Could the test also check that the returned string is the expected one? And check also invalid format strings, to make sure nothing breaks in the future? (I am thinking in degenerate strings like unterminated "%{", or invalid names "%{non-existing-name}")

Good point, will do.

Should I use ${} instead of %{} ?

I don't think it matters much, unless we have other places inside EFL where we do such substitutions.

We might. Nothing exposed yet in the new API, but for path with eina_vpath we have (:something:) which I would add support for whatever we choose here. I would like something that people are used to and generally expect. which (:something:) is not.