diff --git a/.changeset/fix-toc-global-config-zod-v4.md b/.changeset/fix-toc-global-config-zod-v4.md
new file mode 100644
index 00000000000..a9bca42e5c5
--- /dev/null
+++ b/.changeset/fix-toc-global-config-zod-v4.md
@@ -0,0 +1,5 @@
+---
+'@astrojs/starlight': patch
+---
+
+Fixes a regression causing global `tableOfContents` config to be ignored
diff --git a/packages/starlight/__tests__/toc-custom-config/toc-custom-config.test.ts b/packages/starlight/__tests__/toc-custom-config/toc-custom-config.test.ts
new file mode 100644
index 00000000000..a75b99c27d9
--- /dev/null
+++ b/packages/starlight/__tests__/toc-custom-config/toc-custom-config.test.ts
@@ -0,0 +1,55 @@
+import { describe, expect, test, vi } from 'vitest';
+import { routes } from '../../utils/routing';
+import { getRouteDataTestContext } from '../test-utils';
+import { generateRouteData } from '../../utils/routing/data';
+
+vi.mock('astro:content', async () =>
+	(await import('../test-utils')).mockedAstroContent({
+		docs: [
+			['index.mdx', { title: 'Home Page' }],
+			['showcase.mdx', { title: 'ToC Disabled', tableOfContents: false }],
+			[
+				'environmental-impact.md',
+				{ title: 'Explicit update date', tableOfContents: { minHeadingLevel: 2 } },
+			],
+		],
+	})
+);
+
+const headings = [
+	{ depth: 1, slug: 'heading-1', text: 'Heading 1' },
+	{ depth: 2, slug: 'heading-2', text: 'Heading 2' },
+	{ depth: 3, slug: 'heading-3', text: 'Heading 3' },
+	{ depth: 4, slug: 'heading-4', text: 'Heading 4' },
+];
+
+describe('custom table of contents config', () => {
+	test('table of contents heading levels match configuration', () => {
+		const route = routes[0]!;
+		const data = generateRouteData({
+			props: { ...route, headings },
+			context: getRouteDataTestContext(),
+		});
+		expect(data.toc?.minHeadingLevel).toBe(1);
+		expect(data.toc?.maxHeadingLevel).toBe(4);
+	});
+
+	test('table of contents can be disabled by frontmatter', () => {
+		const route = routes[1]!;
+		const data = generateRouteData({
+			props: { ...route, headings },
+			context: getRouteDataTestContext(),
+		});
+		expect(data.toc).toBeUndefined();
+	});
+
+	test('table of contents heading levels can be customised by frontmatter', () => {
+		const route = routes[2]!;
+		const data = generateRouteData({
+			props: { ...route, headings },
+			context: getRouteDataTestContext(),
+		});
+		expect(data.toc?.minHeadingLevel).toBe(2);
+		expect(data.toc?.maxHeadingLevel).toBe(3);
+	});
+});
diff --git a/packages/starlight/__tests__/toc-custom-config/vitest.config.ts b/packages/starlight/__tests__/toc-custom-config/vitest.config.ts
new file mode 100644
index 00000000000..8e08279bfbb
--- /dev/null
+++ b/packages/starlight/__tests__/toc-custom-config/vitest.config.ts
@@ -0,0 +1,9 @@
+import { defineVitestConfig } from '../test-config';
+
+export default defineVitestConfig({
+	title: 'custom ToC config',
+	tableOfContents: {
+		minHeadingLevel: 1,
+		maxHeadingLevel: 4,
+	},
+});
diff --git a/packages/starlight/schema.ts b/packages/starlight/schema.ts
index a90220b87e6..746734f3f78 100644
--- a/packages/starlight/schema.ts
+++ b/packages/starlight/schema.ts
@@ -2,7 +2,7 @@ import { z } from 'astro/zod';
 import type { SchemaContext } from 'astro:content';
 import { HeadConfigSchema } from './schemas/head';
 import { PrevNextLinkConfigSchema } from './schemas/prevNextLink';
-import { TableOfContentsSchema } from './schemas/tableOfContents';
+import { FrontmatterTableOfContentsSchema } from './schemas/tableOfContents';
 import { BadgeConfigSchema } from './schemas/badge';
 import { HeroSchema } from './schemas/hero';
 import { SidebarLinkItemHTMLAttributesSchema } from './schemas/sidebar';
@@ -33,7 +33,7 @@ const StarlightFrontmatterSchema = (context: SchemaContext) =>
 		head: HeadConfigSchema({ source: 'content' }),
 
 		/** Override global table of contents configuration for this page. */
-		tableOfContents: TableOfContentsSchema().optional(),
+		tableOfContents: FrontmatterTableOfContentsSchema(),
 
 		/**
 		 * Set the layout style for this page.
diff --git a/packages/starlight/schemas/tableOfContents.ts b/packages/starlight/schemas/tableOfContents.ts
index b83ea80b958..918a010f9b3 100644
--- a/packages/starlight/schemas/tableOfContents.ts
+++ b/packages/starlight/schemas/tableOfContents.ts
@@ -2,18 +2,20 @@ import { z } from 'astro/zod';
 
 const defaults = { minHeadingLevel: 2, maxHeadingLevel: 3 };
 
-export const TableOfContentsSchema = () =>
-	z
-		.union([
-			z.object({
-				/** The level to start including headings at in the table of contents. Default: 2. */
-				minHeadingLevel: z.int().min(1).max(6).optional().default(2),
-				/** The level to stop including headings at in the table of contents. Default: 3. */
-				maxHeadingLevel: z.int().min(1).max(6).optional().default(3),
-			}),
-			z.boolean().transform((enabled) => (enabled ? defaults : false)),
-		])
-		.default(defaults)
-		.refine((toc) => (toc ? toc.minHeadingLevel <= toc.maxHeadingLevel : true), {
-			error: 'minHeadingLevel must be less than or equal to maxHeadingLevel',
-		});
+const TableOfContentsBaseSchema = z
+	.union([
+		z.object({
+			/** The level to start including headings at in the table of contents. Default: 2. */
+			minHeadingLevel: z.int().min(1).max(6).optional().default(2),
+			/** The level to stop including headings at in the table of contents. Default: 3. */
+			maxHeadingLevel: z.int().min(1).max(6).optional().default(3),
+		}),
+		z.boolean().transform((enabled) => (enabled ? defaults : false)),
+	])
+	.refine((toc) => (toc ? toc.minHeadingLevel <= toc.maxHeadingLevel : true), {
+		error: 'minHeadingLevel must be less than or equal to maxHeadingLevel',
+	});
+
+export const UserConfigTableOfContentsSchema = () => TableOfContentsBaseSchema.default(defaults);
+
+export const FrontmatterTableOfContentsSchema = () => TableOfContentsBaseSchema.optional();
diff --git a/packages/starlight/utils/user-config.ts b/packages/starlight/utils/user-config.ts
index 675adae7484..f896b663485 100644
--- a/packages/starlight/utils/user-config.ts
+++ b/packages/starlight/utils/user-config.ts
@@ -9,7 +9,7 @@ import { PagefindConfigDefaults, PagefindConfigSchema } from '../schemas/pagefin
 import { SidebarItemSchema } from '../schemas/sidebar';
 import { TitleConfigSchema, TitleTransformConfigSchema } from '../schemas/site-title';
 import { SocialLinksSchema } from '../schemas/social';
-import { TableOfContentsSchema } from '../schemas/tableOfContents';
+import { UserConfigTableOfContentsSchema } from '../schemas/tableOfContents';
 import { BuiltInDefaultLocale } from './i18n';
 
 const LocaleSchema = z.object({
@@ -49,7 +49,7 @@ const UserConfigSchema = z.object({
 	tagline: z.string().optional(),
 
 	/** Configure the defaults for the table of contents on each page. */
-	tableOfContents: TableOfContentsSchema(),
+	tableOfContents: UserConfigTableOfContentsSchema(),
 
 	/** Enable and configure “Edit this page” links. */
 	editLink: z