diff --git a/src/libexpr/common-eval-args.cc b/src/libcmd/common-eval-args.cc
similarity index 95%
rename from src/libexpr/common-eval-args.cc
rename to src/libcmd/common-eval-args.cc
index fffca4ac5..ed3a08915 100644
--- a/src/libexpr/common-eval-args.cc
+++ b/src/libcmd/common-eval-args.cc
@@ -7,6 +7,7 @@
 #include "registry.hh"
 #include "flake/flakeref.hh"
 #include "store-api.hh"
+#include "command.hh"
 
 namespace nix {
 
@@ -59,6 +60,9 @@ MixEvalArgs::MixEvalArgs()
             fetchers::Attrs extraAttrs;
             if (to.subdir != "") extraAttrs["dir"] = to.subdir;
             fetchers::overrideRegistry(from.input, to.input, extraAttrs);
+        }},
+        .completer = {[&](size_t, std::string_view prefix) {
+            completeFlakeRef(openStore(), prefix);
         }}
     });
 
diff --git a/src/libexpr/common-eval-args.hh b/src/libcmd/common-eval-args.hh
similarity index 100%
rename from src/libexpr/common-eval-args.hh
rename to src/libcmd/common-eval-args.hh