ext-apps/docs/patterns.md

title
Patterns

MCP Apps Patterns

This document covers common patterns and recipes for building MCP Apps.

Tools that are private to Apps

Set {@link types!McpUiToolMeta.visibility Tool._meta.ui.visibility} to ["app"] to make tools only callable by Apps (hidden from the model). This is useful for UI-driven actions like updating quantities, toggling settings, or other interactions that shouldn't appear in the model's tool list.

{@includeCode ../src/server/index.examples.ts#registerAppTool_appOnlyVisibility}

[TODO] Authenticated calls from App

• Use tool calls / read resources
  • See PDF example to read binaries by chunks to avoid call tool size limitations on platforms like claude.ai
• Pass auth token in _meta (will be loaded again in the future) + refresh token + store in local storage (see Persist data section below)

{@includeCode ./patterns.tsx#authenticatedCalls}

[TODO] Giving errors back to model

• Before app runs: validate inputs in tool call
• After it runs: use updateModelContext

{@includeCode ./patterns.tsx#errorsToModel}

Matching host styling (CSS variables, theme, and fonts)

Use the SDK's style helpers to apply host styling, then reference them in your CSS:

• CSS variables — Use var(--color-background-primary), etc. in your CSS
• Theme — Use [data-theme="dark"] selectors or light-dark() function for theme-aware styles
• Fonts — Use var(--font-sans) or var(--font-mono) with fallbacks (e.g., font-family: var(--font-sans, system-ui, sans-serif))

Vanilla JS:

{@includeCode ./patterns.tsx#hostStylingVanillaJs}

React:

{@includeCode ./patterns.tsx#hostStylingReact}

Entering / Exiting fullscreen

Toggle fullscreen mode by calling {@link app!App.requestDisplayMode requestDisplayMode}:

{@includeCode ../src/app.examples.ts#App_requestDisplayMode_toggle}

Listen for display mode changes via {@link app!App.onhostcontextchanged onhostcontextchanged} to update your UI:

{@includeCode ../src/app.examples.ts#App_onhostcontextchanged_respondToDisplayMode}

[TODO] Persist data (incl. widget state)

• Note: OAI's window.openai.setWidgetState({modelContent, privateContent, imageIds}) has only a partial equivalent in the MCP Apps spec (for now!): App.updateModelContext({content, structuredContent})
• For data persistence / to reload when conversation is reloaded, you must use localStorage / IndexedDb with hostInfo.toolInfo.id as key returned CallToolResult._meta.widgetUUID = randomUUID()

{@includeCode ./patterns.tsx#persistData}

Lowering perceived latency

Use {@link app!App.ontoolinputpartial ontoolinputpartial} to receive streaming tool arguments as they arrive, allowing you to show a loading preview before the complete input is available.

{@includeCode ../src/app.examples.ts#App_ontoolinputpartial_progressiveRendering}

Important

Partial arguments are "healed" JSON — the host closes unclosed brackets/braces to produce valid JSON. This means objects may be incomplete (e.g., the last item in an array may be truncated). Don't rely on partial data for critical operations; use it only for preview UI.

[TODO] Supporting both iframe & MCP Apps in same binary

See recipe: https://github.com/modelcontextprotocol/ext-apps/issues/34

{@includeCode ./patterns.tsx#iframeAndMcpApps}

[TODO] Migrating from OpenAI to MCP Apps

See OpenAI -> MCP Apps migration guide.

Also: Managing State (OpenAI)

{@includeCode ./patterns.tsx#migrateFromOpenai}