Address feedback.

Author: eiriktsarpalis

src/ModelContextProtocol.Core/Client/McpClient.cs

@@ -95,11 +95,16 @@ public McpClient(IClientTransport clientTransport, McpClientOptions? options, IL
     }
 
     /// <inheritdoc/>
-    public ITransport Transport
+    public string? SessionId
     {
         get
         {
-            return _sessionTransport ??  throw new InvalidOperationException("Must have already initialized a session when invoking this property.");
+            if (_sessionTransport is null)
+            {
+                throw new InvalidOperationException("Must have already initialized a session when invoking this property.");
+            }
+
+            return _sessionTransport.SessionId;
         }
     }
 

src/ModelContextProtocol.Core/IMcpEndpoint.cs

@@ -28,10 +28,13 @@ namespace ModelContextProtocol;
 /// </remarks>
 public interface IMcpEndpoint : IAsyncDisposable
 {
-    /// <summary>
-    /// Gets the underlying transport driving the current MCP endpoint.
-    /// </summary>
-    ITransport Transport { get; }
+    /// <summary>Gets an identifier associated with the current MCP session.</summary>
+    /// <remarks>
+    /// Typically populated in transports supporting multiple sessions such as Streamable HTTP or SSE.
+    /// Can return <see langword="null"/> if the session hasn't initialized or if the transport doesn't
+    /// support multiple sessions (as is the case with STDIO).
+    /// </remarks>
+    string? SessionId { get; }
 
     /// <summary>
     /// Sends a JSON-RPC request to the connected endpoint and waits for a response.

src/ModelContextProtocol.Core/Server/DestinationBoundMcpServer.cs

@@ -6,7 +6,7 @@ namespace ModelContextProtocol.Server;
 internal sealed class DestinationBoundMcpServer(McpServer server, ITransport? transport) : IMcpServer
 {
     public string EndpointName => server.EndpointName;
-    public ITransport Transport => transport ?? server.Transport;
+    public string? SessionId => transport?.SessionId ?? server.SessionId;
     public ClientCapabilities? ClientCapabilities => server.ClientCapabilities;
     public Implementation? ClientInfo => server.ClientInfo;
     public McpServerOptions ServerOptions => server.ServerOptions;

src/ModelContextProtocol.Core/Server/McpServer.cs

@@ -97,7 +97,7 @@ void Register<TPrimitive>(McpServerPrimitiveCollection<TPrimitive>? collection,
     }
 
     /// <inheritdoc/>
-    public ITransport Transport => _sessionTransport;
+    public string? SessionId => _sessionTransport.SessionId;
 
     /// <inheritdoc/>
     public ServerCapabilities ServerCapabilities { get; } = new();

tests/ModelContextProtocol.AspNetCore.Tests/HttpServerIntegrationTests.cs

@@ -55,11 +55,11 @@ public async Task Connect_TestServer_ShouldProvideServerFields()
 
         if (ClientTransportOptions.Endpoint.AbsolutePath.EndsWith("/sse"))
         {
-            Assert.Null(client.Transport.SessionId);
+            Assert.Null(client.SessionId);
         }
         else
         {
-            Assert.NotNull(client.Transport.SessionId);
+            Assert.NotNull(client.SessionId);
         }
     }
 

tests/ModelContextProtocol.TestServer/Program.cs

@@ -184,7 +184,7 @@ private static ToolsCapability ConfigureTools()
                 {
                     return new CallToolResponse()
                     {
-                        Content = [new Content() { Text = request.Server.Transport.SessionId, Type = "text" }]
+                        Content = [new Content() { Text = request.Server.SessionId, Type = "text" }]
                     };
                 }
                 else if (request.Params?.Name == "sampleLLM")

tests/ModelContextProtocol.TestSseServer/Program.cs

@@ -182,7 +182,7 @@ static CreateMessageRequestParams CreateRequestSamplingParams(string context, st
                     {
                         return new CallToolResponse()
                         {
-                            Content = [new Content() { Text = request.Server.Transport.SessionId, Type = "text" }]
+                            Content = [new Content() { Text = request.Server.SessionId, Type = "text" }]
                         };
                     }
                     else if (request.Params.Name == "sampleLLM")

tests/ModelContextProtocol.Tests/ClientIntegrationTests.cs

@@ -55,7 +55,7 @@ public async Task Connect_ShouldProvideServerFields(string clientId)
         if (clientId != "everything")   // Note: Comment the below assertion back when the everything server is updated to provide instructions
             Assert.NotNull(client.ServerInstructions);
 
-        Assert.Null(client.Transport.SessionId);
+        Assert.Null(client.SessionId);
     }
 
     [Theory]

tests/ModelContextProtocol.Tests/Server/McpServerTests.cs

@@ -650,7 +650,7 @@ public Task<JsonRpcResponse> SendRequestAsync(JsonRpcRequest request, Cancellati
 
         public ValueTask DisposeAsync() => default;
 
-        public ITransport Transport => throw new NotImplementedException();
+        public string? SessionId => throw new NotImplementedException();
         public Implementation? ClientInfo => throw new NotImplementedException();
         public IServiceProvider? Services => throw new NotImplementedException();
         public LoggingLevel? LoggingLevel => throw new NotImplementedException();