Gate 2026-07-28 result fields on negotiated protocol version (#1721) (#1753)

Co-authored-by: Tarek Mahmoud Sayed <tarekms@ntdev.microsoft.com>
Copilot-Session: 80653343-5dcb-43cb-89b0-8ac8e572c4f7

Author: tarekgh

src/ModelContextProtocol.Core/Server/McpServerImpl.cs

@@ -609,7 +609,10 @@ private void ConfigureInitialize(McpServerOptions options)
                     Instructions = options.ServerInstructions,
                     ServerInfo = options.ServerInfo ?? DefaultImplementation,
                     Capabilities = ServerCapabilities ?? new(),
-                    ResultType = "complete",
+
+                    // resultType is a 2026-07-28 result field. The initialize handshake is only available on
+                    // 2025-11-25 and earlier revisions (2026-07-28+ negotiate via server/discover and throw
+                    // above), so InitializeResult must never carry resultType (issue #1721).
                 };
             },
             McpJsonUtilities.JsonContext.Default.InitializeRequestParams,
@@ -1635,8 +1638,12 @@ private void ConfigureLogging(McpServerOptions options)
                     return InvokeHandlerAsync(setLoggingLevelHandler, request!, jsonRpcRequest, cancellationToken);
                 }
 
-                // Otherwise, consider it handled.
-                return new ValueTask<EmptyResult>(EmptyResult.Instance);
+                // Otherwise, consider it handled. logging/setLevel is a legacy (<= 2025-11-25) method
+                // (2026-07-28+ is rejected above), so the response must not carry the 2026-07-28 resultType
+                // field. Return a fresh EmptyResult rather than the shared EmptyResult.Instance, which is
+                // pre-stamped with resultType="complete" for the 2026-07-28-only subscriptions/listen path
+                // (issue #1721).
+                return new ValueTask<EmptyResult>(new EmptyResult());
             },
             McpJsonUtilities.JsonContext.Default.SetLevelRequestParams,
             McpJsonUtilities.JsonContext.Default.EmptyResult);
@@ -1707,7 +1714,11 @@ private void SetHandler<TParams, TResult>(
             handler = async (request, cancellationToken) =>
             {
                 var result = await innerHandler(request, cancellationToken).ConfigureAwait(false);
-                if (result is ICacheableResult cacheable)
+
+                // ttlMs and cacheScope are 2026-07-28 result fields; only stamp them when the request
+                // was negotiated under that revision or later. Earlier revisions (e.g. 2025-11-25) reject
+                // these as unrecognized keys (issue #1721).
+                if (result is ICacheableResult cacheable && IsJuly2026OrLaterProtocolRequest(request.JsonRpcRequest))
                 {
                     cacheable.TimeToLive ??= TimeSpan.Zero;
                     cacheable.CacheScope ??= CacheScope.Private;
@@ -1723,7 +1734,12 @@ private void SetHandler<TParams, TResult>(
             handler = async (request, cancellationToken) =>
             {
                 var result = await innerHandler(request, cancellationToken).ConfigureAwait(false);
-                if (result is Result protocolResult && protocolResult.ResultType is null)
+
+                // resultType is a 2026-07-28 result field; only stamp it when the request was negotiated
+                // under that revision or later. Earlier revisions (e.g. 2025-11-25) reject it as an
+                // unrecognized key (issue #1721).
+                if (result is Result protocolResult && protocolResult.ResultType is null &&
+                    IsJuly2026OrLaterProtocolRequest(request.JsonRpcRequest))
                 {
                     protocolResult.ResultType = "complete";
                 }
@@ -1750,7 +1766,12 @@ private void SetWithAlternateHandler<TParams, TResult>(
         handler = async (request, cancellationToken) =>
         {
             var result = await innerHandler(request, cancellationToken).ConfigureAwait(false);
-            if (!result.IsAlternate && result.Result is { ResultType: null } immediateResult)
+
+            // resultType is a 2026-07-28 result field; only stamp it when the request was negotiated
+            // under that revision or later. Earlier revisions (e.g. 2025-11-25) reject it as an
+            // unrecognized key (issue #1721).
+            if (!result.IsAlternate && result.Result is { ResultType: null } immediateResult &&
+                IsJuly2026OrLaterProtocolRequest(request.JsonRpcRequest))
             {
                 immediateResult.ResultType = "complete";
             }

tests/ModelContextProtocol.AspNetCore.Tests/RawHttpConformanceTests.cs

@@ -322,6 +322,40 @@ public async Task InitializeHandshake_StillSucceeds_OnDefaultServer()
         Assert.Equal(HttpStatusCode.OK, response.StatusCode);
         var json = await ReadJsonResponseAsync(response, TestContext.Current.CancellationToken);
         Assert.Equal("2025-11-25", json["result"]!["protocolVersion"]!.GetValue<string>());
+
+        // resultType is a 2026-07-28 result field, and the initialize handshake only ever negotiates
+        // 2025-11-25 or earlier. It must not appear on the InitializeResult, or strict 2025-11-25 clients
+        // reject the handshake (issue #1721).
+        Assert.False(json["result"]!.AsObject().ContainsKey("resultType"),
+            "InitializeResult must not carry resultType on a 2025-11-25 handshake.");
+    }
+
+    [Fact]
+    public async Task DownlevelToolsList_On2025_11_25_OmitsResultTypeAndCacheHints()
+    {
+        await StartAsync();
+
+        // A 2025-11-25 client completes the initialize handshake and then sends subsequent requests with
+        // the MCP-Protocol-Version header pinned to the negotiated revision. The server must not decorate
+        // the result with the 2026-07-28-exclusive resultType/ttlMs/cacheScope fields (issue #1721).
+        var initBody = @"{""jsonrpc"":""2.0"",""id"":1,""method"":""initialize"",""params"":{""protocolVersion"":""2025-11-25"",""capabilities"":{},""clientInfo"":{""name"":""initialize-handshake"",""version"":""1.0""}}}";
+        using (var initRequest = new HttpRequestMessage(HttpMethod.Post, "") { Content = JsonContent(initBody) })
+        using (var initResponse = await HttpClient.SendAsync(initRequest, TestContext.Current.CancellationToken))
+        {
+            Assert.Equal(HttpStatusCode.OK, initResponse.StatusCode);
+        }
+
+        var body = @"{""jsonrpc"":""2.0"",""id"":2,""method"":""tools/list"",""params"":{}}";
+        using var request = new HttpRequestMessage(HttpMethod.Post, "") { Content = JsonContent(body) };
+        request.Headers.Add(ProtocolVersionHeader, McpProtocolVersions.November2025ProtocolVersion);
+        using var response = await HttpClient.SendAsync(request, TestContext.Current.CancellationToken);
+
+        Assert.Equal(HttpStatusCode.OK, response.StatusCode);
+        var json = await ReadJsonResponseAsync(response, TestContext.Current.CancellationToken);
+        var result = json["result"]!.AsObject();
+        Assert.False(result.ContainsKey("resultType"), "resultType must be absent on a 2025-11-25 tools/list result.");
+        Assert.False(result.ContainsKey("ttlMs"), "ttlMs must be absent on a 2025-11-25 tools/list result.");
+        Assert.False(result.ContainsKey("cacheScope"), "cacheScope must be absent on a 2025-11-25 tools/list result.");
     }
 
     [Fact]

tests/ModelContextProtocol.Tests/Server/McpServerTests.cs

@@ -283,7 +283,7 @@ await Can_Handle_Requests(
             {
                 var result = JsonSerializer.Deserialize<InitializeResult>(response, McpJsonUtilities.DefaultOptions);
                 Assert.NotNull(result);
-                Assert.Equal("complete", result.ResultType);
+                Assert.Null(result.ResultType);
                 Assert.Equal(expectedAssemblyName.Name, result.ServerInfo.Name);
                 Assert.Equal(expectedAssemblyName.Version?.ToString() ?? "1.0.0", result.ServerInfo.Version);
                 Assert.Equal("2024-11-05", result.ProtocolVersion);
@@ -386,7 +386,7 @@ await Can_Handle_Requests(
             {
                 var result = JsonSerializer.Deserialize<InitializeResult>(response, McpJsonUtilities.DefaultOptions);
                 Assert.NotNull(result);
-                Assert.Equal("complete", result.ResultType);
+                Assert.Null(result.ResultType);
                 Assert.NotNull(result.Capabilities.Extensions);
                 Assert.True(result.Capabilities.Extensions.ContainsKey("io.myext"));
             });
@@ -406,7 +406,7 @@ await Can_Handle_Requests(
             {
                 var result = JsonSerializer.Deserialize<InitializeResult>(response, McpJsonUtilities.DefaultOptions);
                 Assert.NotNull(result);
-                Assert.Equal("complete", result.ResultType);
+                Assert.Null(result.ResultType);
                 Assert.NotNull(result.Capabilities.Experimental);
                 Assert.True(result.Capabilities.Experimental.ContainsKey("customFeature"));
             });
@@ -438,7 +438,7 @@ await Can_Handle_Requests(
             {
                 var result = JsonSerializer.Deserialize<InitializeResult>(response, McpJsonUtilities.DefaultOptions);
                 Assert.NotNull(result);
-                Assert.Equal("complete", result.ResultType);
+                Assert.Null(result.ResultType);
 
                 // Use reflection to verify every public property on ServerCapabilities is non-null.
                 // This catches cases where new capability properties are added but not copied
@@ -485,7 +485,7 @@ await Can_Handle_Requests(
             {
                 var result = JsonSerializer.Deserialize<CompleteResult>(response, McpJsonUtilities.DefaultOptions);
                 Assert.NotNull(result?.Completion);
-                Assert.Equal("complete", result.ResultType);
+                Assert.Null(result.ResultType);
                 Assert.Equal(["test"], result.Completion.Values);
                 Assert.Equal(2, result.Completion.Total);
                 Assert.True(result.Completion.HasMore);
@@ -529,7 +529,7 @@ await transport.SendMessageAsync(new JsonRpcRequest
         Assert.NotNull(response);
         var result = JsonSerializer.Deserialize<CompleteResult>(response.Result, McpJsonUtilities.DefaultOptions);
         Assert.NotNull(result?.Completion);
-        Assert.Equal("complete", result.ResultType);
+        Assert.Null(result.ResultType);
         Assert.Equal(["cat"], result.Completion.Values);
         Assert.Equal(1, result.Completion.Total);
 
@@ -573,7 +573,7 @@ await transport.SendMessageAsync(new JsonRpcRequest
         Assert.NotNull(response);
         var result = JsonSerializer.Deserialize<CompleteResult>(response.Result, McpJsonUtilities.DefaultOptions);
         Assert.NotNull(result?.Completion);
-        Assert.Equal("complete", result.ResultType);
+        Assert.Null(result.ResultType);
         Assert.Empty(result.Completion.Values);
 
         await transport.DisposeAsync();
@@ -623,7 +623,7 @@ await transport.SendMessageAsync(new JsonRpcRequest
         Assert.NotNull(response);
         var result = JsonSerializer.Deserialize<CompleteResult>(response.Result, McpJsonUtilities.DefaultOptions);
         Assert.NotNull(result?.Completion);
-        Assert.Equal("complete", result.ResultType);
+        Assert.Null(result.ResultType);
         Assert.Equal(["us-east-1", "us-west-2"], result.Completion.Values);
         Assert.Equal(2, result.Completion.Total);
 
@@ -679,7 +679,7 @@ await transport.SendMessageAsync(new JsonRpcRequest
         Assert.NotNull(response);
         var result = JsonSerializer.Deserialize<CompleteResult>(response.Result, McpJsonUtilities.DefaultOptions);
         Assert.NotNull(result?.Completion);
-        Assert.Equal("complete", result.ResultType);
+        Assert.Null(result.ResultType);
         // Custom handler values + auto-populated values should be combined
         Assert.Equal(["custom-value", "dog", "cat"], result.Completion.Values);
         Assert.Equal(3, result.Completion.Total);
@@ -727,7 +727,7 @@ await transport.SendMessageAsync(new JsonRpcRequest
         Assert.NotNull(response);
         var result = JsonSerializer.Deserialize<CompleteResult>(response.Result, McpJsonUtilities.DefaultOptions);
         Assert.NotNull(result?.Completion);
-        Assert.Equal("complete", result.ResultType);
+        Assert.Null(result.ResultType);
         Assert.Equal(["a", "b"], result.Completion.Values);
 
         await transport.DisposeAsync();
@@ -766,7 +766,7 @@ await Can_Handle_Requests(
             {
                 var result = JsonSerializer.Deserialize<ListResourceTemplatesResult>(response, McpJsonUtilities.DefaultOptions);
                 Assert.NotNull(result?.ResourceTemplates);
-                Assert.Equal("complete", result.ResultType);
+                Assert.Null(result.ResultType);
                 Assert.NotEmpty(result.ResourceTemplates);
                 Assert.Equal("test", result.ResourceTemplates[0].UriTemplate);
             });
@@ -796,7 +796,7 @@ await Can_Handle_Requests(
             {
                 var result = JsonSerializer.Deserialize<ListResourcesResult>(response, McpJsonUtilities.DefaultOptions);
                 Assert.NotNull(result?.Resources);
-                Assert.Equal("complete", result.ResultType);
+                Assert.Null(result.ResultType);
                 Assert.NotEmpty(result.Resources);
                 Assert.Equal("test", result.Resources[0].Uri);
             });
@@ -832,7 +832,7 @@ await Can_Handle_Requests(
             {
                 var result = JsonSerializer.Deserialize<ReadResourceResult>(response, McpJsonUtilities.DefaultOptions);
                 Assert.NotNull(result?.Contents);
-                Assert.Equal("complete", result.ResultType);
+                Assert.Null(result.ResultType);
                 Assert.NotEmpty(result.Contents);
 
                 TextResourceContents textResource = Assert.IsType<TextResourceContents>(result.Contents[0]);
@@ -870,7 +870,7 @@ await Can_Handle_Requests(
             {
                 var result = JsonSerializer.Deserialize<ListPromptsResult>(response, McpJsonUtilities.DefaultOptions);
                 Assert.NotNull(result?.Prompts);
-                Assert.Equal("complete", result.ResultType);
+                Assert.Null(result.ResultType);
                 Assert.NotEmpty(result.Prompts);
                 Assert.Equal("test", result.Prompts[0].Name);
             });
@@ -900,7 +900,7 @@ await Can_Handle_Requests(
             {
                 var result = JsonSerializer.Deserialize<GetPromptResult>(response, McpJsonUtilities.DefaultOptions);
                 Assert.NotNull(result);
-                Assert.Equal("complete", result.ResultType);
+                Assert.Null(result.ResultType);
                 Assert.Equal("test", result.Description);
             });
     }
@@ -935,7 +935,7 @@ await Can_Handle_Requests(
             {
                 var result = JsonSerializer.Deserialize<ListToolsResult>(response, McpJsonUtilities.DefaultOptions);
                 Assert.NotNull(result);
-                Assert.Equal("complete", result.ResultType);
+                Assert.Null(result.ResultType);
                 Assert.NotEmpty(result.Tools);
                 Assert.Equal("test", result.Tools[0].Name);
             });
@@ -971,7 +971,7 @@ await Can_Handle_Requests(
             {
                 var result = JsonSerializer.Deserialize<CallToolResult>(response, McpJsonUtilities.DefaultOptions);
                 Assert.NotNull(result);
-                Assert.Equal("complete", result.ResultType);
+                Assert.Null(result.ResultType);
                 Assert.NotEmpty(result.Content);
                 Assert.Equal("test", Assert.IsType<TextContentBlock>(result.Content[0]).Text);
             });
@@ -1011,6 +1011,31 @@ await Can_Handle_Requests(
             });
     }
 
+    [Fact]
+    public async Task Can_Handle_SetLoggingLevel_Requests_WithoutHandler_OmitsResultType()
+    {
+        // With no custom SetLoggingLevelHandler configured, the server uses its default logging/setLevel
+        // handler. logging/setLevel is a legacy (<= 2025-11-25) method, so the default handler must also
+        // serialize its result as an empty object {} without the 2026-07-28 resultType field (issue #1721).
+        await Can_Handle_Requests(
+            new ServerCapabilities
+            {
+                Logging = new()
+            },
+            method: RequestMethods.LoggingSetLevel,
+            configureOptions: null,
+            assertResult: (_, response) =>
+            {
+                var result = JsonSerializer.Deserialize<EmptyResult>(response, McpJsonUtilities.DefaultOptions);
+                Assert.NotNull(result);
+                Assert.Null(result.ResultType);
+
+                // The wire response must be exactly {} with no additional properties.
+                var obj = Assert.IsType<JsonObject>(response);
+                Assert.Empty(obj);
+            });
+    }
+
     [Fact]
     public async Task Can_Handle_Call_Tool_Requests_With_McpException()
     {
@@ -1033,7 +1058,7 @@ await Can_Handle_Requests(
             {
                 var result = JsonSerializer.Deserialize<CallToolResult>(response, McpJsonUtilities.DefaultOptions);
                 Assert.NotNull(result);
-                Assert.Equal("complete", result.ResultType);
+                Assert.Null(result.ResultType);
                 Assert.True(result.IsError);
                 Assert.NotEmpty(result.Content);
                 var textContent = Assert.IsType<TextContentBlock>(result.Content[0]);
@@ -1062,7 +1087,7 @@ await Can_Handle_Requests(
             {
                 var result = JsonSerializer.Deserialize<CallToolResult>(response, McpJsonUtilities.DefaultOptions);
                 Assert.NotNull(result);
-                Assert.Equal("complete", result.ResultType);
+                Assert.Null(result.ResultType);
                 Assert.True(result.IsError);
                 Assert.NotEmpty(result.Content);
                 var textContent = Assert.IsType<TextContentBlock>(result.Content[0]);
@@ -1098,7 +1123,7 @@ await Can_Handle_Requests(
             {
                 var result = JsonSerializer.Deserialize<CallToolResult>(response, McpJsonUtilities.DefaultOptions);
                 Assert.NotNull(result);
-                Assert.Equal("complete", result.ResultType);
+                Assert.Null(result.ResultType);
                 Assert.True(result.IsError, "Input validation errors should be returned as tool execution errors (IsError=true), not protocol errors");
                 Assert.NotEmpty(result.Content);
                 var textContent = Assert.IsType<TextContentBlock>(result.Content[0]);
@@ -1359,7 +1384,7 @@ await transport.SendClientMessageAsync(new JsonRpcNotification
         Assert.NotNull(response.Result);
         var initResult = JsonSerializer.Deserialize<InitializeResult>(response.Result, McpJsonUtilities.DefaultOptions);
         Assert.NotNull(initResult);
-        Assert.Equal("complete", initResult.ResultType);
+        Assert.Null(initResult.ResultType);
         Assert.NotNull(initResult.ServerInfo);
 
         await transport.DisposeAsync();