Tests pass locally but fail in CI	Run dotnet build before test execution. Use --no-build only after an explicit build step
Mocking HttpClient is awkward	Mock HttpMessageHandler, not HttpClient directly. See references/test-patterns.md
Full test suite runs are slow	Use --filter for development. Run the full suite only for CI verification

Related Skills

• mcp-csharp-create — Create a new MCP server project
• mcp-csharp-debug — Running and interactive debugging
• mcp-csharp-publish — NuGet, Docker, Azure deployment

Reference Files

• references/test-patterns.md — Complete test code examples: ClientServerTestBase in-memory pattern, WebApplicationFactory for HTTP, MockHttpMessageHandler helper, test categorization, coverage reporting. Load when: writing integration tests or need detailed mock patterns.
• references/evaluations.md — Evaluation format, question design principles, and example eval questions. Load when: user asks about evaluations, eval questions, or measuring tool quality.

More Info

• xUnit documentation — Getting started with xUnit for .NET