I have successfully installed pandoc-include (https://pypi.org/project/pandoc-include/) and I run it calling the filter as usual:

pandoc —filter=pandoc-include test.md -o test.pdf

The filter is a python—panflute filter.

The pandoc-include documentation suggests to add this item to the document header:

pandoc-options:
  - --filter=pandoc-include

but it seems ineffective, since an explicit call of the filter in the command line is necessary.

I also added to ${HOME}/defaults/defaults file this line:

filters:
  -  --pandoc-include

but again nothing happens.
I’m sure I’m missing some basic step, and I am grateful in advance for your kind help.

guido (northern Italy)
Disclaimer: I have already asked the same question in the pandoc-include discussione group.