Align the 2026-07-28 wire with spec PR #3002 and SEP-2575 HTTP statuses (#1752)

Co-authored-by: Stephen Halter <halter73@gmail.com>
Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Co-authored-by: tarekgh <tarek.mahmoud.sayed@gmail.com>

src/ModelContextProtocol.Core/Client/McpClient.Methods.cs

@@ -733,6 +733,12 @@ public abstract partial class McpClient : McpSession
     /// For a simpler API that handles both subscription and notification registration in a single call,
     /// use <see cref="SubscribeToResourceAsync(Uri, Func{ResourceUpdatedNotificationParams, CancellationToken, ValueTask}, RequestOptions?, CancellationToken)"/>.
     /// </para>
+    /// <para>
+    /// The 2026-07-28 protocol revision (SEP-2575) removed <c>resources/subscribe</c> in favor of
+    /// <see cref="RequestMethods.SubscriptionsListen"/> with <c>resourceSubscriptions</c>. On a session that
+    /// negotiated that revision or later, this method throws an <see cref="McpException"/> with
+    /// <see cref="McpErrorCode.MethodNotFound"/>.
+    /// </para>
     /// </remarks>
     public Task SubscribeToResourceAsync(
         SubscribeRequestParams requestParams,
@@ -931,6 +937,12 @@ public abstract partial class McpClient : McpSession
     /// <returns>The result of the request.</returns>
     /// <exception cref="ArgumentNullException"><paramref name="requestParams"/> is <see langword="null"/>.</exception>
     /// <exception cref="McpException">The request failed or the server returned an error response.</exception>
+    /// <remarks>
+    /// The 2026-07-28 protocol revision (SEP-2575) removed <c>resources/unsubscribe</c> in favor of
+    /// <see cref="RequestMethods.SubscriptionsListen"/> with <c>resourceSubscriptions</c>. On a session that
+    /// negotiated that revision or later, this method throws an <see cref="McpException"/> with
+    /// <see cref="McpErrorCode.MethodNotFound"/>.
+    /// </remarks>
     public Task UnsubscribeFromResourceAsync(
         UnsubscribeRequestParams requestParams,
         CancellationToken cancellationToken = default)

src/ModelContextProtocol.Core/Client/McpClient.cs

@@ -29,14 +29,16 @@ public abstract partial class McpClient : McpSession
     /// <remarks>
     /// <para>
     /// This property provides identification details about the connected server, including its name and version.
-    /// It is populated during the initialization handshake and is available after a successful connection.
+    /// It is populated during the initialization handshake or from <c>server/discover</c> result metadata.
     /// </para>
     /// <para>
     /// This information can be useful for logging, debugging, compatibility checks, and displaying server
     /// information to users.
     /// </para>
     /// </remarks>
-    /// <exception cref="InvalidOperationException">The client is not connected.</exception>
+    /// <exception cref="InvalidOperationException">
+    /// The client is not connected, or the server omitted the optional server identity metadata.
+    /// </exception>
     public abstract Implementation ServerInfo { get; }
 
     /// <summary>

src/ModelContextProtocol.Core/Client/McpClientImpl.cs

@@ -170,7 +170,8 @@ internal sealed partial class McpClientImpl : McpClient
     public override ServerCapabilities ServerCapabilities => _serverCapabilities ?? throw new InvalidOperationException("The client is not connected.");
 
     /// <inheritdoc/>
-    public override Implementation ServerInfo => _serverInfo ?? throw new InvalidOperationException("The client is not connected.");
+    public override Implementation ServerInfo => _serverInfo ?? throw new InvalidOperationException(
+        "The client is not connected, or the connected server did not provide optional server identity metadata.");
 
     /// <inheritdoc/>
     public override string? ServerInstructions => _serverInstructions;
@@ -432,15 +433,19 @@ internal sealed partial class McpClientImpl : McpClient
                     }
                     else
                     {
+                        var discoveredServerInfo = GetServerInfoFromDiscover(discoverResult!);
+
                         if (_logger.IsEnabled(LogLevel.Information))
                         {
                             LogServerCapabilitiesReceived(_endpointName,
                                 capabilities: JsonSerializer.Serialize(discoverResult!.Capabilities, McpJsonUtilities.JsonContext.Default.ServerCapabilities),
-                                serverInfo: JsonSerializer.Serialize(discoverResult.ServerInfo, McpJsonUtilities.JsonContext.Default.Implementation));
+                                serverInfo: discoveredServerInfo is null
+                                    ? "(none)"
+                                    : JsonSerializer.Serialize(discoveredServerInfo, McpJsonUtilities.JsonContext.Default.Implementation));
                         }
 
                         _serverCapabilities = discoverResult!.Capabilities;
-                        _serverInfo = discoverResult.ServerInfo;
+                        _serverInfo = discoveredServerInfo;
                         _serverInstructions = discoverResult.Instructions;
                     }
 
@@ -484,6 +489,30 @@ internal sealed partial class McpClientImpl : McpClient
         LogClientConnected(_endpointName);
     }
 
+    /// <summary>
+    /// Resolves the server identity from a <c>server/discover</c> result. The 2026-07-28 revision carries
+    /// <c>serverInfo</c> in the result's <c>_meta/io.modelcontextprotocol/serverInfo</c> field rather than
+    /// the result body. Identity is optional, so a server that omits it yields <see langword="null"/>.
+    /// </summary>
+    private static Implementation? GetServerInfoFromDiscover(DiscoverResult discoverResult)
+    {
+        if (discoverResult.Meta is { } meta &&
+            meta.TryGetPropertyValue(MetaKeys.ServerInfo, out JsonNode? serverInfoNode))
+        {
+            if (serverInfoNode is null)
+            {
+                throw new JsonException(
+                    $"Discover result metadata '{MetaKeys.ServerInfo}' must contain a server implementation.");
+            }
+
+            return JsonSerializer.Deserialize(serverInfoNode, McpJsonUtilities.JsonContext.Default.Implementation)
+                ?? throw new JsonException(
+                    $"Discover result metadata '{MetaKeys.ServerInfo}' must contain a server implementation.");
+        }
+
+        return null;
+    }
+
     /// <summary>
     /// Performs the initialize handshake (initialize request + initialized notification),
     /// records the negotiated protocol version, and stores the server capabilities/info/instructions.

src/ModelContextProtocol.Core/Client/McpClientTool.cs

@@ -135,7 +135,7 @@ public sealed class McpClientTool : AIFunction
         // would lose that information. So, we only do the translation if there is no additional information to preserve.
         if (result.IsError is not true &&
             result.StructuredContent is null &&
-            result.Meta is not { Count: > 0 })
+            !HasApplicationResultMetadata(result.Meta))
         {
             switch (result.Content.Count)
             {
@@ -150,6 +150,26 @@ public sealed class McpClientTool : AIFunction
         return JsonSerializer.SerializeToElement(result, McpJsonUtilities.JsonContext.Default.CallToolResult);
     }
 
+    private static bool HasApplicationResultMetadata(JsonObject? meta)
+    {
+        if (meta is null)
+        {
+            return false;
+        }
+
+        foreach (var property in meta)
+        {
+            // Server identity is protocol metadata already exposed through McpClient.ServerInfo.
+            // It should not force an otherwise simple tool result into its JSON envelope.
+            if (!string.Equals(property.Key, MetaKeys.ServerInfo, StringComparison.Ordinal))
+            {
+                return true;
+            }
+        }
+
+        return false;
+    }
+
     /// <summary>
     /// Invokes the tool on the server.
     /// </summary>