Address PR review feedback

- WriteUnsupportedProtocolVersionErrorAsync: anchor the prose message to the
  July2026 floor ("Starting with protocol version ...") instead of echoing
  client input, while reporting the actual requested MCP-Protocol-Version
  header in the structured UnsupportedProtocolVersionErrorData.Requested field
  for diagnostics/fallback.
- Rename July2026Post_WithUnsupportedProtocolVersionHeader_Returns400_With_Minus32004
  to ...Minus32022 to match the renumbered error code.
- Fix mid-sentence capitalization of "introduced" in two McpServerImpl XML docs.
- Correct MetaKeys.ProtocolVersion remark: header/body version mismatches are
  rejected with HeaderMismatch, not UnsupportedProtocolVersion.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: halter73

src/ModelContextProtocol.AspNetCore/StreamableHttpHandler.cs

@@ -693,16 +693,17 @@ private static bool ValidateProtocolVersionHeader(HttpContext context, [NotNullW
     /// </summary>
     private static Task WriteUnsupportedProtocolVersionErrorAsync(HttpContext context)
     {
+        var requestedProtocolVersion = context.Request.Headers[McpProtocolVersionHeaderName].ToString();
         var errorDetail = new JsonRpcErrorDetail
         {
             Code = (int)McpErrorCode.UnsupportedProtocolVersion,
-            Message = $"Bad Request: The protocol revision '{McpHttpHeaders.July2026ProtocolVersion}' does not support Streamable HTTP sessions and is not supported when the server is configured with sessions (HttpServerTransportOptions.Stateless = false). " +
+            Message = $"Bad Request: Starting with protocol version '{McpHttpHeaders.July2026ProtocolVersion}', Streamable HTTP does not support sessions and is not supported when the server is configured with sessions enabled (HttpServerTransportOptions.Stateless = false). " +
                 "Use the initialize handshake with a protocol version that still supports sessions instead.",
             Data = JsonSerializer.SerializeToNode(
                 new UnsupportedProtocolVersionErrorData
                 {
                     Supported = s_sessionSupportingProtocolVersions,
-                    Requested = McpHttpHeaders.July2026ProtocolVersion,
+                    Requested = requestedProtocolVersion,
                 },
                 GetRequiredJsonTypeInfo<UnsupportedProtocolVersionErrorData>()),
         };

src/ModelContextProtocol.Core/Protocol/MetaKeys.cs

@@ -10,8 +10,8 @@ public static class MetaKeys
     /// </summary>
     /// <remarks>
     /// Introduced by the 2026-07-28 protocol revision (SEP-2575). For HTTP transports, the value MUST
-    /// match the <c>MCP-Protocol-Version</c> header. Servers reject mismatched versions with
-    /// <see cref="McpErrorCode.UnsupportedProtocolVersion"/>.
+    /// match the <c>MCP-Protocol-Version</c> header. Servers reject a header/body mismatch with
+    /// <see cref="McpErrorCode.HeaderMismatch"/>.
     /// </remarks>
     public const string ProtocolVersion = "io.modelcontextprotocol/protocolVersion";
 

src/ModelContextProtocol.Core/Server/McpServerImpl.cs

@@ -391,7 +391,7 @@ private void ConfigureInitialize(McpServerOptions options)
     }
 
     /// <summary>
-    /// Registers the <c>server/discover</c> request handler Introduced by the 2026-07-28 protocol revision (SEP-2575).
+    /// Registers the <c>server/discover</c> request handler introduced by the 2026-07-28 protocol revision (SEP-2575).
     /// </summary>
     /// <remarks>
     /// The handler is registered unconditionally so legacy clients can probe it too. It returns the server's
@@ -421,7 +421,7 @@ private void ConfigureDiscover(McpServerOptions options)
     }
 
     /// <summary>
-    /// Registers the <c>subscriptions/listen</c> request handler Introduced by the 2026-07-28 protocol revision (SEP-2575).
+    /// Registers the <c>subscriptions/listen</c> request handler introduced by the 2026-07-28 protocol revision (SEP-2575).
     /// </summary>
     /// <remarks>
     /// <para>

tests/ModelContextProtocol.AspNetCore.Tests/RawHttpConformanceTests.cs

@@ -136,7 +136,7 @@ public async Task ServerDiscover_RawPost_ReturnsDiscoverResult()
     }
 
     [Fact]
-    public async Task July2026Post_WithUnsupportedProtocolVersionHeader_Returns400_With_Minus32004()
+    public async Task July2026Post_WithUnsupportedProtocolVersionHeader_Returns400_With_Minus32022()
     {
         await StartAsync();