Pass session id's to MCP endpoints. (#466)

* Pass session id's to MCP endpoints.

* Update src/ModelContextProtocol/Client/McpClient.cs

Co-authored-by: Stephen Halter <halter73@gmail.com>

* Update src/ModelContextProtocol.AspNetCore/StreamableHttpHandler.cs

Co-authored-by: Stephen Halter <halter73@gmail.com>

* Make init callback asynchronous.

* Address feedback.

---------

Co-authored-by: Stephen Halter <halter73@gmail.com>

src/ModelContextProtocol.Core/Server/DestinationBoundMcpServer.cs

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

@@ -96,6 +96,10 @@ internal sealed class McpServer : McpEndpoint, IMcpServer
         InitializeSession(transport);
     }
 
+    /// <inheritdoc/>
+    public string? SessionId => _sessionTransport.SessionId;
+
+    /// <inheritdoc/>
     public ServerCapabilities ServerCapabilities { get; } = new();
 
     /// <inheritdoc />

src/ModelContextProtocol.Core/Server/SseResponseStreamTransport.cs

@@ -23,7 +23,8 @@ namespace ModelContextProtocol.Server;
 /// These messages should be passed to <see cref="OnMessageReceivedAsync(JsonRpcMessage, CancellationToken)"/>.
 /// Defaults to "/message".
 /// </param>
-public sealed class SseResponseStreamTransport(Stream sseResponseStream, string? messageEndpoint = "/message") : ITransport
+/// <param name="sessionId">The identifier corresponding to the current MCP session.</param>
+public sealed class SseResponseStreamTransport(Stream sseResponseStream, string? messageEndpoint = "/message", string? sessionId = null) : ITransport
 {
     private readonly SseWriter _sseWriter = new(messageEndpoint);
     private readonly Channel<JsonRpcMessage> _incomingChannel = Channel.CreateBounded<JsonRpcMessage>(new BoundedChannelOptions(1)
@@ -49,6 +50,9 @@ public sealed class SseResponseStreamTransport(Stream sseResponseStream, string?
     /// <inheritdoc/>
     public ChannelReader<JsonRpcMessage> MessageReader => _incomingChannel.Reader;
 
+    /// <inheritdoc/>
+    public string? SessionId { get; } = sessionId;
+
     /// <inheritdoc/>
     public async ValueTask DisposeAsync()
     {

src/ModelContextProtocol.Core/Server/StreamableHttpPostTransport.cs

@@ -18,6 +18,8 @@ internal sealed class StreamableHttpPostTransport(StreamableHttpServerTransport
 
     public ChannelReader<JsonRpcMessage> MessageReader => throw new NotSupportedException("JsonRpcMessage.RelatedTransport should only be used for sending messages.");
 
+    string? ITransport.SessionId => parentTransport.SessionId;
+
     /// <returns>
     /// True, if data was written to the respond body.
     /// False, if nothing was written because the request body did not contain any <see cref="JsonRpcRequest"/> messages to respond to.
@@ -79,10 +81,11 @@ internal sealed class StreamableHttpPostTransport(StreamableHttpServerTransport
         {
             _pendingRequest = request.Id;
 
-            // Store client capabilities so they can be serialized by "stateless" callers for use in later requests.
-            if (parentTransport.Stateless && request.Method == RequestMethods.Initialize)
+            // Invoke the initialize request callback if applicable.
+            if (parentTransport.OnInitRequestReceived is { } onInitRequest && request.Method == RequestMethods.Initialize)
             {
-                parentTransport.InitializeRequest = JsonSerializer.Deserialize(request.Params, McpJsonUtilities.JsonContext.Default.InitializeRequestParams);
+                var initializeRequest = JsonSerializer.Deserialize(request.Params, McpJsonUtilities.JsonContext.Default.InitializeRequestParams);
+                await onInitRequest(initializeRequest).ConfigureAwait(false);
             }
         }
 

src/ModelContextProtocol.Core/Server/StreamableHttpServerTransport.cs

@@ -46,15 +46,18 @@ public sealed class StreamableHttpServerTransport : ITransport
     public bool Stateless { get; init; }
 
     /// <summary>
-    /// Gets the initialize request if it was received by <see cref="HandlePostRequest(IDuplexPipe, CancellationToken)"/> and <see cref="Stateless"/> is set to <see langword="true"/>.
+    /// Gets or sets a callback to be invoked before handling the initialize request.
     /// </summary>
-    public InitializeRequestParams? InitializeRequest { get; internal set; }
+    public Func<InitializeRequestParams?, ValueTask>? OnInitRequestReceived { get; set; }
 
     /// <inheritdoc/>
     public ChannelReader<JsonRpcMessage> MessageReader => _incomingChannel.Reader;
 
     internal ChannelWriter<JsonRpcMessage> MessageWriter => _incomingChannel.Writer;
 
+    /// <inheritdoc/>
+    public string? SessionId { get; set; }
+
     /// <summary>
     /// Handles an optional SSE GET request a client using the Streamable HTTP transport might make by
     /// writing any unsolicited JSON-RPC messages sent via <see cref="SendMessageAsync"/>