Send MCP-Protocol-Version header in Streamable HTTP client transport

Directory.Build.props

@@ -1,6 +1,6 @@
 <Project>
   <PropertyGroup>
-    <LangVersion>13</LangVersion>
+    <LangVersion>preview</LangVersion>
     <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
     <ImplicitUsings>enable</ImplicitUsings>
     <Nullable>enable</Nullable>

src/ModelContextProtocol.Core/Client/StreamableHttpClientSessionTransport.cs

@@ -28,6 +28,7 @@ internal sealed partial class StreamableHttpClientSessionTransport : TransportBa
     private readonly ILogger _logger;
 
     private string? _mcpSessionId;
+    private string? _negotiatedProtocolVersion;
     private Task? _getReceiveTask;
 
     public StreamableHttpClientSessionTransport(
@@ -85,7 +86,7 @@ internal async Task<HttpResponseMessage> SendHttpRequestAsync(JsonRpcMessage mes
             },
         };
 
-        CopyAdditionalHeaders(httpRequestMessage.Headers, _options.AdditionalHeaders, _mcpSessionId);
+        CopyAdditionalHeaders(httpRequestMessage.Headers, _options.AdditionalHeaders, _mcpSessionId, _negotiatedProtocolVersion);
 
         var response = await _httpClient.SendAsync(httpRequestMessage, HttpCompletionOption.ResponseHeadersRead, cancellationToken).ConfigureAwait(false);
 
@@ -119,14 +120,17 @@ internal async Task<HttpResponseMessage> SendHttpRequestAsync(JsonRpcMessage mes
             throw new McpException($"Streamable HTTP POST response completed without a reply to request with ID: {rpcRequest.Id}");
         }
 
-        if (rpcRequest.Method == RequestMethods.Initialize && rpcResponseOrError is JsonRpcResponse)
+        if (rpcRequest.Method == RequestMethods.Initialize && rpcResponseOrError is JsonRpcResponse initResponse)
         {
-            // We've successfully initialized! Copy session-id and start GET request if any.
+            // We've successfully initialized! Copy session-id and protocol version, then start GET request if any.
             if (response.Headers.TryGetValues("mcp-session-id", out var sessionIdValues))
             {
                 _mcpSessionId = sessionIdValues.FirstOrDefault();
             }
 
+            var initializeResult = JsonSerializer.Deserialize(initResponse.Result, McpJsonUtilities.JsonContext.Default.InitializeResult);
+            _negotiatedProtocolVersion = initializeResult?.ProtocolVersion;
+
             _getReceiveTask = ReceiveUnsolicitedMessagesAsync();
         }
 
@@ -170,7 +174,7 @@ 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, _mcpSessionId);
+        CopyAdditionalHeaders(request.Headers, _options.AdditionalHeaders, _mcpSessionId, _negotiatedProtocolVersion);
 
         using var response = await _httpClient.SendAsync(request, HttpCompletionOption.ResponseHeadersRead, _connectionCts.Token).ConfigureAwait(false);
 
@@ -245,13 +249,22 @@ private void LogJsonException(JsonException ex, string data)
         }
     }
 
-    internal static void CopyAdditionalHeaders(HttpRequestHeaders headers, Dictionary<string, string>? additionalHeaders, string? sessionId = null)
+    internal static void CopyAdditionalHeaders(
+        HttpRequestHeaders headers,
+        Dictionary<string, string>? additionalHeaders,
+        string? sessionId = null,
+        string? protocolVersion = null)
     {
         if (sessionId is not null)
         {
             headers.Add("mcp-session-id", sessionId);
         }
 
+        if (protocolVersion is not null)
+        {
+            headers.Add("mcp-protocol-version", protocolVersion);
+        }
+
         if (additionalHeaders is null)
         {
             return;

src/ModelContextProtocol.Core/ModelContextProtocol.Core.csproj

@@ -7,7 +7,6 @@
     <PackageId>ModelContextProtocol.Core</PackageId>
     <Description>Core .NET SDK for the Model Context Protocol (MCP)</Description>
     <PackageReadmeFile>README.md</PackageReadmeFile>
-    <LangVersion>preview</LangVersion>
   </PropertyGroup>
 
   <PropertyGroup Condition="'$(TargetFramework)' != 'netstandard2.0'">

src/ModelContextProtocol/ModelContextProtocol.csproj

@@ -7,7 +7,6 @@
     <PackageId>ModelContextProtocol</PackageId>
     <Description>.NET SDK for the Model Context Protocol (MCP) with hosting and dependency injection extensions.</Description>
     <PackageReadmeFile>README.md</PackageReadmeFile>
-    <LangVersion>preview</LangVersion>
   </PropertyGroup>
 
   <PropertyGroup Condition="'$(TargetFramework)' != 'netstandard2.0'">

tests/ModelContextProtocol.AspNetCore.Tests/MapMcpStreamableHttpTests.cs

@@ -1,5 +1,6 @@
 using Microsoft.AspNetCore.Builder;
 using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.Primitives;
 using ModelContextProtocol.Client;
 
 namespace ModelContextProtocol.AspNetCore.Tests;
@@ -143,4 +144,38 @@ public async Task SseMode_Works_WithSseEndpoint()
 
         Assert.Equal("SseTestServer", mcpClient.ServerInfo.Name);
     }
+
+    [Fact]
+    public async Task StreamableHttpClient_SendsMcpProtocolVersionHeader_AfterInitialization()
+    {
+        var protocolVersionHeaderValues = new List<string?>();
+
+        Builder.Services.AddMcpServer().WithHttpTransport(ConfigureStateless).WithTools<EchoHttpContextUserTools>();
+
+        await using var app = Builder.Build();
+
+        app.Use(next =>
+        {
+            return async context =>
+            {
+                if (!StringValues.IsNullOrEmpty(context.Request.Headers["mcp-session-id"]))
+                {
+                    protocolVersionHeaderValues.Add(context.Request.Headers["mcp-protocol-version"]);
+                }
+
+                await next(context);
+            };
+        });
+
+        app.MapMcp();
+
+        await app.StartAsync(TestContext.Current.CancellationToken);
+
+        await using var mcpClient = await ConnectAsync();
+        await mcpClient.ListToolsAsync(cancellationToken: TestContext.Current.CancellationToken);
+
+        // The header should be included in the GET request, the initialized notification, and the tools/list call.
+        Assert.Equal(3, protocolVersionHeaderValues.Count);
+        Assert.All(protocolVersionHeaderValues, v => Assert.Equal("2025-03-26", v));
+    }
 }

tests/ModelContextProtocol.AspNetCore.Tests/MapMcpTests.cs

@@ -200,7 +200,7 @@ private ClaimsPrincipal CreateUser(string name)
             "TestAuthType", "name", "role"));
 
     [McpServerToolType]
-    private class EchoHttpContextUserTools(IHttpContextAccessor contextAccessor)
+    protected class EchoHttpContextUserTools(IHttpContextAccessor contextAccessor)
     {
         [McpServerTool, Description("Echoes the input back to the client with their user name.")]
         public string EchoWithUserName(string message)

tests/ModelContextProtocol.AspNetCore.Tests/ModelContextProtocol.AspNetCore.Tests.csproj

@@ -4,7 +4,6 @@
     <TargetFrameworks>net9.0;net8.0</TargetFrameworks>
     <ImplicitUsings>enable</ImplicitUsings>
     <Nullable>enable</Nullable>
-    <LangVersion>Latest</LangVersion>
     <IsPackable>false</IsPackable>
     <IsTestProject>true</IsTestProject>
     <RootNamespace>ModelContextProtocol.AspNetCore.Tests</RootNamespace>

tests/ModelContextProtocol.Tests/ModelContextProtocol.Tests.csproj

@@ -5,7 +5,6 @@
     <TargetFrameworks>net9.0;net8.0</TargetFrameworks>
     <ImplicitUsings>enable</ImplicitUsings>
     <Nullable>enable</Nullable>
-    <LangVersion>Latest</LangVersion>
 
     <IsPackable>false</IsPackable>
     <IsTestProject>true</IsTestProject>
@@ -45,7 +44,7 @@
     <PackageReference Include="Moq" />
     <PackageReference Include="OpenTelemetry" />
     <PackageReference Include="OpenTelemetry.Exporter.InMemory" />
-    <PackageReference Include="System.Linq.AsyncEnumerable" />    
+    <PackageReference Include="System.Linq.AsyncEnumerable" />
     <PackageReference Include="xunit.v3" />
     <PackageReference Include="xunit.runner.visualstudio">
       <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>