Send a follow-up conversational turn from an Apps SDK widget to trigger a new model response

Documented steps

1. Trigger the call from an explicit user action (button click), never automatically on render — the inserted message appears in the transcript as if the user asked it.
2. Compose the prompt text from current widget state so the follow-up reflects what the user actually did (e.g. the items they favorited).
3. Call the host method: `await window.openai?.sendFollowupTurn({ prompt: "Draft a tasting itinerary for the pizzerias I favorited." })` as documented in the Custom UX build guide.
4. Verify which spelling your SDK build exposes: the Apps SDK reference page documents `sendFollowUpMessage({ prompt, scrollToBottom })` while the Custom UX guide documents `sendFollowupTurn({ prompt })` — feature-detect rather than hardcoding.
5. Feature-detect defensively, e.g. `const send = window.openai?.sendFollowupTurn ?? window.openai?.sendFollowUpMessage;` before calling.
6. Set `scrollToBottom` when using the `sendFollowUpMessage` form if you want the transcript to scroll to the new turn.
7. At the MCP Apps protocol level this maps to the `ui/message` notification; use that framing if you are targeting non-ChatGPT MCP Apps hosts.
8. Official docs: https://developers.openai.com/apps-sdk/build/custom-ux and https://developers.openai.com/apps-sdk/reference