How to implement a "tool middleware"?

[ouvreboite]

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Discussion Topic

As MCP is using a single HTTP connection to perform several tool calls, I struggle to implement custom "middlewares" for my tools the same way I would do for REST controllers (to push custom metrics, ...).

With controllers, I would use an ASPNET.Core middleware, or an ActionFilter.

I tried creating my own middleware-like logic in a custom CallToolHandler, but I think the ConfigureSessionOptions / RunSessionHandler run before the default CallToolHandler is set. So, no luck there.

This does not work ⬇️

services.AddMcpServer()
          .WithHttpTransport(options => {
              options.ConfigureSessionOptions = (context, serverOptions, cancellation) =>
              {
                  var originalCallToolHandler = serverOptions.Capabilities.Tools.CallToolHandler; //😞 always null
                  serverOptions.Capabilities.Tools.CallToolHandler = (requestContext, token) =>
                  {
                      //do something before calling the tool
                      var result = originalCallToolHandler(requestContext, token);
                      //do something after calling the tool
                      return result;
                  };
                  return Task.CompletedTask;
              };
          })
          .WithToolsFromAssembly();

Maybe I missed something? Is there a way to run custom logic before/after each tool call?

[ouvreboite]

I've given it a bit more though, I've tried to implement an ActionFilter's like pattern but for tools.
Cf #606