Launching an action, the browser provide information on the accept headers, which in this probematic case renders: Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8 when the desired effect if to have turbo inject text/vnd.turbo-stream.html in that place.

However, debugging this has a challenge. Using, on rails 7.1 binding.break at the very beginning of processing by the application_controller

Started POST "/ops/aggregate_items?id=23" for ::1 at 2024-05-30 10:17:22 +0200
Processing by OpsController#aggregate_items as HTML
     5|   before_action :set_clean_host_site
[...]
     8|   def set_clean_host_site
=>   9|  binding.break

the processing format has already been set.

How can one step through that process rails has gone through to determine the processing method
(then I assume I can identify the motive for the unexpected behaviour) ?