Expose a IMcpEndpoint.NegotiatedProtocolVersion property.

Author: eiriktsarpalis

src/ModelContextProtocol.Core/Client/AutoDetectingClientSessionTransport.cs

@@ -46,6 +46,7 @@ public AutoDetectingClientSessionTransport(string endpointName, SseClientTranspo
     public ChannelReader<JsonRpcMessage> MessageReader => _messageChannel.Reader;
 
     string? ITransport.SessionId => ActiveTransport?.SessionId;
+    string? ITransport.NegotiatedProtocolVersion => ActiveTransport?.NegotiatedProtocolVersion;
 
     /// <inheritdoc/>
     public Task SendMessageAsync(JsonRpcMessage message, CancellationToken cancellationToken = default)

src/ModelContextProtocol.Core/Client/McpClient.cs

@@ -107,6 +107,20 @@ public string? SessionId
         }
     }
 
+    /// <inheritdoc/>
+    public string? NegotiatedProtocolVersion
+    {
+        get
+        {
+            if (_sessionTransport is null)
+            {
+                throw new InvalidOperationException("Must have already initialized a session when invoking this property.");
+            }
+
+            return _sessionTransport.NegotiatedProtocolVersion;
+        }
+    }
+
     /// <inheritdoc/>
     public ServerCapabilities ServerCapabilities => _serverCapabilities ?? throw new InvalidOperationException("The client is not connected.");
 

src/ModelContextProtocol.Core/Client/StreamableHttpClientSessionTransport.cs

@@ -1,9 +1,9 @@
 using Microsoft.Extensions.Logging;
 using Microsoft.Extensions.Logging.Abstractions;
+using ModelContextProtocol.Protocol;
 using System.Net.Http.Headers;
 using System.Net.ServerSentEvents;
 using System.Text.Json;
-using ModelContextProtocol.Protocol;
 using System.Threading.Channels;
 
 namespace ModelContextProtocol.Client;
@@ -21,7 +21,6 @@ internal sealed partial class StreamableHttpClientSessionTransport : TransportBa
     private readonly CancellationTokenSource _connectionCts;
     private readonly ILogger _logger;
 
-    private string? _negotiatedProtocolVersion;
     private Task? _getReceiveTask;
 
     private readonly SemaphoreSlim _disposeLock = new(1, 1);
@@ -71,7 +70,7 @@ internal async Task<HttpResponseMessage> SendHttpRequestAsync(JsonRpcMessage mes
             },
         };
 
-        CopyAdditionalHeaders(httpRequestMessage.Headers, _options.AdditionalHeaders, SessionId, _negotiatedProtocolVersion);
+        CopyAdditionalHeaders(httpRequestMessage.Headers, _options.AdditionalHeaders, SessionId, NegotiatedProtocolVersion);
 
         var response = await _httpClient.SendAsync(httpRequestMessage, message, cancellationToken).ConfigureAwait(false);
 
@@ -114,7 +113,7 @@ internal async Task<HttpResponseMessage> SendHttpRequestAsync(JsonRpcMessage mes
             }
 
             var initializeResult = JsonSerializer.Deserialize(initResponse.Result, McpJsonUtilities.JsonContext.Default.InitializeResult);
-            _negotiatedProtocolVersion = initializeResult?.ProtocolVersion;
+            NegotiatedProtocolVersion = initializeResult?.ProtocolVersion;
 
             _getReceiveTask = ReceiveUnsolicitedMessagesAsync();
         }
@@ -173,7 +172,7 @@ private async Task ReceiveUnsolicitedMessagesAsync()
         // Send a GET request to handle any unsolicited messages not sent over a POST response.
         using var request = new HttpRequestMessage(HttpMethod.Get, _options.Endpoint);
         request.Headers.Accept.Add(s_textEventStreamMediaType);
-        CopyAdditionalHeaders(request.Headers, _options.AdditionalHeaders, SessionId, _negotiatedProtocolVersion);
+        CopyAdditionalHeaders(request.Headers, _options.AdditionalHeaders, SessionId, NegotiatedProtocolVersion);
 
         using var response = await _httpClient.SendAsync(request, message: null, _connectionCts.Token).ConfigureAwait(false);
 
@@ -241,7 +240,7 @@ message is JsonRpcMessageWithId rpcResponseOrError &&
     private async Task SendDeleteRequest()
     {
         using var deleteRequest = new HttpRequestMessage(HttpMethod.Delete, _options.Endpoint);
-        CopyAdditionalHeaders(deleteRequest.Headers, _options.AdditionalHeaders, SessionId, _negotiatedProtocolVersion);
+        CopyAdditionalHeaders(deleteRequest.Headers, _options.AdditionalHeaders, SessionId, NegotiatedProtocolVersion);
 
         try
         {

src/ModelContextProtocol.Core/IMcpEndpoint.cs

@@ -36,6 +36,9 @@ public interface IMcpEndpoint : IAsyncDisposable
     /// </remarks>
     string? SessionId { get; }
 
+    /// <summary>Gets the negotiated protocol version for the current MCP session, if available.</summary>
+    string? NegotiatedProtocolVersion { get; }
+
     /// <summary>
     /// Sends a JSON-RPC request to the connected endpoint and waits for a response.
     /// </summary>

src/ModelContextProtocol.Core/Protocol/ITransport.cs

@@ -33,6 +33,9 @@ public interface ITransport : IAsyncDisposable
     /// </remarks>
     string? SessionId { get; }
 
+    /// <summary>Gets the negotiated protocol version for the current MCP session, if available.</summary>
+    string? NegotiatedProtocolVersion { get; }
+
     /// <summary>
     /// Gets a channel reader for receiving messages from the transport.
     /// </summary>

src/ModelContextProtocol.Core/Protocol/TransportBase.cs

@@ -62,6 +62,9 @@ internal TransportBase(string name, Channel<JsonRpcMessage>? messageChannel, ILo
     /// <inheritdoc/>
     public virtual string? SessionId { get; protected set; }
 
+    /// <inheritdoc/>
+    public virtual string? NegotiatedProtocolVersion { get; protected set; }
+
     /// <summary>
     /// Gets the name that identifies this transport endpoint in logs.
     /// </summary>

src/ModelContextProtocol.Core/Server/DestinationBoundMcpServer.cs

@@ -7,6 +7,7 @@ internal sealed class DestinationBoundMcpServer(McpServer server, ITransport? tr
 {
     public string EndpointName => server.EndpointName;
     public string? SessionId => transport?.SessionId ?? server.SessionId;
+    public string? NegotiatedProtocolVersion => transport?.NegotiatedProtocolVersion ?? server.NegotiatedProtocolVersion;
     public ClientCapabilities? ClientCapabilities => server.ClientCapabilities;
     public Implementation? ClientInfo => server.ClientInfo;
     public McpServerOptions ServerOptions => server.ServerOptions;

src/ModelContextProtocol.Core/Server/McpServer.cs

@@ -99,6 +99,9 @@ void Register<TPrimitive>(McpServerPrimitiveCollection<TPrimitive>? collection,
     /// <inheritdoc/>
     public string? SessionId => _sessionTransport.SessionId;
 
+    /// <inheritdoc/>
+    public string? NegotiatedProtocolVersion => _sessionTransport.NegotiatedProtocolVersion;
+
     /// <inheritdoc/>
     public ServerCapabilities ServerCapabilities { get; } = new();
 

src/ModelContextProtocol.Core/Server/SseResponseStreamTransport.cs

@@ -54,6 +54,9 @@ public async Task RunAsync(CancellationToken cancellationToken = default)
     /// <inheritdoc/>
     public string? SessionId { get; } = sessionId;
 
+    /// <inheritdoc/>
+    public string? NegotiatedProtocolVersion => null;
+
     /// <inheritdoc/>
     public async ValueTask DisposeAsync()
     {

src/ModelContextProtocol.Core/Server/StreamableHttpPostTransport.cs

@@ -21,6 +21,7 @@ internal sealed class StreamableHttpPostTransport(StreamableHttpServerTransport
     public ChannelReader<JsonRpcMessage> MessageReader => throw new NotSupportedException("JsonRpcMessage.Context.RelatedTransport should only be used for sending messages.");
 
     string? ITransport.SessionId => parentTransport.SessionId;
+    string? ITransport.NegotiatedProtocolVersion => parentTransport.NegotiatedProtocolVersion;
 
     /// <returns>
     /// True, if data was written to the respond body.