Porting from ChatGPT or Claude

The UI layer of an Allternit Plugin is the open MCP Apps standard. An app built to the standard for ChatGPT or Claude runs in Allternit without changes, and an Allternit app that uses only the standard runs there.

What works unchanged

Your MCP server. Tools, schemas, annotations, instructions, and streamable HTTP transport. See Build: MCP server.
_meta.ui.resourceUri on tools, and ui:// resources with the MIME type text/html;profile=mcp-app.
_meta.ui.csp, permissions, and prefersBorder on the resource.
Tool visibility with _meta.ui.visibility.
The View bridge. ui/initialize, ui/notifications/tool-input, tool-result, host-context-changed, size-changed, ui/open-link, ui/message, ui/update-model-context, ui/request-display-mode, and tools/call and resources/* from the View. See Build: MCP App UI.
Views built with the App class from @modelcontextprotocol/ext-apps.
Theming through hostContext.styles.variables.
OAuth 2.1 with PKCE and client ID metadata documents. See Authentication.

If your View uses `window.openai`

window.openai is specific to ChatGPT. Allternit provides a compatibility shim for it: when your View is served as text/html+skybridge, or your tool or resource _meta carries any openai/* key (or allternit/openaiCompat: true), Allternit injects window.openai and maps it onto the standard bridge, so the View runs unchanged. Views that use the standard bridge or the App class get no shim and need none. The table shows what each member maps to. Moving to the standard method is still the better long-term choice, because it runs in every MCP Apps host.

`window.openai`	Standard / Allternit replacement
`toolInput`	`ui/notifications/tool-input` (`arguments`).
`toolOutput`	`ui/notifications/tool-result`. Read `structuredContent` and `content`.
`toolResponseMetadata`	`_meta` on the tool-result notification.
`callTool(name, args)`	Request `tools/call` with `{ name, arguments }`. The tool’s `_meta.ui.visibility` must include `"app"`.
`sendFollowUpMessage({ prompt })`	Request `ui/message` with a `user` role and text content.
`openExternal({ href })`	Request `ui/open-link` with `{ url }`.
`requestDisplayMode({ mode })`	Request `ui/request-display-mode`. Allternit supports `inline` and `fullscreen` only.
`requestClose()`	Notification `ui/notifications/request-teardown`.
`notifyIntrinsicHeight()`	Notification `ui/notifications/size-changed` with `height`.
`theme`	`hostContext.theme` and `hostContext.styles.variables`.
`displayMode`, `maxHeight`	`hostContext.displayMode` and `hostContext.containerDimensions`.
`locale`, `userAgent`	`hostContext.locale` and `hostContext.userAgent`.
`widgetState`, `setWidgetState()`	Through the shim: the host keeps the state per message and restores it on the next render (256 KB limit), and sends it to the model with `ui/update-model-context`. Without the shim there is no View storage; keep durable state on your server.
`requestModal()`	Not defined by the shim, so feature-detect it. Use fullscreen display mode.
`uploadFile()`, `selectFiles()`, `getFileDownloadUrl()`	Not defined by the shim.
`requestCheckout()`	Not defined by the shim. Use `x-allternit/requestCheckout`, a preview.
`safeArea`	Filled from `hostContext.safeAreaInsets` when the host sends it, otherwise zeros.

To save a result or start an agent task, which the standard does not cover, use the optional window.allternit extensions.

A small compatibility layer

If one View has to run in several hosts, wrap the differences in one place:

const host = {
  callTool: (name, args) =>
    window.openai?.callTool
      ? window.openai.callTool(name, args)
      : request("tools/call", { name, arguments: args }),
  openLink: (url) =>
    window.openai?.openExternal
      ? window.openai.openExternal({ href: url })
      : request("ui/open-link", { url }),
};

request is your JSON-RPC helper from the quickstart, or the equivalent method on the App class.

Server-side changes

MIME type. Allternit renders text/html;profile=mcp-app, plain text/html, and ChatGPT’s older Apps SDK type text/html+skybridge (which also turns on the window.openai shim).
Tool metadata. Allternit reads _meta.ui.resourceUri, the flat _meta["ui/resourceUri"], and the older _meta["openai/outputTemplate"] to find a View. When a resource has no _meta.ui, openai/widgetCSP, openai/widgetDomain and openai/widgetPrefersBorder are read in its place. Other openai/* keys are not read.
Auth. Allternit sends resource and PKCE but no scope parameter. See Authentication.

Where Allternit differs

Area	Allternit
Sandbox	Two nested iframes; the View has an opaque origin. No cookies, `localStorage`, or IndexedDB.
Display modes	`inline` and `fullscreen`. No picture-in-picture.
CSP	Closed by default. Declare every domain the View uses.
Tool permissions	Per-app mode: always ask, before changes, or before important changes. Driven by your tool annotations.
Distribution	Your listing goes through Submit & review, including domain verification and held updates for tool changes.
Extensions	Optional `x-allternit/*` methods. Feature-detect them.
Commerce	A preview; test mode only.

Porting from Claude

Claude and Allternit both implement the standard, so there is little to change. Check the CSP you declare, remove any dependency on browser storage, and confirm your OAuth server accepts Allternit’s client ID metadata document.

Porting to other hosts

An Allternit app that uses only the standard bridge runs in any host that implements the specification. If you use window.allternit, guard every call with detect() and supports(). The extension methods are not part of the standard, and other hosts will not have them.

Concepts

Get Started

Apps SDK

Build

Test & Publish

Guides

Reference

Porting from ChatGPT or Claude

What works unchanged

If your View uses `window.openai`

A small compatibility layer

Server-side changes

Where Allternit differs

Porting from Claude

Porting to other hosts

​What works unchanged

​If your View uses window.openai

​A small compatibility layer

​Server-side changes

​Where Allternit differs

​Porting from Claude

​Porting to other hosts

What works unchanged

If your View uses `window.openai`

A small compatibility layer

Server-side changes

Where Allternit differs

Porting from Claude

Porting to other hosts