Add McpSession.NegotiatedProtocolVersion property.

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 @@ private void RegisterHandlers(ClientCapabilities capabilities, NotificationHandl
     /// <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 @@ public async Task ConnectAsync(CancellationToken cancellationToken = default)
                     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

@@ -38,14 +38,23 @@ 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>
     /// <param name="request">The JSON-RPC request to send.</param>
     /// <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 @@
 
 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 @@ void Register<TPrimitive>(McpServerPrimitiveCollection<TPrimitive>? collection,
     /// <inheritdoc/>
     public override string? SessionId => _sessionTransport.SessionId;
 
+    /// <inheritdoc/>
+    public override string? NegotiatedProtocolVersion => _negotiatedProtocolVersion;
+
     /// <inheritdoc/>
     public ServerCapabilities ServerCapabilities { get; } = new();
 
@@ -212,6 +214,8 @@ private void ConfigureInitialize(McpServerOptions options)
                         McpSessionHandler.LatestProtocolVersion;
                 }
 
+                _negotiatedProtocolVersion = protocolVersion;
+
                 return new InitializeResult
                 {
                     ProtocolVersion = protocolVersion,

tests/ModelContextProtocol.AspNetCore.Tests/HttpServerIntegrationTests.cs

@@ -52,6 +52,7 @@ public async Task Connect_TestServer_ShouldProvideServerFields()
         // Assert
         Assert.NotNull(client.ServerCapabilities);
         Assert.NotNull(client.ServerInfo);
+        Assert.NotNull(client.NegotiatedProtocolVersion);
 
         if (ClientTransportOptions.Endpoint.AbsolutePath.EndsWith("/sse"))
         {

tests/ModelContextProtocol.AspNetCore.Tests/MapMcpStreamableHttpTests.cs

@@ -171,13 +171,13 @@ public async Task StreamableHttpClient_SendsMcpProtocolVersionHeader_AfterInitia
 
         await app.StartAsync(TestContext.Current.CancellationToken);
 
-        await using (var mcpClient = await ConnectAsync(clientOptions: new()
+        await using var mcpClient = await ConnectAsync(clientOptions: new()
         {
             ProtocolVersion = "2025-03-26",
-        }))
-        {
-            await mcpClient.ListToolsAsync(cancellationToken: TestContext.Current.CancellationToken);
-        }
+        });
+
+        Assert.Equal("2025-03-26", mcpClient.NegotiatedProtocolVersion);
+        await mcpClient.ListToolsAsync(cancellationToken: TestContext.Current.CancellationToken);
 
         // The header should be included in the GET request, the initialized notification, the tools/list call, and the delete request.
         Assert.NotEmpty(protocolVersionHeaderValues);

tests/ModelContextProtocol.Tests/Client/McpClientTests.cs

@@ -468,4 +468,13 @@ public async Task AsClientLoggerProvider_MessagesSentToClient()
             ], 
             data.OrderBy(s => s));
     }
+
+    [Theory]
+    [InlineData(null)]
+    [InlineData("2025-03-26")]
+    public async Task ReturnsNegotiatedProtocolVersion(string? protocolVersion)
+    {
+        await using McpClient client = await CreateMcpClientForServer(new() { ProtocolVersion = protocolVersion });
+        Assert.Equal(protocolVersion ?? "2025-06-18", client.NegotiatedProtocolVersion);
+    }
 }

tests/ModelContextProtocol.Tests/ClientIntegrationTests.cs

@@ -52,8 +52,12 @@ public async Task Connect_ShouldProvideServerFields(string clientId)
         // Assert
         Assert.NotNull(client.ServerCapabilities);
         Assert.NotNull(client.ServerInfo);
+        Assert.NotNull(client.NegotiatedProtocolVersion);
+
         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.SessionId);
     }

tests/ModelContextProtocol.Tests/Server/McpServerTests.cs

@@ -41,6 +41,7 @@ public async Task Create_Should_Initialize_With_Valid_Parameters()
 
         // Assert
         Assert.NotNull(server);
+        Assert.Null(server.NegotiatedProtocolVersion);
     }
 
     [Fact]
@@ -232,7 +233,7 @@ await Can_Handle_Requests(
             serverCapabilities: null,
             method: RequestMethods.Ping,
             configureOptions: null,
-            assertResult: response =>
+            assertResult: (_, response) =>
             {
                 JsonObject jObj = Assert.IsType<JsonObject>(response);
                 Assert.Empty(jObj);
@@ -247,13 +248,14 @@ await Can_Handle_Requests(
             serverCapabilities: null,
             method: RequestMethods.Initialize,
             configureOptions: null,
-            assertResult: response =>
+            assertResult: (server, response) =>
             {
                 var result = JsonSerializer.Deserialize<InitializeResult>(response, McpJsonUtilities.DefaultOptions);
                 Assert.NotNull(result);
                 Assert.Equal(expectedAssemblyName.Name, result.ServerInfo.Name);
                 Assert.Equal(expectedAssemblyName.Version?.ToString() ?? "1.0.0", result.ServerInfo.Version);
                 Assert.Equal("2024", result.ProtocolVersion);
+                Assert.Equal("2024", server.NegotiatedProtocolVersion);
             });
     }
 
@@ -279,7 +281,7 @@ await Can_Handle_Requests(
             },
             method: RequestMethods.CompletionComplete,
             configureOptions: null,
-            assertResult: response =>
+            assertResult: (_, response) =>
             {
                 var result = JsonSerializer.Deserialize<CompleteResult>(response, McpJsonUtilities.DefaultOptions);
                 Assert.NotNull(result?.Completion);
@@ -316,7 +318,7 @@ await Can_Handle_Requests(
             },
             RequestMethods.ResourcesTemplatesList,
             configureOptions: null,
-            assertResult: response =>
+            assertResult: (_, response) =>
             {
                 var result = JsonSerializer.Deserialize<ListResourceTemplatesResult>(response, McpJsonUtilities.DefaultOptions);
                 Assert.NotNull(result?.ResourceTemplates);
@@ -345,7 +347,7 @@ await Can_Handle_Requests(
             },
             RequestMethods.ResourcesList,
             configureOptions: null,
-            assertResult: response =>
+            assertResult: (_, response) =>
             {
                 var result = JsonSerializer.Deserialize<ListResourcesResult>(response, McpJsonUtilities.DefaultOptions);
                 Assert.NotNull(result?.Resources);
@@ -380,7 +382,7 @@ await Can_Handle_Requests(
             }, 
             method: RequestMethods.ResourcesRead,
             configureOptions: null,
-            assertResult: response =>
+            assertResult: (_, response) =>
             {
                 var result = JsonSerializer.Deserialize<ReadResourceResult>(response, McpJsonUtilities.DefaultOptions);
                 Assert.NotNull(result?.Contents);
@@ -417,7 +419,7 @@ await Can_Handle_Requests(
             },
             method: RequestMethods.PromptsList,
             configureOptions: null,
-            assertResult: response =>
+            assertResult: (_, response) =>
             {
                 var result = JsonSerializer.Deserialize<ListPromptsResult>(response, McpJsonUtilities.DefaultOptions);
                 Assert.NotNull(result?.Prompts);
@@ -446,7 +448,7 @@ await Can_Handle_Requests(
             },
             method: RequestMethods.PromptsGet,
             configureOptions: null,
-            assertResult: response =>
+            assertResult: (_, response) =>
             {
                 var result = JsonSerializer.Deserialize<GetPromptResult>(response, McpJsonUtilities.DefaultOptions);
                 Assert.NotNull(result);
@@ -480,7 +482,7 @@ await Can_Handle_Requests(
             },
             method: RequestMethods.ToolsList,
             configureOptions: null,
-            assertResult: response =>
+            assertResult: (_, response) =>
             {
                 var result = JsonSerializer.Deserialize<ListToolsResult>(response, McpJsonUtilities.DefaultOptions);
                 Assert.NotNull(result);
@@ -515,7 +517,7 @@ await Can_Handle_Requests(
             }, 
             method: RequestMethods.ToolsCall,
             configureOptions: null,
-            assertResult: response =>
+            assertResult: (_, response) =>
             {
                 var result = JsonSerializer.Deserialize<CallToolResult>(response, McpJsonUtilities.DefaultOptions);
                 Assert.NotNull(result);
@@ -530,7 +532,7 @@ public async Task Can_Handle_Call_Tool_Requests_Throws_Exception_If_No_Handler_A
         await Succeeds_Even_If_No_Handler_Assigned(new ServerCapabilities { Tools = new() }, RequestMethods.ToolsCall, "CallTool handler not configured");
     }
 
-    private async Task Can_Handle_Requests(ServerCapabilities? serverCapabilities, string method, Action<McpServerOptions>? configureOptions, Action<JsonNode?> assertResult)
+    private async Task Can_Handle_Requests(ServerCapabilities? serverCapabilities, string method, Action<McpServerOptions>? configureOptions, Action<McpServer, JsonNode?> assertResult)
     {
         await using var transport = new TestServerTransport();
         var options = CreateOptions(serverCapabilities);
@@ -559,7 +561,7 @@ await transport.SendMessageAsync(
         var response = await receivedMessage.Task.WaitAsync(TimeSpan.FromSeconds(5));
         Assert.NotNull(response);
 
-        assertResult(response.Result);
+        assertResult(server, response.Result);
 
         await transport.DisposeAsync();
         await runTask;
@@ -682,6 +684,7 @@ public override Task<JsonRpcResponse> SendRequestAsync(JsonRpcRequest request, C
         public override ValueTask DisposeAsync() => default;
 
         public override string? SessionId => throw new NotImplementedException();
+        public override string? NegotiatedProtocolVersion => throw new NotImplementedException();
         public override Implementation? ClientInfo => throw new NotImplementedException();
         public override IServiceProvider? Services => throw new NotImplementedException();
         public override LoggingLevel? LoggingLevel => throw new NotImplementedException();