From 203801f864e2a067eb7820332bca941d9af41c64 Mon Sep 17 00:00:00 2001
From: sapphi-red <49056869+sapphi-red@users.noreply.github.com>
Date: Tue, 24 Feb 2026 14:44:35 +0900
Subject: [PATCH 1/2] feat: allow `defineConfig` with callback without `as
 const`

---
 .../vite/src/node/__tests_dts__/config.ts     | 37 ++++++++++++++++++-
 packages/vite/src/node/config.ts              | 12 ++++--
 2 files changed, 44 insertions(+), 5 deletions(-)

diff --git a/packages/vite/src/node/__tests_dts__/config.ts b/packages/vite/src/node/__tests_dts__/config.ts
index a6a6498d3ef304..891ec216d2acbe 100644
--- a/packages/vite/src/node/__tests_dts__/config.ts
+++ b/packages/vite/src/node/__tests_dts__/config.ts
@@ -33,10 +33,45 @@ export type cases1 = [
 
 defineConfig({
   base: '',
-  // @ts-expect-error
+  build: {
+    minify: 'oxc', // `as const` is not needed
+  },
+  // @ts-expect-error --- invalid option should error
   unknownProperty: 1,
 })
 
+defineConfig(() => ({
+  base: '',
+  build: {
+    minify: 'oxc', // `as const` is not needed
+  },
+  unknownProperty: 1, // we cannot catch invalid option for this case, ideally we should
+}))
+
+// @ts-expect-error --- nested invalid option `build.unknown` should error
+defineConfig(() => ({
+  base: '',
+  build: {
+    unknown: 1,
+  },
+}))
+
+defineConfig(async () => ({
+  base: '',
+  build: {
+    minify: 'oxc', // `as const` is not needed
+  },
+  unknownProperty: 1, // we cannot catch invalid option for this case, ideally we should
+}))
+
+// @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..3cbaefbc9037ed 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(config: UserConfig): UserConfig
+export function defineConfig<T extends UserConfigFn>(
+  config: T,
+): T extends UserConfigFnObject
+  ? UserConfigFnObject
+  : T extends UserConfigFnPromise
+    ? UserConfigFnPromise
+    : UserConfigFn
 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: UserConfig): UserConfig
 export function defineConfig(config: UserConfigExport): UserConfigExport
 export function defineConfig(config: UserConfigExport): UserConfigExport {
   return config

From 15cfebed6def8f1c81737c4dc8be67a63d204acd Mon Sep 17 00:00:00 2001
From: sapphi-red <49056869+sapphi-red@users.noreply.github.com>
Date: Tue, 24 Feb 2026 15:21:27 +0900
Subject: [PATCH 2/2] chore: revert

---
 .../vite/src/node/__tests_dts__/config.ts     | 25 +++++++++++++++++--
 packages/vite/src/node/config.ts              | 12 +++------
 2 files changed, 27 insertions(+), 10 deletions(-)

diff --git a/packages/vite/src/node/__tests_dts__/config.ts b/packages/vite/src/node/__tests_dts__/config.ts
index 891ec216d2acbe..35b2542eb20352 100644
--- a/packages/vite/src/node/__tests_dts__/config.ts
+++ b/packages/vite/src/node/__tests_dts__/config.ts
@@ -36,6 +36,13 @@ defineConfig({
   build: {
     minify: 'oxc', // `as const` is not needed
   },
+  server: {
+    proxy: {
+      '/test': {
+        bypass: () => false,
+      },
+    },
+  },
   // @ts-expect-error --- invalid option should error
   unknownProperty: 1,
 })
@@ -43,7 +50,14 @@ defineConfig({
 defineConfig(() => ({
   base: '',
   build: {
-    minify: 'oxc', // `as const` is not needed
+    minify: 'oxc' as const, // ideally we don't want to require `as const` here
+  },
+  server: {
+    proxy: {
+      '/test': {
+        bypass: () => false as const, // ideally we don't want to require `as const` here
+      },
+    },
   },
   unknownProperty: 1, // we cannot catch invalid option for this case, ideally we should
 }))
@@ -59,7 +73,14 @@ defineConfig(() => ({
 defineConfig(async () => ({
   base: '',
   build: {
-    minify: 'oxc', // `as const` is not needed
+    minify: 'oxc' as const, // ideally we don't want to require `as const` here
+  },
+  server: {
+    proxy: {
+      '/test': {
+        bypass: () => false as const, // ideally we don't want to require `as const` here
+      },
+    },
   },
   unknownProperty: 1, // we cannot catch invalid option for this case, ideally we should
 }))
diff --git a/packages/vite/src/node/config.ts b/packages/vite/src/node/config.ts
index 3cbaefbc9037ed..d45c6372f7d77b 100644
--- a/packages/vite/src/node/config.ts
+++ b/packages/vite/src/node/config.ts
@@ -172,15 +172,11 @@ 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,
-): T extends UserConfigFnObject
-  ? UserConfigFnObject
-  : T extends UserConfigFnPromise
-    ? UserConfigFnPromise
-    : UserConfigFn
-export function defineConfig(config: Promise<UserConfig>): Promise<UserConfig>
 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