diff --git a/nix/shell.nix b/nix/shell.nix
index 74e8853d..b0b34f37 100644
--- a/nix/shell.nix
+++ b/nix/shell.nix
@@ -332,6 +332,10 @@ mkShell {
         make -C c_src
       fi
 
+      # Pre-compile test dependencies so mix test works immediately.
+      # The _build/test tree is ephemeral (cleared when nix devShell changes).
+      MIX_ENV=test mix deps.compile 2>/dev/null || true
+
       # Auto-start services on first shell entry.
       # Run in a detached subshell to prevent nix develop from waiting
       # on daemonized PostgreSQL/Redis child processes.