Make supposedly unreachable code less reachable

src/ModelContextProtocol.AspNetCore/McpEndpointRouteBuilderExtensions.cs

@@ -47,11 +47,11 @@ public static IEndpointConventionBuilder MapMcp(this IEndpointRouteBuilder endpo
             {
                 throw new Exception($"Unreachable given good entropy! Session with ID '{sessionId}' has already been created.");
             }
-            await using var server = McpServerFactory.Create(transport, mcpServerOptions.Value, loggerFactory, endpoints.ServiceProvider);
 
             try
             {
                 var transportTask = transport.RunAsync(cancellationToken: requestAborted);
+                await using var server = McpServerFactory.Create(transport, mcpServerOptions.Value, loggerFactory, endpoints.ServiceProvider);
 
                 try
                 {
@@ -85,7 +85,7 @@ public static IEndpointConventionBuilder MapMcp(this IEndpointRouteBuilder endpo
 
             if (!_sessions.TryGetValue(sessionId.ToString(), out var transport))
             {
-                await Results.BadRequest($"Session {sessionId} not found.").ExecuteAsync(context);
+                await Results.BadRequest($"Session ID not found.").ExecuteAsync(context);
                 return;
             }
 

src/ModelContextProtocol/Client/McpClient.cs

@@ -82,29 +82,27 @@ public async Task ConnectAsync(CancellationToken cancellationToken = default)
         {
             // Connect transport
             _sessionTransport = await _clientTransport.ConnectAsync(cancellationToken).ConfigureAwait(false);
-            // We don't want the ConnectAsync token to cancel the session after we've successfully connected.
-            // The base class handles cleaning up the session in DisposeAsync without our help.
-            StartSession(_sessionTransport, fullSessionCancellationToken: CancellationToken.None);
+            StartSession(_sessionTransport);
 
             // Perform initialization sequence
             using var initializationCts = CancellationTokenSource.CreateLinkedTokenSource(cancellationToken);
             initializationCts.CancelAfter(_options.InitializationTimeout);
 
-        try
-        {
-            // Send initialize request
-            var initializeResponse = await SendRequestAsync<InitializeResult>(
-                new JsonRpcRequest
-                {
-                    Method = RequestMethods.Initialize,
-                    Params = new InitializeRequestParams()
+            try
+            {
+                // Send initialize request
+                var initializeResponse = await SendRequestAsync<InitializeResult>(
+                    new JsonRpcRequest
                     {
-                        ProtocolVersion = _options.ProtocolVersion,
-                        Capabilities = _options.Capabilities ?? new ClientCapabilities(),
-                        ClientInfo = _options.ClientInfo
-                    }
-                },
-                initializationCts.Token).ConfigureAwait(false);
+                        Method = RequestMethods.Initialize,
+                        Params = new InitializeRequestParams()
+                        {
+                            ProtocolVersion = _options.ProtocolVersion,
+                            Capabilities = _options.Capabilities ?? new ClientCapabilities(),
+                            ClientInfo = _options.ClientInfo
+                        }
+                    },
+                    initializationCts.Token).ConfigureAwait(false);
 
                 // Store server information
                 _logger.ServerCapabilitiesReceived(EndpointName,

src/ModelContextProtocol/Protocol/Transport/SseResponseStreamTransport.cs

@@ -32,30 +32,30 @@ public sealed class SseResponseStreamTransport(Stream sseResponseStream, string
     /// <returns>A task representing the send loop that writes JSON-RPC messages to the SSE response stream.</returns>
     public Task RunAsync(CancellationToken cancellationToken)
     {
-        void WriteJsonRpcMessageToBuffer(SseItem<IJsonRpcMessage?> item, IBufferWriter<byte> writer)
-        {
-            if (item.EventType == "endpoint")
-            {
-                writer.Write(Encoding.UTF8.GetBytes(messageEndpoint));
-                return;
-            }
-
-            JsonSerializer.Serialize(GetUtf8JsonWriter(writer), item.Data, McpJsonUtilities.DefaultOptions.GetTypeInfo<IJsonRpcMessage?>());
-        }
-
-        IsConnected = true;
-
         // The very first SSE event isn't really an IJsonRpcMessage, but there's no API to write a single item of a different type,
         // so we fib and special-case the "endpoint" event type in the formatter.
         if (!_outgoingSseChannel.Writer.TryWrite(new SseItem<IJsonRpcMessage?>(null, "endpoint")))
         {
             throw new InvalidOperationException($"You must call ${nameof(RunAsync)} before calling ${nameof(SendMessageAsync)}.");
         }
 
+        IsConnected = true;
+
         var sseItems = _outgoingSseChannel.Reader.ReadAllAsync(cancellationToken);
         return _sseWriteTask = SseFormatter.WriteAsync(sseItems, sseResponseStream, WriteJsonRpcMessageToBuffer, cancellationToken);
     }
 
+    private void WriteJsonRpcMessageToBuffer(SseItem<IJsonRpcMessage?> item, IBufferWriter<byte> writer)
+    {
+        if (item.EventType == "endpoint")
+        {
+            writer.Write(Encoding.UTF8.GetBytes(messageEndpoint));
+            return;
+        }
+
+        JsonSerializer.Serialize(GetUtf8JsonWriter(writer), item.Data, McpJsonUtilities.DefaultOptions.GetTypeInfo<IJsonRpcMessage?>());
+    }
+
     /// <inheritdoc/>
     public ChannelReader<IJsonRpcMessage> MessageReader => _incomingChannel.Reader;
 

src/ModelContextProtocol/Server/McpServer.cs

@@ -14,8 +14,8 @@ internal sealed class McpServer : McpJsonRpcEndpoint, IMcpServer
     private readonly EventHandler? _toolsChangedDelegate;
     private readonly EventHandler? _promptsChangedDelegate;
 
-    private ITransport _sessionTransport;
     private string _endpointName;
+    private int _started;
 
     /// <summary>
     /// Creates a new instance of <see cref="McpServer"/>.
@@ -32,7 +32,6 @@ public McpServer(ITransport transport, McpServerOptions options, ILoggerFactory?
         Throw.IfNull(transport);
         Throw.IfNull(options);
 
-        _sessionTransport = transport;
         ServerOptions = options;
         Services = serviceProvider;
         _endpointName = $"Server ({options.ServerInfo.Name} {options.ServerInfo.Version})";
@@ -74,6 +73,8 @@ public McpServer(ITransport transport, McpServerOptions options, ILoggerFactory?
         SetPromptsHandler(options);
         SetResourcesHandler(options);
         SetSetLoggingLevelHandler(options);
+
+        StartSession(transport);
     }
 
     public ServerCapabilities? ServerCapabilities { get; set; }
@@ -96,11 +97,16 @@ public McpServer(ITransport transport, McpServerOptions options, ILoggerFactory?
     /// <inheritdoc />
     public async Task RunAsync(CancellationToken cancellationToken = default)
     {
+        if (Interlocked.Exchange(ref _started, 1) != 0)
+        {
+            throw new InvalidOperationException($"{nameof(RunAsync)} must only be called once.");
+        }
+
         try
         {
-            // Start processing messages
-            StartSession(_sessionTransport, fullSessionCancellationToken: cancellationToken);
-            await MessageProcessingTask.ConfigureAwait(false);
+            using var _ = cancellationToken.Register(static s => ((McpServer)s!).CancelSession(), this);
+            // The McpServer ctor always calls StartSession, so MessageProcessingTask is always set.
+            await MessageProcessingTask!.ConfigureAwait(false);
         }
         finally
         {

src/ModelContextProtocol/Shared/McpJsonRpcEndpoint.cs

@@ -22,7 +22,6 @@ internal abstract class McpJsonRpcEndpoint : IAsyncDisposable
 
     private McpSession? _session;
     private CancellationTokenSource? _sessionCts;
-    private int _started;
 
     private readonly SemaphoreSlim _disposeLock = new(1, 1);
     private bool _disposed;
@@ -61,18 +60,15 @@ public Task SendMessageAsync(IJsonRpcMessage message, CancellationToken cancella
     protected Task? MessageProcessingTask { get; set; }
 
     [MemberNotNull(nameof(MessageProcessingTask))]
-    protected void StartSession(ITransport sessionTransport, CancellationToken fullSessionCancellationToken = default)
+    protected void StartSession(ITransport sessionTransport)
     {
-        if (Interlocked.Exchange(ref _started, 1) != 0)
-        {
-            throw new InvalidOperationException("The MCP session has already stared.");
-        }
-
-        _sessionCts = CancellationTokenSource.CreateLinkedTokenSource(fullSessionCancellationToken);
+        _sessionCts = new CancellationTokenSource();
         _session = new McpSession(sessionTransport, EndpointName, _requestHandlers, _notificationHandlers, _logger);
         MessageProcessingTask = _session.ProcessMessagesAsync(_sessionCts.Token);
     }
 
+    protected void CancelSession() => _sessionCts?.Cancel();
+
     public async ValueTask DisposeAsync()
     {
         using var _ = await _disposeLock.LockAsync().ConfigureAwait(false);

tests/ModelContextProtocol.Tests/Server/McpServerFactoryTests.cs

@@ -1,8 +1,6 @@
-using ModelContextProtocol.Protocol.Transport;
-using ModelContextProtocol.Protocol.Types;
+using ModelContextProtocol.Protocol.Types;
 using ModelContextProtocol.Server;
 using ModelContextProtocol.Tests.Utils;
-using Moq;
 
 namespace ModelContextProtocol.Tests.Server;
 
@@ -25,7 +23,8 @@ public McpServerFactoryTests(ITestOutputHelper testOutputHelper)
     public async Task Create_Should_Initialize_With_Valid_Parameters()
     {
         // Arrange & Act
-        await using IMcpServer server = McpServerFactory.Create(Mock.Of<ITransport>(), _options, LoggerFactory);
+        await using var transport = new TestServerTransport();
+        await using IMcpServer server = McpServerFactory.Create(transport, _options, LoggerFactory);
 
         // Assert
         Assert.NotNull(server);
@@ -39,9 +38,10 @@ public void Create_Throws_For_Null_ServerTransport()
     }
 
     [Fact]
-    public void Create_Throws_For_Null_Options()
+    public async Task Create_Throws_For_Null_Options()
     {
         // Arrange, Act & Assert
-        Assert.Throws<ArgumentNullException>("serverOptions", () => McpServerFactory.Create(Mock.Of<ITransport>(), null!, LoggerFactory));
+        await using var transport = new TestServerTransport();
+        Assert.Throws<ArgumentNullException>("serverOptions", () => McpServerFactory.Create(transport, null!, LoggerFactory));
     }
 }