Add McpSessions.NegotiatedProtocolVersion property. (#794)

src/ModelContextProtocol.Core/Client/McpClientImpl.cs

@@ -26,6 +26,7 @@ internal sealed partial class McpClientImpl : McpClient
     private ServerCapabilities? _serverCapabilities;
     private Implementation? _serverInfo;
     private string? _serverInstructions;
+    private string? _negotiatedProtocolVersion;
 
     private bool _disposed;
 
@@ -112,6 +113,9 @@ internal sealed partial class McpClientImpl : McpClient
     /// <inheritdoc/>
     public override string? SessionId => _transport.SessionId;
 
+    /// <inheritdoc/>
+    public override string? NegotiatedProtocolVersion => _negotiatedProtocolVersion;
+
     /// <inheritdoc/>
     public override ServerCapabilities ServerCapabilities => _serverCapabilities ?? throw new InvalidOperationException("The client is not connected.");
 
@@ -177,6 +181,8 @@ internal sealed partial class McpClientImpl : McpClient
                     throw new McpException($"Server protocol version mismatch. Expected {requestProtocol}, got {initializeResponse.ProtocolVersion}");
                 }
 
+                _negotiatedProtocolVersion = initializeResponse.ProtocolVersion;
+
                 // Send initialized notification
                 await this.SendNotificationAsync(
                     NotificationMethods.InitializedNotification,

src/ModelContextProtocol.Core/McpSession.cs

@@ -38,6 +38,15 @@ public abstract partial class McpSession : IMcpEndpoint, IAsyncDisposable
     /// </remarks>
     public abstract string? SessionId { get; }
 
+    /// <summary>
+    /// Gets the negotiated protocol version for the current MCP session.
+    /// </summary>
+    /// <remarks>
+    /// Returns the protocol version negotiated during session initialization,
+    /// or <see langword="null"/> if initialization hasn't yet occurred.
+    /// </remarks>
+    public abstract string? NegotiatedProtocolVersion { get; }
+
     /// <summary>
     /// Sends a JSON-RPC request to the connected session and waits for a response.
     /// </summary>
@@ -45,7 +54,7 @@ public abstract partial class McpSession : IMcpEndpoint, IAsyncDisposable
     /// <param name="cancellationToken">The <see cref="CancellationToken"/> to monitor for cancellation requests. The default is <see cref="CancellationToken.None"/>.</param>
     /// <returns>A task containing the session's response.</returns>
     /// <exception cref="InvalidOperationException">The transport is not connected, or another error occurs during request processing.</exception>
-    /// <exception cref="McpException">An error occured during request processing.</exception>
+    /// <exception cref="McpException">An error occurred during request processing.</exception>
     /// <remarks>
     /// This method provides low-level access to send raw JSON-RPC requests. For most use cases,
     /// consider using the strongly-typed methods that provide a more convenient API.

src/ModelContextProtocol.Core/Server/DestinationBoundMcpServer.cs

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

src/ModelContextProtocol.Core/Server/McpServerImpl.cs

@@ -7,8 +7,6 @@ using System.Text.Json.Serialization.Metadata;
 
 namespace ModelContextProtocol.Server;
 
-// TODO: Fix merge conflicts in this file.
-
 /// <inheritdoc />
 internal sealed partial class McpServerImpl : McpServer
 {
@@ -31,6 +29,7 @@ internal sealed partial class McpServerImpl : McpServer
     private Implementation? _clientInfo;
 
     private readonly string _serverOnlyEndpointName;
+    private string? _negotiatedProtocolVersion;
     private string _endpointName;
     private int _started;
 
@@ -116,6 +115,9 @@ internal sealed partial class McpServerImpl : McpServer
     /// <inheritdoc/>
     public override string? SessionId => _sessionTransport.SessionId;
 
+    /// <inheritdoc/>
+    public override string? NegotiatedProtocolVersion => _negotiatedProtocolVersion;
+
     /// <inheritdoc/>
     public ServerCapabilities ServerCapabilities { get; } = new();
 
@@ -212,6 +214,8 @@ internal sealed partial class McpServerImpl : McpServer
                         McpSessionHandler.LatestProtocolVersion;
                 }
 
+                _negotiatedProtocolVersion = protocolVersion;
+
                 return new InitializeResult
                 {
                     ProtocolVersion = protocolVersion,