Tighten draft protocol negotiation boundaries (#1692)

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Tarek Mahmoud Sayed <tarekms@ntdev.microsoft.com>

src/ModelContextProtocol.Core/Protocol/DiscoverResult.cs

@@ -8,16 +8,19 @@ namespace ModelContextProtocol.Protocol;
 /// <remarks>
 /// <para>
 /// Introduced by the 2026-07-28 protocol revision (SEP-2575) as the canonical way for a client
-/// to learn what a server supports without performing the legacy <c>initialize</c> handshake.
+/// to learn what a server supports without performing the <c>initialize</c> handshake.
 /// </para>
 /// </remarks>
 public sealed class DiscoverResult : Result, ICacheableResult
 {
     /// <summary>
-    /// Gets or sets the list of MCP protocol version strings that the server supports.
+    /// Gets or sets the list of MCP protocol version strings the server supports for subsequent
+    /// per-request metadata requests.
     /// </summary>
     /// <remarks>
-    /// The client should choose a version from this list for use in subsequent requests.
+    /// The client should choose a version from this list for subsequent requests that carry the
+    /// 2026-07-28-style per-request <c>_meta</c> envelope. Versions that require the
+    /// <c>initialize</c> handshake are negotiated through <c>initialize</c> instead.
     /// </remarks>
     [JsonPropertyName("supportedVersions")]
     public required IList<string> SupportedVersions { get; set; }

src/ModelContextProtocol.Core/Protocol/InitializeRequestParams.cs

@@ -22,16 +22,17 @@ namespace ModelContextProtocol.Protocol;
 public sealed class InitializeRequestParams : RequestParams
 {
     /// <summary>
-    /// Gets or sets the version of the Model Context Protocol that the client wants to use.
+    /// Gets or sets the initialize-handshake Model Context Protocol version that the client wants to use.
     /// </summary>
     /// <remarks>
     /// <para>
     /// Protocol version is specified using a date-based versioning scheme in the format "YYYY-MM-DD".
-    /// The client and server must agree on a protocol version to communicate successfully.
+    /// The client and server must agree on an initialize-capable protocol version to communicate successfully.
     /// </para>
     /// <para>
-    /// During initialization, the server will check if it supports this requested version. If there's a
-    /// mismatch, the server will reject the connection with a version mismatch error.
+    /// During initialization, the server will check if it supports this requested version. Protocol
+    /// revisions starting with <c>2026-07-28</c> do not use <c>initialize</c>; clients select them with
+    /// <c>server/discover</c> and per-request metadata instead.
     /// </para>
     /// <para>
     /// See the <see href="https://spec.modelcontextprotocol.io/specification/">protocol specification</see> for version details.

src/ModelContextProtocol.Core/Protocol/InitializeResult.cs

@@ -22,11 +22,11 @@ namespace ModelContextProtocol.Protocol;
 public sealed class InitializeResult : Result
 {
     /// <summary>
-    /// Gets or sets the version of the Model Context Protocol that the server will use for this session.
+    /// Gets or sets the initialize-handshake Model Context Protocol version that the server will use for this session.
     /// </summary>
     /// <remarks>
     /// <para>
-    /// This is the protocol version the server has agreed to use, which should match the client's
+    /// This is the initialize-capable protocol version the server has agreed to use, which should match the client's
     /// requested version. If there's a mismatch, the client should throw an exception to prevent
     /// communication issues due to incompatible protocol versions.
     /// </para>

src/ModelContextProtocol.Core/Protocol/JsonRpcMessage.cs

@@ -217,7 +217,7 @@ public abstract class JsonRpcMessage
                 // Per JSON-RPC 2.0, when an error occurs before the request id can be determined
                 // (e.g. parse error or invalid request), the server MUST respond with id=null.
                 // Accept null-id error responses so callers can recognize the structured signal
-                // (e.g. an HTTP 400 body whose JSON-RPC envelope carries a non-modern error code).
+                // (e.g. an HTTP 400 body whose JSON-RPC envelope carries a non-SEP-2575 error code).
                 return new JsonRpcError
                 {
                     Id = id,

src/ModelContextProtocol.Core/Protocol/RequestMethods.cs

@@ -159,15 +159,15 @@ public static class RequestMethods
     /// <remarks>
     /// <para>
     /// This RPC is introduced in the 2026-07-28 protocol revision (SEP-2575) as the canonical way for a client
-    /// to learn what a server supports without performing the legacy <c>initialize</c> handshake.
+    /// to learn what a server supports without performing the <c>initialize</c> handshake.
     /// </para>
     /// <para>
     /// The server's response includes its supported protocol versions, capabilities, implementation
     /// information, and optional usage instructions.
     /// </para>
     /// <para>
-    /// Servers SHOULD implement this method. Legacy clients MAY ignore it. Clients on the 2026-07-28 revision
-    /// typically call this once during connection establishment.
+    /// Servers SHOULD implement this method. Initialize-handshake clients MAY ignore it. Clients on the
+    /// 2026-07-28 revision typically call this once during connection establishment.
     /// </para>
     /// </remarks>
     public const string ServerDiscover = "server/discover";
@@ -179,7 +179,7 @@ public static class RequestMethods
     /// <remarks>
     /// <para>
     /// This RPC is introduced in the 2026-07-28 protocol revision (SEP-2575) and replaces the unsolicited
-    /// HTTP GET endpoint and the legacy <see cref="ResourcesSubscribe"/> / <see cref="ResourcesUnsubscribe"/>
+    /// HTTP GET endpoint and the initialize-handshake <see cref="ResourcesSubscribe"/> / <see cref="ResourcesUnsubscribe"/>
     /// request methods.
     /// </para>
     /// <para>