The "/ajax" page:
This "page" is only intended to be used by higher-level pages which have certain Ajax-driven features in common. It is not intended to be used by clients and NONE of its HTTP interfaces are considered documented/stable/supported - they may change on any given build of fossil.
The exact response type depends on the route which gets called. In the case of an initialization error it emits a JSON-format response as documented for ajax_route_error(). Individual routes may emit errors in different formats, e.g. HTML.