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}

Reading large amounts of data via chunked tool calls

Some host platforms have size limits on tool call responses, so large files (PDFs, images, etc.) cannot be sent in a single response. Use an app-only tool with chunked responses to bypass these limits while keeping the data out of model context.

Server-side: Register an app-only tool that returns data in chunks with pagination metadata:

{@includeCode ./patterns.tsx#chunkedDataServer}

Client-side: Loop calling the tool until all chunks are received:

{@includeCode ./patterns.tsx#chunkedDataClient}

Giving errors back to model

Server-side: Tool handler validates inputs and returns { isError: true, content: [...] }. The model receives this error through the normal tool call response.

Client-side: If a runtime error occurs (e.g., API failure, permission denied, resource unavailable), use {@link app!App.updateModelContext updateModelContext} to inform the model:

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

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}

Passing contextual information from the App to the Model

Use {@link app!App.updateModelContext updateModelContext} to keep the model informed about what the user is viewing or interacting with. Structure the content with YAML frontmatter for easy parsing:

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

Persisting widget state

To persist widget state across conversation reloads (e.g., current page in a PDF viewer, camera position in a map), use localStorage with a stable identifier provided by the server.

Server-side: Tool handler generates a unique widgetUUID and returns it in CallToolResult._meta.widgetUUID:

{@includeCode ./patterns.tsx#persistDataServer}

Client-side: Receive the UUID in {@link app!App.ontoolresult ontoolresult} and use it as the storage key:

{@includeCode ./patterns.tsx#persistData}

Note: For model-visible state (informing the LLM about what the user is viewing), use {@link app!App.updateModelContext updateModelContext} instead. Widget state persistence is for UI state that should survive page reloads but doesn't need to be seen by the model.

Pausing computation-heavy widgets when out of view

Widgets with animations, WebGL rendering, or polling can consume significant CPU/GPU even when scrolled out of view. Use IntersectionObserver to pause expensive operations when the widget isn't visible:

{@includeCode ./patterns.tsx#visibilityBasedPause}

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}