diff --git a/.changeset/great-plums-study.md b/.changeset/great-plums-study.md
new file mode 100644
index 0000000000..17765e1809
--- /dev/null
+++ b/.changeset/great-plums-study.md
@@ -0,0 +1,9 @@
+---
+'@astrojs/starlight': minor
+---
+
+Makes sidebar entry parsing stricter in Starlight config
+
+**⚠️ Potentially breaking change:** Previously Starlight would accept a sidebar entry that matched one of its expected shapes, even if it included additional properties. For example, including both `link` and `items` was considered valid, with `items` being ignored. Now, it is an error to include more than one of `link`, `items`, or `autogenerate` in a sidebar entry.
+
+If you see errors after updating, look for sidebar entries in the Starlight configuration in `astro.config.mjs` that include too many keys and remove the one that was previously ignored.
diff --git a/packages/starlight/__tests__/basics/config-errors.test.ts b/packages/starlight/__tests__/basics/config-errors.test.ts
index e48ea1a74e..1639bd72e9 100644
--- a/packages/starlight/__tests__/basics/config-errors.test.ts
+++ b/packages/starlight/__tests__/basics/config-errors.test.ts
@@ -165,7 +165,7 @@ test('errors with bad sidebar config', () => {
 			Invalid config passed to starlight integration
 		Hint:
 			**sidebar.0**: Did not match union.
-			> Expected type \`{ link: string } | { items: array } | { autogenerate: object }\`
+			> Expected type \`{ link: string;  } | { items: array;  } | { autogenerate: object;  }\`
 			> Received \`{ "label": "Example", "href": "/" }\`"
 	`
 	);
@@ -190,7 +190,57 @@ test('errors with bad nested sidebar config', () => {
 			Invalid config passed to starlight integration
 		Hint:
 			**sidebar.0.items.1**: Did not match union.
-			> Expected type \`{ link: string } | { items: array } | { autogenerate: object }\`
+			> Expected type \`{ link: string } | { items: array;  } | { autogenerate: object;  }\`
 			> Received \`{ "label": "Example", "items": [ { "label": "Nested Example 1", "link": "/" }, { "label": "Nested Example 2", "link": true } ] }\`"
 	`);
 });
+
+test('errors with sidebar entry that includes `link` and `items`', () => {
+	expect(() =>
+		parseStarlightConfigWithFriendlyErrors({
+			title: 'Test',
+			sidebar: [
+				{ label: 'Parent', link: '/parent', items: [{ label: 'Child', link: '/parent/child' }] },
+			],
+		})
+	).toThrowErrorMatchingInlineSnapshot(`
+		"[AstroUserError]:
+			Invalid config passed to starlight integration
+		Hint:
+			**sidebar.0**: Unrecognized key(s) in object: 'items'"
+	`);
+});
+
+test('errors with sidebar entry that includes `link` and `autogenerate`', () => {
+	expect(() =>
+		parseStarlightConfigWithFriendlyErrors({
+			title: 'Test',
+			sidebar: [{ label: 'Parent', link: '/parent', autogenerate: { directory: 'test' } }],
+		})
+	).toThrowErrorMatchingInlineSnapshot(`
+		"[AstroUserError]:
+			Invalid config passed to starlight integration
+		Hint:
+			**sidebar.0**: Unrecognized key(s) in object: 'autogenerate'"
+	`);
+});
+
+test('errors with sidebar entry that includes `items` and `autogenerate`', () => {
+	expect(() =>
+		parseStarlightConfigWithFriendlyErrors({
+			title: 'Test',
+			sidebar: [
+				{
+					label: 'Parent',
+					items: [{ label: 'Child', link: '/parent/child' }],
+					autogenerate: { directory: 'test' },
+				},
+			],
+		})
+	).toThrowErrorMatchingInlineSnapshot(`
+		"[AstroUserError]:
+			Invalid config passed to starlight integration
+		Hint:
+			**sidebar.0**: Unrecognized key(s) in object: 'autogenerate'"
+	`);
+});
diff --git a/packages/starlight/schemas/sidebar.ts b/packages/starlight/schemas/sidebar.ts
index 86eb7f87e4..41b1c3ae29 100644
--- a/packages/starlight/schemas/sidebar.ts
+++ b/packages/starlight/schemas/sidebar.ts
@@ -33,7 +33,7 @@ const SidebarLinkItemSchema = SidebarBaseSchema.extend({
 	link: z.string(),
 	/** HTML attributes to add to the link item. */
 	attrs: SidebarLinkItemHTMLAttributesSchema(),
-});
+}).strict();
 export type SidebarLinkItem = z.infer<typeof SidebarLinkItemSchema>;
 
 const AutoSidebarGroupSchema = SidebarGroupSchema.extend({
@@ -50,7 +50,7 @@ const AutoSidebarGroupSchema = SidebarGroupSchema.extend({
 		/** How many directories deep to include from this directory in the sidebar. Default: `Infinity`. */
 		// depth: z.number().optional(),
 	}),
-});
+}).strict();
 export type AutoSidebarGroup = z.infer<typeof AutoSidebarGroupSchema>;
 
 type ManualSidebarGroupInput = z.input<typeof SidebarGroupSchema> & {
@@ -80,7 +80,7 @@ const ManualSidebarGroupSchema: z.ZodType<
 	items: z.lazy(() =>
 		z.union([SidebarLinkItemSchema, ManualSidebarGroupSchema, AutoSidebarGroupSchema]).array()
 	),
-});
+}).strict();
 
 export const SidebarItemSchema = z.union([
 	SidebarLinkItemSchema,