a1ae100ef0
- Expand `App_ontoolinputpartial_progressiveRendering` example in `app.examples.ts` to show full state management pattern with `toolInputs` and `toolInputsPartial` variables - Add healed JSON caveat to `ontoolinputpartial` JSDoc in `app.ts` - Update `patterns.md` to reference the expanded example and document the streaming preview pattern - Remove empty `lowerPerceivedLatency` placeholder from `patterns.tsx` Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
84 lines
3.6 KiB
Markdown
84 lines
3.6 KiB
Markdown
---
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title: Patterns
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---
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# MCP Apps Patterns
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This document covers common patterns and recipes for building MCP Apps.
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## Tools that are private to Apps
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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.
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{@includeCode ../src/server/index.examples.ts#registerAppTool_appOnlyVisibility}
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## [TODO] Authenticated calls from App
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- Use tool calls / read resources
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- See [PDF example](https://github.com/modelcontextprotocol/ext-apps/blob/main/examples/pdf-viewer) to read binaries by chunks to avoid call tool size limitations on platforms like claude.ai
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- Pass auth token in `_meta` (will be loaded again in the future) + refresh token + store in local storage (see Persist data section below)
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{@includeCode ./patterns.tsx#authenticatedCalls}
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## [TODO] Giving errors back to model
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- Before app runs: validate inputs in tool call
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- After it runs: use `updateModelContext`
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{@includeCode ./patterns.tsx#errorsToModel}
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## Matching host styling (CSS variables, theme, and fonts)
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Use the SDK's style helpers to apply host styling, then reference them in your CSS:
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- **CSS variables** — Use `var(--color-background-primary)`, etc. in your CSS
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- **Theme** — Use `[data-theme="dark"]` selectors or `light-dark()` function for theme-aware styles
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- **Fonts** — Use `var(--font-sans)` or `var(--font-mono)` with fallbacks (e.g., `font-family: var(--font-sans, system-ui, sans-serif)`)
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**Vanilla JS:**
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{@includeCode ./patterns.tsx#hostStylingVanillaJs}
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**React:**
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{@includeCode ./patterns.tsx#hostStylingReact}
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## Entering / Exiting fullscreen
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Toggle fullscreen mode by calling {@link app!App.requestDisplayMode requestDisplayMode}:
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{@includeCode ../src/app.examples.ts#App_requestDisplayMode_toggle}
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Listen for display mode changes via {@link app!App.onhostcontextchanged onhostcontextchanged} to update your UI:
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{@includeCode ../src/app.examples.ts#App_onhostcontextchanged_respondToDisplayMode}
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## [TODO] Persist data (incl. widget state)
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- 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})`
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- 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()`
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{@includeCode ./patterns.tsx#persistData}
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## Lowering perceived latency
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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.
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{@includeCode ../src/app.examples.ts#App_ontoolinputpartial_progressiveRendering}
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> [!IMPORTANT]
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> 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.
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## [TODO] Supporting both iframe & MCP Apps in same binary
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See recipe: https://github.com/modelcontextprotocol/ext-apps/issues/34
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{@includeCode ./patterns.tsx#iframeAndMcpApps}
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## [TODO] Migrating from OpenAI to MCP Apps
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See [OpenAI -> MCP Apps](https://docs.google.com/document/d/13ROImOR9B8xc32yhqsFyC9Hh3_H63JFORDIyjyIPcU4/edit) migration guide.
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Also: [Managing State](https://platform.openai.com/docs/actions/managing-state) (OpenAI)
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{@includeCode ./patterns.tsx#migrateFromOpenai}
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