diff --git a/app/code/Magento/SalesRule/Helper/CartFixedDiscount.php b/app/code/Magento/SalesRule/Helper/CartFixedDiscount.php
index a5287e57b3e7..a217e5a83b64 100644
--- a/app/code/Magento/SalesRule/Helper/CartFixedDiscount.php
+++ b/app/code/Magento/SalesRule/Helper/CartFixedDiscount.php
@@ -275,6 +275,92 @@ public function getAvailableDiscountAmount(
         return $availableDiscountAmount;
     }
 
+    /**
+     * Initial cart-fixed balance for shipping discount processing.
+     * Single: remaining after item allocation on quote.
+     * Multi: full rule amount (legacy).
+     *
+     * @param Quote $quote
+     * @param Rule $rule
+     * @param string|int $ruleId
+     * @param bool $isMultiShipping
+     * @return float
+     */
+    public function getCartFixedShippingRuleBalance(
+        Quote $quote,
+        Rule $rule,
+        $ruleId,
+        bool $isMultiShipping
+    ): float {
+        if ($isMultiShipping) {
+            return (float) $rule->getDiscountAmount();
+        }
+        $quoteCartRules = $quote->getCartFixedRules();
+        if (is_array($quoteCartRules) && array_key_exists($ruleId, $quoteCartRules)) {
+            return (float) $quoteCartRules[$ruleId];
+        }
+        return (float) $rule->getDiscountAmount();
+    }
+
+    /**
+     * Single-shipping only: remaining balance capped by shipping discount tax basis.
+     *
+     * @param Quote $quote
+     * @param float $availableRuleBalance
+     * @param float $shippingAmountForDiscount
+     * @param float $baseShippingAmountForDiscount
+     * @param float $appliedShippingDiscount
+     * @param float $baseAppliedShippingDiscount
+     * @return array{0: float, 1: float} [quote currency, base currency]
+     */
+    public function calculateSingleShippingCartFixedDiscount(
+        Quote $quote,
+        float $availableRuleBalance,
+        float $shippingAmountForDiscount,
+        float $baseShippingAmountForDiscount,
+        float $appliedShippingDiscount,
+        float $baseAppliedShippingDiscount
+    ): array {
+        if ($availableRuleBalance <= 0.0) {
+            return [0.0, 0.0];
+        }
+
+        $baseDiscountAmount = max(
+            0.0,
+            min(
+                $availableRuleBalance,
+                $baseShippingAmountForDiscount - $baseAppliedShippingDiscount
+            )
+        );
+        $discountAmount = max(
+            0.0,
+            min(
+                (float) $this->priceCurrency->convert($baseDiscountAmount, $quote->getStore()),
+                $shippingAmountForDiscount - $appliedShippingDiscount
+            )
+        );
+
+        return [$discountAmount, $baseDiscountAmount];
+    }
+
+    /**
+     * Sync remaining cart-fixed balance back onto the quote (single shipping only).
+     *
+     * @param Quote $quote
+     * @param string|int $ruleId
+     * @param float $remaining
+     * @return void
+     */
+    public function syncQuoteCartFixedRuleBalance(Quote $quote, $ruleId, float $remaining): void
+    {
+        $quoteCartRules = $quote->getCartFixedRules();
+        if (!is_array($quoteCartRules)) {
+            $quoteCartRules = [];
+        }
+        $quoteCartRules[$ruleId] = $remaining;
+        $quote->setCartFixedRules($quoteCartRules);
+    }
+
     /**
      * Get configuration setting "Apply Discount On Prices Including Tax" value
      *
diff --git a/app/code/Magento/SalesRule/Model/Validator.php b/app/code/Magento/SalesRule/Model/Validator.php
index 93f7d7a712f0..f66413afc113 100644
--- a/app/code/Magento/SalesRule/Model/Validator.php
+++ b/app/code/Magento/SalesRule/Model/Validator.php
@@ -33,6 +33,7 @@
  * @method int getCustomerGroupId()
  * @method Validator setCustomerGroupId($id)
  * @SuppressWarnings(PHPMD.CouplingBetweenObjects)
+ * @SuppressWarnings(PHPMD.ExcessiveClassComplexity)
  */
 class Validator extends \Magento\Framework\Model\AbstractModel implements ResetAfterRequestInterface
 {
@@ -480,42 +481,13 @@ public function processShippingAmount(Address $address)
                     $baseDiscountAmount = $rule->getDiscountAmount();
                     break;
                 case Rule::CART_FIXED_ACTION:
-                    $cartRules = $address->getCartFixedRules();
-                    $quoteAmount = $this->priceCurrency->convert($rule->getDiscountAmount(), $quote->getStore());
-                    $isAppliedToShipping = (int) $rule->getApplyToShipping();
-                    $ruleId = $rule->getId() ?? '';
-                    if (!isset($cartRules[$ruleId])) {
-                        $cartRules[$ruleId] = $rule->getDiscountAmount();
-                    }
-                    if ($cartRules[$ruleId] > 0) {
-                        $shippingQuoteAmount = (float) $address->getShippingAmount();
-                        $quoteBaseSubtotal = (float) $quote->getBaseSubtotal();
-                        $isMultiShipping = $this->cartFixedDiscountHelper->checkMultiShippingQuote($quote);
-                        if ($isAppliedToShipping) {
-                            $quoteBaseSubtotal = ($quote->getIsMultiShipping() && $isMultiShipping) ?
-                                $this->cartFixedDiscountHelper->getQuoteTotalsForMultiShipping($quote) :
-                                $this->cartFixedDiscountHelper->getQuoteTotalsForRegularShipping(
-                                    $address,
-                                    $quoteBaseSubtotal,
-                                    $shippingQuoteAmount
-                                );
-                            $discountAmount = $this->cartFixedDiscountHelper->
-                            getShippingDiscountAmount(
-                                $rule,
-                                $shippingQuoteAmount,
-                                $quoteBaseSubtotal
-                            );
-                            $baseDiscountAmount = $discountAmount;
-                        } else {
-                            $discountAmount = min($shippingQuoteAmount, $quoteAmount);
-                            $baseDiscountAmount = min(
-                                $baseShippingAmount - $address->getBaseShippingDiscountAmount(),
-                                $cartRules[$ruleId]
-                            );
-                        }
-                        $cartRules[$ruleId] -= $baseDiscountAmount;
-                    }
-                    $address->setCartFixedRules($cartRules);
+                    [$discountAmount, $baseDiscountAmount] = $this->processCartFixedShippingAmount(
+                        $address,
+                        $quote,
+                        $rule,
+                        (float) $shippingAmount,
+                        (float) $baseShippingAmount
+                    );
                     break;
                 case Rule::BUY_X_GET_Y_ACTION:
                     $allQtyDiscount = $this->getDiscountQtyAllItemsBuyXGetYAction($quote, $rule);
@@ -562,6 +534,93 @@ public function processShippingAmount(Address $address)
         return $this;
     }
 
+    /**
+     * Apply cart-fixed rule discount to shipping for one address.
+     *
+     * Single shipping: remaining quote balance capped by shipping tax-basis amounts.
+     * Multi shipping: legacy proportional share of full rule amount.
+     *
+     * @param Address $address
+     * @param Quote $quote
+     * @param Rule $rule
+     * @param float $shippingAmount Shipping amount for discount (quote currency)
+     * @param float $baseShippingAmount Shipping amount for discount (base currency)
+     * @return array{0: float, 1: float} [quote currency discount, base currency discount]
+     */
+    private function processCartFixedShippingAmount(
+        Address $address,
+        Quote $quote,
+        Rule $rule,
+        float $shippingAmount,
+        float $baseShippingAmount
+    ): array {
+        $discountAmount = 0.0;
+        $baseDiscountAmount = 0.0;
+        $cartRules = $address->getCartFixedRules();
+        if (!is_array($cartRules)) {
+            $cartRules = [];
+        }
+        $quoteAmount = $this->priceCurrency->convert($rule->getDiscountAmount(), $quote->getStore());
+        $isAppliedToShipping = (int) $rule->getApplyToShipping();
+        $ruleId = $rule->getId() ?? '';
+        $isMultiShipping = $quote->getIsMultiShipping()
+            && $this->cartFixedDiscountHelper->checkMultiShippingQuote($quote);
+
+        if (!isset($cartRules[$ruleId])) {
+            $cartRules[$ruleId] = $this->cartFixedDiscountHelper->getCartFixedShippingRuleBalance(
+                $quote,
+                $rule,
+                $ruleId,
+                $isMultiShipping
+            );
+        }
+        if ($cartRules[$ruleId] > 0) {
+            $shippingQuoteAmount = (float) $address->getShippingAmount();
+            if ($isAppliedToShipping) {
+                if ($isMultiShipping) {
+                    // HEAD multi path: proportional share of full rule.
+                    $quoteBaseSubtotal = $this->cartFixedDiscountHelper
+                        ->getQuoteTotalsForMultiShipping($quote);
+                    $discountAmount = $this->cartFixedDiscountHelper->getShippingDiscountAmount(
+                        $rule,
+                        $shippingQuoteAmount,
+                        $quoteBaseSubtotal
+                    );
+                    $baseDiscountAmount = $discountAmount;
+                } else {
+                    // Single-ship: remaining balance + tax-basis cap.
+                    [$discountAmount, $baseDiscountAmount] = $this->cartFixedDiscountHelper
+                        ->calculateSingleShippingCartFixedDiscount(
+                            $quote,
+                            (float) $cartRules[$ruleId],
+                            $shippingAmount,
+                            $baseShippingAmount,
+                            (float) $address->getShippingDiscountAmount(),
+                            (float) $address->getBaseShippingDiscountAmount()
+                        );
+                }
+            } else {
+                // HEAD else branch (dead when apply-to-shipping is filtered above; keep for BC).
+                $discountAmount = min($shippingQuoteAmount, $quoteAmount);
+                $baseDiscountAmount = min(
+                    $baseShippingAmount - $address->getBaseShippingDiscountAmount(),
+                    $cartRules[$ruleId]
+                );
+            }
+            $cartRules[$ruleId] -= $baseDiscountAmount;
+        }
+        $address->setCartFixedRules($cartRules);
+        if (!$isMultiShipping) {
+            $this->cartFixedDiscountHelper->syncQuoteCartFixedRuleBalance(
+                $quote,
+                $ruleId,
+                (float) ($cartRules[$ruleId] ?? 0)
+            );
+        }
+
+        return [(float) $discountAmount, (float) $baseDiscountAmount];
+    }
+
     /**
      * Calculate quote totals for each rule and save results
      *
diff --git a/app/code/Magento/SalesRule/Test/Unit/Helper/CartFixedDiscountTest.php b/app/code/Magento/SalesRule/Test/Unit/Helper/CartFixedDiscountTest.php
index 9f50fc084688..9c2bd64d835c 100644
--- a/app/code/Magento/SalesRule/Test/Unit/Helper/CartFixedDiscountTest.php
+++ b/app/code/Magento/SalesRule/Test/Unit/Helper/CartFixedDiscountTest.php
@@ -9,14 +9,24 @@
 
 use Magento\Framework\App\Config\ScopeConfigInterface;
 use Magento\Framework\Pricing\PriceCurrencyInterface;
+use Magento\Framework\TestFramework\Unit\Helper\MockCreationTrait;
 use Magento\Quote\Model\Cart\ShippingMethodConverter;
+use Magento\Quote\Model\Quote;
 use Magento\SalesRule\Helper\CartFixedDiscount;
 use Magento\SalesRule\Model\DeltaPriceRound;
-use PHPUnit\Framework\TestCase;
+use Magento\SalesRule\Model\Rule;
+use Magento\Store\Model\Store;
+use PHPUnit\Framework\Attributes\DataProvider;
 use PHPUnit\Framework\MockObject\MockObject;
+use PHPUnit\Framework\TestCase;
 
+/**
+ * Unit tests for cart-fixed discount helpers (single-ship remaining/cap + multi balance seed).
+ */
 class CartFixedDiscountTest extends TestCase
 {
+    use MockCreationTrait;
+
     /**
      * @var DeltaPriceRound|MockObject
      */
@@ -37,6 +47,11 @@ class CartFixedDiscountTest extends TestCase
      */
     private ScopeConfigInterface $scopeConfig;
 
+    /**
+     * @var CartFixedDiscount
+     */
+    private CartFixedDiscount $cartFixedDiscount;
+
     /**
      * @inheritDoc
      */
@@ -45,9 +60,18 @@ protected function setUp(): void
         parent::setUp();
 
         $this->deltaPriceRound = $this->createMock(DeltaPriceRound::class);
-        $this->priceCurrency = $this->createMock(PriceCurrencyInterface::class);
+        $this->priceCurrency = $this->createPartialMockWithReflection(
+            \Magento\Directory\Model\PriceCurrency::class,
+            ['convert', 'roundPrice']
+        );
         $this->shippingMethodConverter = $this->createMock(ShippingMethodConverter::class);
         $this->scopeConfig = $this->createMock(ScopeConfigInterface::class);
+        $this->cartFixedDiscount = new CartFixedDiscount(
+            $this->deltaPriceRound,
+            $this->priceCurrency,
+            $this->shippingMethodConverter,
+            $this->scopeConfig
+        );
     }
 
     /**
@@ -55,34 +79,203 @@ protected function setUp(): void
      */
     public function testGetDiscountedAmountProportionally(): void
     {
-        $ruleDiscount = 5;
-        $qty = 2.0;
-        $baseItemPrice = 10.0;
-        $baseItemDiscountAmount = 0.0;
-        $baseRuleTotalsDiscount = 10;
-        $discountType = 'fixed';
-        $expected = 5.0;
-
-        $cartFixedDiscount = new CartFixedDiscount(
-            $this->deltaPriceRound,
-            $this->priceCurrency,
-            $this->shippingMethodConverter,
-            $this->scopeConfig
-        );
         $this->deltaPriceRound->expects($this->once())
             ->method('round')
-            ->with(5, $discountType)
-            ->willReturn($expected);
+            ->with(5, 'fixed')
+            ->willReturn(5.0);
+        $this->assertSame(
+            5.0,
+            $this->cartFixedDiscount->getDiscountedAmountProportionally(5, 2.0, 10.0, 0.0, 10, 'fixed')
+        );
+    }
+
+    /**
+     * Single shipping seeds balance from quote remaining after item allocation.
+     *
+     * @return void
+     */
+    public function testGetCartFixedShippingRuleBalanceSingleUsesQuoteRemaining(): void
+    {
+        $rule = $this->createPartialMockWithReflection(Rule::class, ['getDiscountAmount']);
+        $rule->method('getDiscountAmount')->willReturn(56.0);
+        $quote = $this->createPartialMockWithReflection(Quote::class, ['getCartFixedRules']);
+        $quote->method('getCartFixedRules')->willReturn([7 => 5.0]);
+
+        $this->assertSame(
+            5.0,
+            $this->cartFixedDiscount->getCartFixedShippingRuleBalance($quote, $rule, 7, false)
+        );
+    }
+
+    /**
+     * Single shipping falls back to full rule amount when quote has no remaining key.
+     *
+     * @return void
+     */
+    public function testGetCartFixedShippingRuleBalanceSingleFallsBackToRuleAmount(): void
+    {
+        $rule = $this->createPartialMockWithReflection(Rule::class, ['getDiscountAmount']);
+        $rule->method('getDiscountAmount')->willReturn(56.0);
+        $quote = $this->createPartialMockWithReflection(Quote::class, ['getCartFixedRules']);
+        $quote->method('getCartFixedRules')->willReturn([]);
+
+        $this->assertSame(
+            56.0,
+            $this->cartFixedDiscount->getCartFixedShippingRuleBalance($quote, $rule, 7, false)
+        );
+    }
+
+    /**
+     * Multi shipping always seeds address balance with the full rule amount (legacy).
+     *
+     * @return void
+     */
+    public function testGetCartFixedShippingRuleBalanceMultiUsesFullRule(): void
+    {
+        $rule = $this->createPartialMockWithReflection(Rule::class, ['getDiscountAmount']);
+        $rule->method('getDiscountAmount')->willReturn(56.0);
+        $quote = $this->createPartialMockWithReflection(Quote::class, ['getCartFixedRules']);
+        $quote->method('getCartFixedRules')->willReturn([7 => 5.0]);
+
         $this->assertSame(
-            $expected,
-            $cartFixedDiscount->getDiscountedAmountProportionally(
-                $ruleDiscount,
-                $qty,
-                $baseItemPrice,
-                $baseItemDiscountAmount,
-                $baseRuleTotalsDiscount,
-                $discountType
-            )
+            56.0,
+            $this->cartFixedDiscount->getCartFixedShippingRuleBalance($quote, $rule, 7, true)
+        );
+    }
+
+    /**
+     * Single-ship cart-fixed shipping discount uses remaining balance and shipping cap.
+     *
+     * @param float $availableRuleBalance
+     * @param float $shippingAmountForDiscount
+     * @param float $baseShippingAmountForDiscount
+     * @param float $appliedShippingDiscount
+     * @param float $baseAppliedShippingDiscount
+     * @param float $expectedQuoteAmount
+     * @param float $expectedBaseAmount
+     * @return void
+     */
+    #[DataProvider('calculateSingleShippingCartFixedDiscountDataProvider')]
+    public function testCalculateSingleShippingCartFixedDiscountUsesRemainingAndCaps(
+        float $availableRuleBalance,
+        float $shippingAmountForDiscount,
+        float $baseShippingAmountForDiscount,
+        float $appliedShippingDiscount,
+        float $baseAppliedShippingDiscount,
+        float $expectedQuoteAmount,
+        float $expectedBaseAmount
+    ): void {
+        $store = $this->createMock(Store::class);
+        $quote = $this->createPartialMockWithReflection(Quote::class, ['getStore']);
+        $quote->method('getStore')->willReturn($store);
+
+        // Identity convert for unit math (quote currency == base).
+        $this->priceCurrency->method('convert')->willReturnCallback(static function ($amount) {
+            return $amount;
+        });
+
+        [$amount, $baseAmount] = $this->cartFixedDiscount->calculateSingleShippingCartFixedDiscount(
+            $quote,
+            $availableRuleBalance,
+            $shippingAmountForDiscount,
+            $baseShippingAmountForDiscount,
+            $appliedShippingDiscount,
+            $baseAppliedShippingDiscount
+        );
+
+        $this->assertSame($expectedQuoteAmount, $amount);
+        $this->assertSame($expectedBaseAmount, $baseAmount);
+    }
+
+    /**
+     * @return array<string, array<int, float>>
+     */
+    public static function calculateSingleShippingCartFixedDiscountDataProvider(): array
+    {
+        return [
+            'remaining equals shipping' => [
+                5.0,
+                5.0,
+                5.0,
+                0.0,
+                0.0,
+                5.0,
+                5.0,
+            ],
+            'remaining larger than shipping is capped' => [
+                10.0,
+                5.0,
+                5.0,
+                0.0,
+                0.0,
+                5.0,
+                5.0,
+            ],
+            'remaining reduced by already applied shipping discount' => [
+                5.0,
+                5.0,
+                5.0,
+                2.0,
+                2.0,
+                3.0,
+                3.0,
+            ],
+            'clamps to zero when applied exceeds shipping' => [
+                5.0,
+                5.0,
+                5.0,
+                6.0,
+                6.0,
+                0.0,
+                0.0,
+            ],
+            'zero remaining yields zero discount' => [
+                0.0,
+                5.0,
+                5.0,
+                0.0,
+                0.0,
+                0.0,
+                0.0,
+            ],
+        ];
+    }
+
+    /**
+     * Sync writes remaining cart-fixed balance onto the quote ledger.
+     *
+     * @return void
+     */
+    public function testSyncQuoteCartFixedRuleBalance(): void
+    {
+        $quote = $this->createPartialMockWithReflection(
+            Quote::class,
+            ['getCartFixedRules', 'setCartFixedRules']
         );
+        $quote->method('getCartFixedRules')->willReturn([7 => 10.0]);
+        $quote->expects($this->once())
+            ->method('setCartFixedRules')
+            ->with([7 => 3.0]);
+
+        $this->cartFixedDiscount->syncQuoteCartFixedRuleBalance($quote, 7, 3.0);
+    }
+
+    /**
+     * Sync initializes quote cart fixed rules when storage is missing/non-array.
+     *
+     * @return void
+     */
+    public function testSyncQuoteCartFixedRuleBalanceWhenRulesMissing(): void
+    {
+        $quote = $this->createPartialMockWithReflection(
+            Quote::class,
+            ['getCartFixedRules', 'setCartFixedRules']
+        );
+        $quote->method('getCartFixedRules')->willReturn(null);
+        $quote->expects($this->once())
+            ->method('setCartFixedRules')
+            ->with([11 => 4.5]);
+
+        $this->cartFixedDiscount->syncQuoteCartFixedRuleBalance($quote, 11, 4.5);
     }
 }
diff --git a/app/code/Magento/SalesRule/Test/Unit/Model/ValidatorTest.php b/app/code/Magento/SalesRule/Test/Unit/Model/ValidatorTest.php
index b2266033c603..c5ede855922f 100644
--- a/app/code/Magento/SalesRule/Test/Unit/Model/ValidatorTest.php
+++ b/app/code/Magento/SalesRule/Test/Unit/Model/ValidatorTest.php
@@ -125,7 +125,10 @@ protected function setUp(): void
                 'getCustomAttributesCodes',
                 'getShippingAmountForDiscount',
                 'getBaseShippingAmountForDiscount',
-                'setCartFixedRules'
+                'getShippingAmount',
+                'getBaseShippingAmount',
+                'getCartFixedRules',
+                'setCartFixedRules',
             ]
         );
 
@@ -161,7 +164,11 @@ protected function setUp(): void
                 'getQuoteTotalsForMultiShipping',
                 'getQuoteTotalsForRegularShipping',
                 'getBaseRuleTotals',
-                'getAvailableDiscountAmount'])
+                'getAvailableDiscountAmount',
+                'getCartFixedShippingRuleBalance',
+                'calculateSingleShippingCartFixedDiscount',
+                'syncQuoteCartFixedRuleBalance',
+            ])
             ->disableOriginalConstructor()
             ->getMock();
         /** @var Validator|MockObject $validator */
@@ -705,6 +712,7 @@ public static function dataProviderForFullShippingDiscount(): array
      * @param float $quoteBaseSubTotal
      *
      * @return Address|MockObject
+     * @SuppressWarnings(PHPMD.ExcessiveMethodLength)
      */
     protected function setupAddressMock(
         float $shippingAmount = 0.0,
@@ -720,7 +728,10 @@ protected function setupAddressMock(
                 'getExtensionAttributes',
                 'isVirtual',
                 'setAppliedRuleIds',
-                'getBaseSubtotal'
+                'getBaseSubtotal',
+                'getCartFixedRules',
+                'setCartFixedRules',
+                'getIsMultiShipping'
             ]
         );
         $cartExtensionMock = $this->createPartialMockWithReflection(
@@ -742,6 +753,15 @@ protected function setupAddressMock(
         $quoteMock->method('getBaseSubtotal')
             ->willReturn($quoteBaseSubTotal);
 
+        $quoteMock->method('getCartFixedRules')
+            ->willReturn([]);
+
+        $quoteMock->method('setCartFixedRules')
+            ->willReturnSelf();
+
+        $quoteMock->method('getIsMultiShipping')
+            ->willReturn(false);
+
         $this->cartFixedDiscountHelper
             ->method('getQuoteTotalsForRegularShipping')
             ->willReturn($quoteBaseSubTotal);
@@ -750,6 +770,26 @@ protected function setupAddressMock(
             ->method('getShippingDiscountAmount')
             ->willReturn($shippingAmount);
 
+        // Single-ship path: dual gate is false when either flag is false.
+        $this->cartFixedDiscountHelper
+            ->method('checkMultiShippingQuote')
+            ->willReturn(false);
+
+        $this->cartFixedDiscountHelper
+            ->method('getCartFixedShippingRuleBalance')
+            ->willReturnCallback(static function (...$args) {
+                /** @var Rule $rule */
+                $rule = $args[1];
+                return (float) $rule->getDiscountAmount();
+            });
+
+        $this->cartFixedDiscountHelper
+            ->method('calculateSingleShippingCartFixedDiscount')
+            ->willReturn([$shippingAmount, $shippingAmount]);
+
+        $this->cartFixedDiscountHelper
+            ->method('syncQuoteCartFixedRuleBalance');
+
         $quoteMock->method('getExtensionAttributes')
             ->willReturn($cartExtensionMock);
 
@@ -759,6 +799,18 @@ protected function setupAddressMock(
         $this->addressMock->method('getBaseShippingAmountForDiscount')
             ->willReturn($shippingAmount);
 
+        $this->addressMock->method('getShippingAmount')
+            ->willReturn($shippingAmount);
+
+        $this->addressMock->method('getBaseShippingAmount')
+            ->willReturn($shippingAmount);
+
+        $this->addressMock->method('getCartFixedRules')
+            ->willReturn([]);
+
+        $this->addressMock->method('setCartFixedRules')
+            ->willReturnSelf();
+
         $this->addressMock->method('getQuote')
             ->willReturn($quoteMock);
 
@@ -861,4 +913,221 @@ public static function dataProviderNonShippingRuleStopProcessing(): array
             'no stop processing allows subsequent rules' => [false, true, 10.0],
         ];
     }
+
+    /**
+     * Cart-fixed single shipping: seed remaining, cap via helper, sync quote ledger.
+     *
+     * @return void
+     * @SuppressWarnings(PHPMD.ExcessiveMethodLength)
+     */
+    public function testProcessShippingAmountCartFixedSingleShippingUsesRemainingHelper(): void
+    {
+        $shippingAmount = 5.0;
+        $remainingBalance = 5.0;
+        $ruleId = 7;
+        $discountQuote = 5.0;
+        $discountBase = 5.0;
+
+        $ruleMock = $this->createPartialMockWithReflection(
+            Rule::class,
+            ['getApplyToShipping', 'getDiscountAmount', 'getSimpleAction', 'getId', 'getRuleId']
+        );
+        $ruleMock->method('getApplyToShipping')->willReturn(true);
+        $ruleMock->method('getDiscountAmount')->willReturn(56.0);
+        $ruleMock->method('getSimpleAction')->willReturn(Rule::CART_FIXED_ACTION);
+        $ruleMock->method('getId')->willReturn($ruleId);
+        $ruleMock->method('getRuleId')->willReturn($ruleId);
+
+        $iterator = new \ArrayIterator([$ruleMock]);
+        $this->ruleCollection->method('getIterator')->willReturn($iterator);
+        $this->utility->method('canProcessRule')->willReturn(true);
+
+        $this->priceCurrency->method('convert')->willReturn(56.0);
+        $this->priceCurrency->method('roundPrice')
+            ->willReturnCallback(static function ($price) {
+                return round((float) $price, 2);
+            });
+
+        $storeMock = $this->createMock(Store::class);
+        $quoteMock = $this->createPartialMockWithReflection(
+            Quote::class,
+            [
+                'getStore',
+                'getExtensionAttributes',
+                'isVirtual',
+                'setAppliedRuleIds',
+                'getBaseSubtotal',
+                'getCartFixedRules',
+                'setCartFixedRules',
+                'getIsMultiShipping',
+            ]
+        );
+        $quoteMock->method('getStore')->willReturn($storeMock);
+        $quoteMock->method('setAppliedRuleIds')->willReturnSelf();
+        $quoteMock->method('isVirtual')->willReturn(false);
+        $quoteMock->method('getBaseSubtotal')->willReturn(51.0);
+        $quoteMock->method('getCartFixedRules')->willReturn([$ruleId => $remainingBalance]);
+        $quoteMock->method('setCartFixedRules')->willReturnSelf();
+        $quoteMock->method('getIsMultiShipping')->willReturn(false);
+        $quoteMock->method('getExtensionAttributes')->willReturn(null);
+
+        $this->addressMock->method('getQuote')->willReturn($quoteMock);
+        $this->addressMock->method('getCustomAttributesCodes')->willReturn([]);
+        $this->addressMock->method('getShippingAmountForDiscount')->willReturn($shippingAmount);
+        $this->addressMock->method('getBaseShippingAmountForDiscount')->willReturn($shippingAmount);
+        $this->addressMock->method('getShippingAmount')->willReturn($shippingAmount);
+        $this->addressMock->method('getBaseShippingAmount')->willReturn($shippingAmount);
+        $this->addressMock->method('getCartFixedRules')->willReturn([]);
+        $this->addressMock->method('setCartFixedRules')->willReturnSelf();
+
+        // Dual gate short-circuits when getIsMultiShipping() is false, so
+        // checkMultiShippingQuote may not be invoked; stub only if it is.
+        $this->cartFixedDiscountHelper
+            ->method('checkMultiShippingQuote')
+            ->willReturn(false);
+
+        $this->cartFixedDiscountHelper
+            ->expects($this->once())
+            ->method('getCartFixedShippingRuleBalance')
+            ->with($quoteMock, $ruleMock, $ruleId, false)
+            ->willReturn($remainingBalance);
+
+        $this->cartFixedDiscountHelper
+            ->expects($this->once())
+            ->method('calculateSingleShippingCartFixedDiscount')
+            ->with(
+                $quoteMock,
+                $remainingBalance,
+                $shippingAmount,
+                $shippingAmount,
+                0.0,
+                0.0
+            )
+            ->willReturn([$discountQuote, $discountBase]);
+
+        $this->cartFixedDiscountHelper
+            ->expects($this->once())
+            ->method('syncQuoteCartFixedRuleBalance')
+            ->with($quoteMock, $ruleId, 0.0);
+
+        // Multi proportional helpers must not be used on the single-ship path.
+        $this->cartFixedDiscountHelper
+            ->expects($this->never())
+            ->method('getQuoteTotalsForMultiShipping');
+        $this->cartFixedDiscountHelper
+            ->expects($this->never())
+            ->method('getShippingDiscountAmount');
+
+        $this->model->init(
+            $this->model->getWebsiteId(),
+            $this->model->getCustomerGroupId(),
+            $this->model->getCouponCode()
+        );
+
+        $this->assertInstanceOf(Validator::class, $this->model->processShippingAmount($this->addressMock));
+    }
+
+    /**
+     * Cart-fixed multi shipping keeps HEAD proportional helpers (not single-ship calculator).
+     *
+     * @return void
+     */
+    public function testProcessShippingAmountCartFixedMultiShippingUsesLegacyProportional(): void
+    {
+        $shippingAmount = 5.0;
+        $ruleId = 9;
+        $ruleDiscount = 30.0;
+        $proportionalDiscount = 3.75;
+
+        $ruleMock = $this->createPartialMockWithReflection(
+            Rule::class,
+            ['getApplyToShipping', 'getDiscountAmount', 'getSimpleAction', 'getId', 'getRuleId']
+        );
+        $ruleMock->method('getApplyToShipping')->willReturn(true);
+        $ruleMock->method('getDiscountAmount')->willReturn($ruleDiscount);
+        $ruleMock->method('getSimpleAction')->willReturn(Rule::CART_FIXED_ACTION);
+        $ruleMock->method('getId')->willReturn($ruleId);
+        $ruleMock->method('getRuleId')->willReturn($ruleId);
+
+        $iterator = new \ArrayIterator([$ruleMock]);
+        $this->ruleCollection->method('getIterator')->willReturn($iterator);
+        $this->utility->method('canProcessRule')->willReturn(true);
+
+        $this->priceCurrency->method('convert')->willReturn($ruleDiscount);
+        $this->priceCurrency->method('roundPrice')
+            ->willReturnCallback(static function ($price) {
+                return round((float) $price, 2);
+            });
+
+        $storeMock = $this->createMock(Store::class);
+        $quoteMock = $this->createPartialMockWithReflection(
+            Quote::class,
+            [
+                'getStore',
+                'getExtensionAttributes',
+                'isVirtual',
+                'setAppliedRuleIds',
+                'getBaseSubtotal',
+                'getCartFixedRules',
+                'setCartFixedRules',
+                'getIsMultiShipping',
+            ]
+        );
+        $quoteMock->method('getStore')->willReturn($storeMock);
+        $quoteMock->method('setAppliedRuleIds')->willReturnSelf();
+        $quoteMock->method('isVirtual')->willReturn(false);
+        $quoteMock->method('getBaseSubtotal')->willReturn(30.0);
+        $quoteMock->method('getCartFixedRules')->willReturn([]);
+        $quoteMock->method('setCartFixedRules')->willReturnSelf();
+        $quoteMock->method('getIsMultiShipping')->willReturn(true);
+        $quoteMock->method('getExtensionAttributes')->willReturn(null);
+
+        $this->addressMock->method('getQuote')->willReturn($quoteMock);
+        $this->addressMock->method('getCustomAttributesCodes')->willReturn([]);
+        $this->addressMock->method('getShippingAmountForDiscount')->willReturn($shippingAmount);
+        $this->addressMock->method('getBaseShippingAmountForDiscount')->willReturn($shippingAmount);
+        $this->addressMock->method('getShippingAmount')->willReturn($shippingAmount);
+        $this->addressMock->method('getBaseShippingAmount')->willReturn($shippingAmount);
+        $this->addressMock->method('getCartFixedRules')->willReturn([]);
+        $this->addressMock->method('setCartFixedRules')->willReturnSelf();
+
+        $this->cartFixedDiscountHelper
+            ->expects($this->atLeastOnce())
+            ->method('checkMultiShippingQuote')
+            ->with($quoteMock)
+            ->willReturn(true);
+
+        $this->cartFixedDiscountHelper
+            ->expects($this->once())
+            ->method('getCartFixedShippingRuleBalance')
+            ->with($quoteMock, $ruleMock, $ruleId, true)
+            ->willReturn($ruleDiscount);
+
+        $this->cartFixedDiscountHelper
+            ->expects($this->once())
+            ->method('getQuoteTotalsForMultiShipping')
+            ->with($quoteMock)
+            ->willReturn(40.0);
+
+        $this->cartFixedDiscountHelper
+            ->expects($this->once())
+            ->method('getShippingDiscountAmount')
+            ->willReturn($proportionalDiscount);
+
+        // Single-ship calculator and quote sync must not run on multi path.
+        $this->cartFixedDiscountHelper
+            ->expects($this->never())
+            ->method('calculateSingleShippingCartFixedDiscount');
+        $this->cartFixedDiscountHelper
+            ->expects($this->never())
+            ->method('syncQuoteCartFixedRuleBalance');
+
+        $this->model->init(
+            $this->model->getWebsiteId(),
+            $this->model->getCustomerGroupId(),
+            $this->model->getCouponCode()
+        );
+
+        $this->assertInstanceOf(Validator::class, $this->model->processShippingAmount($this->addressMock));
+    }
 }
diff --git a/dev/tests/integration/testsuite/Magento/SalesRule/Model/Rule/Action/Discount/CartFixedShippingTaxTest.php b/dev/tests/integration/testsuite/Magento/SalesRule/Model/Rule/Action/Discount/CartFixedShippingTaxTest.php
new file mode 100644
index 000000000000..3b224df6ee29
--- /dev/null
+++ b/dev/tests/integration/testsuite/Magento/SalesRule/Model/Rule/Action/Discount/CartFixedShippingTaxTest.php
@@ -0,0 +1,672 @@
+<?php
+/**
+ * Copyright 2026 Adobe
+ * All Rights Reserved.
+ */
+declare(strict_types=1);
+
+namespace Magento\SalesRule\Model\Rule\Action\Discount;
+
+use Magento\Catalog\Test\Fixture\Product as ProductFixture;
+use Magento\Checkout\Api\Data\TotalsInformationInterface;
+use Magento\Checkout\Model\TotalsInformationManagement;
+use Magento\Quote\Api\CartRepositoryInterface;
+use Magento\Quote\Api\CouponManagementInterface;
+use Magento\Quote\Model\Quote;
+use Magento\Quote\Model\Quote\Address;
+use Magento\Quote\Model\Quote\AddressFactory;
+use Magento\Quote\Test\Fixture\AddProductToCart as AddProductToCartFixture;
+use Magento\Quote\Test\Fixture\GuestCart as GuestCartFixture;
+use Magento\SalesRule\Model\Rule;
+use Magento\SalesRule\Test\Fixture\Rule as RuleFixture;
+use Magento\Tax\Test\Fixture\TaxRate as TaxRateFixture;
+use Magento\Tax\Test\Fixture\TaxRule as TaxRuleFixture;
+use Magento\TestFramework\Fixture\Config;
+use Magento\TestFramework\Fixture\DataFixture;
+use Magento\TestFramework\Fixture\DataFixtureStorageManager;
+use Magento\TestFramework\Fixture\DbIsolation;
+use Magento\TestFramework\Helper\Bootstrap;
+use PHPUnit\Framework\TestCase;
+
+/**
+ * Single-shipping cart-fixed discount + shipping VAT regressions.
+ *
+ * Multi-shipping cart-fixed confidence is covered by CartFixedTest (unchanged multi path).
+ *
+ * Ticket matrix (EU-style B2C):
+ * - price/shipping/discount tax: including tax
+ * - shipping tax class: taxable goods
+ * - cross-border trade: enabled
+ * - flatrate per order: 5.00
+ * - product 51 taxable, cart-fixed 56 apply_to_shipping
+ *
+ * @magentoAppArea frontend
+ * @SuppressWarnings(PHPMD.CouplingBetweenObjects)
+ */
+class CartFixedShippingTaxTest extends TestCase
+{
+    private const EPSILON = 0.01;
+
+    private const COUPON_CODE = 'CART_FIXED_SHIP_TAX';
+
+    /**
+     * @var CartRepositoryInterface
+     */
+    private $quoteRepository;
+
+    /**
+     * @var CouponManagementInterface
+     */
+    private $couponManagement;
+
+    /**
+     * @inheritdoc
+     */
+    protected function setUp(): void
+    {
+        $objectManager = Bootstrap::getObjectManager();
+        $this->quoteRepository = $objectManager->get(CartRepositoryInterface::class);
+        $this->couponManagement = $objectManager->get(CouponManagementInterface::class);
+    }
+
+    /**
+     * The bug path: coupon applied before shipping is set (incl tax, discount on incl).
+     *
+     * Expected: full rule −56, shipping discount 5.00 (incl VAT), grand total 0.
+     * Buggy path used excl shipping basis → shipping discount 4.17 and residual 0.83.
+     */
+    #[
+        DbIsolation(true),
+        Config('tax/classes/shipping_tax_class', '2', 'store', 'default'),
+        Config('tax/calculation/price_includes_tax', '1', 'store', 'default'),
+        Config('tax/calculation/based_on', 'shipping', 'store', 'default'),
+        Config('tax/calculation/shipping_includes_tax', '1', 'store', 'default'),
+        Config('tax/calculation/discount_tax', '1', 'store', 'default'),
+        Config('tax/calculation/apply_after_discount', '0', 'store', 'default'),
+        Config('tax/calculation/cross_border_trade_enabled', '1', 'store', 'default'),
+        Config('carriers/flatrate/active', '1', 'store', 'default'),
+        Config('carriers/flatrate/price', '5', 'store', 'default'),
+        Config('carriers/flatrate/type', 'O', 'store', 'default'),
+        DataFixture(
+            TaxRateFixture::class,
+            ['tax_country_id' => 'US', 'tax_region_id' => 0, 'tax_postcode' => '*', 'rate' => 20],
+            'taxRate'
+        ),
+        DataFixture(
+            TaxRuleFixture::class,
+            [
+                'customer_tax_class_ids' => [3],
+                'product_tax_class_ids' => [2],
+                'tax_rate_ids' => ['$taxRate.id$'],
+            ]
+        ),
+        DataFixture(ProductFixture::class, ['price' => 51, 'tax_class_id' => 2], 'product'),
+        DataFixture(
+            RuleFixture::class,
+            [
+                'simple_action' => Rule::CART_FIXED_ACTION,
+                'discount_amount' => 56,
+                'apply_to_shipping' => 1,
+                'stop_rules_processing' => 0,
+                'coupon_code' => self::COUPON_CODE,
+            ],
+            'rule'
+        ),
+        DataFixture(GuestCartFixture::class, as: 'cart'),
+        DataFixture(
+            AddProductToCartFixture::class,
+            ['cart_id' => '$cart.id$', 'product_id' => '$product.id$', 'qty' => 1]
+        ),
+    ]
+    public function testCartFixedApplyToShippingCouponBeforeShippingInclTax(): void
+    {
+        $quote = $this->getCartQuote();
+        $this->couponManagement->set((int) $quote->getId(), self::COUPON_CODE);
+        $quote = $this->getCartQuote();
+        $this->assignCaliforniaShipping($quote);
+        $quote->setTotalsCollectedFlag(false);
+        $quote->collectTotals();
+        $this->quoteRepository->save($quote);
+        $quote = $this->getCartQuote();
+        $address = $quote->getShippingAddress();
+
+        $shippingDiscount = (float) $address->getShippingDiscountAmount();
+
+        $this->assertGreaterThan(0.0, (float) $address->getTaxAmount(), 'Expected tax to apply');
+        $this->assertEqualsWithDelta(0.0, (float) $quote->getGrandTotal(), self::EPSILON);
+        $this->assertEqualsWithDelta(-56.0, (float) $address->getDiscountAmount(), self::EPSILON);
+        // Explicit regression: must cover shipping incl VAT (5), not excl (4.17).
+        $this->assertEqualsWithDelta(5.0, $shippingDiscount, self::EPSILON);
+        $this->assertNotEqualsWithDelta(
+            4.17,
+            $shippingDiscount,
+            self::EPSILON,
+            'Shipping discount must not use excl-tax basis when discount_tax=1'
+        );
+    }
+
+    /**
+     * Control path: shipping method first, then coupon — same finals as coupon-first.
+     */
+    #[
+        DbIsolation(true),
+        Config('tax/classes/shipping_tax_class', '2', 'store', 'default'),
+        Config('tax/calculation/price_includes_tax', '1', 'store', 'default'),
+        Config('tax/calculation/based_on', 'shipping', 'store', 'default'),
+        Config('tax/calculation/shipping_includes_tax', '1', 'store', 'default'),
+        Config('tax/calculation/discount_tax', '1', 'store', 'default'),
+        Config('tax/calculation/apply_after_discount', '0', 'store', 'default'),
+        Config('tax/calculation/cross_border_trade_enabled', '1', 'store', 'default'),
+        Config('carriers/flatrate/active', '1', 'store', 'default'),
+        Config('carriers/flatrate/price', '5', 'store', 'default'),
+        Config('carriers/flatrate/type', 'O', 'store', 'default'),
+        DataFixture(
+            TaxRateFixture::class,
+            ['tax_country_id' => 'US', 'tax_region_id' => 0, 'tax_postcode' => '*', 'rate' => 20],
+            'taxRate'
+        ),
+        DataFixture(
+            TaxRuleFixture::class,
+            [
+                'customer_tax_class_ids' => [3],
+                'product_tax_class_ids' => [2],
+                'tax_rate_ids' => ['$taxRate.id$'],
+            ]
+        ),
+        DataFixture(ProductFixture::class, ['price' => 51, 'tax_class_id' => 2], 'product'),
+        DataFixture(
+            RuleFixture::class,
+            [
+                'simple_action' => Rule::CART_FIXED_ACTION,
+                'discount_amount' => 56,
+                'apply_to_shipping' => 1,
+                'stop_rules_processing' => 0,
+                'coupon_code' => self::COUPON_CODE,
+            ],
+            'rule'
+        ),
+        DataFixture(GuestCartFixture::class, as: 'cart'),
+        DataFixture(
+            AddProductToCartFixture::class,
+            ['cart_id' => '$cart.id$', 'product_id' => '$product.id$', 'qty' => 1]
+        ),
+    ]
+    public function testCartFixedApplyToShippingShippingBeforeCouponInclTax(): void
+    {
+        $quote = $this->getCartQuote();
+        $this->assignCaliforniaShipping($quote);
+        $quote->collectTotals();
+        $this->quoteRepository->save($quote);
+        $this->couponManagement->set((int) $quote->getId(), self::COUPON_CODE);
+        $quote = $this->getCartQuote();
+        $address = $quote->getShippingAddress();
+
+        $shippingDiscount = (float) $address->getShippingDiscountAmount();
+
+        $this->assertEqualsWithDelta(0.0, (float) $quote->getGrandTotal(), self::EPSILON);
+        $this->assertEqualsWithDelta(-56.0, (float) $address->getDiscountAmount(), self::EPSILON);
+        $this->assertEqualsWithDelta(5.0, $shippingDiscount, self::EPSILON);
+        $this->assertNotEqualsWithDelta(4.17, $shippingDiscount, self::EPSILON);
+    }
+
+    /**
+     * Cart estimate style.
+     *
+     * Models: guest cart with coupon, then TotalsInformationManagement::calculate with
+     * destination + flatrate (shipping estimation / "estimate shipping and tax" style
+     * recollect without a full checkout place-order flow).
+     */
+    #[
+        DbIsolation(true),
+        Config('tax/classes/shipping_tax_class', '2', 'store', 'default'),
+        Config('tax/calculation/price_includes_tax', '1', 'store', 'default'),
+        Config('tax/calculation/based_on', 'shipping', 'store', 'default'),
+        Config('tax/calculation/shipping_includes_tax', '1', 'store', 'default'),
+        Config('tax/calculation/discount_tax', '1', 'store', 'default'),
+        Config('tax/calculation/apply_after_discount', '0', 'store', 'default'),
+        Config('tax/calculation/cross_border_trade_enabled', '1', 'store', 'default'),
+        Config('carriers/flatrate/active', '1', 'store', 'default'),
+        Config('carriers/flatrate/price', '5', 'store', 'default'),
+        Config('carriers/flatrate/type', 'O', 'store', 'default'),
+        DataFixture(
+            TaxRateFixture::class,
+            ['tax_country_id' => 'US', 'tax_region_id' => 0, 'tax_postcode' => '*', 'rate' => 20],
+            'taxRate'
+        ),
+        DataFixture(
+            TaxRuleFixture::class,
+            [
+                'customer_tax_class_ids' => [3],
+                'product_tax_class_ids' => [2],
+                'tax_rate_ids' => ['$taxRate.id$'],
+            ]
+        ),
+        DataFixture(ProductFixture::class, ['price' => 51, 'tax_class_id' => 2], 'product'),
+        DataFixture(
+            RuleFixture::class,
+            [
+                'simple_action' => Rule::CART_FIXED_ACTION,
+                'discount_amount' => 56,
+                'apply_to_shipping' => 1,
+                'stop_rules_processing' => 0,
+                'coupon_code' => self::COUPON_CODE,
+            ],
+            'rule'
+        ),
+        DataFixture(GuestCartFixture::class, as: 'cart'),
+        DataFixture(
+            AddProductToCartFixture::class,
+            ['cart_id' => '$cart.id$', 'product_id' => '$product.id$', 'qty' => 1]
+        ),
+    ]
+    public function testTicket32468CartEstimateStyle(): void
+    {
+        $quote = $this->getCartQuote();
+        $cartId = (int) $quote->getId();
+        $this->couponManagement->set($cartId, self::COUPON_CODE);
+
+        $objectManager = Bootstrap::getObjectManager();
+        /** @var Address $address */
+        $address = $objectManager->get(AddressFactory::class)->create();
+        $address->setAddressType(Address::ADDRESS_TYPE_SHIPPING)
+            ->setCountryId('US')
+            ->setRegionId(12)
+            ->setRegion('California')
+            ->setPostcode('90210')
+            ->setCity('Los Angeles');
+        $addressInformation = $objectManager->create(
+            TotalsInformationInterface::class,
+            [
+                'data' => [
+                    'address' => $address,
+                    'shipping_method_code' => 'flatrate',
+                    'shipping_carrier_code' => 'flatrate',
+                ],
+            ]
+        );
+
+        /** @var TotalsInformationManagement $totalsManagement */
+        $totalsManagement = $objectManager->get(TotalsInformationManagement::class);
+        $totals = $totalsManagement->calculate($cartId, $addressInformation);
+
+        $this->assertEqualsWithDelta(0.0, (float) $totals->getGrandTotal(), self::EPSILON);
+        $this->assertEqualsWithDelta(-56.0, (float) $totals->getDiscountAmount(), self::EPSILON);
+
+        // Persist path: re-load quote after estimate-style collect and assert shipping share.
+        $quote = $this->getCartQuote();
+        $shippingDiscount = (float) $quote->getShippingAddress()->getShippingDiscountAmount();
+        $this->assertEqualsWithDelta(5.0, $shippingDiscount, self::EPSILON);
+        $this->assertNotEqualsWithDelta(4.17, $shippingDiscount, self::EPSILON);
+    }
+
+    /**
+     * Tax applied after discount (apply_after_discount=1) still fully covers 51+5 with rule 56.
+     *
+     * Ticket notes both apply_after_discount 0 and 1 reproduce the shipping VAT miss without the fix.
+     */
+    #[
+        DbIsolation(true),
+        Config('tax/classes/shipping_tax_class', '2', 'store', 'default'),
+        Config('tax/calculation/price_includes_tax', '1', 'store', 'default'),
+        Config('tax/calculation/based_on', 'shipping', 'store', 'default'),
+        Config('tax/calculation/shipping_includes_tax', '1', 'store', 'default'),
+        Config('tax/calculation/discount_tax', '1', 'store', 'default'),
+        Config('tax/calculation/apply_after_discount', '1', 'store', 'default'),
+        Config('tax/calculation/cross_border_trade_enabled', '1', 'store', 'default'),
+        Config('carriers/flatrate/active', '1', 'store', 'default'),
+        Config('carriers/flatrate/price', '5', 'store', 'default'),
+        Config('carriers/flatrate/type', 'O', 'store', 'default'),
+        DataFixture(
+            TaxRateFixture::class,
+            ['tax_country_id' => 'US', 'tax_region_id' => 0, 'tax_postcode' => '*', 'rate' => 20],
+            'taxRate'
+        ),
+        DataFixture(
+            TaxRuleFixture::class,
+            [
+                'customer_tax_class_ids' => [3],
+                'product_tax_class_ids' => [2],
+                'tax_rate_ids' => ['$taxRate.id$'],
+            ]
+        ),
+        DataFixture(ProductFixture::class, ['price' => 51, 'tax_class_id' => 2], 'product'),
+        DataFixture(
+            RuleFixture::class,
+            [
+                'simple_action' => Rule::CART_FIXED_ACTION,
+                'discount_amount' => 56,
+                'apply_to_shipping' => 1,
+                'stop_rules_processing' => 0,
+                'coupon_code' => self::COUPON_CODE,
+            ],
+            'rule'
+        ),
+        DataFixture(GuestCartFixture::class, as: 'cart'),
+        DataFixture(
+            AddProductToCartFixture::class,
+            ['cart_id' => '$cart.id$', 'product_id' => '$product.id$', 'qty' => 1]
+        ),
+    ]
+    public function testCartFixedApplyToShippingCouponBeforeShippingTaxAfterDiscount(): void
+    {
+        $quote = $this->getCartQuote();
+        $this->couponManagement->set((int) $quote->getId(), self::COUPON_CODE);
+        $quote = $this->getCartQuote();
+        $this->assignCaliforniaShipping($quote);
+        $quote->setTotalsCollectedFlag(false);
+        $quote->collectTotals();
+        $this->quoteRepository->save($quote);
+        $quote = $this->getCartQuote();
+        $address = $quote->getShippingAddress();
+
+        $this->assertEqualsWithDelta(0.0, (float) $quote->getGrandTotal(), self::EPSILON);
+        $this->assertEqualsWithDelta(-56.0, (float) $address->getDiscountAmount(), self::EPSILON);
+        $this->assertEqualsWithDelta(5.0, (float) $address->getShippingDiscountAmount(), self::EPSILON);
+    }
+
+    /**
+     * Discount on excl tax (discount_tax=0): shipping discount uses excl shipping basis.
+     *
+     * Expected: shipping disc ≈ 4.17 (5/1.20), not full 5.00 VAT-inclusive cover.
+     * Total discount is limited by excl-tax item+ship basis (≈ 46.67), not rule amount 56.
+     */
+    #[
+        DbIsolation(true),
+        Config('tax/classes/shipping_tax_class', '2', 'store', 'default'),
+        Config('tax/calculation/price_includes_tax', '1', 'store', 'default'),
+        Config('tax/calculation/based_on', 'shipping', 'store', 'default'),
+        Config('tax/calculation/shipping_includes_tax', '1', 'store', 'default'),
+        Config('tax/calculation/discount_tax', '0', 'store', 'default'),
+        Config('tax/calculation/apply_after_discount', '0', 'store', 'default'),
+        Config('tax/calculation/cross_border_trade_enabled', '1', 'store', 'default'),
+        Config('carriers/flatrate/active', '1', 'store', 'default'),
+        Config('carriers/flatrate/price', '5', 'store', 'default'),
+        Config('carriers/flatrate/type', 'O', 'store', 'default'),
+        DataFixture(
+            TaxRateFixture::class,
+            ['tax_country_id' => 'US', 'tax_region_id' => 0, 'tax_postcode' => '*', 'rate' => 20],
+            'taxRate'
+        ),
+        DataFixture(
+            TaxRuleFixture::class,
+            [
+                'customer_tax_class_ids' => [3],
+                'product_tax_class_ids' => [2],
+                'tax_rate_ids' => ['$taxRate.id$'],
+            ]
+        ),
+        DataFixture(ProductFixture::class, ['price' => 51, 'tax_class_id' => 2], 'product'),
+        DataFixture(
+            RuleFixture::class,
+            [
+                'simple_action' => Rule::CART_FIXED_ACTION,
+                'discount_amount' => 56,
+                'apply_to_shipping' => 1,
+                'stop_rules_processing' => 0,
+                'coupon_code' => self::COUPON_CODE,
+            ],
+            'rule'
+        ),
+        DataFixture(GuestCartFixture::class, as: 'cart'),
+        DataFixture(
+            AddProductToCartFixture::class,
+            ['cart_id' => '$cart.id$', 'product_id' => '$product.id$', 'qty' => 1]
+        ),
+    ]
+    public function testCartFixedApplyToShippingDiscountOnExclTax(): void
+    {
+        $quote = $this->getCartQuote();
+        $this->couponManagement->set((int) $quote->getId(), self::COUPON_CODE);
+        $quote = $this->getCartQuote();
+        $this->assignCaliforniaShipping($quote);
+        $quote->setTotalsCollectedFlag(false);
+        $quote->collectTotals();
+        $this->quoteRepository->save($quote);
+        $quote = $this->getCartQuote();
+        $address = $quote->getShippingAddress();
+
+        $this->assertGreaterThan(0.0, (float) $address->getTaxAmount(), 'Tax must apply for excl-discount assertions');
+        // Shipping amount stored excl when shipping_includes_tax display path resolves excl basis.
+        $this->assertEqualsWithDelta(4.17, (float) $address->getShippingAmount(), self::EPSILON);
+        $this->assertEqualsWithDelta(
+            4.17,
+            (float) $address->getShippingDiscountAmount(),
+            self::EPSILON,
+            'With discount_tax=0 shipping discount must not cover full incl-tax 5.00'
+        );
+        $this->assertEqualsWithDelta(-46.67, (float) $address->getDiscountAmount(), self::EPSILON);
+        // Must not exceed rule amount even when tax basis is excl.
+        $this->assertLessThanOrEqual(56.0 + self::EPSILON, abs((float) $address->getDiscountAmount()));
+    }
+
+    /**
+     * Partial remaining: rule 53 on items+ship basis 56 → shipping share ≈ 53 * 5/56 ≈ 4.73.
+     */
+    #[
+        DbIsolation(true),
+        Config('tax/classes/shipping_tax_class', '2', 'store', 'default'),
+        Config('tax/calculation/price_includes_tax', '1', 'store', 'default'),
+        Config('tax/calculation/based_on', 'shipping', 'store', 'default'),
+        Config('tax/calculation/shipping_includes_tax', '1', 'store', 'default'),
+        Config('tax/calculation/discount_tax', '1', 'store', 'default'),
+        Config('tax/calculation/apply_after_discount', '0', 'store', 'default'),
+        Config('tax/calculation/cross_border_trade_enabled', '1', 'store', 'default'),
+        Config('carriers/flatrate/active', '1', 'store', 'default'),
+        Config('carriers/flatrate/price', '5', 'store', 'default'),
+        Config('carriers/flatrate/type', 'O', 'store', 'default'),
+        DataFixture(
+            TaxRateFixture::class,
+            ['tax_country_id' => 'US', 'tax_region_id' => 0, 'tax_postcode' => '*', 'rate' => 20],
+            'taxRate'
+        ),
+        DataFixture(
+            TaxRuleFixture::class,
+            [
+                'customer_tax_class_ids' => [3],
+                'product_tax_class_ids' => [2],
+                'tax_rate_ids' => ['$taxRate.id$'],
+            ]
+        ),
+        DataFixture(ProductFixture::class, ['price' => 51, 'tax_class_id' => 2], 'product'),
+        DataFixture(
+            RuleFixture::class,
+            [
+                'simple_action' => Rule::CART_FIXED_ACTION,
+                'discount_amount' => 53,
+                'apply_to_shipping' => 1,
+                'stop_rules_processing' => 0,
+                'coupon_code' => self::COUPON_CODE,
+            ],
+            'rule'
+        ),
+        DataFixture(GuestCartFixture::class, as: 'cart'),
+        DataFixture(
+            AddProductToCartFixture::class,
+            ['cart_id' => '$cart.id$', 'product_id' => '$product.id$', 'qty' => 1]
+        ),
+    ]
+    public function testCartFixedPartialRemainingAppliedToShipping(): void
+    {
+        $quote = $this->getCartQuote();
+        $this->assignCaliforniaShipping($quote);
+        $quote->collectTotals();
+        $this->quoteRepository->save($quote);
+        $this->couponManagement->set((int) $quote->getId(), self::COUPON_CODE);
+        $quote = $this->getCartQuote();
+        $address = $quote->getShippingAddress();
+
+        // Fixed 53 with items+ship basis 56 → shipping share ≈ 53 * 5/56 ≈ 4.73
+        $this->assertEqualsWithDelta(4.73, (float) $address->getShippingDiscountAmount(), self::EPSILON);
+        $this->assertEqualsWithDelta(-53.0, (float) $address->getDiscountAmount(), self::EPSILON);
+        $this->assertLessThanOrEqual(53.0 + self::EPSILON, abs((float) $address->getDiscountAmount()));
+    }
+
+    /**
+     * Money invariant: cart-fixed total discount never exceeds the rule amount.
+     *
+     * Rule 100 with product 51 + ship 5 (incl basis 56): discount must be ≤ 100 and
+     * practically capped by cart value (≈ 56), shipping still fully covered at 5.00.
+     */
+    #[
+        DbIsolation(true),
+        Config('tax/classes/shipping_tax_class', '2', 'store', 'default'),
+        Config('tax/calculation/price_includes_tax', '1', 'store', 'default'),
+        Config('tax/calculation/based_on', 'shipping', 'store', 'default'),
+        Config('tax/calculation/shipping_includes_tax', '1', 'store', 'default'),
+        Config('tax/calculation/discount_tax', '1', 'store', 'default'),
+        Config('tax/calculation/apply_after_discount', '0', 'store', 'default'),
+        Config('tax/calculation/cross_border_trade_enabled', '1', 'store', 'default'),
+        Config('carriers/flatrate/active', '1', 'store', 'default'),
+        Config('carriers/flatrate/price', '5', 'store', 'default'),
+        Config('carriers/flatrate/type', 'O', 'store', 'default'),
+        DataFixture(
+            TaxRateFixture::class,
+            ['tax_country_id' => 'US', 'tax_region_id' => 0, 'tax_postcode' => '*', 'rate' => 20],
+            'taxRate'
+        ),
+        DataFixture(
+            TaxRuleFixture::class,
+            [
+                'customer_tax_class_ids' => [3],
+                'product_tax_class_ids' => [2],
+                'tax_rate_ids' => ['$taxRate.id$'],
+            ]
+        ),
+        DataFixture(ProductFixture::class, ['price' => 51, 'tax_class_id' => 2], 'product'),
+        DataFixture(
+            RuleFixture::class,
+            [
+                'simple_action' => Rule::CART_FIXED_ACTION,
+                'discount_amount' => 100,
+                'apply_to_shipping' => 1,
+                'stop_rules_processing' => 0,
+                'coupon_code' => self::COUPON_CODE,
+            ],
+            'rule'
+        ),
+        DataFixture(GuestCartFixture::class, as: 'cart'),
+        DataFixture(
+            AddProductToCartFixture::class,
+            ['cart_id' => '$cart.id$', 'product_id' => '$product.id$', 'qty' => 1]
+        ),
+    ]
+    public function testCartFixedDiscountNeverExceedsRuleAmount(): void
+    {
+        $ruleAmount = 100.0;
+        $quote = $this->getCartQuote();
+        $this->couponManagement->set((int) $quote->getId(), self::COUPON_CODE);
+        $quote = $this->getCartQuote();
+        $this->assignCaliforniaShipping($quote);
+        $quote->setTotalsCollectedFlag(false);
+        $quote->collectTotals();
+        $this->quoteRepository->save($quote);
+        $quote = $this->getCartQuote();
+        $address = $quote->getShippingAddress();
+
+        $discountAmount = abs((float) $address->getDiscountAmount());
+        $shippingDiscount = (float) $address->getShippingDiscountAmount();
+
+        $this->assertLessThanOrEqual(
+            $ruleAmount + self::EPSILON,
+            $discountAmount,
+            'Cart-fixed total discount must never exceed the rule amount'
+        );
+        // Cart basis is 51 + 5 = 56; oversize rule must not invent extra discount.
+        $this->assertLessThanOrEqual(56.0 + self::EPSILON, $discountAmount);
+        $this->assertEqualsWithDelta(5.0, $shippingDiscount, self::EPSILON);
+        $this->assertEqualsWithDelta(0.0, (float) $quote->getGrandTotal(), self::EPSILON);
+        $this->assertEqualsWithDelta(-56.0, (float) $address->getDiscountAmount(), self::EPSILON);
+    }
+
+    /**
+     * Catalog excl tax, shipping incl tax, discount on incl tax (stable mixed config).
+     */
+    #[
+        DbIsolation(true),
+        Config('tax/classes/shipping_tax_class', '2', 'store', 'default'),
+        Config('tax/calculation/price_includes_tax', '0', 'store', 'default'),
+        Config('tax/calculation/based_on', 'shipping', 'store', 'default'),
+        Config('tax/calculation/shipping_includes_tax', '1', 'store', 'default'),
+        Config('tax/calculation/discount_tax', '1', 'store', 'default'),
+        Config('tax/calculation/apply_after_discount', '0', 'store', 'default'),
+        Config('tax/calculation/cross_border_trade_enabled', '1', 'store', 'default'),
+        Config('carriers/flatrate/active', '1', 'store', 'default'),
+        Config('carriers/flatrate/price', '5', 'store', 'default'),
+        Config('carriers/flatrate/type', 'O', 'store', 'default'),
+        DataFixture(
+            TaxRateFixture::class,
+            ['tax_country_id' => 'US', 'tax_region_id' => 0, 'tax_postcode' => '*', 'rate' => 20],
+            'taxRate'
+        ),
+        DataFixture(
+            TaxRuleFixture::class,
+            [
+                'customer_tax_class_ids' => [3],
+                'product_tax_class_ids' => [2],
+                'tax_rate_ids' => ['$taxRate.id$'],
+            ]
+        ),
+        DataFixture(ProductFixture::class, ['price' => 42.5, 'tax_class_id' => 2], 'product'),
+        DataFixture(
+            RuleFixture::class,
+            [
+                'simple_action' => Rule::CART_FIXED_ACTION,
+                'discount_amount' => 56,
+                'apply_to_shipping' => 1,
+                'stop_rules_processing' => 0,
+                'coupon_code' => self::COUPON_CODE,
+            ],
+            'rule'
+        ),
+        DataFixture(GuestCartFixture::class, as: 'cart'),
+        DataFixture(
+            AddProductToCartFixture::class,
+            ['cart_id' => '$cart.id$', 'product_id' => '$product.id$', 'qty' => 1]
+        ),
+    ]
+    public function testCartFixedMixedCatalogExclShippingInclDiscountIncl(): void
+    {
+        $quote = $this->getCartQuote();
+        $this->couponManagement->set((int) $quote->getId(), self::COUPON_CODE);
+        $quote = $this->getCartQuote();
+        $this->assignCaliforniaShipping($quote);
+        $quote->setTotalsCollectedFlag(false);
+        $quote->collectTotals();
+        $this->quoteRepository->save($quote);
+        $quote = $this->getCartQuote();
+        $address = $quote->getShippingAddress();
+
+        $this->assertEqualsWithDelta(0.0, (float) $quote->getGrandTotal(), self::EPSILON);
+        $this->assertEqualsWithDelta(-56.0, (float) $address->getDiscountAmount(), self::EPSILON);
+        $this->assertEqualsWithDelta(5.0, (float) $address->getShippingDiscountAmount(), self::EPSILON);
+        $this->assertLessThanOrEqual(56.0 + self::EPSILON, abs((float) $address->getDiscountAmount()));
+    }
+
+    private function getCartQuote(): Quote
+    {
+        $cart = DataFixtureStorageManager::getStorage()->get('cart');
+        return $this->quoteRepository->get((int) $cart->getId());
+    }
+
+    private function assignCaliforniaShipping(Quote $quote): void
+    {
+        $addressData = [
+            'firstname' => 'Test',
+            'lastname' => 'User',
+            'street' => ['123 Test St'],
+            'city' => 'Los Angeles',
+            'region' => 'CA',
+            'region_id' => 12,
+            'postcode' => '90210',
+            'country_id' => 'US',
+            'telephone' => '5555555555',
+            'email' => 'cart-fixed-shipping-tax@example.com',
+        ];
+        $quote->getBillingAddress()->addData($addressData);
+        $quote->getShippingAddress()
+            ->addData($addressData)
+            ->setCollectShippingRates(true)
+            ->setShippingMethod('flatrate_flatrate')
+            ->setSameAsBilling(1);
+    }
+}