diff --git a/src/libstore/include/nix/store/globals.hh b/src/libstore/include/nix/store/globals.hh
index fcfc2e94a..041300bed 100644
--- a/src/libstore/include/nix/store/globals.hh
+++ b/src/libstore/include/nix/store/globals.hh
@@ -1248,7 +1248,7 @@ public:
         R"(
           Helper programs that execute derivations.
 
-          The program is passed a JSON document that describes the build environment as the final argument.
+          The program is passed a JSON document that describes the build environment on standard input.
           The JSON document looks like this:
 
             {
diff --git a/src/libstore/unix/build/external-derivation-builder.cc b/src/libstore/unix/build/external-derivation-builder.cc
index 1906ddd70..9fe0eb19f 100644
--- a/src/libstore/unix/build/external-derivation-builder.cc
+++ b/src/libstore/unix/build/external-derivation-builder.cc
@@ -4,6 +4,11 @@ struct ExternalDerivationBuilder : DerivationBuilderImpl
 {
     Settings::ExternalBuilder externalBuilder;
 
+    /**
+     * Pipe for talking to the spawned builder.
+     */
+    Pipe toBuilder;
+
     ExternalDerivationBuilder(
         Store & store,
         std::unique_ptr<DerivationBuilderCallbacks> miscMethods,
@@ -83,23 +88,22 @@ struct ExternalDerivationBuilder : DerivationBuilderImpl
         json.emplace("realStoreDir", getLocalStore(store).config->realStoreDir.get());
         json.emplace("system", drv.platform);
 
-        // FIXME: maybe write this JSON into the builder's stdin instead....?
-        auto jsonFile = topTmpDir + "/build.json";
-        writeFile(jsonFile, json.dump());
+        toBuilder.create();
 
         pid = startProcess([&]() {
             openSlave();
             try {
                 commonChildInit();
 
+                if (dup2(toBuilder.readSide.get(), STDIN_FILENO) == -1)
+                    throw SysError("duping to-builder read side to builder's stdin");
+
                 Strings args = {externalBuilder.program};
 
                 if (!externalBuilder.args.empty()) {
                     args.insert(args.end(), externalBuilder.args.begin(), externalBuilder.args.end());
                 }
 
-                args.insert(args.end(), jsonFile);
-
                 debug("executing external builder: %s", concatStringsSep(" ", args));
                 execv(externalBuilder.program.c_str(), stringsToCharPtrs(args).data());
 
@@ -109,6 +113,9 @@ struct ExternalDerivationBuilder : DerivationBuilderImpl
                 _exit(1);
             }
         });
+
+        writeFull(toBuilder.writeSide.get(), json.dump());
+        toBuilder.close();
     }
 };