diff --git a/nix/checks.ulu.nix b/nix/checks.ulu.nix
index ecb0f6a..e0f6f72 100644
--- a/nix/checks.ulu.nix
+++ b/nix/checks.ulu.nix
@@ -1,10 +1,45 @@
-{ ... }:
+{ self, ... }:
 {
   perSystem =
-    { pkgs, self', ... }:
+    {
+      pkgs,
+      lib,
+      self',
+      ...
+    }:
     let
       valw = self'.packages.default;
 
+      # The home-manager module, evaluated with stand-ins for the two
+      # home-manager options it sets (no home-manager input needed).
+      homeModule =
+        settings:
+        (lib.evalModules {
+          modules = [
+            self.homeModules.default
+            (
+              { lib, ... }:
+              {
+                options.home.packages = lib.mkOption {
+                  type = lib.types.listOf lib.types.package;
+                  default = [ ];
+                };
+                options.xdg.configFile = lib.mkOption {
+                  type = lib.types.attrsOf (
+                    lib.types.submodule { options.source = lib.mkOption { type = lib.types.path; }; }
+                  );
+                  default = { };
+                };
+                config._module.args.pkgs = pkgs;
+                config.programs.valw = {
+                  enable = true;
+                  inherit settings;
+                };
+              }
+            )
+          ];
+        }).config;
+
       # Reuses the package's vendored dependencies, runs `cmd` instead of the build.
       cargoCheck =
         name: tools: cmd:
@@ -19,6 +54,35 @@
     in
     {
       checks = {
+        home-module =
+          let
+            plain = homeModule { };
+            set = homeModule {
+              preview.timeout_secs = 3;
+              sound.volume = 0.4;
+            };
+          in
+          assert lib.assertMsg (plain.home.packages == [ valw ]) "the module installs valw";
+          assert lib.assertMsg (plain.xdg.configFile == { }) "no settings, no config file";
+          pkgs.runCommand "valw-home-module" { } ''
+            grep -qx 'timeout_secs = 3' ${set.xdg.configFile."valw/config.toml".source}
+            grep -qx 'volume = 0.4' ${set.xdg.configFile."valw/config.toml".source}
+            touch $out
+          '';
+
+        home-module-rejects =
+          let
+            fails =
+              settings:
+              pkgs.testers.testBuildFailure (homeModule settings).xdg.configFile."valw/config.toml".source;
+          in
+          pkgs.runCommand "valw-home-module-rejects" { } ''
+            grep -q 'sound.volume must be between 0 and 1' ${fails { sound.volume = 2; }}/testBuildFailure.log
+            grep -q 'unknown config key: preview.timout_secs' ${
+              fails { preview.timout_secs = 3; }
+            }/testBuildFailure.log
+            touch $out
+          '';
         clippy = cargoCheck "clippy" [ pkgs.clippy ] "cargo clippy --all-targets --offline -- -D warnings";
         nextest = cargoCheck "nextest" [ pkgs.cargo-nextest ] "cargo nextest run --offline";
 
diff --git a/nix/devshell.ulu.nix b/nix/devshell.ulu.nix
index f066634..291a44e 100644
--- a/nix/devshell.ulu.nix
+++ b/nix/devshell.ulu.nix
@@ -1,7 +1,12 @@
 { ... }:
 {
   perSystem =
-    { pkgs, lib, self', ... }:
+    {
+      pkgs,
+      lib,
+      self',
+      ...
+    }:
     {
       devShells.default = pkgs.mkShell {
         inputsFrom = [ self'.packages.default ];
diff --git a/nix/home-module.ulu.nix b/nix/home-module.ulu.nix
new file mode 100644
index 0000000..66311fb
--- /dev/null
+++ b/nix/home-module.ulu.nix
@@ -0,0 +1,57 @@
+{ withSystem, ... }:
+{
+  flake.homeModules.default =
+    {
+      config,
+      lib,
+      pkgs,
+      ...
+    }:
+    let
+      cfg = config.programs.valw;
+      toml = pkgs.formats.toml { };
+      # valw checks its own config: a typo or a bad value fails the build.
+      checked =
+        settings:
+        let
+          file = toml.generate "valw-config.toml" settings;
+        in
+        pkgs.runCommand "valw-config.toml" { } ''
+          ${lib.getExe cfg.package} __check-config ${file}
+          cp ${file} $out
+        '';
+    in
+    {
+      options.programs.valw = {
+        enable = lib.mkEnableOption "valw, macOS-style screenshots for niri";
+        package = lib.mkOption {
+          type = lib.types.package;
+          default = withSystem pkgs.stdenv.hostPlatform.system ({ config, ... }: config.packages.default);
+          defaultText = lib.literalExpression "valw.packages.\${system}.default";
+          description = "The valw package to install and to check the settings with.";
+        };
+        settings = lib.mkOption {
+          inherit (toml) type;
+          default = { };
+          example = lib.literalExpression ''
+            {
+              preview.timeout_secs = 3;
+              sound.volume = 0.4;
+              capture.window_shadow = true;
+            }
+          '';
+          description = ''
+            valw's config (`$XDG_CONFIG_HOME/valw/config.toml`), checked by
+            valw at build time. Empty means valw's defaults.
+          '';
+        };
+      };
+
+      config = lib.mkIf cfg.enable {
+        home.packages = [ cfg.package ];
+        xdg.configFile."valw/config.toml" = lib.mkIf (cfg.settings != { }) {
+          source = checked cfg.settings;
+        };
+      };
+    };
+}
diff --git a/nix/package.ulu.nix b/nix/package.ulu.nix
index c5cd557..5f32eed 100644
--- a/nix/package.ulu.nix
+++ b/nix/package.ulu.nix
@@ -16,6 +16,9 @@
               ".jj"
               # Plan ledgers and review notes; not part of the build.
               ".superpowers"
+              # Documentation: a doc change shouldn't rebuild the binary.
+              "docs"
+              "README.md"
               "result"
               "target"
             ]);