Ensure all API interfaces have clearly documented contracts specifying expected request/response formats, data encoding standards, and return types. This prevents integration issues and enables reliable external consumption. When designing callback functions or data exchange formats, explicitly document: - Expected response structure and types - Data encoding methods (e.g., base64 variants, URL encoding) - Backwards compatibility requirements Example from callback responses: ```typescript const handleUIAction = useCallback(async (result: UIActionResult) => { // Process the action... // IMPORTANT: Always return documented response format const response = { type: 'ui-message-response', payload: result, }; return response; }, []); ``` Consider adding TypeScript interfaces or runtime validation to enforce documented contracts. For data formats like deeplinks, establish official standards (e.g., "use URL_SAFE_NO_PAD base64") and implement backwards-compatible decoding to handle legacy formats during transitions.