diff --git a/.tangled/workflows/test.yml b/.tangled/workflows/test.yml
index c3d7e84..6e64e9e 100644
--- a/.tangled/workflows/test.yml
+++ b/.tangled/workflows/test.yml
@@ -5,12 +5,15 @@ when:
 
 engine: nixery
 
+# nixpkgs-unstable: default nixpkgs gleam is 1.13, too old for our deps and its
+# formatter. bun is needed because gleam.toml sets tools.lustre.bin.bun=system.
 dependencies:
-  nixpkgs:
+  nixpkgs/nixpkgs-unstable:
     - gleam
     - erlang
     - rebar3
     - nodejs
+    - bun
     - gnumake
 
 steps: