Merge branch 'main' into copilot/fix-comment-issue-1338

Author: stephentoub

docs/list-of-diagnostics.md

@@ -23,4 +23,4 @@ If you use experimental APIs, you will get one of the diagnostics shown below. T
 
 | Diagnostic ID | Description |
 | :------------ | :---------- |
-| `MCPEXP001` | MCP task-related APIs are experimental. Tasks provide a mechanism for asynchronous long-running operations that can be polled for status and results. See [MCP Tasks specification](https://modelcontextprotocol.io/specification/draft/basic/utilities/tasks) for details. |
+| `MCPEXP001` | MCP experimental APIs including Tasks and Extensions. Tasks provide a mechanism for asynchronous long-running operations that can be polled for status and results (see [MCP Tasks specification](https://modelcontextprotocol.io/specification/draft/basic/utilities/tasks)). Extensions provide a framework for extending the Model Context Protocol while maintaining interoperability (see [SEP-2133](https://github.com/modelcontextprotocol/modelcontextprotocol/pull/2133)). |

src/Common/Experimentals.cs

@@ -35,4 +35,25 @@ internal static class Experimentals
     /// URL for the experimental MCP Tasks feature.
     /// </summary>
     public const string Tasks_Url = "https://github.com/modelcontextprotocol/csharp-sdk/blob/main/docs/list-of-diagnostics.md#mcpexp001";
+
+    /// <summary>
+    /// Diagnostic ID for the experimental MCP Extensions feature.
+    /// </summary>
+    /// <remarks>
+    /// This uses the same diagnostic ID as <see cref="Tasks_DiagnosticId"/> because both
+    /// Tasks and Extensions are covered by the same MCPEXP001 diagnostic for experimental
+    /// MCP features. Having separate constants improves code clarity while maintaining a
+    /// single diagnostic suppression point.
+    /// </remarks>
+    public const string Extensions_DiagnosticId = "MCPEXP001";
+
+    /// <summary>
+    /// Message for the experimental MCP Extensions feature.
+    /// </summary>
+    public const string Extensions_Message = "The Extensions feature is part of a future MCP specification version that has not yet been ratified and is subject to change.";
+
+    /// <summary>
+    /// URL for the experimental MCP Extensions feature.
+    /// </summary>
+    public const string Extensions_Url = "https://github.com/modelcontextprotocol/csharp-sdk/blob/main/docs/list-of-diagnostics.md#mcpexp001";
 }

src/Directory.Build.props

@@ -5,8 +5,8 @@
     <PackageProjectUrl>https://modelcontextprotocol.github.io/csharp-sdk</PackageProjectUrl>
     <RepositoryUrl>https://github.com/modelcontextprotocol/csharp-sdk</RepositoryUrl>
     <RepositoryType>git</RepositoryType>
-    <VersionPrefix>1.0.0</VersionPrefix>
-    <VersionSuffix>rc.1</VersionSuffix>
+    <VersionPrefix>0.9.0</VersionPrefix>
+    <VersionSuffix>preview.2</VersionSuffix>
     <Authors>ModelContextProtocol</Authors>
     <Copyright>© Anthropic and Contributors.</Copyright>
     <PackageTags>ModelContextProtocol;mcp;ai;llm</PackageTags>

src/ModelContextProtocol.Core/Protocol/ClientCapabilities.cs

@@ -93,4 +93,31 @@ public McpTasksCapability? Tasks
     [JsonInclude]
     [JsonPropertyName("tasks")]
     internal McpTasksCapability? TasksCore { get; set; }
+
+    /// <summary>
+    /// Gets or sets optional MCP extensions that the client supports.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// Keys are extension identifiers in reverse domain notation with an extension name
+    /// (e.g., <c>"io.modelcontextprotocol/oauth-client-credentials"</c>), and values are
+    /// per-extension settings objects. An empty object indicates support with no additional settings.
+    /// </para>
+    /// <para>
+    /// Extensions provide a framework for extending the Model Context Protocol while maintaining
+    /// interoperability. Clients advertise extension support via this field during the initialization handshake.
+    /// </para>
+    /// </remarks>
+    [Experimental(Experimentals.Extensions_DiagnosticId, UrlFormat = Experimentals.Extensions_Url)]
+    [JsonIgnore]
+    public IDictionary<string, object>? Extensions
+    {
+        get => ExtensionsCore;
+        set => ExtensionsCore = value;
+    }
+
+    // See ExperimentalInternalPropertyTests.cs before modifying this property.
+    [JsonInclude]
+    [JsonPropertyName("extensions")]
+    internal IDictionary<string, object>? ExtensionsCore { get; set; }
 }

src/ModelContextProtocol.Core/Protocol/Root.cs

@@ -1,5 +1,5 @@
 using System.Diagnostics.CodeAnalysis;
-using System.Text.Json;
+using System.Text.Json.Nodes;
 using System.Text.Json.Serialization;
 
 namespace ModelContextProtocol.Protocol;
@@ -35,5 +35,5 @@ public sealed class Root
     /// This is reserved by the protocol for future use.
     /// </remarks>
     [JsonPropertyName("_meta")]
-    public JsonElement? Meta { get; set; }
+    public JsonObject? Meta { get; set; }
 }

src/ModelContextProtocol.Core/Protocol/ServerCapabilities.cs

@@ -92,4 +92,31 @@ public McpTasksCapability? Tasks
     [JsonInclude]
     [JsonPropertyName("tasks")]
     internal McpTasksCapability? TasksCore { get; set; }
+
+    /// <summary>
+    /// Gets or sets optional MCP extensions that the server supports.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// Keys are extension identifiers in reverse domain notation with an extension name
+    /// (e.g., <c>"io.modelcontextprotocol/apps"</c>), and values are per-extension settings
+    /// objects. An empty object indicates support with no additional settings.
+    /// </para>
+    /// <para>
+    /// Extensions provide a framework for extending the Model Context Protocol while maintaining
+    /// interoperability. Servers advertise extension support via this field during the initialization handshake.
+    /// </para>
+    /// </remarks>
+    [Experimental(Experimentals.Extensions_DiagnosticId, UrlFormat = Experimentals.Extensions_Url)]
+    [JsonIgnore]
+    public IDictionary<string, object>? Extensions
+    {
+        get => ExtensionsCore;
+        set => ExtensionsCore = value;
+    }
+
+    // See ExperimentalInternalPropertyTests.cs before modifying this property.
+    [JsonInclude]
+    [JsonPropertyName("extensions")]
+    internal IDictionary<string, object>? ExtensionsCore { get; set; }
 }

tests/ModelContextProtocol.AspNetCore.Tests/ClientConformanceTests.cs

@@ -132,8 +132,13 @@ public async Task RunConformanceTest(string scenario)
 
     /// <summary>
     /// Checks if the conformance test output indicates that all checks passed with only
-    /// warnings (no actual failures). The conformance runner exits with code 1 for warnings,
-    /// but warnings represent acceptable behavior (e.g., timing tolerances in CI environments).
+    /// warnings or known CI-timing failures. The conformance runner exits with code 1 for
+    /// warnings/failures, but some represent acceptable behavior in CI environments:
+    /// - Warnings (e.g., slightly late reconnects) are always acceptable.
+    /// - "Reconnected very late" failures are acceptable when the actual delay is within a
+    ///   reasonable bound, as CI machines may introduce network/scheduling latency that pushes
+    ///   the observed reconnect time past the conformance test's strict threshold even though
+    ///   the client correctly honored the retry field.
     /// </summary>
     private static bool HasOnlyWarnings(string output, string error)
     {
@@ -142,9 +147,39 @@ private static bool HasOnlyWarnings(string output, string error)
         // If there are 0 failures but warnings > 0, the test behavior is acceptable.
         var combined = output + error;
         var match = Regex.Match(combined, @"(?<failed>\d+) failed, (?<warnings>\d+) warnings");
-        return match.Success
-            && match.Groups["failed"].Value == "0"
+        if (!match.Success)
+        {
+            return false;
+        }
+
+        if (match.Groups["failed"].Value == "0"
             && int.TryParse(match.Groups["warnings"].Value, out var warnings)
-            && warnings > 0;
+            && warnings > 0)
+        {
+            return true;
+        }
+
+        // Also accept cases where all failures are "reconnected very late" timing failures.
+        // These occur in CI when OS/network overhead between the server closing the SSE stream
+        // and the client detecting it pushes the total reconnect time past the conformance
+        // test's VERY_LATE_MULTIPLIER threshold (2x the retry value), even though the client
+        // correctly waited the retry interval after detecting the stream close.
+        // We require the actual delay to be < 10x the expected retry value to avoid masking
+        // genuine bugs where the client ignores the retry field entirely.
+        if (int.TryParse(match.Groups["failed"].Value, out var failed) && failed > 0)
+        {
+            var lateReconnectMatches = Regex.Matches(combined, @"Client reconnected very late \((\d+)ms instead of (\d+)ms\)");
+            if (lateReconnectMatches.Count == failed
+                && lateReconnectMatches.Cast<Match>().All(m =>
+                    int.TryParse(m.Groups[1].Value, out var actual)
+                    && int.TryParse(m.Groups[2].Value, out var expected)
+                    && expected > 0
+                    && actual < expected * 10))
+            {
+                return true;
+            }
+        }
+
+        return false;
     }
 }

tests/ModelContextProtocol.Tests/Protocol/ClientCapabilitiesTests.cs

@@ -21,7 +21,11 @@ public static void ClientCapabilities_SerializationRoundTrip_PreservesAllPropert
                 Form = new FormElicitationCapability(),
                 Url = new UrlElicitationCapability()
             },
-            Tasks = new McpTasksCapability()
+            Tasks = new McpTasksCapability(),
+            Extensions = new Dictionary<string, object>
+            {
+                ["io.modelcontextprotocol/test"] = new object()
+            }
         };
 
         string json = JsonSerializer.Serialize(original, McpJsonUtilities.DefaultOptions);
@@ -37,6 +41,8 @@ public static void ClientCapabilities_SerializationRoundTrip_PreservesAllPropert
         Assert.NotNull(deserialized.Elicitation.Form);
         Assert.NotNull(deserialized.Elicitation.Url);
         Assert.NotNull(deserialized.Tasks);
+        Assert.NotNull(deserialized.Extensions);
+        Assert.True(deserialized.Extensions.ContainsKey("io.modelcontextprotocol/test"));
     }
 
     [Fact]
@@ -53,5 +59,42 @@ public static void ClientCapabilities_SerializationRoundTrip_WithMinimalProperti
         Assert.Null(deserialized.Sampling);
         Assert.Null(deserialized.Elicitation);
         Assert.Null(deserialized.Tasks);
+        Assert.Null(deserialized.Extensions);
+    }
+
+    [Fact]
+    public static void ClientCapabilities_Extensions_DeserializesFromJson()
+    {
+        string json = """
+            {
+                "extensions": {
+                    "io.modelcontextprotocol/oauth-client-credentials": {},
+                    "io.modelcontextprotocol/test-extension": {
+                        "setting1": "value1",
+                        "setting2": 42
+                    }
+                }
+            }
+            """;
+
+        var deserialized = JsonSerializer.Deserialize<ClientCapabilities>(json, McpJsonUtilities.DefaultOptions);
+
+        Assert.NotNull(deserialized);
+        Assert.NotNull(deserialized.Extensions);
+        Assert.Equal(2, deserialized.Extensions.Count);
+        Assert.True(deserialized.Extensions.ContainsKey("io.modelcontextprotocol/oauth-client-credentials"));
+        Assert.True(deserialized.Extensions.ContainsKey("io.modelcontextprotocol/test-extension"));
+    }
+
+    [Fact]
+    public static void ClientCapabilities_Extensions_EmptyObjectDeserializesAsEmptyDictionary()
+    {
+        string json = """{"extensions": {}}""";
+
+        var deserialized = JsonSerializer.Deserialize<ClientCapabilities>(json, McpJsonUtilities.DefaultOptions);
+
+        Assert.NotNull(deserialized);
+        Assert.NotNull(deserialized.Extensions);
+        Assert.Empty(deserialized.Extensions);
     }
 }

tests/ModelContextProtocol.Tests/Protocol/RootTests.cs

@@ -1,5 +1,6 @@
 using ModelContextProtocol.Protocol;
 using System.Text.Json;
+using System.Text.Json.Nodes;
 
 namespace ModelContextProtocol.Tests.Protocol;
 
@@ -12,7 +13,7 @@ public static void Root_SerializationRoundTrip_PreservesAllProperties()
         {
             Uri = "file:///home/user/project",
             Name = "My Project",
-            Meta = JsonDocument.Parse("""{"custom":"data"}""").RootElement.Clone()
+            Meta = new JsonObject { ["custom"] = "data" }
         };
 
         string json = JsonSerializer.Serialize(original, McpJsonUtilities.DefaultOptions);
@@ -22,7 +23,7 @@ public static void Root_SerializationRoundTrip_PreservesAllProperties()
         Assert.Equal("file:///home/user/project", deserialized.Uri);
         Assert.Equal("My Project", deserialized.Name);
         Assert.NotNull(deserialized.Meta);
-        Assert.Equal("data", deserialized.Meta.Value.GetProperty("custom").GetString());
+        Assert.Equal("data", (string?)deserialized.Meta["custom"]);
     }
 
     [Fact]

tests/ModelContextProtocol.Tests/Protocol/ServerCapabilitiesTests.cs

@@ -15,7 +15,11 @@ public static void ServerCapabilities_SerializationRoundTrip_PreservesAllPropert
             Resources = new ResourcesCapability { Subscribe = true, ListChanged = true },
             Tools = new ToolsCapability { ListChanged = false },
             Completions = new CompletionsCapability(),
-            Tasks = new McpTasksCapability()
+            Tasks = new McpTasksCapability(),
+            Extensions = new Dictionary<string, object>
+            {
+                ["io.modelcontextprotocol/apps"] = new object()
+            }
         };
 
         string json = JsonSerializer.Serialize(original, McpJsonUtilities.DefaultOptions);
@@ -32,6 +36,8 @@ public static void ServerCapabilities_SerializationRoundTrip_PreservesAllPropert
         Assert.False(deserialized.Tools.ListChanged);
         Assert.NotNull(deserialized.Completions);
         Assert.NotNull(deserialized.Tasks);
+        Assert.NotNull(deserialized.Extensions);
+        Assert.True(deserialized.Extensions.ContainsKey("io.modelcontextprotocol/apps"));
     }
 
     [Fact]
@@ -50,5 +56,42 @@ public static void ServerCapabilities_SerializationRoundTrip_WithMinimalProperti
         Assert.Null(deserialized.Tools);
         Assert.Null(deserialized.Completions);
         Assert.Null(deserialized.Tasks);
+        Assert.Null(deserialized.Extensions);
+    }
+
+    [Fact]
+    public static void ServerCapabilities_Extensions_DeserializesFromJson()
+    {
+        string json = """
+            {
+                "extensions": {
+                    "io.modelcontextprotocol/apps": {},
+                    "io.modelcontextprotocol/custom": {
+                        "option": 42,
+                        "enabled": true
+                    }
+                }
+            }
+            """;
+
+        var deserialized = JsonSerializer.Deserialize<ServerCapabilities>(json, McpJsonUtilities.DefaultOptions);
+
+        Assert.NotNull(deserialized);
+        Assert.NotNull(deserialized.Extensions);
+        Assert.Equal(2, deserialized.Extensions.Count);
+        Assert.True(deserialized.Extensions.ContainsKey("io.modelcontextprotocol/apps"));
+        Assert.True(deserialized.Extensions.ContainsKey("io.modelcontextprotocol/custom"));
+    }
+
+    [Fact]
+    public static void ServerCapabilities_Extensions_EmptyObjectDeserializesAsEmptyDictionary()
+    {
+        string json = """{"extensions": {}}""";
+
+        var deserialized = JsonSerializer.Deserialize<ServerCapabilities>(json, McpJsonUtilities.DefaultOptions);
+
+        Assert.NotNull(deserialized);
+        Assert.NotNull(deserialized.Extensions);
+        Assert.Empty(deserialized.Extensions);
     }
 }