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 cb85ef5e3..132dafc2e 100644
--- a/src/ModelContextProtocol.Core/Protocol/ClientCapabilities.cs
+++ b/src/ModelContextProtocol.Core/Protocol/ClientCapabilities.cs
@@ -1,4 +1,5 @@
 using System.ComponentModel;
+using System.Diagnostics.CodeAnalysis;
 using System.Text.Json.Serialization;
 using ModelContextProtocol.Client;
 using ModelContextProtocol.Server;
@@ -82,4 +83,23 @@ public sealed class ClientCapabilities
     /// </remarks>
     [JsonPropertyName("tasks")]
     public McpTasksCapability? Tasks { 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. Both clients and servers advertise extension support via this field during
+    /// the initialization handshake.
+    /// </para>
+    /// </remarks>
+    [JsonPropertyName("extensions")]
+    [Experimental(Experimentals.Extensions_DiagnosticId, UrlFormat = Experimentals.Extensions_Url)] // SEP-2133: https://github.com/modelcontextprotocol/modelcontextprotocol/pull/2133
+    public IDictionary<string, object>? Extensions { get; set; }
 }
diff --git a/src/ModelContextProtocol.Core/Protocol/ServerCapabilities.cs b/src/ModelContextProtocol.Core/Protocol/ServerCapabilities.cs
index 499819662..aec553609 100644
--- a/src/ModelContextProtocol.Core/Protocol/ServerCapabilities.cs
+++ b/src/ModelContextProtocol.Core/Protocol/ServerCapabilities.cs
@@ -1,4 +1,5 @@
 using System.ComponentModel;
+using System.Diagnostics.CodeAnalysis;
 using System.Text.Json.Serialization;
 using ModelContextProtocol.Server;
 
@@ -81,4 +82,23 @@ public sealed class ServerCapabilities
     /// </remarks>
     [JsonPropertyName("tasks")]
     public McpTasksCapability? Tasks { 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. Both clients and servers advertise extension support via this field during
+    /// the initialization handshake.
+    /// </para>
+    /// </remarks>
+    [JsonPropertyName("extensions")]
+    [Experimental(Experimentals.Extensions_DiagnosticId, UrlFormat = Experimentals.Extensions_Url)] // SEP-2133: https://github.com/modelcontextprotocol/modelcontextprotocol/pull/2133
+    public IDictionary<string, object>? Extensions { get; set; }
 }
diff --git a/tests/ModelContextProtocol.Tests/Protocol/ClientCapabilitiesTests.cs b/tests/ModelContextProtocol.Tests/Protocol/ClientCapabilitiesTests.cs
new file mode 100644
index 000000000..b7b2766a7
--- /dev/null
+++ b/tests/ModelContextProtocol.Tests/Protocol/ClientCapabilitiesTests.cs
@@ -0,0 +1,138 @@
+using ModelContextProtocol.Protocol;
+using System.Text.Json;
+
+namespace ModelContextProtocol.Tests.Protocol;
+
+public static class ClientCapabilitiesTests
+{
+    [Fact]
+    public static void ExtensionsProperty_SerializationRoundTrip()
+    {
+        // Arrange - Use raw JSON instead of objects for source generation compatibility
+        string json = """
+            {
+                "extensions": {
+                    "io.modelcontextprotocol/oauth-client-credentials": {},
+                    "io.modelcontextprotocol/test-extension": {
+                        "setting1": "value1",
+                        "setting2": 42
+                    }
+                }
+            }
+            """;
+
+        // Act - Deserialize from JSON
+        var deserialized = JsonSerializer.Deserialize<ClientCapabilities>(json, McpJsonUtilities.DefaultOptions);
+
+        // Assert
+        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"));
+
+        // Act - Serialize back to JSON
+        string roundtrippedJson = JsonSerializer.Serialize(deserialized, McpJsonUtilities.DefaultOptions);
+
+        // Assert - Deserialize again to verify
+        var deserialized2 = JsonSerializer.Deserialize<ClientCapabilities>(roundtrippedJson, McpJsonUtilities.DefaultOptions);
+        Assert.NotNull(deserialized2);
+        Assert.NotNull(deserialized2.Extensions);
+        Assert.Equal(2, deserialized2.Extensions.Count);
+    }
+
+    [Fact]
+    public static void ExtensionsProperty_DeserializesCorrectly()
+    {
+        // Arrange
+        string json = """
+            {
+                "extensions": {
+                    "io.modelcontextprotocol/test": {}
+                }
+            }
+            """;
+
+        // Act
+        var capabilities = JsonSerializer.Deserialize<ClientCapabilities>(json, McpJsonUtilities.DefaultOptions);
+
+        // Assert
+        Assert.NotNull(capabilities);
+        Assert.NotNull(capabilities.Extensions);
+        Assert.Single(capabilities.Extensions);
+        Assert.True(capabilities.Extensions.ContainsKey("io.modelcontextprotocol/test"));
+    }
+
+    [Fact]
+    public static void WithoutExtensions_DeserializesWithNullExtensions()
+    {
+        // Arrange
+        string json = "{}";
+
+        // Act
+        var capabilities = JsonSerializer.Deserialize<ClientCapabilities>(json, McpJsonUtilities.DefaultOptions);
+
+        // Assert
+        Assert.NotNull(capabilities);
+        Assert.Null(capabilities.Extensions);
+    }
+
+    [Fact]
+    public static void WithEmptyExtensions_DeserializesAsEmptyDictionary()
+    {
+        // Arrange
+        string json = """
+            {
+                "extensions": {}
+            }
+            """;
+
+        // Act
+        var capabilities = JsonSerializer.Deserialize<ClientCapabilities>(json, McpJsonUtilities.DefaultOptions);
+
+        // Assert
+        Assert.NotNull(capabilities);
+        Assert.NotNull(capabilities.Extensions);
+        Assert.Empty(capabilities.Extensions);
+    }
+
+    [Fact]
+    public static void ExtensionsWithComplexValues_RoundTrips()
+    {
+        // Arrange
+        string json = """
+            {
+                "extensions": {
+                    "io.modelcontextprotocol/complex": {
+                        "stringValue": "test",
+                        "numberValue": 123,
+                        "boolValue": true,
+                        "arrayValue": [1, 2, 3]
+                    }
+                }
+            }
+            """;
+
+        // Act - Deserialize from JSON
+        var deserialized = JsonSerializer.Deserialize<ClientCapabilities>(json, McpJsonUtilities.DefaultOptions);
+
+        // Assert
+        Assert.NotNull(deserialized);
+        Assert.NotNull(deserialized.Extensions);
+        Assert.Single(deserialized.Extensions);
+        Assert.True(deserialized.Extensions.ContainsKey("io.modelcontextprotocol/complex"));
+
+        // Verify the complex value can be accessed as JsonElement
+        var complexValue = deserialized.Extensions["io.modelcontextprotocol/complex"];
+        Assert.NotNull(complexValue);
+
+        // Act - Serialize back to JSON
+        string roundtrippedJson = JsonSerializer.Serialize(deserialized, McpJsonUtilities.DefaultOptions);
+
+        // Assert - Verify it can deserialize again
+        var deserialized2 = JsonSerializer.Deserialize<ClientCapabilities>(roundtrippedJson, McpJsonUtilities.DefaultOptions);
+        Assert.NotNull(deserialized2);
+        Assert.NotNull(deserialized2.Extensions);
+        Assert.Single(deserialized2.Extensions);
+    }
+}
diff --git a/tests/ModelContextProtocol.Tests/Protocol/ServerCapabilitiesTests.cs b/tests/ModelContextProtocol.Tests/Protocol/ServerCapabilitiesTests.cs
new file mode 100644
index 000000000..30f1e7bf7
--- /dev/null
+++ b/tests/ModelContextProtocol.Tests/Protocol/ServerCapabilitiesTests.cs
@@ -0,0 +1,138 @@
+using ModelContextProtocol.Protocol;
+using System.Text.Json;
+
+namespace ModelContextProtocol.Tests.Protocol;
+
+public static class ServerCapabilitiesTests
+{
+    [Fact]
+    public static void ExtensionsProperty_SerializationRoundTrip()
+    {
+        // Arrange - Use raw JSON instead of objects for source generation compatibility
+        string json = """
+            {
+                "extensions": {
+                    "io.modelcontextprotocol/apps": {},
+                    "io.modelcontextprotocol/custom": {
+                        "option": 42,
+                        "enabled": true
+                    }
+                }
+            }
+            """;
+
+        // Act - Deserialize from JSON
+        var deserialized = JsonSerializer.Deserialize<ServerCapabilities>(json, McpJsonUtilities.DefaultOptions);
+
+        // Assert
+        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"));
+
+        // Act - Serialize back to JSON
+        string roundtrippedJson = JsonSerializer.Serialize(deserialized, McpJsonUtilities.DefaultOptions);
+
+        // Assert - Deserialize again to verify
+        var deserialized2 = JsonSerializer.Deserialize<ServerCapabilities>(roundtrippedJson, McpJsonUtilities.DefaultOptions);
+        Assert.NotNull(deserialized2);
+        Assert.NotNull(deserialized2.Extensions);
+        Assert.Equal(2, deserialized2.Extensions.Count);
+    }
+
+    [Fact]
+    public static void ExtensionsProperty_DeserializesCorrectly()
+    {
+        // Arrange
+        string json = """
+            {
+                "extensions": {
+                    "io.modelcontextprotocol/test": {}
+                }
+            }
+            """;
+
+        // Act
+        var capabilities = JsonSerializer.Deserialize<ServerCapabilities>(json, McpJsonUtilities.DefaultOptions);
+
+        // Assert
+        Assert.NotNull(capabilities);
+        Assert.NotNull(capabilities.Extensions);
+        Assert.Single(capabilities.Extensions);
+        Assert.True(capabilities.Extensions.ContainsKey("io.modelcontextprotocol/test"));
+    }
+
+    [Fact]
+    public static void WithoutExtensions_DeserializesWithNullExtensions()
+    {
+        // Arrange
+        string json = "{}";
+
+        // Act
+        var capabilities = JsonSerializer.Deserialize<ServerCapabilities>(json, McpJsonUtilities.DefaultOptions);
+
+        // Assert
+        Assert.NotNull(capabilities);
+        Assert.Null(capabilities.Extensions);
+    }
+
+    [Fact]
+    public static void WithEmptyExtensions_DeserializesAsEmptyDictionary()
+    {
+        // Arrange
+        string json = """
+            {
+                "extensions": {}
+            }
+            """;
+
+        // Act
+        var capabilities = JsonSerializer.Deserialize<ServerCapabilities>(json, McpJsonUtilities.DefaultOptions);
+
+        // Assert
+        Assert.NotNull(capabilities);
+        Assert.NotNull(capabilities.Extensions);
+        Assert.Empty(capabilities.Extensions);
+    }
+
+    [Fact]
+    public static void ExtensionsWithComplexValues_RoundTrips()
+    {
+        // Arrange
+        string json = """
+            {
+                "extensions": {
+                    "io.modelcontextprotocol/complex": {
+                        "stringValue": "test",
+                        "numberValue": 456,
+                        "boolValue": false,
+                        "arrayValue": ["a", "b", "c"]
+                    }
+                }
+            }
+            """;
+
+        // Act - Deserialize from JSON
+        var deserialized = JsonSerializer.Deserialize<ServerCapabilities>(json, McpJsonUtilities.DefaultOptions);
+
+        // Assert
+        Assert.NotNull(deserialized);
+        Assert.NotNull(deserialized.Extensions);
+        Assert.Single(deserialized.Extensions);
+        Assert.True(deserialized.Extensions.ContainsKey("io.modelcontextprotocol/complex"));
+
+        // Verify the complex value can be accessed as JsonElement
+        var complexValue = deserialized.Extensions["io.modelcontextprotocol/complex"];
+        Assert.NotNull(complexValue);
+
+        // Act - Serialize back to JSON
+        string roundtrippedJson = JsonSerializer.Serialize(deserialized, McpJsonUtilities.DefaultOptions);
+
+        // Assert - Verify it can deserialize again
+        var deserialized2 = JsonSerializer.Deserialize<ServerCapabilities>(roundtrippedJson, McpJsonUtilities.DefaultOptions);
+        Assert.NotNull(deserialized2);
+        Assert.NotNull(deserialized2.Extensions);
+        Assert.Single(deserialized2.Extensions);
+    }
+}