Example of the new feature. Initially the mirror was empty so on the first request it returned 404. After that it tried to fetch substitute from upstream, then on the second request it served the expected content.

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   hako（重生版） (hako@boiledscript.com)'s status on Tuesday, 16-Jun-2026 02:54:56 JST hako（重生版）

   in reply to
   mirror-substitutes now adds a new on-demand operation mode. I have also refreshed README of the repository.
   
   In contrast to the approach that requires a preknown list of Guix / Nix store items and can be regulated by the operator, this new one mirrors substitutes on user request. (The scope is still limited to specified upstream servers but you as the operator can't select the content.)
   
   The benefit it brings is to only mirror substitutes that are actually used. This usually reduces transfer and storage requirement on a Guix / Nix mirror.
   
   #guix