diff --git a/CHANGELOG.md b/CHANGELOG.md
index b4058a7729bf..4b4256ce9377 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -13,6 +13,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Canonicalization: match arbitrary hex colors against theme colors case-insensitively (e.g. `bg-[#fff]` and `bg-[#FFF]` → `bg-white`) ([#20298](https://github.com/tailwindlabs/tailwindcss/pull/20298))
 - Prevent Preflight from overriding Firefox's native `iframe:focus-visible` outline styles ([#20292](https://github.com/tailwindlabs/tailwindcss/pull/20292))
 - Prevent `theme('colors.foo')` in JS plugins from returning an internal disambiguation object when a CSS theme key shares a prefix with a sibling key like `--color-foo-bar` ([#20299](https://github.com/tailwindlabs/tailwindcss/pull/20299))
+- Ensure fractional opacity modifiers work with named shadow sizes like `shadow-sm/12.5`, `text-shadow-sm/12.5`, `drop-shadow-sm/12.5`, and `inset-shadow-sm/12.5` ([#20302](https://github.com/tailwindlabs/tailwindcss/pull/20302))
 
 ## [4.3.2] - 2026-06-26
 
diff --git a/packages/tailwindcss/src/utilities.test.ts b/packages/tailwindcss/src/utilities.test.ts
index e404a0a90dc2..5201878f2e43 100644
--- a/packages/tailwindcss/src/utilities.test.ts
+++ b/packages/tailwindcss/src/utilities.test.ts
@@ -24053,6 +24053,7 @@ test('filter', async () => {
         'invert-[var(--value)]',
         'drop-shadow',
         'drop-shadow/25',
+        'drop-shadow/12.5',
         'drop-shadow-xl',
         'drop-shadow-multi',
         'drop-shadow-[0_0_red]',
@@ -24150,6 +24151,13 @@ test('filter', async () => {
       filter: var(--tw-blur, ) var(--tw-brightness, ) var(--tw-contrast, ) var(--tw-grayscale, ) var(--tw-hue-rotate, ) var(--tw-invert, ) var(--tw-saturate, ) var(--tw-sepia, ) var(--tw-drop-shadow, );
     }
 
+    .drop-shadow\\/12\\.5 {
+      --tw-drop-shadow-alpha: 12.5%;
+      --tw-drop-shadow-size: drop-shadow(0 1px 1px var(--tw-drop-shadow-color, oklab(0% 0 0 / .125)));
+      --tw-drop-shadow: drop-shadow(var(--drop-shadow));
+      filter: var(--tw-blur, ) var(--tw-brightness, ) var(--tw-contrast, ) var(--tw-grayscale, ) var(--tw-hue-rotate, ) var(--tw-invert, ) var(--tw-saturate, ) var(--tw-sepia, ) var(--tw-drop-shadow, );
+    }
+
     .drop-shadow\\/25 {
       --tw-drop-shadow-alpha: 25%;
       --tw-drop-shadow-size: drop-shadow(0 1px 1px var(--tw-drop-shadow-color, oklab(0% 0 0 / .25)));
@@ -26901,6 +26909,7 @@ test('text-shadow', async () => {
         'text-shadow-[shadow:var(--value)]',
 
         'text-shadow-sm/25',
+        'text-shadow-sm/12.5',
         'text-shadow-[12px_12px_#0088cc]/25',
         'text-shadow-[12px_12px_var(--value)]/25',
         'text-shadow-[10px_10px]/25',
@@ -26979,6 +26988,11 @@ test('text-shadow', async () => {
       text-shadow: 12px 12px var(--tw-text-shadow-color, oklab(59.9824% -.067 -.124 / .25));
     }
 
+    .text-shadow-sm\\/12\\.5 {
+      --tw-text-shadow-alpha: 12.5%;
+      text-shadow: 0px 1px 2px var(--tw-text-shadow-color, oklab(0% 0 0 / .125)), 0px 2px 2px var(--tw-text-shadow-color, oklab(0% 0 0 / .125));
+    }
+
     .text-shadow-sm\\/25 {
       --tw-text-shadow-alpha: 25%;
       text-shadow: 0px 1px 2px var(--tw-text-shadow-color, oklab(0% 0 0 / .25)), 0px 2px 2px var(--tw-text-shadow-color, oklab(0% 0 0 / .25));
@@ -27266,6 +27280,7 @@ test('shadow', async () => {
         'shadow-[shadow:var(--value)]',
 
         'shadow-sm/25',
+        'shadow-sm/12.5',
         'shadow-[12px_12px_#0088cc]/25',
         'shadow-[12px_12px_var(--value)]/25',
         'shadow-[10px_10px]/25',
@@ -27368,6 +27383,12 @@ test('shadow', async () => {
       box-shadow: var(--tw-inset-shadow), var(--tw-inset-ring-shadow), var(--tw-ring-offset-shadow), var(--tw-ring-shadow), var(--tw-shadow);
     }
 
+    .shadow-sm\\/12\\.5 {
+      --tw-shadow-alpha: 12.5%;
+      --tw-shadow: 0 1px 3px 0 var(--tw-shadow-color, oklab(0% 0 0 / .125)), 0 1px 2px -1px var(--tw-shadow-color, oklab(0% 0 0 / .125));
+      box-shadow: var(--tw-inset-shadow), var(--tw-inset-ring-shadow), var(--tw-ring-offset-shadow), var(--tw-ring-shadow), var(--tw-shadow);
+    }
+
     .shadow-sm\\/25 {
       --tw-shadow-alpha: 25%;
       --tw-shadow: 0 1px 3px 0 var(--tw-shadow-color, oklab(0% 0 0 / .25)), 0 1px 2px -1px var(--tw-shadow-color, oklab(0% 0 0 / .25));
@@ -27742,6 +27763,7 @@ test('inset-shadow', async () => {
         'inset-shadow-[12px_12px_#0088cc,12px_12px_var(--value,#0088cc)]',
 
         'inset-shadow-sm/25',
+        'inset-shadow-sm/12.5',
         'inset-shadow-[12px_12px_#0088cc]/25',
         'inset-shadow-[12px_12px_var(--value)]/25',
         'inset-shadow-[10px_10px]/25',
@@ -27857,6 +27879,12 @@ test('inset-shadow', async () => {
       box-shadow: var(--tw-inset-shadow), var(--tw-inset-ring-shadow), var(--tw-ring-offset-shadow), var(--tw-ring-shadow), var(--tw-shadow);
     }
 
+    .inset-shadow-sm\\/12\\.5 {
+      --tw-inset-shadow-alpha: 12.5%;
+      --tw-inset-shadow: inset 0 1px 1px var(--tw-inset-shadow-color, oklab(0% 0 0 / .125));
+      box-shadow: var(--tw-inset-shadow), var(--tw-inset-ring-shadow), var(--tw-ring-offset-shadow), var(--tw-ring-shadow), var(--tw-shadow);
+    }
+
     .inset-shadow-sm\\/25 {
       --tw-inset-shadow-alpha: 25%;
       --tw-inset-shadow: inset 0 1px 1px var(--tw-inset-shadow-color, oklab(0% 0 0 / .25));
diff --git a/packages/tailwindcss/src/utilities.ts b/packages/tailwindcss/src/utilities.ts
index c9fb0cc439c1..5da9e63e173f 100644
--- a/packages/tailwindcss/src/utilities.ts
+++ b/packages/tailwindcss/src/utilities.ts
@@ -4618,7 +4618,7 @@ export function createUtilities(theme: Theme) {
         if (candidate.modifier.kind === 'arbitrary') {
           alpha = candidate.modifier.value
         } else {
-          if (isPositiveInteger(candidate.modifier.value)) {
+          if (isValidOpacityValue(candidate.modifier.value)) {
             alpha = `${candidate.modifier.value}%`
           }
         }
@@ -5427,7 +5427,7 @@ export function createUtilities(theme: Theme) {
       if (candidate.modifier.kind === 'arbitrary') {
         alpha = candidate.modifier.value
       } else {
-        if (isPositiveInteger(candidate.modifier.value)) {
+        if (isValidOpacityValue(candidate.modifier.value)) {
           alpha = `${candidate.modifier.value}%`
         }
       }
@@ -5573,7 +5573,7 @@ export function createUtilities(theme: Theme) {
         if (candidate.modifier.kind === 'arbitrary') {
           alpha = candidate.modifier.value
         } else {
-          if (isPositiveInteger(candidate.modifier.value)) {
+          if (isValidOpacityValue(candidate.modifier.value)) {
             alpha = `${candidate.modifier.value}%`
           }
         }
@@ -5699,7 +5699,7 @@ export function createUtilities(theme: Theme) {
         if (candidate.modifier.kind === 'arbitrary') {
           alpha = candidate.modifier.value
         } else {
-          if (isPositiveInteger(candidate.modifier.value)) {
+          if (isValidOpacityValue(candidate.modifier.value)) {
             alpha = `${candidate.modifier.value}%`
           }
         }