diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 3a99438..1f15d8b 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -1,3 +1,7 @@
 test:
   script:
     - nix-build --no-out-link --show-trace tests
+
+nixpkgs-fmt:
+  script:
+    - ./check-fmt
diff --git a/check-fmt b/check-fmt
new file mode 100755
index 0000000..4c41da4
--- /dev/null
+++ b/check-fmt
@@ -0,0 +1,10 @@
+#!/usr/bin/env nix-shell
+#!nix-shell -i bash
+#
+# Just because the nixpkgs-fmt error message is not super readable. Used by
+# CI.
+
+if ! nixpkgs-fmt --check . ; then
+  echo 'run `nixpkgs-fmt .` to fix it'
+  exit 1
+fi
diff --git a/shell.nix b/shell.nix
index 7e13b49..8610a3d 100644
--- a/shell.nix
+++ b/shell.nix
@@ -6,5 +6,6 @@ in
 pkgs.mkShell {
   buildInputs = [
     poetry
+    pkgs.nixpkgs-fmt
   ];
 }