modelcontextprotocol · pja-ant · Jan 26, 2026 · Jan 22, 2026 · Jan 22, 2026 · Jan 23, 2026
@@ -14,13 +14,14 @@ Specification Enhancement Proposals (SEPs) are the primary mechanism for proposi
 
 ## Summary
 
-- **Draft**: 1
+- **Draft**: 2
 - **Final**: 21
 
 ## All SEPs
 
 | SEP                                                                                 | Title                                                                         | Status                             | Type            | Created    |
 | ----------------------------------------------------------------------------------- | ----------------------------------------------------------------------------- | ---------------------------------- | --------------- | ---------- |
+| [SEP-2133](/community/seps/2133-extensions)                                         | Extensions                                                                    | <Badge color="gray">Draft</Badge>  | Standards Track | 2025-01-21 |
 | [SEP-2085](/community/seps/2085-governance-succession-and-amendment)                | Governance Succession and Amendment Procedures                                | <Badge color="gray">Draft</Badge>  | Process         | 2025-12-05 |
 | [SEP-1850](/community/seps/1850-pr-based-sep-workflow)                              | PR-Based SEP Workflow                                                         | <Badge color="green">Final</Badge> | Process         | 2025-11-20 |
 | [SEP-1730](/community/seps/1730-sdks-tiering-system)                                | SDKs Tiering System                                                           | <Badge color="green">Final</Badge> | Standards Track | 2025-10-29 |

@@ -376,11 +376,18 @@
               {
                 "group": "Draft",
                 "pages": [
-                  "community/seps/2085-governance-succession-and-amendment"
+                  "community/seps/2085-governance-succession-and-amendment",
+                  "community/seps/2133-extensions"
                 ]
               }
             ]
           },
+          {
+            "group": "Extensions",
+            "pages": [
+              "extensions"
+            ]
+          },
           {
             "group": "Roadmap",
             "pages": [

@@ -0,0 +1,68 @@
+---
+title: Extensions
+description: Optional extensions to the Model Context Protocol
+---
+
+# MCP Extensions
+
+MCP extensions are optional additions to the specification that define capabilities beyond the core protocol. Extensions enable functionality that may be modular (e.g., distinct features like authentication), specialized (e.g., industry-specific logic), or experimental (e.g., features being incubated for potential core inclusion).
+
+Extensions are identified using a unique _extension identifier_ with the format: `{vendor-prefix}/{extension-name}`, e.g. `io.modelcontextprotocol/oauth-client-credentials`. Official extensions use the `io.modelcontextprotocol` vendor prefix.
+
+## Official Extension Repositories
+
+Official extensions live inside the [MCP GitHub org](https://github.com/modelcontextprotocol/) in repositories with the `ext-` prefix.
+
+### ext-auth
+
+**Repository:** [github.com/modelcontextprotocol/ext-auth](https://github.com/modelcontextprotocol/ext-auth)
+
+Extensions for supplementary authorization mechanisms beyond the core specification.
+
+| Extension                        | Description                                                                | Specification                                                                                                               |
+| -------------------------------- | -------------------------------------------------------------------------- | --------------------------------------------------------------------------------------------------------------------------- |
+| OAuth Client Credentials         | OAuth 2.0 client credentials flow for machine-to-machine authentication    | [Link](https://github.com/modelcontextprotocol/ext-auth/blob/main/specification/draft/oauth-client-credentials.mdx)         |
+| Enterprise-Managed Authorization | Framework for enterprise environments requiring centralized access control | [Link](https://github.com/modelcontextprotocol/ext-auth/blob/main/specification/draft/enterprise-managed-authorization.mdx) |
+
+### ext-apps
+
+**Repository:** [github.com/modelcontextprotocol/ext-apps](https://github.com/modelcontextprotocol/ext-apps)
+
+Extensions for interactive UI elements in conversational MCP clients.
+
+| Extension | Description                                                                                                      | Specification                                                                                   |
+| --------- | ---------------------------------------------------------------------------------------------------------------- | ----------------------------------------------------------------------------------------------- |
+| MCP Apps  | Allows MCP Servers to display interactive UI elements (charts, forms, video players) inline within conversations | [Link](https://github.com/modelcontextprotocol/ext-apps/blob/main/specification/draft/apps.mdx) |
+
+## Creating Extensions
+
+The lifecycle for official extensions is similar to a SEP, but delegated to extension repository maintainers:
+
+1. **Propose**: Author creates a SEP in the main MCP repository using the [standard SEP guidelines](/community/sep-guidelines) with type **Extensions Track**.
+2. **Review**: Extension SEPs are reviewed by the relevant extension repository maintainers.
+3. **Implement**: Extension SEPs **MUST** have at least one reference implementation in an official SDK before being accepted.
+4. **Publish**: Once approved, the author produces a PR that introduces the extension to the extension repository.
+5. **Adopt**: Approved extensions **MAY** be implemented in additional clients, servers, and SDKs.
+
+### Requirements
+
+- Extension specifications **MUST** use RFC 2119 language (MUST, SHOULD, MAY)
+- Extensions **SHOULD** have an associated working group or interest group
+
+### SDK Implementation
+
+SDKs **MAY** implement extensions. Where implemented:
+
+- Extensions **MUST** be disabled by default and require explicit opt-in
+- SDK documentation **SHOULD** list supported extensions
+- SDK maintainers have full autonomy over which extensions they support
+- Extension support is not required for protocol conformance
+
+### Evolution
+
+Extensions evolve independently of the core protocol. Updates to extensions are managed by the extension repository maintainers and do not require core maintainer review.
+
+Extensions **MUST** consider backwards compatibility in their design:
+
+- Extensions **SHOULD** maintain backwards compatibility through capability flags or versioning within the extension settings object, rather than creating a new extension identifier
+- When backwards-incompatible changes are unavoidable, a new extension identifier **MUST** be used (e.g., `io.modelcontextprotocol/my-extension-v2`)
@@ -196,13 +196,15 @@ Key capabilities include:
 | Client   | `sampling`     | Support for LLM [sampling](/specification/draft/client/sampling) requests                |
 | Client   | `elicitation`  | Support for server [elicitation](/specification/draft/client/elicitation) requests       |
 | Client   | `tasks`        | Support for [task-augmented](/specification/draft/basic/utilities/tasks) client requests |
+| Client   | `extensions`   | Support for optional [extensions](/extensions) beyond the core protocol                  |
 | Client   | `experimental` | Describes support for non-standard experimental features                                 |
 | Server   | `prompts`      | Offers [prompt templates](/specification/draft/server/prompts)                           |
 | Server   | `resources`    | Provides readable [resources](/specification/draft/server/resources)                     |
 | Server   | `tools`        | Exposes callable [tools](/specification/draft/server/tools)                              |
 | Server   | `logging`      | Emits structured [log messages](/specification/draft/server/utilities/logging)           |
 | Server   | `completions`  | Supports argument [autocompletion](/specification/draft/server/utilities/completion)     |
 | Server   | `tasks`        | Support for [task-augmented](/specification/draft/basic/utilities/tasks) server requests |
+| Server   | `extensions`   | Support for optional [extensions](/extensions) beyond the core protocol                  |
 | Server   | `experimental` | Describes support for non-standard experimental features                                 |
 
 Capability objects can describe sub-capabilities like:
@@ -211,6 +213,42 @@ Capability objects can describe sub-capabilities like:
   tools)
 - `subscribe`: Support for subscribing to individual items' changes (resources only)
 
+#### Extension Negotiation
+
+Clients and servers can also negotiate support for optional [extensions](/extensions) beyond the core protocol. Extensions are advertised in the `extensions` field of capabilities, which is a map of extension identifiers to per-extension settings objects.
+
+Example client capabilities with extensions:
+
+```json
+{
+  "capabilities": {
+    "roots": {},
+    "extensions": {
+      "io.modelcontextprotocol/apps": {
+        "mimeTypes": ["text/html;profile=mcp-app"]
+      }
+    }
+  }
+}
+```
+
+Example server capabilities with extensions:
+
+```json
+{
+  "capabilities": {
+    "tools": {},
+    "extensions": {
+      "io.modelcontextprotocol/apps": {}
+    }
+  }
+}
+```
+
+Each extension specifies the schema of its settings object; an empty object indicates support with no additional settings.
+
+If one party supports an extension but the other does not, the supporting party **MUST** either revert to core protocol behavior or reject the request with an appropriate error. Extensions **SHOULD** document their expected fallback behavior.
+
 ### Operation
 
 During the operation phase, the client and server exchange messages according to the

@@ -13,7 +13,7 @@ N/A
 
 ## Minor changes
 
-N/A
+1. Add `extensions` field to `ClientCapabilities` and `ServerCapabilities` to support optional [extensions](/extensions) beyond the core protocol.
 
 ## Other schema changes
 

@@ -0,0 +1,7 @@
+{
+  "extensions": {
+    "io.modelcontextprotocol/apps": {
+      "mimeTypes": ["text/html;profile=mcp-app"]
+    }
+  }
+}
@@ -0,0 +1,5 @@
+{
+  "extensions": {
+    "io.modelcontextprotocol/apps": {}
+  }
+}
@@ -548,6 +548,15 @@ export interface ClientCapabilities {
       };
     };
   };
+  /**
+   * Optional MCP extensions that the client supports. Keys are extension identifiers
+   * (e.g., "io.modelcontextprotocol/oauth-client-credentials"), and values are
+   * per-extension settings objects. An empty object indicates support with no settings.
+   *
+   * @example Extensions — UI extension with MIME type support
+   * {@includeCode ./examples/ClientCapabilities/extensions-ui-mime-types.json}
+   */
+  extensions?: { [key: string]: object };
 }
 
 /**
@@ -656,6 +665,15 @@ export interface ServerCapabilities {
       };
     };
   };
+  /**
+   * Optional MCP extensions that the server supports. Keys are extension identifiers
+   * (e.g., "io.modelcontextprotocol/apps"), and values are per-extension settings
+   * objects. An empty object indicates support with no settings.
+   *
+   * @example Extensions — UI extension support
+   * {@includeCode ./examples/ServerCapabilities/extensions-ui.json}
+   */
+  extensions?: { [key: string]: object };
 }
 
 /**
-Original file line number
+Diff line change
@@ Expand Up / @@ -13,7 +13,7 @@ N/A @@
     ## Minor changes
-    N/A
+. Add `extensions` field to `ClientCapabilities` and `ServerCapabilities` to support optional [extensions](/extensions) beyond the core protocol.
     ## Other schema changes
@@ Expand Down @@