How it renders across platforms

The contract is the same. Only the presentation changes — the server has no say in it.

Component	iPhone	iPad	Mac
page	stack + grouped list	list-detail, 2 columns	list-detail, 3 columns
section	section	section	section
row	row / navigation	same	same + hover
metric	row with a large value	grid possible	grid possible
table	list + lazy loading	table: columns, sorting	table + selection + menu
field	form controls	same	grouped form
button	button with a role	same	same + keyboard shortcut
chart	chart	same	same
toolbar	action bar	action bar	bar + app menu

What to keep in mind when authoring the contract

A table on iPhone shows as a list: the first two columns matter, the rest collapse — put the important one on the left.
metric tiles — wrap several in a section with columns: 2.
Swipes (swipe_actions) exist only on iOS; on Mac they become a context menu — don’t make them the only way to perform an action.
More than 5 tabs won’t fit on iPhone — keep it to five.

The app opens straight into the active service.
The tab bar is shown only if the manifest has tabs: the service’s tabs + a fixed “Settings” tab. Without tabs — a single root_page.
Switching and adding services, and tokens, live in the client’s “Settings” screen.
Push: the payload carries service_id + page_endpoint → tapping makes the service active and opens the page.