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diff --git a/CHANGELOG.md b/CHANGELOG.md
index 261e67188b24..85901f90c98b 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -15,6 +15,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Fix `inset-shadow-*` suggestions in IntelliSense ([#15471](https://github.com/tailwindlabs/tailwindcss/pull/15471))
 - Only compile arbitrary values ending in `]` ([#15503](https://github.com/tailwindlabs/tailwindcss/pull/15503))
 - Improve performance and memory usage ([#15529](https://github.com/tailwindlabs/tailwindcss/pull/15529))
+- _Upgrade (experimental)_: Do not extract class names from functions (e.g. `shadow` in `filter: 'drop-shadow(…)'`) ([#15566](https://github.com/tailwindlabs/tailwindcss/pull/15566))
 
 ### Changed
 
@@ -782,4 +783,3 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ## [4.0.0-alpha.1] - 2024-03-06
 
 - First 4.0.0-alpha.1 release
-
diff --git a/integrations/upgrade/js-config.test.ts b/integrations/upgrade/js-config.test.ts
index f2643143a124..e9ad772f6803 100644
--- a/integrations/upgrade/js-config.test.ts
+++ b/integrations/upgrade/js-config.test.ts
@@ -137,6 +137,7 @@ test(
       'src/test.js': ts`
         export default {
           'shouldNotMigrate': !border.test + '',
+          'filter': 'drop-shadow(0 0 0.5rem #000)',
         }
       `,
       'src/index.html': html`
@@ -288,6 +289,7 @@ test(
       --- src/test.js ---
       export default {
         'shouldNotMigrate': !border.test + '',
+        'filter': 'drop-shadow(0 0 0.5rem #000)',
       }
       "
     `)
diff --git a/packages/@tailwindcss-upgrade/src/template/codemods/important.test.ts b/packages/@tailwindcss-upgrade/src/template/codemods/important.test.ts
index 60d78aa53675..6667d1e3ac47 100644
--- a/packages/@tailwindcss-upgrade/src/template/codemods/important.test.ts
+++ b/packages/@tailwindcss-upgrade/src/template/codemods/important.test.ts
@@ -38,31 +38,31 @@ test('does not match false positives', async () => {
   ).toEqual('!border')
 })
 
-test('does not match false positives', async () => {
+test('does not replace classes in invalid positions', async () => {
   let designSystem = await __unstable__loadDesignSystem('@import "tailwindcss";', {
     base: __dirname,
   })
 
-  function shouldNotDetect(example: string, candidate = '!border') {
+  function shouldNotReplace(example: string, candidate = '!border') {
     expect(
       important(designSystem, {}, candidate, {
         contents: example,
         start: example.indexOf(candidate),
         end: example.indexOf(candidate) + candidate.length,
       }),
-    ).toEqual('!border')
+    ).toEqual(candidate)
   }
 
-  shouldNotDetect(`let notBorder = !border    \n`)
-  shouldNotDetect(`{ "foo": !border.something + ""}\n`)
-  shouldNotDetect(`<div v-if="something && !border"></div>\n`)
-  shouldNotDetect(`<div v-else-if="something && !border"></div>\n`)
-  shouldNotDetect(`<div v-show="something && !border"></div>\n`)
-  shouldNotDetect(`<div v-if="!border || !border"></div>\n`)
-  shouldNotDetect(`<div v-else-if="!border || !border"></div>\n`)
-  shouldNotDetect(`<div v-show="!border || !border"></div>\n`)
-  shouldNotDetect(`<div v-if="!border"></div>\n`)
-  shouldNotDetect(`<div v-else-if="!border"></div>\n`)
-  shouldNotDetect(`<div v-show="!border"></div>\n`)
-  shouldNotDetect(`<div x-if="!border"></div>\n`)
+  shouldNotReplace(`let notBorder = !border    \n`)
+  shouldNotReplace(`{ "foo": !border.something + ""}\n`)
+  shouldNotReplace(`<div v-if="something && !border"></div>\n`)
+  shouldNotReplace(`<div v-else-if="something && !border"></div>\n`)
+  shouldNotReplace(`<div v-show="something && !border"></div>\n`)
+  shouldNotReplace(`<div v-if="!border || !border"></div>\n`)
+  shouldNotReplace(`<div v-else-if="!border || !border"></div>\n`)
+  shouldNotReplace(`<div v-show="!border || !border"></div>\n`)
+  shouldNotReplace(`<div v-if="!border"></div>\n`)
+  shouldNotReplace(`<div v-else-if="!border"></div>\n`)
+  shouldNotReplace(`<div v-show="!border"></div>\n`)
+  shouldNotReplace(`<div x-if="!border"></div>\n`)
 })
diff --git a/packages/@tailwindcss-upgrade/src/template/codemods/legacy-classes.test.ts b/packages/@tailwindcss-upgrade/src/template/codemods/legacy-classes.test.ts
index 5d29c1b71b6b..f301deaa9dde 100644
--- a/packages/@tailwindcss-upgrade/src/template/codemods/legacy-classes.test.ts
+++ b/packages/@tailwindcss-upgrade/src/template/codemods/legacy-classes.test.ts
@@ -39,3 +39,33 @@ test.each([
 
   expect(await legacyClasses(designSystem, {}, candidate)).toEqual(result)
 })
+
+test('does not replace classes in invalid positions', async () => {
+  let designSystem = await __unstable__loadDesignSystem('@import "tailwindcss";', {
+    base: __dirname,
+  })
+
+  async function shouldNotReplace(example: string, candidate = 'shadow') {
+    expect(
+      await legacyClasses(designSystem, {}, candidate, {
+        contents: example,
+        start: example.indexOf(candidate),
+        end: example.indexOf(candidate) + candidate.length,
+      }),
+    ).toEqual(candidate)
+  }
+
+  await shouldNotReplace(`let notShadow = shadow    \n`)
+  await shouldNotReplace(`{ "foo": shadow.something + ""}\n`)
+  await shouldNotReplace(`<div v-if="something && shadow"></div>\n`)
+  await shouldNotReplace(`<div v-else-if="something && shadow"></div>\n`)
+  await shouldNotReplace(`<div v-show="something && shadow"></div>\n`)
+  await shouldNotReplace(`<div v-if="shadow || shadow"></div>\n`)
+  await shouldNotReplace(`<div v-else-if="shadow || shadow"></div>\n`)
+  await shouldNotReplace(`<div v-show="shadow || shadow"></div>\n`)
+  await shouldNotReplace(`<div v-if="shadow"></div>\n`)
+  await shouldNotReplace(`<div v-else-if="shadow"></div>\n`)
+  await shouldNotReplace(`<div v-show="shadow"></div>\n`)
+  await shouldNotReplace(`<div x-if="shadow"></div>\n`)
+  await shouldNotReplace(`<div style="{{filter:" drop-shadow(30px 10px 4px #4444dd }>\n`)
+})
diff --git a/packages/@tailwindcss-upgrade/src/template/is-safe-migration.ts b/packages/@tailwindcss-upgrade/src/template/is-safe-migration.ts
index 046ee10e6df4..e0408d3e57b8 100644
--- a/packages/@tailwindcss-upgrade/src/template/is-safe-migration.ts
+++ b/packages/@tailwindcss-upgrade/src/template/is-safe-migration.ts
@@ -29,19 +29,24 @@ export function isSafeMigration(location: { contents: string; start: number; end
     currentLineAfterCandidate += char
   }
 
-  // Heuristic 1: Require the candidate to be inside quotes
+  // Heuristic: Require the candidate to be inside quotes
   let isQuoteBeforeCandidate = QUOTES.some((quote) => currentLineBeforeCandidate.includes(quote))
   let isQuoteAfterCandidate = QUOTES.some((quote) => currentLineAfterCandidate.includes(quote))
   if (!isQuoteBeforeCandidate || !isQuoteAfterCandidate) {
     return false
   }
 
-  // Heuristic 2: Disallow object access immediately following the candidate
+  // Heuristic: Disallow object access immediately following the candidate
   if (currentLineAfterCandidate[0] === '.') {
     return false
   }
 
-  // Heuristic 3: Disallow logical operators preceding or following the candidate
+  // Heuristic: Disallow function call expressions immediately following the candidate
+  if (currentLineAfterCandidate.trim().startsWith('(')) {
+    return false
+  }
+
+  // Heuristic: Disallow logical operators preceding or following the candidate
   for (let operator of LOGICAL_OPERATORS) {
     if (
       currentLineAfterCandidate.trim().startsWith(operator) ||
@@ -51,7 +56,7 @@ export function isSafeMigration(location: { contents: string; start: number; end
     }
   }
 
-  // Heuristic 4: Disallow conditional template syntax
+  // Heuristic: Disallow conditional template syntax
   for (let rule of CONDITIONAL_TEMPLATE_SYNTAX) {
     if (rule.test(currentLineBeforeCandidate)) {
       return false
