diff --git a/docs/list-of-diagnostics.md b/docs/list-of-diagnostics.md
index 386b90bc2..573d97fea 100644
--- a/docs/list-of-diagnostics.md
+++ b/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. |
\ No newline at end of file
+| `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)). |
\ No newline at end of file
diff --git a/src/Common/Experimentals.cs b/src/Common/Experimentals.cs
index 2ccde0796..ec2c7c550 100644
--- a/src/Common/Experimentals.cs
+++ b/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";
 }
diff --git a/src/ModelContextProtocol.Core/Protocol/ClientCapabilities.cs b/src/ModelContextProtocol.Core/Protocol/ClientCapabilities.cs
index 9841e3da0..77b2bef9f 100644
--- a/src/ModelContextProtocol.Core/Protocol/ClientCapabilities.cs
+++ b/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; }
 }
diff --git a/src/ModelContextProtocol.Core/Protocol/ServerCapabilities.cs b/src/ModelContextProtocol.Core/Protocol/ServerCapabilities.cs
index d4e55653e..d4e23a66f 100644
--- a/src/ModelContextProtocol.Core/Protocol/ServerCapabilities.cs
+++ b/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; }
 }
diff --git a/tests/ModelContextProtocol.Tests/Protocol/ClientCapabilitiesTests.cs b/tests/ModelContextProtocol.Tests/Protocol/ClientCapabilitiesTests.cs
index 188326a62..cacb7e84e 100644
--- a/tests/ModelContextProtocol.Tests/Protocol/ClientCapabilitiesTests.cs
+++ b/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);
     }
 }
diff --git a/tests/ModelContextProtocol.Tests/Protocol/ServerCapabilitiesTests.cs b/tests/ModelContextProtocol.Tests/Protocol/ServerCapabilitiesTests.cs
index ca698ef2b..a6f8265f1 100644
--- a/tests/ModelContextProtocol.Tests/Protocol/ServerCapabilitiesTests.cs
+++ b/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);
     }
 }