> ## Documentation Index
> Fetch the complete documentation index at: https://docs.skybridge.tech/llms.txt
> Use this file to discover all available pages before exploring further.

# Apps SDK and MCP Apps

> How ChatGPT and other hosts run your views, and what Skybridge unifies

MCP Apps is the open contract for rendering interactive views inside an AI client. ChatGPT also exposes the Apps SDK: an optional layer of `window.openai` APIs for its own capabilities, on top of that same contract. Skybridge targets the MCP Apps baseline and reaches the Apps SDK only where you opt into a ChatGPT feature, so one server and one set of views run on both.

## MCP Apps

MCP Apps extends the [Model Context Protocol](https://modelcontextprotocol.io) with a UI contract: a tool result can carry a view, and the view talks to the host over a JSON-RPC `ui/*` bridge to read the tool's data, call other tools, sync state, and request layout. It is the portable baseline, so a view built against it runs on any host that implements the spec. The full contract is in the [MCP Apps specification](https://github.com/modelcontextprotocol/ext-apps/blob/main/specification/2026-01-26/apps.mdx).

## Apps SDK

ChatGPT implements MCP Apps and layers the Apps SDK on top: `window.openai`, a set of APIs for ChatGPT-only capabilities such as [file params](/api-reference/file-ref) and the [open-in-app URL](/api-reference/use-set-open-in-app-url). OpenAI's guidance is to use the MCP Apps `ui/*` bridge by default and reach for `window.openai` only for those extras. The [Apps SDK reference](https://developers.openai.com/apps-sdk/reference) documents the surface, and [MCP Apps in ChatGPT](https://developers.openai.com/apps-sdk/mcp-apps-in-chatgpt/) covers how the two fit together.

## In Skybridge

You write one MCP server and one set of views. Skybridge detects the runtime when the view loads and routes each call to the right layer: the MCP Apps bridge everywhere, `window.openai` only for ChatGPT features. Where a capability exists on one host and not the other, it surfaces as ChatGPT versus Claude in the [compatibility overview](/api-reference/overview). When you need a raw value the cross-host hooks don't expose, [`useAppsSdkContext`](/api-reference/use-apps-sdk-context) and [`useMcpAppContext`](/api-reference/use-mcp-app-context) are the escape hatches to each layer.

### Feature Mapping

Each hook, and the host API it routes to on either runtime.

| Skybridge                                                             | Apps SDK (`window.openai`)                        | MCP Apps (`ext-apps`)                                         |
| --------------------------------------------------------------------- | ------------------------------------------------- | ------------------------------------------------------------- |
| [`useToolInfo`](/api-reference/use-tool-info)                         | `toolInput`, `toolOutput`, `toolResponseMetadata` | `ui/notifications/tool-input`, `ui/notifications/tool-result` |
| [`useViewState`](/api-reference/use-view-state)                       | `viewState`, `setViewState()`                     | `ui/update-model-context`                                     |
| [`useCallTool`](/api-reference/use-call-tool)                         | `callTool()`                                      | `tools/call`                                                  |
| [`useSendFollowUpMessage`](/api-reference/use-send-follow-up-message) | `sendFollowUpMessage()`                           | `ui/message`                                                  |
| [`useOpenExternal`](/api-reference/use-open-external)                 | `openExternal()`                                  | `ui/open-link`                                                |
| [`useDisplayMode`](/api-reference/use-display-mode)                   | `displayMode`, `requestDisplayMode()`             | `ui/request-display-mode`                                     |
| [`useLayout`](/api-reference/use-layout)                              | `theme`, `maxHeight`, `safeArea`                  | `ui/notifications/host-context-changed`                       |
| [`useUser`](/api-reference/use-user)                                  | `locale`, `userAgent`                             | `ui/notifications/host-context-changed`                       |
| [`useRequestModal`](/api-reference/use-request-modal)                 | `requestModal()`                                  | Polyfilled in-iframe                                          |
| [`useRequestClose`](/api-reference/use-request-close)                 | `requestClose()`                                  | `ui/notifications/request-teardown`                           |
| [`useRequestSize`](/api-reference/use-request-size)                   | Not supported                                     | `ui/notifications/size-changed`                               |
| [`useFiles`](/api-reference/use-files)                                | `uploadFile()`, `getFileDownloadUrl()`            | Not supported                                                 |
| [`useSetOpenInAppUrl`](/api-reference/use-set-open-in-app-url)        | `setOpenInAppUrl()`                               | Not supported                                                 |

<CardGroup cols={3}>
  <Card title="Compatibility" icon="house" href="/api-reference/overview">
    Which hosts each API runs on
  </Card>

  <Card title="useAppsSdkContext" icon="panel-top" href="/api-reference/use-apps-sdk-context">
    Read a raw ChatGPT (Apps SDK) value
  </Card>

  <Card title="useMcpAppContext" icon="smartphone" href="/api-reference/use-mcp-app-context">
    Read a raw MCP Apps value
  </Card>
</CardGroup>