diff --git a/src/libfetchers/git.cc b/src/libfetchers/git.cc
index c4dfc27a2..5a9679199 100644
--- a/src/libfetchers/git.cc
+++ b/src/libfetchers/git.cc
@@ -435,7 +435,17 @@ struct GitInputScheme : InputScheme
         //
         // See: https://discourse.nixos.org/t/57783 and #9708
         //
-        repoInfo.url = repoInfo.isLocal ? std::filesystem::absolute(url.path).string() : url.to_string();
+        if (repoInfo.isLocal) {
+            if (!isAbsolute(url.path)) {
+                warn(
+                    "Fetching Git repository '%s', which uses a path relative to the current directory. "
+                    "This is not supported and will stop working in a future release. "
+                    "See https://github.com/NixOS/nix/issues/12281 for details.",
+                    url);
+            }
+            repoInfo.url = std::filesystem::absolute(url.path).string();
+        } else
+            repoInfo.url = url.to_string();
 
         // If this is a local directory and no ref or revision is
         // given, then allow the use of an unclean working tree.