diff --git a/meat.nu b/meat.nu
index cacf97a..a350b15 100755
--- a/meat.nu
+++ b/meat.nu
@@ -42,19 +42,42 @@ def nix-conf-attr []: nothing -> string {
   $"nixosConfigurations.(hn).config.environment.etc.\"nix/nix.conf\""
 }
 
-def nix-build-nom [out: string, build_args: list<string>] {
-  if ($env.MONITOR? | is-empty) {
-    ^nix-build --out-link $out ...$build_args
-  } else {
-    ^nix-build --log-format internal-json --out-link $out ...$build_args out+err>| ^$env.MONITOR --json
+def monitor-available []: nothing -> bool {
+  let monitor = $env.MONITOR? | default ""
+  (not ($monitor | is-empty)) and (not (which $monitor | is-empty))
+}
+
+def monitor-build-args [build_args: list<string>]: nothing -> list<string> {
+  $build_args | each { |arg|
+    if ($arg | str ends-with ".drv") { $"($arg)^*" } else { $arg }
+  }
+}
+
+def nix-build-monitored [out: string, build_args: list<string>] {
+  if (monitor-available) {
+    let monitor_args = monitor-build-args $build_args
+    match ($env.MONITOR | path basename) {
+      "nom" => { ^$env.MONITOR build ...$monitor_args --out-link $out }
+      "rom" => { ^$env.MONITOR build ...$monitor_args -- --out-link $out }
+      $name => {
+        error make { msg: $"unsupported Nix build monitor '($name)'; expected nom or rom" }
+      }
+    }
+  } else {
+    ^nix-build ...$build_args --out-link $out
   }
-  # Piping to MONITOR masks nix-build's exit code, so detect failure by the
-  # absence of the out-link.
   if not ($out | path exists) {
     error make { msg: "nix-build produced no output" }
   }
 }
 
+def ensure-system-build [build: string] {
+  let switch = $"($build)/bin/switch-to-configuration"
+  if not ($switch | path exists) {
+    error make { msg: $"build output is not a NixOS system closure: missing ($switch)" }
+  }
+}
+
 def differ-step [build: string] {
   if ($env.DIFFER? | is-empty) {
     meat-print "  (no DIFFER set; skipping diff)"
@@ -74,7 +97,8 @@ def do-build [extras: list<string> = []] {
   let start = date now
   let drv = ^nix-instantiate $"($env.MEATS)/entry.nix" -A (system-attr) ...$extras | lines | last | str trim
   meat-print $"MASTICATION TAKING ((date now) - $start)"
-  nix-build-nom $build [$drv]
+  nix-build-monitored $build [$drv]
+  ensure-system-build $build
   differ-step $build
   activate $build
   try { rm $build }
@@ -241,12 +265,15 @@ def cmd-ritual [] {
     let build = $"($tmpdir)/build"
     try {
       warm-pins
-      nix-build-nom $nix_conf [$"($meats)/entry.nix" "-A" (nix-conf-attr)]
+      let nix_conf_drv = ^nix-instantiate $"($meats)/entry.nix" -A (nix-conf-attr) | lines | last | str trim
+      nix-build-monitored $nix_conf [$nix_conf_drv]
       meat-print "CONSUMING MEATS.."
       with-env { NIX_USER_CONF_FILES: $nix_conf } {
-        nix-build-nom $build [$"($meats)/entry.nix" "-A" (system-attr)]
+        let build_drv = ^nix-instantiate $"($meats)/entry.nix" -A (system-attr) | lines | last | str trim
+        nix-build-monitored $build [$build_drv]
       }
-      ^$env.DIFFER /nix/var/nix/profiles/system $build
+      ensure-system-build $build
+      differ-step $build
       activate $build
       try { rm $nix_conf }
       try { rm $build }