diff --git a/core/src/magic.sld b/core/src/magic.sld
index 7c2b4b7..53d8b37 100644
--- a/core/src/magic.sld
+++ b/core/src/magic.sld
@@ -25,6 +25,7 @@
     store-path-devirtualise
 
     store-path-register-fallback
+    store-path-register-post-build
 
     ca-thread-count drv-resolve-ca
 
@@ -464,6 +465,11 @@
          (mutex-lock! pending-count-mutex)
          (unless (eq? pending-count 'error)
            (set! pending-count (- pending-count 1)))
+
+         ; Notify the derivation that it is built.
+         (let* ((meta (derivation-meta ca-drv))
+                (post-build (and meta (assoc 'post-build meta))))
+          (when post-build ((cdr post-build) (pending-item-resolved-paths item))))
          (mutex-unlock! pending-count-mutex)
          (mutex-lock! pending-mutex)
          (set-pending-item-awaiting-count! item 'built)
@@ -593,6 +599,11 @@
      (set-derivation-meta! (store-path-drv path) (cons (cons 'fallback wrap-fallback) (or (derivation-meta (store-path-drv path)) '())))
      path)
 
+   ; Note: this post-build hook is called with a mutex taken.
+   (define (store-path-register-post-build path callback)
+     (set-derivation-meta! (store-path-drv path) (cons (cons 'post-build callback) (or (derivation-meta (store-path-drv path)) '())))
+     path)
+
    (register-build-step '((zilch core magic) build) #t
     (lambda items
       (printf "received build info: ~S\n" items)