Intercepting routes
The links below open each photo as a modal over this page. Load the same URL directly and you get the photo as a full page instead. The URL does not change between the two; the Next-Url request header, which the client sets to the page it is navigating from, is what selects the modal.
Every photo prints render: FULL-PAGE or render: INTERCEPTED-MODAL, and that text is in the RSC payload, so curl can tell which render it got.
What to check
In a browser. Click a photo: modal. Refresh: full page. Back: the modal closes.
Vary. curl -sI localhost:3000/intercept/photo/1 | grep -i vary lists next-url. The same command on /ssg does not.
Two payloads, two URLs. Send RSC: 1 to /intercept/photo/1 with and without Next-Url: /intercept. Each gets a 307 to a different _rsc value. Following each redirect with the same headers returns INTERCEPTED-MODAL for the first and FULL-PAGE for the second.