From 49fe17b59712a755925c2feafa59eb595bbbbadd Mon Sep 17 00:00:00 2001
From: sapphi-red <49056869+sapphi-red@users.noreply.github.com>
Date: Wed, 22 Apr 2026 11:44:00 +0900
Subject: [PATCH 1/2] refactor: remove format sniffing module resolution from
 JS resolver

---
 packages/vite/src/node/plugins/resolve.ts | 55 +----------------------
 1 file changed, 1 insertion(+), 54 deletions(-)

diff --git a/packages/vite/src/node/plugins/resolve.ts b/packages/vite/src/node/plugins/resolve.ts
index b6d79ffcb68d58..ddf603084dd15b 100644
--- a/packages/vite/src/node/plugins/resolve.ts
+++ b/packages/vite/src/node/plugins/resolve.ts
@@ -4,7 +4,6 @@ import colors from 'picocolors'
 import type { PartialResolvedId } from 'rolldown'
 import { viteResolvePlugin } from 'rolldown/experimental'
 import { exports, imports } from 'resolve.exports'
-import { hasESMSyntax } from 'mlly'
 import type { Plugin } from '../plugin'
 import {
   CLIENT_ENTRY,
@@ -932,12 +931,7 @@ export function resolvePackageEntry(
     // fallback to mainFields if still not resolved
     if (!entryPoint) {
       for (const field of options.mainFields) {
-        if (field === 'browser') {
-          entryPoint = tryResolveBrowserEntry(dir, data, options)
-          if (entryPoint) {
-            break
-          }
-        } else if (typeof data[field] === 'string') {
+        if (typeof data[field] === 'string') {
           entryPoint = data[field]
           break
         }
@@ -1109,53 +1103,6 @@ function resolveDeepImport(
   }
 }
 
-function tryResolveBrowserEntry(
-  dir: string,
-  data: PackageData['data'],
-  options: InternalResolveOptions,
-) {
-  // handle edge case with browser and module field semantics
-
-  // check browser field
-  // https://github.com/defunctzombie/package-browser-field-spec
-  const browserEntry =
-    typeof data.browser === 'string'
-      ? data.browser
-      : isObject(data.browser) && data.browser['.']
-  if (browserEntry) {
-    // check if the package also has a "module" field.
-    if (
-      !options.isRequire &&
-      options.mainFields.includes('module') &&
-      typeof data.module === 'string' &&
-      data.module !== browserEntry
-    ) {
-      // if both are present, we may have a problem: some package points both
-      // to ESM, with "module" targeting Node.js, while some packages points
-      // "module" to browser ESM and "browser" to UMD/IIFE.
-      // the heuristics here is to actually read the browser entry when
-      // possible and check for hints of ESM. If it is not ESM, prefer "module"
-      // instead; Otherwise, assume it's ESM and use it.
-      const resolvedBrowserEntry = tryFsResolve(
-        path.join(dir, browserEntry),
-        options,
-      )
-      if (resolvedBrowserEntry) {
-        const content = fs.readFileSync(resolvedBrowserEntry, 'utf-8')
-        if (hasESMSyntax(content)) {
-          // likely ESM, prefer browser
-          return browserEntry
-        } else {
-          // non-ESM, UMD or IIFE or CJS(!!! e.g. firebase 7.x), prefer module
-          return data.module
-        }
-      }
-    } else {
-      return browserEntry
-    }
-  }
-}
-
 /**
  * given a relative path in pkg dir,
  * return a relative path in pkg dir,

From d95f1029d0df49b7c34430fd6cc282bb3ae49779 Mon Sep 17 00:00:00 2001
From: sapphi-red <49056869+sapphi-red@users.noreply.github.com>
Date: Wed, 22 Apr 2026 11:47:05 +0900
Subject: [PATCH 2/2] refactor: fix

---
 packages/vite/src/node/plugins/resolve.ts | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/packages/vite/src/node/plugins/resolve.ts b/packages/vite/src/node/plugins/resolve.ts
index ddf603084dd15b..964a1082492068 100644
--- a/packages/vite/src/node/plugins/resolve.ts
+++ b/packages/vite/src/node/plugins/resolve.ts
@@ -934,6 +934,12 @@ export function resolvePackageEntry(
         if (typeof data[field] === 'string') {
           entryPoint = data[field]
           break
+        } else if (field === 'browser') {
+          const browser = data[field]
+          if (isObject(browser) && browser['.']) {
+            entryPoint = browser['.']
+            break
+          }
         }
       }
     }