diff --git a/.changeset/add-extensions-capability.md b/.changeset/add-extensions-capability.md
new file mode 100644
index 000000000..c5d4d200f
--- /dev/null
+++ b/.changeset/add-extensions-capability.md
@@ -0,0 +1,5 @@
+---
+'@modelcontextprotocol/sdk': minor
+---
+
+Add `extensions` field to `ClientCapabilities` and `ServerCapabilities` to allow servers and clients to advertise extension support per SEP-2133
diff --git a/src/spec.types.ts b/src/spec.types.ts
index 07a1cceff..2640152a2 100644
--- a/src/spec.types.ts
+++ b/src/spec.types.ts
@@ -383,6 +383,10 @@ export interface ClientCapabilities {
       };
     };
   };
+  /**
+   * Extensions that the client supports. Keys are extension identifiers (vendor-prefix/extension-name).
+   */
+  extensions?: { [key: string]: object };
 }
 
 /**
@@ -461,6 +465,10 @@ export interface ServerCapabilities {
       };
     };
   };
+  /**
+   * Extensions that the server supports. Keys are extension identifiers (vendor-prefix/extension-name).
+   */
+  extensions?: { [key: string]: object };
 }
 
 /**
diff --git a/src/types.ts b/src/types.ts
index 87899a38f..835eac89f 100644
--- a/src/types.ts
+++ b/src/types.ts
@@ -511,7 +511,11 @@ export const ClientCapabilitiesSchema = z.object({
     /**
      * Present if the client supports task creation.
      */
-    tasks: ClientTasksCapabilitySchema.optional()
+    tasks: ClientTasksCapabilitySchema.optional(),
+    /**
+     * Extensions that the client supports. Keys are extension identifiers (vendor-prefix/extension-name).
+     */
+    extensions: z.record(z.string(), AssertObjectSchema).optional()
 });
 
 export const InitializeRequestParamsSchema = BaseRequestParamsSchema.extend({
@@ -589,7 +593,11 @@ export const ServerCapabilitiesSchema = z.object({
     /**
      * Present if the server supports task creation.
      */
-    tasks: ServerTasksCapabilitySchema.optional()
+    tasks: ServerTasksCapabilitySchema.optional(),
+    /**
+     * Extensions that the server supports. Keys are extension identifiers (vendor-prefix/extension-name).
+     */
+    extensions: z.record(z.string(), AssertObjectSchema).optional()
 });
 
 /**
diff --git a/test/server/mcp.test.ts b/test/server/mcp.test.ts
index 961af234a..575d6a300 100644
--- a/test/server/mcp.test.ts
+++ b/test/server/mcp.test.ts
@@ -198,6 +198,60 @@ describe.each(zodTestMatrix)('$zodVersionLabel', (entry: ZodMatrixEntry) => {
                 message: 'Completed step 3 of 3'
             });
         });
+
+        /***
+         * Test: Extensions capability registration
+         */
+        test('should register and advertise server extensions capability', async () => {
+            const mcpServer = new McpServer({
+                name: 'test server',
+                version: '1.0'
+            });
+            const client = new Client({
+                name: 'test client',
+                version: '1.0'
+            });
+
+            mcpServer.server.registerCapabilities({
+                extensions: {
+                    'io.modelcontextprotocol/test-extension': { listChanged: true }
+                }
+            });
+
+            const [clientTransport, serverTransport] = InMemoryTransport.createLinkedPair();
+            await Promise.all([client.connect(clientTransport), mcpServer.connect(serverTransport)]);
+
+            const capabilities = client.getServerCapabilities();
+            expect(capabilities?.extensions).toBeDefined();
+            expect(capabilities?.extensions?.['io.modelcontextprotocol/test-extension']).toEqual({ listChanged: true });
+        });
+
+        test('should advertise client extensions capability to server', async () => {
+            const mcpServer = new McpServer({
+                name: 'test server',
+                version: '1.0'
+            });
+            const client = new Client(
+                {
+                    name: 'test client',
+                    version: '1.0'
+                },
+                {
+                    capabilities: {
+                        extensions: {
+                            'io.modelcontextprotocol/test-extension': { streaming: true }
+                        }
+                    }
+                }
+            );
+
+            const [clientTransport, serverTransport] = InMemoryTransport.createLinkedPair();
+            await Promise.all([client.connect(clientTransport), mcpServer.connect(serverTransport)]);
+
+            const capabilities = mcpServer.server.getClientCapabilities();
+            expect(capabilities?.extensions).toBeDefined();
+            expect(capabilities?.extensions?.['io.modelcontextprotocol/test-extension']).toEqual({ streaming: true });
+        });
     });
 
     describe('ResourceTemplate', () => {