Hi Thierry,

On Fri, Jul 28, 2023 at 11:59 AM Joseph Wright
<[log in to unmask]> wrote:
>
> Older LaTeX used \openin + \ifeof to test for file existence; we now use
> \(pdf)filesize, which is expandable and reports "0" for a non-existent
> file. Both methods should find "foo.tex" from "foo" in the same way.

A wild guess:  maybe your implementation of |\pdffilesize| doesn't try
to guess the extension as |\input| does, thus the difference you are
seeing.  Because as others have already said, |\input{docstrip}| works
in both older and newer (web2c-based) LaTeX.

Sorry if this isn't useful:  can't run any code right now.

Best,
Phelype