diff --git a/Directory.Build.props b/Directory.Build.props
index 159c4a8f..bca37592 100644
--- a/Directory.Build.props
+++ b/Directory.Build.props
@@ -1,6 +1,6 @@
 <Project>
   <PropertyGroup>
-    <LangVersion>13</LangVersion>
+    <LangVersion>preview</LangVersion>
     <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
     <ImplicitUsings>enable</ImplicitUsings>
     <Nullable>enable</Nullable>
diff --git a/src/ModelContextProtocol.AspNetCore/HttpServerTransportOptions.cs b/src/ModelContextProtocol.AspNetCore/HttpServerTransportOptions.cs
index 7741193e..b88cfb76 100644
--- a/src/ModelContextProtocol.AspNetCore/HttpServerTransportOptions.cs
+++ b/src/ModelContextProtocol.AspNetCore/HttpServerTransportOptions.cs
@@ -28,7 +28,7 @@ public class HttpServerTransportOptions
     /// </summary>
     /// <remarks>
     /// If <see langword="true"/>, the "/sse" endpoint will be disabled, and client information will be round-tripped as part
-    /// of the "mcp-session-id" header instead of stored in memory. Unsolicited server-to-client messages and all server-to-client
+    /// of the "MCP-Session-Id" header instead of stored in memory. Unsolicited server-to-client messages and all server-to-client
     /// requests are also unsupported, because any responses may arrive at another ASP.NET Core application process.
     /// Client sampling and roots capabilities are also disabled in stateless mode, because the server cannot make requests.
     /// Defaults to <see langword="false"/>.
diff --git a/src/ModelContextProtocol.AspNetCore/StreamableHttpHandler.cs b/src/ModelContextProtocol.AspNetCore/StreamableHttpHandler.cs
index a38fa7c6..aeac38bf 100644
--- a/src/ModelContextProtocol.AspNetCore/StreamableHttpHandler.cs
+++ b/src/ModelContextProtocol.AspNetCore/StreamableHttpHandler.cs
@@ -26,6 +26,7 @@ internal sealed class StreamableHttpHandler(
     ILoggerFactory loggerFactory,
     IServiceProvider applicationServices)
 {
+    private const string McpSessionIdHeaderName = "Mcp-Session-Id";
     private static readonly JsonTypeInfo<JsonRpcError> s_errorTypeInfo = GetRequiredJsonTypeInfo<JsonRpcError>();
 
     public ConcurrentDictionary<string, HttpMcpSession<StreamableHttpServerTransport>> Sessions { get; } = new(StringComparer.Ordinal);
@@ -70,8 +71,8 @@ await WriteJsonRpcErrorAsync(context,
         }
         finally
         {
-            // Stateless sessions are 1:1 with HTTP requests and are outlived by the MCP session tracked by the mcp-session-id.
-            // Non-stateless sessions are 1:1 with the mcp-session-id and outlive the POST request.
+            // Stateless sessions are 1:1 with HTTP requests and are outlived by the MCP session tracked by the Mcp-Session-Id.
+            // Non-stateless sessions are 1:1 with the Mcp-Session-Id and outlive the POST request.
             // Non-stateless sessions get disposed by a DELETE request or the IdleTrackingBackgroundService.
             if (HttpServerTransportOptions.Stateless)
             {
@@ -90,7 +91,7 @@ await WriteJsonRpcErrorAsync(context,
             return;
         }
 
-        var sessionId = context.Request.Headers["mcp-session-id"].ToString();
+        var sessionId = context.Request.Headers[McpSessionIdHeaderName].ToString();
         var session = await GetSessionAsync(context, sessionId);
         if (session is null)
         {
@@ -117,7 +118,7 @@ await WriteJsonRpcErrorAsync(context,
 
     public async Task HandleDeleteRequestAsync(HttpContext context)
     {
-        var sessionId = context.Request.Headers["mcp-session-id"].ToString();
+        var sessionId = context.Request.Headers[McpSessionIdHeaderName].ToString();
         if (Sessions.TryRemove(sessionId, out var session))
         {
             await session.DisposeAsync();
@@ -157,14 +158,14 @@ await WriteJsonRpcErrorAsync(context,
             return null;
         }
 
-        context.Response.Headers["mcp-session-id"] = session.Id;
+        context.Response.Headers[McpSessionIdHeaderName] = session.Id;
         context.Features.Set(session.Server);
         return session;
     }
 
     private async ValueTask<HttpMcpSession<StreamableHttpServerTransport>?> GetOrCreateSessionAsync(HttpContext context)
     {
-        var sessionId = context.Request.Headers["mcp-session-id"].ToString();
+        var sessionId = context.Request.Headers[McpSessionIdHeaderName].ToString();
 
         if (string.IsNullOrEmpty(sessionId))
         {
@@ -188,11 +189,11 @@ private async ValueTask<HttpMcpSession<StreamableHttpServerTransport>> StartNewS
             {
                 SessionId = sessionId,
             };
-            context.Response.Headers["mcp-session-id"] = sessionId;
+            context.Response.Headers[McpSessionIdHeaderName] = sessionId;
         }
         else
         {
-            // "(uninitialized stateless id)" is not written anywhere. We delay writing the mcp-session-id
+            // "(uninitialized stateless id)" is not written anywhere. We delay writing the MCP-Session-Id
             // until after we receive the initialize request with the client info we need to serialize.
             sessionId = "(uninitialized stateless id)";
             transport = new()
@@ -204,7 +205,7 @@ private async ValueTask<HttpMcpSession<StreamableHttpServerTransport>> StartNewS
 
         var session = await CreateSessionAsync(context, transport, sessionId);
 
-        // The HttpMcpSession is not stored between requests in stateless mode. Instead, the session is recreated from the mcp-session-id.
+        // The HttpMcpSession is not stored between requests in stateless mode. Instead, the session is recreated from the MCP-Session-Id.
         if (!HttpServerTransportOptions.Stateless)
         {
             if (!Sessions.TryAdd(sessionId, session))
@@ -299,7 +300,7 @@ private void ScheduleStatelessSessionIdWrite(HttpContext context, StreamableHttp
 
             var sessionJson = JsonSerializer.Serialize(statelessId, StatelessSessionIdJsonContext.Default.StatelessSessionId);
             transport.SessionId = Protector.Protect(sessionJson);
-            context.Response.Headers["mcp-session-id"] = transport.SessionId;
+            context.Response.Headers[McpSessionIdHeaderName] = transport.SessionId;
             return ValueTask.CompletedTask;
         };
     }
diff --git a/src/ModelContextProtocol.Core/Client/StreamableHttpClientSessionTransport.cs b/src/ModelContextProtocol.Core/Client/StreamableHttpClientSessionTransport.cs
index 981185dd..d4d03480 100644
--- a/src/ModelContextProtocol.Core/Client/StreamableHttpClientSessionTransport.cs
+++ b/src/ModelContextProtocol.Core/Client/StreamableHttpClientSessionTransport.cs
@@ -27,6 +27,7 @@ internal sealed partial class StreamableHttpClientSessionTransport : TransportBa
     private readonly CancellationTokenSource _connectionCts;
     private readonly ILogger _logger;
 
+    private string? _negotiatedProtocolVersion;
     private Task? _getReceiveTask;
 
     public StreamableHttpClientSessionTransport(
@@ -84,7 +85,7 @@ internal async Task<HttpResponseMessage> SendHttpRequestAsync(JsonRpcMessage mes
             },
         };
 
-        CopyAdditionalHeaders(httpRequestMessage.Headers, _options.AdditionalHeaders, SessionId);
+        CopyAdditionalHeaders(httpRequestMessage.Headers, _options.AdditionalHeaders, SessionId, _negotiatedProtocolVersion);
 
         var response = await _httpClient.SendAsync(httpRequestMessage, HttpCompletionOption.ResponseHeadersRead, cancellationToken).ConfigureAwait(false);
 
@@ -118,14 +119,17 @@ 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.
-            if (response.Headers.TryGetValues("mcp-session-id", out var sessionIdValues))
+            // 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))
             {
                 SessionId = sessionIdValues.FirstOrDefault();
             }
 
+            var initializeResult = JsonSerializer.Deserialize(initResponse.Result, McpJsonUtilities.JsonContext.Default.InitializeResult);
+            _negotiatedProtocolVersion = initializeResult?.ProtocolVersion;
+
             _getReceiveTask = ReceiveUnsolicitedMessagesAsync();
         }
 
@@ -169,7 +173,7 @@ 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);
+        CopyAdditionalHeaders(request.Headers, _options.AdditionalHeaders, SessionId, _negotiatedProtocolVersion);
 
         using var response = await _httpClient.SendAsync(request, HttpCompletionOption.ResponseHeadersRead, _connectionCts.Token).ConfigureAwait(false);
 
@@ -244,11 +248,20 @@ private void LogJsonException(JsonException ex, string data)
         }
     }
 
-    internal static void CopyAdditionalHeaders(HttpRequestHeaders headers, IDictionary<string, string>? additionalHeaders, string? sessionId = null)
+    internal static void CopyAdditionalHeaders(
+        HttpRequestHeaders headers,
+        IDictionary<string, string>? additionalHeaders,
+        string? sessionId = null,
+        string? protocolVersion = null)
     {
         if (sessionId is not null)
         {
-            headers.Add("mcp-session-id", sessionId);
+            headers.Add("Mcp-Session-Id", sessionId);
+        }
+
+        if (protocolVersion is not null)
+        {
+            headers.Add("MCP-Protocol-Version", protocolVersion);
         }
 
         if (additionalHeaders is null)
diff --git a/src/ModelContextProtocol.Core/ModelContextProtocol.Core.csproj b/src/ModelContextProtocol.Core/ModelContextProtocol.Core.csproj
index e9edb169..928c76d9 100644
--- a/src/ModelContextProtocol.Core/ModelContextProtocol.Core.csproj
+++ b/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'">
diff --git a/src/ModelContextProtocol/ModelContextProtocol.csproj b/src/ModelContextProtocol/ModelContextProtocol.csproj
index 74604158..994f3dcc 100644
--- a/src/ModelContextProtocol/ModelContextProtocol.csproj
+++ b/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'">
diff --git a/tests/ModelContextProtocol.AspNetCore.Tests/MapMcpStreamableHttpTests.cs b/tests/ModelContextProtocol.AspNetCore.Tests/MapMcpStreamableHttpTests.cs
index b2f65f82..b1c1a89d 100644
--- a/tests/ModelContextProtocol.AspNetCore.Tests/MapMcpStreamableHttpTests.cs
+++ b/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));
+    }
 }
diff --git a/tests/ModelContextProtocol.AspNetCore.Tests/MapMcpTests.cs b/tests/ModelContextProtocol.AspNetCore.Tests/MapMcpTests.cs
index d9d98b74..2690352f 100644
--- a/tests/ModelContextProtocol.AspNetCore.Tests/MapMcpTests.cs
+++ b/tests/ModelContextProtocol.AspNetCore.Tests/MapMcpTests.cs
@@ -195,7 +195,7 @@ 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)
diff --git a/tests/ModelContextProtocol.AspNetCore.Tests/ModelContextProtocol.AspNetCore.Tests.csproj b/tests/ModelContextProtocol.AspNetCore.Tests/ModelContextProtocol.AspNetCore.Tests.csproj
index 76187ee5..bbcac5f5 100644
--- a/tests/ModelContextProtocol.AspNetCore.Tests/ModelContextProtocol.AspNetCore.Tests.csproj
+++ b/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>
diff --git a/tests/ModelContextProtocol.Tests/ModelContextProtocol.Tests.csproj b/tests/ModelContextProtocol.Tests/ModelContextProtocol.Tests.csproj
index d88ec985..6ddad70f 100644
--- a/tests/ModelContextProtocol.Tests/ModelContextProtocol.Tests.csproj
+++ b/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>