nixd should work now

.nixd.json

@@ -0,0 +1,18 @@
+{
+  "eval": {
+    "target": {
+      "args": ["-f", "default.nix"],
+      "installable": "nixosConfigurations.sakotop.config.system.build.toplevel"
+    }
+  },
+  "formatting": {
+    "command": "nixpkgs-fmt"
+  },
+  "options": {
+    "enable": true,
+    "target": {
+      "args": [],
+      "installable": ".#nixosConfigurations.sakotop.options"
+    }
+  }
+}

default.nix

@@ -0,0 +1,10 @@
+(import
+  (
+    let lock = builtins.fromJSON (builtins.readFile ./flake.lock); in
+    fetchTarball {
+      url = "https://github.com/edolstra/flake-compat/archive/${lock.nodes.flake-compat.locked.rev}.tar.gz";
+      sha256 = lock.nodes.flake-compat.locked.narHash;
+    }
+  )
+  { src = ./.; }
+).defaultNix

flake.nix

@@ -9,6 +9,11 @@
       nixpkgs-unstable.url = "github:nixos/nixpkgs/nixos-unstable";
       home-manager.url = "github:nix-community/home-manager/release-23.05";
       home-manager.inputs.nixpkgs.follows = "nixpkgs";
+      # flake-compat for nixd
+      flake-compat = {
+        url = "github:edolstra/flake-compat";
+        flake = false;
+      };
       # TODO(sako):: add agenix or sops-nix
       # https://github.com/ryantm/agenix#install-via-flakes
       # https://github.com/Mic92/sops-nix