How to do things during initialization phase?

[jph33]

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• Protocol Specification
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Your Question

Hi!
I am developing a remote server with C# sdk and i am looking to do "things" during initialization phase of an http server. Someone can help me on how to do that? Is there any handler that can be implemented?
Thanks for all your answers.

[ryux1]

There is no separate typed initialize handler to replace, but the current SDK gives you two hooks depending on what “during initialization” means.

For legacy handshake clients (2025-11-25 and earlier), an incoming message filter can run logic before the built-in initialize handler and observe the later notifications/initialized acknowledgement:

builder.Services.AddMcpServer()
    .WithMessageFilters(filters =>
    {
        filters.AddIncomingFilter(next => async (context, cancellationToken) =>
        {
            if (context.JsonRpcMessage is JsonRpcRequest
                { Method: RequestMethods.Initialize })
            {
                // Validate or prepare handshake-scoped state here.
            }

            await next(context, cancellationToken);

            if (context.JsonRpcMessage is JsonRpcNotification
                { Method: NotificationMethods.InitializedNotification })
            {
                // The client has acknowledged initialization.
            }
        });
    });

Call next; the SDK owns protocol negotiation and the InitializeResult. Avoid long-running work before it, because that delays the handshake response.

If you mean work for the lifetime of an HTTP MCP session, configure WithHttpTransport(options => options.RunSessionHandler = ...) instead. That callback receives the HttpContext, McpServer, and session cancellation token, and must normally await server.RunAsync(cancellationToken). It is experimental, and its frequency depends on HttpServerSessionMode (once per stateful session versus once per request in stateless mode).

One important current-version caveat: protocol 2026-07-28 removed the initialize/initialized handshake. It uses server/discover plus per-request metadata, and ASP.NET Core defaults to stateless HTTP for that revision. Therefore new server logic should not depend on a one-time initialization phase unless you deliberately support an older stateful handshake revision. Application-wide startup belongs in normal ASP.NET Core DI/hosted-service startup.

Relevant current sources:

• message filters
• RunSessionHandler semantics
• built-in initialize handling and 2026 boundary