Unable to resolve service for type 'ModelContextProtocol.Server.IMcpServer'

[scholtz]

Describe the bug
I follow the main repo readme, and i am getting the "Unable to resolve service for type 'ModelContextProtocol.Server.IMcpServer'" error when trying to access the IMcpServer.

To Reproduce
Steps to reproduce the behavior:

1. build mcp server
2. connect from mcp tool
3. see the exception in the mcp server (on first usage)

Startup.cs:

using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;
using Microsoft.Extensions.Logging;
using ModelContextProtocol.Server;
using System.ComponentModel;

var builder = Host.CreateApplicationBuilder(args);
builder.Logging.AddConsole(consoleLogOptions =>
{
    // Configure all logs to go to stderr
    consoleLogOptions.LogToStandardErrorThreshold = LogLevel.Trace;
});
builder.Services
    .AddMcpServer()
    .WithHttpTransport()
    .WithToolsFromAssembly();
await builder.Build().RunAsync();

mcptool.cs

[McpServerTool(Name = "SummarizeContentFromUrl"), Description("Summarizes content downloaded from a specific URI")]
public static async Task<string> SummarizeDownloadedContent(
    IMcpServer thisServer,
    HttpClient httpClient,
    [Description("The url from which to download the content to summarize")] string url,
    CancellationToken cancellationToken)
{
    string content = await httpClient.GetStringAsync(url);

    ChatMessage[] messages =
    [
        new(ChatRole.User, "Briefly summarize the following downloaded content:"),
        new(ChatRole.User, content),
    ];
    
    ChatOptions options = new()
    {
        MaxOutputTokens = 256,
        Temperature = 0.3f,
    };

    return $"Summary: {await thisServer.AsSamplingChatClient().GetResponseAsync(messages, options, cancellationToken)}";
}

Expected behavior
A clear and concise description of what you expected to happen.

Logs

System.InvalidOperationException: Unable to resolve service for type 'ModelContextProtocol.Server.IMcpServer' while attempting to activate 'MyClass..'
   at Microsoft.Extensions.DependencyInjection.ActivatorUtilities.ConstructorMatcher.CreateInstance(IServiceProvider provider)
   at Microsoft.Extensions.DependencyInjection.ActivatorUtilities.CreateInstance(IServiceProvider provider, Type instanceType, Object[] parameters)
   at Microsoft.Extensions.DependencyInjection.McpServerBuilderExtensions.CreateTarget(IServiceProvider services, Type type)
   at Microsoft.Extensions.DependencyInjection.McpServerBuilderExtensions.<>c__DisplayClass3_1.<WithTools>b__2(RequestContext`1 r)
   at ModelContextProtocol.Server.AIFunctionMcpServerTool.<>c__DisplayClass4_0.<Create>b__0(AIFunctionArguments args)
   at Microsoft.Extensions.AI.AIFunctionFactory.ReflectionAIFunction.InvokeCoreAsync(AIFunctionArguments arguments, CancellationToken cancellationToken)
   at Microsoft.Extensions.AI.AIFunctionFactory.ReflectionAIFunction.InvokeCoreAsync(AIFunctionArguments arguments, CancellationToken cancellationToken)
   at ModelContextProtocol.Server.AIFunctionMcpServerTool.InvokeAsync(RequestContext`1 request, CancellationToken cancellationToken)
ModelContextProtocol.Server.McpServer: Information: Server (MyClass 1.0.0.0), Client (Visual Studio Code (via mcp-remote 0.1.17) 1.101.2) method 'tools/call' request handler completed.

Additional context

Please either update the documentation on how to properly register IMcpServer dependency, or remove it from it and create example on how to fetch the current session id by the tool.

Is there any way how can I help to fix this issue?

Thanks

[stephentoub]

Can you confirm that's your whole repro? You don't have a type called MyClass somewhere, attributed with [McpServerToolType], that's taking an IMcpServer as a constructor parameter (which is not supported right now and which is not in our samples, or if it is that should be fixed)?

[scholtz]

@stephentoub actually my whole repo is here, but it has much more info than this, so i tried to focus only on part which seems to be wrong..

https://github.com/scholtz/AlgorandGoogleDriveAccount/blob/c908268aacb8278b7ce28d2f08f3f997f757493d/AlgorandGoogleDriveAccount/MCP/BiatecMCPGoogle.cs#L27

it does not work with dependency injection in class constructor nor with IMcpEndpoint nor IMcpServer

[stephentoub]

it does not work with dependency injection in class constructor

Dependency injection does work with the constructor... that's the only thing that works with the class constructor. The IMcpServer is not coming from the DI container, and hence doesn't work. We use ActivatorUtilities.CreateInstance, and have not done work to enable injecting other things. For methods, IMcpServer, CancellationToken, and a few other types are manually handled to support binding them to the arguments of the methods.

IMcpEndpoint isn't currently special-cased like IMcpServer is. It could be made to work, but it would still be an IMcpServer, and IMO it would be confusing as to what you were getting, because IMcpClient is also an IMcpEndpoint. Can you just change your method parameter to be IMcpServer instead of IMcpEndpoint?

[scholtz]

i have created sample minimalistic code how i believe the session id should be fetched - https://github.com/modelcontextprotocol/csharp-sdk/pull/565/files#diff-a13341123b305c423b53c2c6939d2269b1d15ddf3c0077419d78c8e404039dbc

Can you point me to any other way how I can fetch the session id? If you use ActivatorUtilities.CreateInstance and not allow injecting other things perhaps it may be good point for redesign some internal working so that the coding is more simple.

Is there any way how I can help with it?

[scholtz]

@stephentoub btw why did you remove the bug tag? i believe if something is documented as base use case in the main readme file and it is not working, it should be considered as bug..

[stephentoub]

i believe if something is documented as base use case in the main readme file and it is not working, it should be considered as bug..

Where is it documented in the README that you can use IMcpServer as a parameter to the ctor or that you can use IMcpEndpoint as a parameter to anything?

[scholtz]

@stephentoub Now i understand.. Thanks.. I moved the IMcpServer from constructor to method.. i updated the GetSession sample, its working now, so feel free to merge it so that others has easier onboarding.. or do you need anything else to be changed there?

#565