From 2c8aae6a4a44304cc24519ee322cd2f04fd13123 Mon Sep 17 00:00:00 2001
From: Vishnu Vardhan P <vishnuuu24@gmail.com>
Date: Tue, 24 Feb 2026 15:47:09 +0530
Subject: [PATCH 1/2] fix(types): improve defineConfig callback inference

---
 .../vite/src/node/__tests_dts__/config.ts     | 22 +++++++++++++++++++
 packages/vite/src/node/config.ts              | 10 ++++++---
 2 files changed, 29 insertions(+), 3 deletions(-)

diff --git a/packages/vite/src/node/__tests_dts__/config.ts b/packages/vite/src/node/__tests_dts__/config.ts
index a6a6498d3ef304..f3fbe24e4588be 100644
--- a/packages/vite/src/node/__tests_dts__/config.ts
+++ b/packages/vite/src/node/__tests_dts__/config.ts
@@ -37,6 +37,28 @@ defineConfig({
   unknownProperty: 1,
 })
 
+defineConfig(() => ({
+  base: '',
+  build: {
+    minify: 'oxc',
+  },
+}))
+
+defineConfig(async () => ({
+  base: '',
+  build: {
+    minify: 'terser',
+  },
+}))
+
+// @ts-expect-error --- nested invalid option `build.unknown` should error
+defineConfig(async () => ({
+  base: '',
+  build: {
+    unknown: 1,
+  },
+}))
+
 mergeConfig(defineConfig({}), defineConfig({}))
 mergeConfig(
   // @ts-expect-error
diff --git a/packages/vite/src/node/config.ts b/packages/vite/src/node/config.ts
index d45c6372f7d77b..72f01d49443e34 100644
--- a/packages/vite/src/node/config.ts
+++ b/packages/vite/src/node/config.ts
@@ -172,11 +172,15 @@ export type UserConfigExport =
  * accepts a direct {@link UserConfig} object, or a function that returns it.
  * The function receives a {@link ConfigEnv} object.
  */
+export function defineConfig<T extends UserConfigFn>(
+  config: T & UserConfigFn,
+): T extends UserConfigFnObject
+  ? UserConfigFnObject
+  : T extends UserConfigFnPromise
+    ? UserConfigFnPromise
+    : UserConfigFn
 export function defineConfig(config: UserConfig): UserConfig
 export function defineConfig(config: Promise<UserConfig>): Promise<UserConfig>
-export function defineConfig(config: UserConfigFnObject): UserConfigFnObject
-export function defineConfig(config: UserConfigFnPromise): UserConfigFnPromise
-export function defineConfig(config: UserConfigFn): UserConfigFn
 export function defineConfig(config: UserConfigExport): UserConfigExport
 export function defineConfig(config: UserConfigExport): UserConfigExport {
   return config

From c8eceeebf125e8dc44ee99e50fa4cba3acc6e6ed Mon Sep 17 00:00:00 2001
From: Vishnu Vardhan P <vishnuuu24@gmail.com>
Date: Tue, 24 Feb 2026 16:07:03 +0530
Subject: [PATCH 2/2] test(playground): preserve false literal in proxy bypass

---
 playground/proxy-bypass/vite.config.js | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/playground/proxy-bypass/vite.config.js b/playground/proxy-bypass/vite.config.js
index 2b190df0ff8250..202b32811b1b39 100644
--- a/playground/proxy-bypass/vite.config.js
+++ b/playground/proxy-bypass/vite.config.js
@@ -9,7 +9,7 @@ export default defineConfig({
       '/nonExistentApp': {
         target: 'http://localhost:9607',
         bypass: () => {
-          return false
+          return /** @type {false} */ (false)
         },
       },
       '/proxyError': {