Fix trailing slash inconsistency in sidebar links (#2918)

12 files changed, 226 insertions, 37 deletions

diff --git a/.changeset/large-chairs-refuse.md b/.changeset/large-chairs-refuse.md
new file mode 100644
index 00000000..28f53b08
--- /dev/null
+++ b/.changeset/large-chairs-refuse.md
@@ -0,0 +1,5 @@
+---
+'@astrojs/starlight': patch
+---
+
+Fixes a trailing slash inconsistency in generated sidebar links when using the [`trailingSlash: 'ignore'`](https://docs.astro.build/en/reference/configuration-reference/#trailingslash) Astro option (the default) between [internal](https://starlight.astro.build/guides/sidebar/#internal-links) and [auto-generated](https://starlight.astro.build/guides/sidebar/#autogenerated-groups) links. Starlight behavior for this configuration value is to use a trailing slash as many common hosting providers redirect to URLs with a trailing slash by default.
diff --git a/packages/starlight/__tests__/basics/starlight-page-route-data.test.ts b/packages/starlight/__tests__/basics/starlight-page-route-data.test.ts
index b9a23232..972dbfe4 100644
--- a/packages/starlight/__tests__/basics/starlight-page-route-data.test.ts
+++ b/packages/starlight/__tests__/basics/starlight-page-route-data.test.ts
@@ -238,7 +238,7 @@ test('uses provided sidebar if any', async () => {
 		  {
 		    "attrs": {},
 		    "badge": undefined,
-		    "href": "/reference/frontmatter",
+		    "href": "/reference/frontmatter/",
 		    "isCurrent": false,
 		    "label": "Frontmatter Reference",
 		    "type": "link",
diff --git a/packages/starlight/__tests__/i18n-sidebar/i18n-sidebar-fallback-slug.test.ts b/packages/starlight/__tests__/i18n-sidebar/i18n-sidebar-fallback-slug.test.ts
index 750c4244..725a312d 100644
--- a/packages/starlight/__tests__/i18n-sidebar/i18n-sidebar-fallback-slug.test.ts
+++ b/packages/starlight/__tests__/i18n-sidebar/i18n-sidebar-fallback-slug.test.ts
@@ -30,7 +30,7 @@ describe('getSidebar', () => {
 			  {
 			    "attrs": {},
 			    "badge": undefined,
-			    "href": "/getting-started",
+			    "href": "/getting-started/",
 			    "isCurrent": false,
 			    "label": "Getting Started",
 			    "type": "link",
@@ -41,7 +41,7 @@ describe('getSidebar', () => {
 			      "text": "New",
 			      "variant": "default",
 			    },
-			    "href": "/manual-setup",
+			    "href": "/manual-setup/",
 			    "isCurrent": false,
 			    "label": "Do it yourself",
 			    "type": "link",
@@ -52,7 +52,7 @@ describe('getSidebar', () => {
 			      "text": "Eco-friendly",
 			      "variant": "success",
 			    },
-			    "href": "/environmental-impact",
+			    "href": "/environmental-impact/",
 			    "isCurrent": false,
 			    "label": "Eco-friendly docs",
 			    "type": "link",
@@ -64,7 +64,7 @@ describe('getSidebar', () => {
 			      {
 			        "attrs": {},
 			        "badge": undefined,
-			        "href": "/guides/pages",
+			        "href": "/guides/pages/",
 			        "isCurrent": false,
 			        "label": "Pages",
 			        "type": "link",
@@ -75,7 +75,7 @@ describe('getSidebar', () => {
 			          "text": "Deprecated",
 			          "variant": "default",
 			        },
-			        "href": "/guides/authoring-content",
+			        "href": "/guides/authoring-content/",
 			        "isCurrent": false,
 			        "label": "Authoring Content in Markdown",
 			        "type": "link",
@@ -87,7 +87,7 @@ describe('getSidebar', () => {
 			  {
 			    "attrs": {},
 			    "badge": undefined,
-			    "href": "/resources/plugins",
+			    "href": "/resources/plugins/",
 			    "isCurrent": false,
 			    "label": "Plugins and Integrations",
 			    "type": "link",
@@ -101,7 +101,7 @@ describe('getSidebar', () => {
 			  {
 			    "attrs": {},
 			    "badge": undefined,
-			    "href": "/fr",
+			    "href": "/fr/",
 			    "isCurrent": true,
 			    "label": "Starlight 🌟 Build documentation sites with Astro",
 			    "type": "link",
@@ -109,7 +109,7 @@ describe('getSidebar', () => {
 			  {
 			    "attrs": {},
 			    "badge": undefined,
-			    "href": "/fr/getting-started",
+			    "href": "/fr/getting-started/",
 			    "isCurrent": false,
 			    "label": "Getting Started",
 			    "type": "link",
@@ -120,7 +120,7 @@ describe('getSidebar', () => {
 			      "text": "Nouveau",
 			      "variant": "default",
 			    },
-			    "href": "/fr/manual-setup",
+			    "href": "/fr/manual-setup/",
 			    "isCurrent": false,
 			    "label": "Fait maison",
 			    "type": "link",
@@ -131,7 +131,7 @@ describe('getSidebar', () => {
 			      "text": "Écologique",
 			      "variant": "success",
 			    },
-			    "href": "/fr/environmental-impact",
+			    "href": "/fr/environmental-impact/",
 			    "isCurrent": false,
 			    "label": "Eco-friendly docs",
 			    "type": "link",
@@ -143,7 +143,7 @@ describe('getSidebar', () => {
 			      {
 			        "attrs": {},
 			        "badge": undefined,
-			        "href": "/fr/guides/pages",
+			        "href": "/fr/guides/pages/",
 			        "isCurrent": false,
 			        "label": "Pages",
 			        "type": "link",
@@ -154,7 +154,7 @@ describe('getSidebar', () => {
 			          "text": "Deprecated",
 			          "variant": "default",
 			        },
-			        "href": "/fr/guides/authoring-content",
+			        "href": "/fr/guides/authoring-content/",
 			        "isCurrent": false,
 			        "label": "Authoring Content in Markdown",
 			        "type": "link",
@@ -166,7 +166,7 @@ describe('getSidebar', () => {
 			  {
 			    "attrs": {},
 			    "badge": undefined,
-			    "href": "/fr/resources/plugins",
+			    "href": "/fr/resources/plugins/",
 			    "isCurrent": false,
 			    "label": "Plugins and Integrations",
 			    "type": "link",
diff --git a/packages/starlight/__tests__/i18n-sidebar/i18n-sidebar.test.ts b/packages/starlight/__tests__/i18n-sidebar/i18n-sidebar.test.ts
index ee62e268..9d7e005e 100644
--- a/packages/starlight/__tests__/i18n-sidebar/i18n-sidebar.test.ts
+++ b/packages/starlight/__tests__/i18n-sidebar/i18n-sidebar.test.ts
@@ -43,7 +43,7 @@ describe('getSidebar', () => {
 			  {
 			    "attrs": {},
 			    "badge": undefined,
-			    "href": "/getting-started",
+			    "href": "/getting-started/",
 			    "isCurrent": false,
 			    "label": "Getting Started",
 			    "type": "link",
@@ -54,7 +54,7 @@ describe('getSidebar', () => {
 			      "text": "New",
 			      "variant": "default",
 			    },
-			    "href": "/manual-setup",
+			    "href": "/manual-setup/",
 			    "isCurrent": false,
 			    "label": "Do it yourself",
 			    "type": "link",
@@ -65,7 +65,7 @@ describe('getSidebar', () => {
 			      "text": "Eco-friendly",
 			      "variant": "success",
 			    },
-			    "href": "/environmental-impact",
+			    "href": "/environmental-impact/",
 			    "isCurrent": false,
 			    "label": "Eco-friendly docs",
 			    "type": "link",
@@ -82,7 +82,7 @@ describe('getSidebar', () => {
 			          "text": "Test",
 			          "variant": "default",
 			        },
-			        "href": "/guides/pages",
+			        "href": "/guides/pages/",
 			        "isCurrent": false,
 			        "label": "Pages Guide",
 			        "type": "link",
@@ -93,7 +93,7 @@ describe('getSidebar', () => {
 			          "text": "Deprecated",
 			          "variant": "default",
 			        },
-			        "href": "/guides/authoring-content",
+			        "href": "/guides/authoring-content/",
 			        "isCurrent": false,
 			        "label": "Authoring Content in Markdown",
 			        "type": "link",
@@ -105,7 +105,7 @@ describe('getSidebar', () => {
 			  {
 			    "attrs": {},
 			    "badge": undefined,
-			    "href": "/resources/plugins",
+			    "href": "/resources/plugins/",
 			    "isCurrent": false,
 			    "label": "Plugins and Integrations",
 			    "type": "link",
@@ -119,7 +119,7 @@ describe('getSidebar', () => {
 			  {
 			    "attrs": {},
 			    "badge": undefined,
-			    "href": "/fr",
+			    "href": "/fr/",
 			    "isCurrent": true,
 			    "label": "Starlight 🌟 Construire des sites de documentation avec Astro",
 			    "type": "link",
@@ -127,7 +127,7 @@ describe('getSidebar', () => {
 			  {
 			    "attrs": {},
 			    "badge": undefined,
-			    "href": "/fr/getting-started",
+			    "href": "/fr/getting-started/",
 			    "isCurrent": false,
 			    "label": "Mise en route",
 			    "type": "link",
@@ -138,7 +138,7 @@ describe('getSidebar', () => {
 			      "text": "Nouveau",
 			      "variant": "default",
 			    },
-			    "href": "/fr/manual-setup",
+			    "href": "/fr/manual-setup/",
 			    "isCurrent": false,
 			    "label": "Fait maison",
 			    "type": "link",
@@ -149,7 +149,7 @@ describe('getSidebar', () => {
 			      "text": "Écologique",
 			      "variant": "success",
 			    },
-			    "href": "/fr/environmental-impact",
+			    "href": "/fr/environmental-impact/",
 			    "isCurrent": false,
 			    "label": "Documents écologiques",
 			    "type": "link",
@@ -161,7 +161,7 @@ describe('getSidebar', () => {
 			      {
 			        "attrs": {},
 			        "badge": undefined,
-			        "href": "/fr/guides/pages",
+			        "href": "/fr/guides/pages/",
 			        "isCurrent": false,
 			        "label": "Pages",
 			        "type": "link",
@@ -172,7 +172,7 @@ describe('getSidebar', () => {
 			          "text": "Deprecated",
 			          "variant": "default",
 			        },
-			        "href": "/fr/guides/authoring-content",
+			        "href": "/fr/guides/authoring-content/",
 			        "isCurrent": false,
 			        "label": "Création de contenu en Markdown",
 			        "type": "link",
@@ -184,7 +184,7 @@ describe('getSidebar', () => {
 			  {
 			    "attrs": {},
 			    "badge": undefined,
-			    "href": "/fr/resources/plugins",
+			    "href": "/fr/resources/plugins/",
 			    "isCurrent": false,
 			    "label": "Modules d'extension et outils",
 			    "type": "link",
@@ -198,7 +198,7 @@ describe('getSidebar', () => {
 			  {
 			    "attrs": {},
 			    "badge": undefined,
-			    "href": "/fr",
+			    "href": "/fr/",
 			    "isCurrent": false,
 			    "label": "Starlight 🌟 Construire des sites de documentation avec Astro",
 			    "type": "link",
@@ -206,7 +206,7 @@ describe('getSidebar', () => {
 			  {
 			    "attrs": {},
 			    "badge": undefined,
-			    "href": "/fr/getting-started",
+			    "href": "/fr/getting-started/",
 			    "isCurrent": true,
 			    "label": "Mise en route",
 			    "type": "link",
@@ -217,7 +217,7 @@ describe('getSidebar', () => {
 			      "text": "Nouveau",
 			      "variant": "default",
 			    },
-			    "href": "/fr/manual-setup",
+			    "href": "/fr/manual-setup/",
 			    "isCurrent": false,
 			    "label": "Fait maison",
 			    "type": "link",
@@ -228,7 +228,7 @@ describe('getSidebar', () => {
 			      "text": "Écologique",
 			      "variant": "success",
 			    },
-			    "href": "/fr/environmental-impact",
+			    "href": "/fr/environmental-impact/",
 			    "isCurrent": false,
 			    "label": "Documents écologiques",
 			    "type": "link",
@@ -240,7 +240,7 @@ describe('getSidebar', () => {
 			      {
 			        "attrs": {},
 			        "badge": undefined,
-			        "href": "/fr/guides/pages",
+			        "href": "/fr/guides/pages/",
 			        "isCurrent": false,
 			        "label": "Pages",
 			        "type": "link",
@@ -251,7 +251,7 @@ describe('getSidebar', () => {
 			          "text": "Deprecated",
 			          "variant": "default",
 			        },
-			        "href": "/fr/guides/authoring-content",
+			        "href": "/fr/guides/authoring-content/",
 			        "isCurrent": false,
 			        "label": "Création de contenu en Markdown",
 			        "type": "link",
@@ -263,7 +263,7 @@ describe('getSidebar', () => {
 			  {
 			    "attrs": {},
 			    "badge": undefined,
-			    "href": "/fr/resources/plugins",
+			    "href": "/fr/resources/plugins/",
 			    "isCurrent": false,
 			    "label": "Modules d'extension et outils",
 			    "type": "link",
@@ -273,12 +273,12 @@ describe('getSidebar', () => {
 	});
 	test('uses label from config for internal links', () => {
 		const sidebar = getSidebar('/', undefined);
-		const entry = sidebar.find((item) => item.type === 'link' && item.href === '/manual-setup');
+		const entry = sidebar.find((item) => item.type === 'link' && item.href === '/manual-setup/');
 		expect(entry?.label).toBe('Do it yourself');
 	});
 	test('uses translation from config for internal links', () => {
 		const sidebar = getSidebar('/fr', 'fr');
-		const entry = sidebar.find((item) => item.type === 'link' && item.href === '/fr/manual-setup');
+		const entry = sidebar.find((item) => item.type === 'link' && item.href === '/fr/manual-setup/');
 		expect(entry?.label).toBe('Fait maison');
 	});
 	test('uses intermediate sidebars cached by locales', async () => {
diff --git a/packages/starlight/__tests__/sidebar/navigation-attributes.test.ts b/packages/starlight/__tests__/sidebar/navigation-attributes.test.ts
index 2dda3324..60d24a04 100644
--- a/packages/starlight/__tests__/sidebar/navigation-attributes.test.ts
+++ b/packages/starlight/__tests__/sidebar/navigation-attributes.test.ts
@@ -6,6 +6,8 @@ vi.mock('astro:content', async () =>
 		docs: [
 			['index.mdx', { title: 'Home Page' }],
 			['environmental-impact.md', { title: 'Eco-friendly docs' }],
+			['resources/plugins.mdx', { title: 'Plugins' }],
+			['resources/themes.mdx', { title: 'Themes' }],
 			[
 				'reference/frontmatter.md',
 				{
@@ -73,6 +75,30 @@ describe('getSidebar', () => {
 			    "type": "group",
 			  },
 			  {
+			    "badge": undefined,
+			    "collapsed": false,
+			    "entries": [
+			      {
+			        "attrs": {},
+			        "badge": undefined,
+			        "href": "/resources/plugins/",
+			        "isCurrent": false,
+			        "label": "Plugins",
+			        "type": "link",
+			      },
+			      {
+			        "attrs": {},
+			        "badge": undefined,
+			        "href": "/resources/themes/",
+			        "isCurrent": false,
+			        "label": "Themes",
+			        "type": "link",
+			      },
+			    ],
+			    "label": "Resources",
+			    "type": "group",
+			  },
+			  {
 			    "badge": {
 			      "text": "Experimental",
 			      "variant": "default",
diff --git a/packages/starlight/__tests__/sidebar/navigation-badges.test.ts b/packages/starlight/__tests__/sidebar/navigation-badges.test.ts
index d0963c2c..1a4e9e25 100644
--- a/packages/starlight/__tests__/sidebar/navigation-badges.test.ts
+++ b/packages/starlight/__tests__/sidebar/navigation-badges.test.ts
@@ -6,6 +6,8 @@ vi.mock('astro:content', async () =>
 		docs: [
 			['index.mdx', { title: 'Home Page' }],
 			['environmental-impact.md', { title: 'Eco-friendly docs' }],
+			['resources/plugins.mdx', { title: 'Plugins' }],
+			['resources/themes.mdx', { title: 'Themes' }],
 			[
 				'reference/configuration.mdx',
 				{
@@ -80,6 +82,30 @@ describe('getSidebar', () => {
 			    "type": "group",
 			  },
 			  {
+			    "badge": undefined,
+			    "collapsed": false,
+			    "entries": [
+			      {
+			        "attrs": {},
+			        "badge": undefined,
+			        "href": "/resources/plugins/",
+			        "isCurrent": false,
+			        "label": "Plugins",
+			        "type": "link",
+			      },
+			      {
+			        "attrs": {},
+			        "badge": undefined,
+			        "href": "/resources/themes/",
+			        "isCurrent": false,
+			        "label": "Themes",
+			        "type": "link",
+			      },
+			    ],
+			    "label": "Resources",
+			    "type": "group",
+			  },
+			  {
 			    "badge": {
 			      "text": "Experimental",
 			      "variant": "default",
diff --git a/packages/starlight/__tests__/sidebar/navigation-hidden.test.ts b/packages/starlight/__tests__/sidebar/navigation-hidden.test.ts
index ee5cc0c5..d2d92024 100644
--- a/packages/starlight/__tests__/sidebar/navigation-hidden.test.ts
+++ b/packages/starlight/__tests__/sidebar/navigation-hidden.test.ts
@@ -6,6 +6,8 @@ vi.mock('astro:content', async () =>
 		docs: [
 			['index.mdx', { title: 'Home Page' }],
 			['environmental-impact.md', { title: 'Eco-friendly docs' }],
+			['resources/plugins.mdx', { title: 'Plugins' }],
+			['resources/themes.mdx', { title: 'Themes' }],
 			['reference/configuration.mdx', { title: 'Config Reference' }],
 			['reference/frontmatter.md', { title: 'Frontmatter Reference', sidebar: { hidden: true } }],
 			['api/v1/users.md', { title: 'Users API' }],
@@ -69,6 +71,30 @@ describe('getSidebar', () => {
 			    "type": "group",
 			  },
 			  {
+			    "badge": undefined,
+			    "collapsed": false,
+			    "entries": [
+			      {
+			        "attrs": {},
+			        "badge": undefined,
+			        "href": "/resources/plugins/",
+			        "isCurrent": false,
+			        "label": "Plugins",
+			        "type": "link",
+			      },
+			      {
+			        "attrs": {},
+			        "badge": undefined,
+			        "href": "/resources/themes/",
+			        "isCurrent": false,
+			        "label": "Themes",
+			        "type": "link",
+			      },
+			    ],
+			    "label": "Resources",
+			    "type": "group",
+			  },
+			  {
 			    "badge": {
 			      "text": "Experimental",
 			      "variant": "default",
diff --git a/packages/starlight/__tests__/sidebar/navigation-order.test.ts b/packages/starlight/__tests__/sidebar/navigation-order.test.ts
index cce9a694..31e64413 100644
--- a/packages/starlight/__tests__/sidebar/navigation-order.test.ts
+++ b/packages/starlight/__tests__/sidebar/navigation-order.test.ts
@@ -6,6 +6,8 @@ vi.mock('astro:content', async () =>
 		docs: [
 			['index.mdx', { title: 'Home Page' }],
 			['environmental-impact.md', { title: 'Eco-friendly docs' }],
+			['resources/plugins.mdx', { title: 'Plugins' }],
+			['resources/themes.mdx', { title: 'Themes' }],
 			['reference/configuration.mdx', { title: 'Config Reference' }],
 			['reference/frontmatter.md', { title: 'Frontmatter Reference', sidebar: { order: 1 } }],
 			['api/v1/users.md', { title: 'Users API' }],
@@ -69,6 +71,30 @@ describe('getSidebar', () => {
 			    "type": "group",
 			  },
 			  {
+			    "badge": undefined,
+			    "collapsed": false,
+			    "entries": [
+			      {
+			        "attrs": {},
+			        "badge": undefined,
+			        "href": "/resources/plugins/",
+			        "isCurrent": false,
+			        "label": "Plugins",
+			        "type": "link",
+			      },
+			      {
+			        "attrs": {},
+			        "badge": undefined,
+			        "href": "/resources/themes/",
+			        "isCurrent": false,
+			        "label": "Themes",
+			        "type": "link",
+			      },
+			    ],
+			    "label": "Resources",
+			    "type": "group",
+			  },
+			  {
 			    "badge": {
 			      "text": "Experimental",
 			      "variant": "default",
diff --git a/packages/starlight/__tests__/sidebar/navigation-unicode.test.ts b/packages/starlight/__tests__/sidebar/navigation-unicode.test.ts
index 02c16bb4..c5bb2643 100644
--- a/packages/starlight/__tests__/sidebar/navigation-unicode.test.ts
+++ b/packages/starlight/__tests__/sidebar/navigation-unicode.test.ts
@@ -6,6 +6,8 @@ vi.mock('astro:content', async () =>
 		docs: [
 			['index.mdx', { title: 'Home Page' }],
 			['environmental-impact.md', { title: 'Eco-friendly docs' }],
+			['resources/plugins.mdx', { title: 'Plugins' }],
+			['resources/themes.mdx', { title: 'Themes' }],
 			['reference/configuration.mdx', { title: 'Config Reference' }],
 			['reference/frontmatter.md', { title: 'Frontmatter Reference' }],
 			['api/v1/用户.md', { title: 'Path with non-ASCII characters' }],
@@ -69,6 +71,30 @@ describe('getSidebar', () => {
 			    "type": "group",
 			  },
 			  {
+			    "badge": undefined,
+			    "collapsed": false,
+			    "entries": [
+			      {
+			        "attrs": {},
+			        "badge": undefined,
+			        "href": "/resources/plugins/",
+			        "isCurrent": false,
+			        "label": "Plugins",
+			        "type": "link",
+			      },
+			      {
+			        "attrs": {},
+			        "badge": undefined,
+			        "href": "/resources/themes/",
+			        "isCurrent": false,
+			        "label": "Themes",
+			        "type": "link",
+			      },
+			    ],
+			    "label": "Resources",
+			    "type": "group",
+			  },
+			  {
 			    "badge": {
 			      "text": "Experimental",
 			      "variant": "default",
diff --git a/packages/starlight/__tests__/sidebar/navigation.test.ts b/packages/starlight/__tests__/sidebar/navigation.test.ts
index f113d431..a2291819 100644
--- a/packages/starlight/__tests__/sidebar/navigation.test.ts
+++ b/packages/starlight/__tests__/sidebar/navigation.test.ts
@@ -1,11 +1,14 @@
 import { describe, expect, test, vi } from 'vitest';
 import { getSidebar } from '../../utils/navigation';
+import type { SidebarEntry } from '../../utils/routing/types';
 
 vi.mock('astro:content', async () =>
 	(await import('../test-utils')).mockedAstroContent({
 		docs: [
 			['index.mdx', { title: 'Home Page' }],
 			['environmental-impact.md', { title: 'Eco-friendly docs' }],
+			['resources/plugins.mdx', { title: 'Plugins' }],
+			['resources/themes.mdx', { title: 'Themes' }],
 			['reference/configuration.mdx', { title: 'Config Reference' }],
 			['reference/frontmatter.md', { title: 'Frontmatter Reference' }],
 			['reference/frontmatter/foo.mdx', { title: 'Foo' }],
@@ -70,6 +73,30 @@ describe('getSidebar', () => {
 			    "type": "group",
 			  },
 			  {
+			    "badge": undefined,
+			    "collapsed": false,
+			    "entries": [
+			      {
+			        "attrs": {},
+			        "badge": undefined,
+			        "href": "/resources/plugins/",
+			        "isCurrent": false,
+			        "label": "Plugins",
+			        "type": "link",
+			      },
+			      {
+			        "attrs": {},
+			        "badge": undefined,
+			        "href": "/resources/themes/",
+			        "isCurrent": false,
+			        "label": "Themes",
+			        "type": "link",
+			      },
+			    ],
+			    "label": "Resources",
+			    "type": "group",
+			  },
+			  {
 			    "badge": {
 			      "text": "Experimental",
 			      "variant": "default",
@@ -147,4 +174,21 @@ describe('getSidebar', () => {
 			]
 		`);
 	});
+
+	test("ensures trailing slash consistency between internal and auto-generated sidebar links when using `trailingSlash: 'ignore'`", () => {
+		const sidebar = getSidebar('/', undefined);
+		const internalLinksGroup = sidebar.at(2);
+		const autoGeneratedLinksGroup = sidebar.at(3);
+
+		function includesOnlyLinksWithTrailingSlash(entry: SidebarEntry | undefined): boolean {
+			return (
+				entry !== undefined &&
+				((entry.type === 'link' && entry.href.endsWith('/')) ||
+					(entry.type === 'group' && entry.entries.every(includesOnlyLinksWithTrailingSlash)))
+			);
+		}
+
+		expect(includesOnlyLinksWithTrailingSlash(internalLinksGroup)).toBe(true);
+		expect(includesOnlyLinksWithTrailingSlash(autoGeneratedLinksGroup)).toBe(true);
+	});
 });
diff --git a/packages/starlight/__tests__/sidebar/vitest.config.ts b/packages/starlight/__tests__/sidebar/vitest.config.ts
index 8eceebbc..205238c9 100644
--- a/packages/starlight/__tests__/sidebar/vitest.config.ts
+++ b/packages/starlight/__tests__/sidebar/vitest.config.ts
@@ -25,6 +25,11 @@ export default defineVitestConfig({
 				},
 			],
 		},
+		// A group containing internal links using the `slug` property or its shorthand string syntax.
+		{
+			label: 'Resources',
+			items: ['resources/plugins', { slug: 'resources/themes' }],
+		},
 		// A group linking to all pages in the `reference` directory.
 		{
 			label: 'Reference',
diff --git a/packages/starlight/utils/navigation.ts b/packages/starlight/utils/navigation.ts
index cfcd2e83..2870d619 100644
--- a/packages/starlight/utils/navigation.ts
+++ b/packages/starlight/utils/navigation.ts
@@ -165,7 +165,12 @@ function linkFromInternalSidebarLinkItem(
 		frontmatter.title;
 	const badge = item.badge ?? frontmatter.sidebar?.badge;
 	const attrs = { ...frontmatter.sidebar?.attrs, ...item.attrs };
-	return makeSidebarLink(route.slug, label, getSidebarBadge(badge, locale, label), attrs);
+	return makeSidebarLink(
+		slugToPathname(route.slug),
+		label,
+		getSidebarBadge(badge, locale, label),
+		attrs
+	);
 }
 
 /** Process sidebar link options to create a link entry. */