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.ts#authenticatedCalls}

[TODO] Giving errors back to model

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

{@includeCode ./patterns.ts#errorsToModel}

Support native host styling using CSS variables

Hosts provide CSS variables (e.g., --color-background-primary) that match their theme. Apply these to the document, then use them in your styles via var().

Vanilla JS:

{@includeCode ../src/styles.examples.ts#applyHostStyleVariables_fromHostContext}

React:

{@includeCode ../src/react/useHostStyles.examples.tsx#useHostStyles_basicUsage}

[TODO] Reacting to light/dark theme changes

{@includeCode ./patterns.ts#lightDarkTheme}

[TODO] Support fullscreen / exit fullscreen

{@includeCode ./patterns.ts#fullscreen}

[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.ts#persistData}

[TODO] Lower perceived latency / manage loading time

Leverage partial inputs to show widgets as possible.

Beware of partial JSON being partial (but healed), so some of your objects may not be complete (e.g. in a list you may need to skip the last item if your code validates input schemas strictly).

{@includeCode ./patterns.ts#lowerPerceivedLatency}

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

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

{@includeCode ./patterns.ts#iframeAndMcpApps}

[TODO] Migrating from OpenAI to MCP Apps

See OpenAI -> MCP Apps migration guide.

Also: Managing State (OpenAI)

{@includeCode ./patterns.ts#migrateFromOpenai}