doi2url.lua

-- Time-stamp: <2024-04-20T21:07:41+0200 mpiotrow>

--[[ When converting from biblatex to bibtex, Pandoc only considers
   URL fields: DOIs get lost in the conversion (see
   <https://github.com/jgm/pandoc/blob/main/src/Text/Pandoc/Citeproc/BibTeX.hs>).

   This filter copies the values of DOI fields to URL fields, so that
   they are output to bibtex. ]]

function Pandoc(d)
   refs = pandoc.utils.references(d)

   local i, entry = next(refs, nil)

   while i do
      if refs[i].DOI then
         refs[i].URL = "https://doi.org/" .. refs[i].DOI
      end

      i, entry = next(refs, i) -- get next index
   end

   d.meta.references = refs
   return d
end

Shouldn't this be fixed in Pandoc directly instead?

I agree, but I needed a solution now ;-) The change would be trivial, but I’m not building Pandoc from source, so I can’t make a PR, just add another enhancement request to the 1000+ issues…