diff --git a/src/libstore/include/nix/store/globals.hh b/src/libstore/include/nix/store/globals.hh
index 385f8cd7a..1b59bd6fc 100644
--- a/src/libstore/include/nix/store/globals.hh
+++ b/src/libstore/include/nix/store/globals.hh
@@ -1419,7 +1419,8 @@ public:
               "system": "aarch64-linux",
               "tmpDir": "/private/tmp/nix-build-hello-2.12.2.drv-0/build",
               "tmpDirInSandbox": "/build",
-              "topTmpDir": "/private/tmp/nix-build-hello-2.12.2.drv-0"
+              "topTmpDir": "/private/tmp/nix-build-hello-2.12.2.drv-0",
+              "version": 1
             }
         )",
         {},   // aliases
diff --git a/src/libstore/unix/build/external-derivation-builder.cc b/src/libstore/unix/build/external-derivation-builder.cc
index 12ac77542..71cfd1a62 100644
--- a/src/libstore/unix/build/external-derivation-builder.cc
+++ b/src/libstore/unix/build/external-derivation-builder.cc
@@ -49,6 +49,7 @@ struct ExternalDerivationBuilder : DerivationBuilderImpl
 
         auto json = nlohmann::json::object();
 
+        json.emplace("version", 1);
         json.emplace("builder", drv.builder);
         {
             auto l = nlohmann::json::array();