diff --git a/.changeset/wild-boats-wait.md b/.changeset/wild-boats-wait.md
new file mode 100644
index 0000000000..6300e839ea
--- /dev/null
+++ b/.changeset/wild-boats-wait.md
@@ -0,0 +1,5 @@
+---
+'astro': patch
+---
+
+Returns the updated config in the integration `astro:config:setup` hook's `updateConfig()` API
diff --git a/packages/astro/src/@types/astro.ts b/packages/astro/src/@types/astro.ts
index 91d8d181e6..499170f253 100644
--- a/packages/astro/src/@types/astro.ts
+++ b/packages/astro/src/@types/astro.ts
@@ -28,7 +28,7 @@ import type { DevOverlayToggle } from '../runtime/client/dev-overlay/ui-library/
 import type { DevOverlayTooltip } from '../runtime/client/dev-overlay/ui-library/tooltip.js';
 import type { DevOverlayWindow } from '../runtime/client/dev-overlay/ui-library/window.js';
 import type { AstroComponentFactory, AstroComponentInstance } from '../runtime/server/index.js';
-import type { OmitIndexSignature, Simplify } from '../type-utils.js';
+import type { DeepPartial, OmitIndexSignature, Simplify } from '../type-utils.js';
 import type { SUPPORTED_MARKDOWN_FILE_EXTENSIONS } from './../core/constants.js';
 
 export { type AstroIntegrationLogger };
@@ -2335,7 +2335,7 @@ export interface AstroIntegration {
 			config: AstroConfig;
 			command: 'dev' | 'build' | 'preview';
 			isRestart: boolean;
-			updateConfig: (newConfig: Record<string, any>) => void;
+			updateConfig: (newConfig: DeepPartial<AstroConfig>) => AstroConfig;
 			addRenderer: (renderer: AstroRenderer) => void;
 			addWatchFile: (path: URL | string) => void;
 			injectScript: (stage: InjectedScriptStage, content: string) => void;
diff --git a/packages/astro/src/integrations/index.ts b/packages/astro/src/integrations/index.ts
index 8b40e5825f..ca2b89b303 100644
--- a/packages/astro/src/integrations/index.ts
+++ b/packages/astro/src/integrations/index.ts
@@ -118,6 +118,7 @@ export async function runHookConfigSetup({
 				},
 				updateConfig: (newConfig) => {
 					updatedConfig = mergeConfig(updatedConfig, newConfig) as AstroConfig;
+					return { ...updatedConfig };
 				},
 				injectRoute: (injectRoute) => {
 					updatedSettings.injectedRoutes.push(injectRoute);
@@ -374,6 +375,7 @@ export async function runHookBuildSetup({
 					target,
 					updateConfig: (newConfig) => {
 						updatedConfig = mergeConfig(updatedConfig, newConfig);
+						return { ...updatedConfig };
 					},
 					logger: getLogger(integration, logger),
 				}),
diff --git a/packages/astro/src/type-utils.ts b/packages/astro/src/type-utils.ts
index d777a9f287..98f2c9864f 100644
--- a/packages/astro/src/type-utils.ts
+++ b/packages/astro/src/type-utils.ts
@@ -30,3 +30,13 @@ export type ValueOf<T> = T[keyof T];
 
 // Gets the type of the values of a Map
 export type MapValue<T> = T extends Map<any, infer V> ? V : never;
+
+// Allow the user to create a type where all keys are optional.
+// Useful for functions where props are merged.
+export type DeepPartial<T> = {
+	[P in keyof T]?: T[P] extends (infer U)[]
+		? DeepPartial<U>[]
+		: T[P] extends object | undefined
+		? DeepPartial<T[P]>
+		: T[P];
+};
diff --git a/packages/astro/test/units/integrations/api.test.js b/packages/astro/test/units/integrations/api.test.js
index a4a76975a3..4aafec3757 100644
--- a/packages/astro/test/units/integrations/api.test.js
+++ b/packages/astro/test/units/integrations/api.test.js
@@ -30,6 +30,33 @@ describe('Integration API', () => {
 		expect(updatedViteConfig).to.haveOwnProperty('define');
 	});
 
+	it('runHookBuildSetup should return updated config', async () => {
+		let updatedInternalConfig;
+		const updatedViteConfig = await runHookBuildSetup({
+			config: {
+				integrations: [
+					{
+						name: 'test',
+						hooks: {
+							'astro:build:setup'({ updateConfig }) {
+								updatedInternalConfig = updateConfig({
+									define: {
+										foo: 'bar',
+									},
+								});
+							},
+						},
+					},
+				],
+			},
+			vite: {},
+			logger: defaultLogger,
+			pages: new Map(),
+			target: 'server',
+		});
+		expect(updatedViteConfig).to.be.deep.equal(updatedInternalConfig);
+	});
+
 	it('runHookConfigSetup can update Astro config', async () => {
 		const site = 'https://test.com/';
 		const updatedSettings = await runHookConfigSetup({
diff --git a/packages/integrations/solid/src/index.ts b/packages/integrations/solid/src/index.ts
index 127d9ddb66..8707c006f2 100644
--- a/packages/integrations/solid/src/index.ts
+++ b/packages/integrations/solid/src/index.ts
@@ -1,4 +1,4 @@
-import type { AstroIntegration, AstroRenderer } from 'astro';
+import type { AstroConfig, AstroIntegration, AstroRenderer } from 'astro';
 import solid, { type Options as ViteSolidPluginOptions } from 'vite-plugin-solid';
 
 async function getViteConfiguration(isDev: boolean, { include, exclude }: Options = {}) {
@@ -34,7 +34,7 @@ async function getViteConfiguration(isDev: boolean, { include, exclude }: Option
 		ssr: {
 			external: ['babel-preset-solid'],
 		},
-	};
+	} satisfies AstroConfig['vite'];
 }
 
 function getRenderer(): AstroRenderer {