diff --git a/core/default.nix b/core/default.nix
index 127727e..c092ec5 100644
--- a/core/default.nix
+++ b/core/default.nix
@@ -20,6 +20,8 @@
     trace
     json
 
+    (callPackage ../planner {})
+
     libsodium # TODO(puck): don't propagate this
   ];
 }
diff --git a/core/src/magic.sld b/core/src/magic.sld
index f1e3702..0f7544f 100644
--- a/core/src/magic.sld
+++ b/core/src/magic.sld
@@ -5,6 +5,7 @@
   (import
     (scheme base) (scheme file)
     (zilch lib hash) (zilch nix daemon) (zilch nix drv) (zilch nix path)
+    (zilch planner step)
     (zilch zexpr)
     (srfi 128) (srfi 132) (srfi 146) (srfi 152)
     (chicken base) (chicken format) socket)
@@ -250,6 +251,25 @@
        (daemon-wop-build-paths (*daemon*) (list->vector to-build)))
      val)
 
+   (register-build-step '((zilch core magic) build) #t
+    (lambda items
+      (printf "received build info: ~S\n" items)
+      (define all-paths '())
+      (for-each
+        (lambda (item)
+          (define outputs (list-ref item 1))
+          (define drv-path (list-ref item 2))
+          (set! all-paths (append (map (lambda (o) (string-append drv-path "!" o)) outputs) all-paths)))
+        items)
+      (daemon-wop-build-paths (*daemon*) (list->vector all-paths))
+      (define output '())
+      (for-each
+        (lambda (item)
+          (define output-map (daemon-wop-query-derivation-output-map (*daemon*) (list-ref item 2)))
+          (set! output (cons (cons (car item) output-map) output)))
+        items)
+      output))
+
    ;; Ensures the `<store-path>` exists, then opens an input port to allow reading from it.
    (define (store-path-open path)
      (increment-counter 2)