Address PR feedback: move SupportedProtocolVersions to ProtocolVersions class, use nullable out param

- Move SupportedProtocolVersions from McpSession to new ProtocolVersions
  static class in Protocol namespace (following RequestMethods/NotificationMethods pattern)
- Revert McpSession.cs to original state
- Change out string to out string? with null on success
- Add ProtocolVersionsTests for the new public property

Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

Author: Copilot

src/ModelContextProtocol.AspNetCore/StreamableHttpHandler.cs

@@ -35,7 +35,7 @@ public async Task HandlePostRequestAsync(HttpContext context)
     {
         if (!ValidateProtocolVersionHeader(context, out var errorMessage))
         {
-            await WriteJsonRpcErrorAsync(context, errorMessage, StatusCodes.Status400BadRequest);
+            await WriteJsonRpcErrorAsync(context, errorMessage!, StatusCodes.Status400BadRequest);
             return;
         }
 
@@ -83,7 +83,7 @@ public async Task HandleGetRequestAsync(HttpContext context)
     {
         if (!ValidateProtocolVersionHeader(context, out var errorMessage))
         {
-            await WriteJsonRpcErrorAsync(context, errorMessage, StatusCodes.Status400BadRequest);
+            await WriteJsonRpcErrorAsync(context, errorMessage!, StatusCodes.Status400BadRequest);
             return;
         }
 
@@ -186,7 +186,7 @@ public async Task HandleDeleteRequestAsync(HttpContext context)
     {
         if (!ValidateProtocolVersionHeader(context, out var errorMessage))
         {
-            await WriteJsonRpcErrorAsync(context, errorMessage, StatusCodes.Status400BadRequest);
+            await WriteJsonRpcErrorAsync(context, errorMessage!, StatusCodes.Status400BadRequest);
             return;
         }
 
@@ -413,17 +413,17 @@ internal static Task RunSessionAsync(HttpContext httpContext, McpServer session,
     /// Validates the MCP-Protocol-Version header if present. A missing header is allowed for backwards compatibility,
     /// but an invalid or unsupported value must be rejected with 400 Bad Request per the MCP spec.
     /// </summary>
-    private static bool ValidateProtocolVersionHeader(HttpContext context, out string errorMessage)
+    private static bool ValidateProtocolVersionHeader(HttpContext context, out string? errorMessage)
     {
         var protocolVersionHeader = context.Request.Headers[McpProtocolVersionHeaderName].ToString();
         if (!string.IsNullOrEmpty(protocolVersionHeader) &&
-            !McpSession.SupportedProtocolVersions.Contains(protocolVersionHeader))
+            !ProtocolVersions.SupportedVersions.Contains(protocolVersionHeader))
         {
             errorMessage = $"Bad Request: The MCP-Protocol-Version header value '{protocolVersionHeader}' is not supported.";
             return false;
         }
 
-        errorMessage = "";
+        errorMessage = null;
         return true;
     }
 

src/ModelContextProtocol.Core/McpSession.cs

@@ -28,11 +28,6 @@ namespace ModelContextProtocol;
 /// </remarks>
 public abstract partial class McpSession : IAsyncDisposable
 {
-    /// <summary>
-    /// Gets the protocol versions supported by this implementation of MCP.
-    /// </summary>
-    public static IReadOnlyList<string> SupportedProtocolVersions { get; } = McpSessionHandler.SupportedProtocolVersions;
-
     /// <summary>Gets an identifier associated with the current MCP session.</summary>
     /// <remarks>
     /// Typically populated in transports supporting multiple sessions, such as Streamable HTTP or SSE.

src/ModelContextProtocol.Core/Protocol/ProtocolVersions.cs

@@ -0,0 +1,12 @@
+namespace ModelContextProtocol.Protocol;
+
+/// <summary>
+/// Provides the protocol versions supported by this implementation of MCP.
+/// </summary>
+public static class ProtocolVersions
+{
+    /// <summary>
+    /// Gets the protocol versions supported by this implementation of MCP.
+    /// </summary>
+    public static IReadOnlyList<string> SupportedVersions { get; } = McpSessionHandler.SupportedProtocolVersions;
+}

tests/ModelContextProtocol.Tests/Protocol/ProtocolVersionsTests.cs

@@ -0,0 +1,26 @@
+using ModelContextProtocol.Protocol;
+
+namespace ModelContextProtocol.Tests.Protocol;
+
+public class ProtocolVersionsTests
+{
+    [Fact]
+    public void SupportedVersions_IsNotEmpty()
+    {
+        Assert.NotEmpty(ProtocolVersions.SupportedVersions);
+    }
+
+    [Fact]
+    public void SupportedVersions_ContainsExpectedVersions()
+    {
+        Assert.Contains("2024-11-05", ProtocolVersions.SupportedVersions);
+        Assert.Contains("2025-03-26", ProtocolVersions.SupportedVersions);
+        Assert.Contains("2025-06-18", ProtocolVersions.SupportedVersions);
+    }
+
+    [Fact]
+    public void SupportedVersions_ReturnsSameInstance()
+    {
+        Assert.Same(ProtocolVersions.SupportedVersions, ProtocolVersions.SupportedVersions);
+    }
+}