Migrate OpenAI App to MCP

Migrate existing OpenAI Apps SDK applications to the MCP Apps SDK (

@modelcontextprotocol/ext-apps

). The MCP Apps SDK provides a standardized, open protocol for interactive UIs in conversational clients.

Best Practices

Use your package manager to add dependencies (e.g.,

npm install

,

pnpm add

,

yarn add

) instead of manually writing version numbers. This lets the package manager resolve the latest compatible versions. Never specify version numbers from memory.

Preemptively add a final todo item with this exact wording: "Re-read the 'Before Finishing' checklist in this skill and address each checkbox individually, stating what you did for each one, before marking this todo complete."

Getting Reference Code

Clone the SDK repository for complete migration documentation and working examples:

git

clone

--branch

"v

$(

npm

view @modelcontextprotocol/ext-apps version

)

"

--depth

1

https://github.com/modelcontextprotocol/ext-apps.git /tmp/mcp-ext-apps

Migration Reference Guide

Read the migration reference guide with "before/after" mapping tables:

/tmp/mcp-ext-apps/docs/migrate_from_openai_apps.md

API Reference (Source Files)

Read JSDoc documentation directly from

/tmp/mcp-ext-apps/src/*

:

File

Contents

src/app.ts

App

class, handlers, lifecycle

src/server/index.ts

registerAppTool

,

registerAppResource

src/spec.types.ts

Type definitions

src/react/useApp.tsx

useApp

hook for React apps

src/react/use.ts

Other

use*

hooks for React apps

Front-End Framework Examples

See

/tmp/mcp-ext-apps/examples/basic-server-{framework}/

for basic SDK usage examples organized by front-end framework:

Template

Key Files

basic-server-vanillajs/

server.ts

,

src/mcp-app.ts

,

mcp-app.html

basic-server-react/

server.ts

,

src/mcp-app.tsx

(uses

useApp

hook)

basic-server-vue/

server.ts

,

src/App.vue

basic-server-svelte/

server.ts

,

src/App.svelte

basic-server-preact/

server.ts

,

src/mcp-app.tsx

basic-server-solid/

server.ts

,

src/mcp-app.tsx

CSP Investigation

MCP Apps HTML is served as an MCP resource, not as a web page, and runs in a sandboxed iframe with no same-origin server.

Every

origin must be declared in CSP—including the origin serving your JS/CSS bundles (

localhost

in dev, your CDN in production). Missing origins fail silently.

Before writing any migration code

, build the app and investigate all origins it references:

Build the app using the existing build command

Search the resulting HTML, CSS, and JS for

every

origin (not just "external" origins—every network request will need CSP approval)

For each origin found, trace back to source:

If it comes from a constant → universal (same in dev and prod)

If it comes from an env var or conditional → note the mechanism and identify both dev and prod values

Check for third-party libraries that may make their own requests (analytics, error tracking, etc.)

Document your findings

as three lists, and note for each origin whether it's universal, dev-only, or prod-only:

resourceDomains

origins serving images, fonts, styles, scripts

connectDomains

origins for API/fetch requests

frameDomains

origins for nested iframes If no origins are found, the app may not need custom CSP domains. CORS Configuration MCP clients make cross-origin requests. If using Express, app.use(cors()) handles this. For raw HTTP servers, configure standard CORS and additionally: Allow headers: mcp-session-id , mcp-protocol-version , last-event-id Expose headers: mcp-session-id Key Conceptual Changes Server-Side Use registerAppTool() and registerAppResource() helpers instead of raw server.registerTool() / server.registerResource() . These helpers handle the MCP Apps metadata format automatically. See /tmp/mcp-ext-apps/docs/migrate_from_openai_apps.md for server-side mapping tables. Client-Side The fundamental paradigm shift: OpenAI uses a synchronous global object ( window.openai.toolInput , window.openai.theme ) that's pre-populated before your code runs. MCP Apps uses an App instance with async event handlers. Key differences: Create an App instance and register handlers ( ontoolinput , ontoolresult , onhostcontextchanged ) before calling connect() . (Events may fire immediately after connection, so handlers must be registered first.) Access tool data via handlers: app.ontoolinput for window.openai.toolInput , app.ontoolresult for window.openai.toolOutput . Access host environment (theme, locale, etc.) via app.getHostContext() . For React apps, the useApp hook manages this lifecycle automatically—see basic-server-react/ for the pattern. See /tmp/mcp-ext-apps/docs/migrate_from_openai_apps.md for client-side mapping tables. Features Not Yet Available in MCP Apps These OpenAI features don't have MCP equivalents yet: Server-side: OpenAI Feature Status/Workaround _meta["openai/toolInvocation/invoking"] / _meta["openai/toolInvocation/invoked"] Progress indicators not yet available _meta["openai/widgetDescription"] Use app.updateModelContext() for dynamic context Client-side: OpenAI Feature Status/Workaround window.openai.widgetState / setWidgetState() Use localStorage or server-side state window.openai.uploadFile() / getFileDownloadUrl() File operations not yet available window.openai.requestModal() / requestClose() Modal management not yet available window.openai.view Not yet available Before Finishing Slow down and carefully follow each item in this checklist: Search for and migrate any remaining server-side OpenAI patterns: Pattern Indicates "openai/ Old metadata keys → _meta.ui.* text/html+skybridge Old MIME type → RESOURCE_MIME_TYPE constant text/html;profile=mcp-app New MIME type, but prefer RESOURCE_MIME_TYPE constant _domains" or _domains: snake_case CSP → camelCase ( connect_domains → connectDomains ) Search for and migrate any remaining client-side OpenAI patterns: Pattern Indicates window.openai.toolInput Old global → params.arguments in ontoolinput handler window.openai.toolOutput Old global → params.structuredContent in ontoolresult window.openai Old global API → App instance methods For each origin from your CSP investigation, show where it appears in the registerAppResource() CSP config. Every origin from the CSP investigation (universal, dev-only, prod-only) must be included in the CSP config—MCP Apps HTML runs in a sandboxed iframe with no same-origin server . If an origin was not included in the CSP config, add it now. For each conditional (dev-only, prod-only) origin from your CSP investigation, show the code where the same configuration setting (env var, config file, etc.) controls both the runtime URL and the CSP entry. If the CSP has a hardcoded origin that should be conditional, fix it now—the app must be production-ready. Testing Using basic-host Test the migrated app with the basic-host example:

Terminal 1: Build and run your server

npm run build && npm run serve

Terminal 2: Run basic-host (from cloned repo)

cd /tmp/mcp-ext-apps/examples/basic-host npm install SERVERS = '["http://localhost:3001/mcp"]' npm run start

Open http://localhost:8080

Verify Runtime Behavior Once the app loads in basic-host, confirm: App loads without console errors ontoolinput handler fires with tool arguments ontoolresult handler fires with tool result