diff --git a/CHANGELOG.md b/CHANGELOG.md
index 0a3dd0c..bd07373 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,6 +6,12 @@ This project follows [Semantic Versioning 2.0.0](https://semver.org/spec/v2.0.0.
 
 ---
 
+## [Unreleased]
+
+### Fixed
+
+- **A multi-line `className` hid its flex tokens from the row and column composers, and this project's own style guide is what put them there.** `.claude/rules/widgets.md` and `SKILL.md` both instruct a className covering 3+ concerns to be a triple-quoted string with one concern per line, so `flex-1` at the end of a line arrives at the composition helpers as `flex-1\n`. Three of them split on a single space and therefore never matched it. Two consequences, both reproduced: a `flex-1` child of an `overflow-hidden` row got wrapped a second time and threw `Incorrect use of ParentDataWidget` (`_selfWrapsInFlex`), and a `w-24 shrink-0` child in a crowded row shrank to its 50pt flex share instead of holding 96pt (`_hasShrinkZero`). The third site, `_hasExplicitCrossWidth`, is corrected for consistency with its own documented "in ANY state or breakpoint variant" intent, but no observable failure could be produced for it: a stretched column child still renders at the width it asked for, so the miss costs a redundant wrapper rather than a wrong layout. All five token scans in `WDiv` now share one hoisted `_whitespaceRegex`, which also stops the two that already split on whitespace from allocating a fresh `RegExp` on every pass through the composition path. (`lib/src/widgets/w_div.dart`, `test/widgets/w_div/multiline_classname_test.dart`)
+
 ## [1.4.0] - 2026-08-21
 
 ### Added
diff --git a/lib/src/widgets/w_div.dart b/lib/src/widgets/w_div.dart
index c8ce8a5..d8f03d0 100644
--- a/lib/src/widgets/w_div.dart
+++ b/lib/src/widgets/w_div.dart
@@ -787,7 +787,7 @@ class WDiv extends StatelessWidget {
   /// tokens and prefixed variants like `md:shrink-0` / `md:flex-none`.
   static bool _hasShrinkZero(String? className) {
     if (className == null || className.isEmpty) return false;
-    for (final token in className.split(' ')) {
+    for (final token in className.split(_whitespaceRegex)) {
       if (token == 'shrink-0' ||
           token.endsWith(':shrink-0') ||
           token == 'flex-none' ||
@@ -879,7 +879,7 @@ class WDiv extends StatelessWidget {
   /// stretch wrap (a parse without those states active would miss them).
   static bool _hasExplicitCrossWidth(String? className) {
     if (className == null || className.isEmpty) return false;
-    for (final raw in className.split(' ')) {
+    for (final raw in className.split(_whitespaceRegex)) {
       if (raw.isEmpty) continue;
       final token = raw.contains(':') ? raw.split(':').last : raw;
       if (token.startsWith('w-') ||
@@ -966,6 +966,15 @@ class WDiv extends StatelessWidget {
   /// has been stripped.
   static final RegExp _numericFlexRegex = RegExp(r'^flex-[0-9]+$');
 
+  /// Splits a raw `className` into tokens.
+  ///
+  /// Any whitespace, not a single space: this project writes a className with
+  /// 3+ concerns as a triple-quoted string with one concern per line (see
+  /// `.claude/rules/widgets.md`), so a token that ends a line arrives as
+  /// `flex-1\n` and a single-space split never matches it. Hoisted to a field
+  /// because every caller runs inside the row/column composition path.
+  static final RegExp _whitespaceRegex = RegExp(r'\s+');
+
   /// Whether a child's className makes it self-wrap in `Expanded`/`Flexible`
   /// (i.e. sets `styles.flex` or `styles.flexFit`, see the composition pipeline
   /// at the bottom of `_buildCompositionPipeline`). Such a child must never be
@@ -978,7 +987,7 @@ class WDiv extends StatelessWidget {
   /// `Flexible`), so they are absent here.
   static bool _selfWrapsInFlex(String? className) {
     if (className == null || className.isEmpty) return false;
-    for (final raw in className.split(' ')) {
+    for (final raw in className.split(_whitespaceRegex)) {
       if (raw.isEmpty) continue;
       final token = raw.contains(':') ? raw.split(':').last : raw;
       if (token == 'grow' ||
@@ -1003,7 +1012,7 @@ class WDiv extends StatelessWidget {
   /// responsive intent at breakpoints where it does not apply.
   static bool _hasBareFullWidth(String? className) {
     if (className == null || className.isEmpty) return false;
-    for (final raw in className.split(RegExp(r'\s+'))) {
+    for (final raw in className.split(_whitespaceRegex)) {
       if (raw == 'w-full') return true;
     }
     return false;
@@ -1046,7 +1055,7 @@ class WDiv extends StatelessWidget {
       return true;
     }
 
-    for (final token in className.split(RegExp(r'\s+'))) {
+    for (final token in className.split(_whitespaceRegex)) {
       if (token.isEmpty || token.contains(':')) continue;
       if (token == 'grow' ||
           token == 'flex-grow' ||
diff --git a/test/widgets/w_div/multiline_classname_test.dart b/test/widgets/w_div/multiline_classname_test.dart
new file mode 100644
index 0000000..4246401
--- /dev/null
+++ b/test/widgets/w_div/multiline_classname_test.dart
@@ -0,0 +1,91 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:fluttersdk_wind/fluttersdk_wind.dart';
+
+/// The composition helpers scan a child's raw `className` for tokens, and this
+/// project's own convention writes any className with 3+ concerns as a
+/// triple-quoted string with one concern per line (`.claude/rules/widgets.md`).
+///
+/// A scan that splits on a single space therefore sees `flex-1\n` rather than
+/// `flex-1` for every token that ends a line, and misses it. Each helper fails
+/// differently when that happens, so each gets its own case here: the wrap is
+/// applied twice, or applied where it should not be, or skipped where it should
+/// not be.
+void main() {
+  setUp(() {
+    WindParser.clearCache();
+  });
+
+  /// Pumps [child] inside a fixed-width Wind surface.
+  Future<void> pumpAt(WidgetTester tester, double width, Widget child) {
+    return tester.pumpWidget(
+      MaterialApp(
+        home: WindTheme(
+          data: WindThemeData(),
+          child: Align(
+            alignment: Alignment.topLeft,
+            child: SizedBox(width: width, child: child),
+          ),
+        ),
+      ),
+    );
+  }
+
+  testWidgets(
+    'a multi-line flex-1 child is not wrapped a second time',
+    (tester) async {
+      // `_selfWrapsInFlex` guards against exactly this: the child already
+      // carries its own Expanded, so a parent Flexible on top of it throws
+      // "Incorrect use of ParentDataWidget". `overflow-hidden` wraps
+      // unconditionally, which is the shortest route to the double wrap.
+      await pumpAt(
+        tester,
+        400,
+        const WDiv(
+          className: 'flex flex-row items-center overflow-hidden',
+          children: [
+            WDiv(
+              className: '''
+                flex-1
+                bg-white dark:bg-gray-800
+              ''',
+              child: SizedBox(height: 20),
+            ),
+            WDiv(child: SizedBox(width: 24, height: 24)),
+          ],
+        ),
+      );
+
+      expect(tester.takeException(), isNull);
+    },
+  );
+
+  testWidgets(
+    'a multi-line shrink-0 child keeps its intrinsic width',
+    (tester) async {
+      // `shrink-0` is the caller saying "never shrink me". Missing the token
+      // wraps the child in a Flexible, whose share in a crowded row is smaller
+      // than the width it asked for.
+      await pumpAt(
+        tester,
+        100,
+        const WDiv(
+          className: 'flex flex-row items-center overflow-hidden',
+          children: [
+            WDiv(
+              key: Key('fixed'),
+              className: '''
+                w-24 shrink-0
+                bg-white dark:bg-gray-800
+              ''',
+              child: SizedBox(height: 20),
+            ),
+            WText('Very Long Value Text That Cannot Fit', className: 'text-sm'),
+          ],
+        ),
+      );
+
+      expect(tester.getSize(find.byKey(const Key('fixed'))).width, 96);
+    },
+  );
+}