diff --git a/.changeset/tidy-jars-listen.md b/.changeset/tidy-jars-listen.md
new file mode 100644
index 000000000..dca1debd4
--- /dev/null
+++ b/.changeset/tidy-jars-listen.md
@@ -0,0 +1,7 @@
+---
+'@cloudfour/patterns': patch
+---
+
+Load `src/index-with-dependencies.scss` with `@use` rather than `@import`, which Dart Sass 3 removes. This was the last `@import` in the repo.
+
+Compiled output loses seven duplicate `@font-face` rules and nothing else. `@use` loads each stylesheet once, so the fonts configured at the top of the file are emitted a single time; under `@import` they came out twice, because `./index` loads `base/fonts` as well and an `@import` does not share the module graph with the file importing it. Every surviving rule is byte-for-byte one of the originals, in the same order, and all other CSS is byte-for-byte identical — 1,245 bytes smaller in the Storybook build.
diff --git a/eslint.config.mjs b/eslint.config.mjs
index 72af2b46f..9ed127713 100644
--- a/eslint.config.mjs
+++ b/eslint.config.mjs
@@ -28,13 +28,7 @@ export default [
     languageOptions: {
       parserOptions: {
         tsconfigRootDir: import.meta.dirname,
-        projectService: {
-          // Our tsconfig covers `src`, because that is what ships. These files
-          // configure the tooling around it and belong to no project, so the type
-          // aware rules read them through an inferred one instead of failing to
-          // find them at all.
-          allowDefaultProject: ['vitest.config.mts', 'vitest.setup.*.ts'],
-        },
+        projectService: true,
       },
     },
   },
diff --git a/src/components/button-swap/button-swap.browser.test.ts b/src/components/button-swap/button-swap.browser.test.ts
index 7672ea017..7d077c055 100644
--- a/src/components/button-swap/button-swap.browser.test.ts
+++ b/src/components/button-swap/button-swap.browser.test.ts
@@ -7,12 +7,14 @@ import template from './button-swap.twig';
 
 const body = () => page.elementLocator(document.body);
 
-test('Swap UI state when clicked', async () => {
+test('Render both states, exposing only the initial one', async () => {
   document.body.innerHTML = template();
 
   // This package ships the markup for Button Swap but not the script that swaps it:
-  // that lives in the consuming site. So what this asserts is the initial state --
-  // only the unsubscribed button is exposed, and it is not the slashed variant.
+  // that lives in the consuming site. So there is no swap to exercise here -- what
+  // the template has to get right is that both states are present and only the
+  // unsubscribed one reaches the accessibility tree. The snapshot covers the second
+  // half: `hidden` keeps the swapped state out of it.
   await expect.element(body()).toMatchAriaInlineSnapshot(`
     - status: Currently unsubscribed from notifications
     - button "Get notifications"
@@ -21,6 +23,12 @@ test('Swap UI state when clicked', async () => {
   const firstBtn = page.getByRole('button');
   await expect.element(firstBtn).toBeVisible();
   await expect.element(firstBtn).not.toHaveClass('is-slashed');
+
+  const swapped = document.querySelector(
+    '.js-button-swap__swapped-button-wrapper',
+  );
+  expect(swapped).not.toBeNull();
+  expect(swapped).toHaveAttribute('hidden');
 });
 
 test('Set custom messages and labels', async () => {
diff --git a/src/index-with-dependencies.scss b/src/index-with-dependencies.scss
index d8ccdb293..f08129319 100644
--- a/src/index-with-dependencies.scss
+++ b/src/index-with-dependencies.scss
@@ -12,6 +12,9 @@
   $dir: '/src/assets/fonts'
 );
 
-// stylelint-disable at-rule-disallowed-list
-@import '../node_modules/@wordpress/block-library/build-style/style';
-@import './index';
+/// `@use` rather than `@import`, which Dart Sass 3 removes. Because `@use` loads each
+/// stylesheet once, the `@font-face` rules from `base/fonts` above are emitted a single
+/// time here; under `@import` they came out twice, since `./index` loads `base/fonts`
+/// too and an `@import` does not share the module graph with the file importing it.
+@use '../node_modules/@wordpress/block-library/build-style/style';
+@use './index';
diff --git a/tsconfig.json b/tsconfig.json
index fae584e2c..74af31df5 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -24,7 +24,14 @@
     "skipLibCheck": true,
     "resolveJsonModule": true
   },
-  "include": ["src"],
+  // `src` is what ships. The root-level globs pick up the tooling that configures the
+  // build and tests around it: those files belong to no project otherwise, so the
+  // type-aware lint rules either fail to find them or fall back to an inferred
+  // project that ignores the strictness settings above. Globs rather than a list, so
+  // the next root-level TypeScript file is covered without anyone remembering to add
+  // it. Nothing outside `ts-dist/src` reaches `dist`, so this does not widen what we
+  // publish.
+  "include": ["src", "*.ts", "*.mts", "*.cts"],
   "exclude": [
     "src/prototypes",
     "src/vendor/highlight/demo/samples",
diff --git a/twing/vite-plugin-twig.mjs b/twing/vite-plugin-twig.mjs
index 054861ca6..153bafa07 100644
--- a/twing/vite-plugin-twig.mjs
+++ b/twing/vite-plugin-twig.mjs
@@ -64,15 +64,22 @@ export const twigInputs =
 export const twigPlugin = () => ({
   name: 'cloudfour-twig',
 
+  /** @param {string} id */
   resolveId(id) {
     if (id === VIRTUAL_ID) return RESOLVED_VIRTUAL_ID;
   },
 
+  /** @param {string} id */
   load(id) {
     if (id === RESOLVED_VIRTUAL_ID) return environmentModule;
   },
 
-  transform(code, id) {
+  /**
+   * @param {string} _code The module source, which we never read: the transform
+   *   rewrites `.twig` imports into a render wrapper generated from the id alone.
+   * @param {string} id
+   */
+  transform(_code, id) {
     // Let `?raw` imports through untouched -- the environment module needs the source.
     if (!id.endsWith('.twig')) return;