Make init callback asynchronous.

Author: eiriktsarpalis

src/ModelContextProtocol.AspNetCore/StreamableHttpHandler.cs

@@ -293,13 +293,14 @@ private void ScheduleStatelessSessionIdWrite(HttpContext context, StreamableHttp
         {
             var statelessId = new StatelessSessionId
             {
-                ClientInfo = initRequestParams.ClientInfo,
+                ClientInfo = initRequestParams?.ClientInfo,
                 UserIdClaim = GetUserIdClaim(context.User),
             };
 
             var sessionJson = JsonSerializer.Serialize(statelessId, StatelessSessionIdJsonContext.Default.StatelessSessionId);
             transport.SessionId = Protector.Protect(sessionJson);
             context.Response.Headers["mcp-session-id"] = transport.SessionId;
+            return ValueTask.CompletedTask;
         };
     }
 

src/ModelContextProtocol/Server/StreamableHttpPostTransport.cs

@@ -85,7 +85,7 @@ private async ValueTask OnMessageReceivedAsync(JsonRpcMessage? message, Cancella
             if (parentTransport.OnInitRequestReceived is { } onInitRequest && request.Method == RequestMethods.Initialize)
             {
                 var initializeRequest = JsonSerializer.Deserialize(request.Params, McpJsonUtilities.JsonContext.Default.InitializeRequestParams);
-                onInitRequest(initializeRequest!);
+                await onInitRequest(initializeRequest).ConfigureAwait(false);
             }
         }
 

src/ModelContextProtocol/Server/StreamableHttpServerTransport.cs

@@ -48,7 +48,7 @@ public sealed class StreamableHttpServerTransport : ITransport
     /// <summary>
     /// Gets or sets a callback to be invoked before handling the initialize request.
     /// </summary>
-    public Action<InitializeRequestParams>? OnInitRequestReceived { get; set; }
+    public Func<InitializeRequestParams?, ValueTask>? OnInitRequestReceived { get; set; }
 
     /// <inheritdoc/>
     public ChannelReader<JsonRpcMessage> MessageReader => _incomingChannel.Reader;