diff --git a/.github/workflows/publish.yml b/.github/workflows/publish.yml
index fc7a078..ea40046 100644
--- a/.github/workflows/publish.yml
+++ b/.github/workflows/publish.yml
@@ -31,7 +31,7 @@ jobs:
 
       - name: Check Nix formatting
         run: |
-          nix develop --command nixpkgs-fmt --check flake.nix
+          git ls-files '*.nix' | xargs nix develop --command nixfmt --check
 
       - name: nix flake check
         run: |
@@ -40,7 +40,7 @@ jobs:
       - name: Check external links in README
         run: |
           nix develop --command \
-            lychee README.md
+            check-external-links
 
       - name: check image label
         run: |
diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..d09b708
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,3 @@
+# Link checker cache
+.lycheecache
+
diff --git a/README.md b/README.md
index ef9240a..288826b 100644
--- a/README.md
+++ b/README.md
@@ -85,4 +85,4 @@ data "aws_ami" "detsys_nixos" {
 [private-flakes]: https://docs.determinate.systems/flakehub/private-flakes
 [ssm]: https://aws.amazon.com/systems-manager
 [sts]: https://docs.aws.amazon.com/STS/latest/APIReference/welcome.html
-[terraform]: https://terraform.io
+[terraform]: https://developer.hashicorp.com/terraform
diff --git a/flake.lock b/flake.lock
index 12d7d35..baa41cf 100644
--- a/flake.lock
+++ b/flake.lock
@@ -320,12 +320,12 @@
     },
     "nixpkgs_5": {
       "locked": {
-        "lastModified": 1756787288,
-        "narHash": "sha256-rw/PHa1cqiePdBxhF66V7R+WAP8WekQ0mCDG4CFqT8Y=",
-        "rev": "d0fc30899600b9b3466ddb260fd83deb486c32f1",
-        "revCount": 855242,
+        "lastModified": 1757487488,
+        "narHash": "sha256-zwE/e7CuPJUWKdvvTCB7iunV4E/+G0lKfv4kk/5Izdg=",
+        "rev": "ab0f3607a6c7486ea22229b92ed2d355f1482ee0",
+        "revCount": 859555,
         "type": "tarball",
-        "url": "https://api.flakehub.com/f/pinned/nixos/nixpkgs/0.1.855242%2Brev-d0fc30899600b9b3466ddb260fd83deb486c32f1/01990df1-e511-7b09-83a4-f9910966d674/source.tar.gz"
+        "url": "https://api.flakehub.com/f/pinned/nixos/nixpkgs/0.1.859555%2Brev-ab0f3607a6c7486ea22229b92ed2d355f1482ee0/01993860-4c2b-73a3-9c67-955d87cd3025/source.tar.gz"
       },
       "original": {
         "type": "tarball",
diff --git a/flake.nix b/flake.nix
index 94935e8..03960bf 100644
--- a/flake.nix
+++ b/flake.nix
@@ -7,18 +7,30 @@
     nixos-amis.url = "https://flakehub.com/f/NixOS/amis/0.1.*";
   };
 
-  outputs = { self, ... }@inputs:
+  outputs =
+    { self, ... }@inputs:
     let
-      linuxSystems = [ "x86_64-linux" "aarch64-linux" ];
-      allSystems = linuxSystems ++ [ "x86_64-darwin" "aarch64-darwin" ];
+      linuxSystems = [
+        "x86_64-linux"
+        "aarch64-linux"
+      ];
+      allSystems = linuxSystems ++ [
+        "x86_64-darwin"
+        "aarch64-darwin"
+      ];
 
-      forSystems = systems: f: inputs.nixpkgs.lib.genAttrs systems (system: f {
-        inherit system;
-        pkgs = import inputs.nixpkgs {
-          inherit system;
-        };
-        lib = inputs.nixpkgs.lib;
-      });
+      forSystems =
+        systems: f:
+        inputs.nixpkgs.lib.genAttrs systems (
+          system:
+          f {
+            inherit system;
+            pkgs = import inputs.nixpkgs {
+              inherit system;
+            };
+            lib = inputs.nixpkgs.lib;
+          }
+        );
 
       forLinuxSystems = forSystems linuxSystems;
       forAllSystems = forSystems allSystems;
@@ -27,52 +39,90 @@
       # Update this, and the changelog *and* usage examples in the README, for breaking changes to the AMIs
       epoch = builtins.toString 1;
 
-      nixosConfigurations = forLinuxSystems ({ system, pkgs, lib, ... }: lib.nixosSystem {
-        inherit system;
-        modules = [
-          "${inputs.nixpkgs}/nixos/maintainers/scripts/ec2/amazon-image.nix"
-          inputs.determinate.nixosModules.default
-          ({ config, ... }: {
+      nixosConfigurations = forLinuxSystems (
+        {
+          system,
+          pkgs,
+          lib,
+          ...
+        }:
+        lib.nixosSystem {
+          inherit system;
+          modules = [
+            "${inputs.nixpkgs}/nixos/maintainers/scripts/ec2/amazon-image.nix"
+            inputs.determinate.nixosModules.default
+            (
+              { config, ... }:
+              {
 
-            system.nixos.tags = lib.mkForce [ ];
-            environment.systemPackages = [
-              inputs.fh.packages.${system}.default
-              pkgs.git
-            ];
+                system.nixos.tags = lib.mkForce [ ];
+                environment.systemPackages = [
+                  inputs.fh.packages.${system}.default
+                  pkgs.git
+                ];
 
-            virtualisation.diskSize = lib.mkForce (4 * 1024);
+                virtualisation.diskSize = lib.mkForce (4 * 1024);
 
-            assertions =
-              [{
-                assertion = ((
-                  builtins.match
-                    "^[0-9][0-9]\.[0-9][0-9]\..*"
-                    config.system.nixos.label
-                ) != null);
-                message = "nixos image label is incorrect";
-              }];
-          })
-        ];
-      });
+                assertions = [
+                  {
+                    assertion = ((builtins.match "^[0-9][0-9]\.[0-9][0-9]\..*" config.system.nixos.label) != null);
+                    message = "nixos image label is incorrect";
+                  }
+                ];
+              }
+            )
+          ];
+        }
+      );
 
-      diskImages = forLinuxSystems ({ system, ... }: {
-        aws = self.nixosConfigurations.${system}.config.system.build.amazonImage;
-      });
+      diskImages = forLinuxSystems (
+        { system, ... }:
+        {
+          aws = self.nixosConfigurations.${system}.config.system.build.amazonImage;
+        }
+      );
 
-      devShells = forAllSystems ({ system, pkgs, lib, ... }: {
-        default = pkgs.mkShell {
-          packages = with pkgs; [
-            lychee
-            nixpkgs-fmt
-          ] ++ lib.optionals (builtins.elem system linuxSystems) [
-            inputs.nixos-amis.packages.${system}.upload-ami
-          ];
-        };
-      });
+      devShells = forAllSystems (
+        {
+          system,
+          pkgs,
+          lib,
+          ...
+        }:
+        {
+          default = pkgs.mkShell {
+            packages =
+              with pkgs;
+              [
+                lychee
+                (writeShellApplication {
+                  name = "check-external-links";
+                  runtimeInputs = with pkgs; [ lychee ];
+                  text = ''
+                    lychee \
+                      --cache \
+                      --max-concurrency 2 \
+                      --verbose \
+                      README.me
+                  '';
+                })
+                self.formatter.${system}
+              ]
+              ++ lib.optionals (builtins.elem system linuxSystems) [
+                inputs.nixos-amis.packages.${system}.upload-ami
+              ];
+          };
+        }
+      );
+
+      formatter = forAllSystems ({ pkgs, ... }: pkgs.nixfmt-rfc-style);
 
-      apps = forLinuxSystems ({ system, ... }: {
-        smoke-test = inputs.nixos-amis.apps.${system}.smoke-test;
-      });
+      apps = forLinuxSystems (
+        { system, ... }:
+        {
+          smoke-test = inputs.nixos-amis.apps.${system}.smoke-test;
+        }
+      );
 
       schemas = inputs.flake-schemas.schemas // {
         diskImages = {
diff --git a/result b/result
new file mode 120000
index 0000000..5714715
--- /dev/null
+++ b/result
@@ -0,0 +1 @@
+/nix/store/93rkfkf9arhvawzk6y9fq59m7bd0gij3-nix-shell
\ No newline at end of file