diff --git a/src/Entity/Bucket.php b/src/Entity/Bucket.php
index 4ce047f..ec09345 100644
--- a/src/Entity/Bucket.php
+++ b/src/Entity/Bucket.php
@@ -5,16 +5,19 @@ namespace App\Entity;
 use App\Enum\BucketAllocationType;
 use App\Enum\BucketType;
 use App\Repository\BucketRepository;
+use App\Validator\BufferOnlyForFixedLimit;
 use App\Validator\CompatibleAllocationType;
 use App\Validator\SingleOverflowPerScenario;
 use Doctrine\ORM\Mapping as ORM;
 use Symfony\Component\Uid\UuidV7;
+use Symfony\Component\Validator\Constraints as Assert;
 
 #[ORM\Entity(repositoryClass: BucketRepository::class)]
 #[ORM\Table(name: 'bucket')]
 #[ORM\UniqueConstraint(name: 'unique_scenario_priority', columns: ['scenario_id', 'priority'])]
 #[CompatibleAllocationType]
 #[SingleOverflowPerScenario]
+#[BufferOnlyForFixedLimit]
 class Bucket
 {
     #[ORM\Id]
@@ -47,6 +50,7 @@ class Bucket
     private int $startingAmount = 0;
 
     #[ORM\Column(type: 'decimal', precision: 5, scale: 2, options: ['default' => '0.00'])]
+    #[Assert\GreaterThanOrEqual(0)]
     private string $bufferMultiplier = '0.00';
 
     public function __construct()
diff --git a/src/Validator/BufferOnlyForFixedLimit.php b/src/Validator/BufferOnlyForFixedLimit.php
new file mode 100644
index 0000000..a4a5db4
--- /dev/null
+++ b/src/Validator/BufferOnlyForFixedLimit.php
@@ -0,0 +1,18 @@
+<?php
+
+namespace App\Validator;
+
+use Symfony\Component\Validator\Constraint;
+
+#[\Attribute(\Attribute::TARGET_CLASS)]
+final class BufferOnlyForFixedLimit extends Constraint
+{
+    public string $message = 'The buffer multiplier can only be set for fixed-limit buckets.';
+
+    public const string BUFFER_NOT_ALLOWED_ERROR = 'BUFFER_NOT_ALLOWED_ERROR';
+
+    public function getTargets(): string
+    {
+        return self::CLASS_CONSTRAINT;
+    }
+}
diff --git a/src/Validator/BufferOnlyForFixedLimitValidator.php b/src/Validator/BufferOnlyForFixedLimitValidator.php
new file mode 100644
index 0000000..140fcb6
--- /dev/null
+++ b/src/Validator/BufferOnlyForFixedLimitValidator.php
@@ -0,0 +1,34 @@
+<?php
+
+namespace App\Validator;
+
+use App\Entity\Bucket;
+use App\Enum\BucketAllocationType;
+use Symfony\Component\Validator\Constraint;
+use Symfony\Component\Validator\ConstraintValidator;
+use Symfony\Component\Validator\Exception\UnexpectedTypeException;
+
+final class BufferOnlyForFixedLimitValidator extends ConstraintValidator
+{
+    public function validate(mixed $value, Constraint $constraint): void
+    {
+        if (!$constraint instanceof BufferOnlyForFixedLimit) {
+            throw new UnexpectedTypeException($constraint, BufferOnlyForFixedLimit::class);
+        }
+
+        if (!$value instanceof Bucket) {
+            return;
+        }
+
+        if (BucketAllocationType::FIXED_LIMIT === $value->getAllocationType()) {
+            return; // buffer allowed
+        }
+
+        if (0.0 !== (float) $value->getBufferMultiplier()) {
+            $this->context->buildViolation($constraint->message)
+                ->atPath('bufferMultiplier')
+                ->setCode(BufferOnlyForFixedLimit::BUFFER_NOT_ALLOWED_ERROR)
+                ->addViolation();
+        }
+    }
+}
diff --git a/tests/Validator/BufferOnlyForFixedLimitTest.php b/tests/Validator/BufferOnlyForFixedLimitTest.php
new file mode 100644
index 0000000..ee75c29
--- /dev/null
+++ b/tests/Validator/BufferOnlyForFixedLimitTest.php
@@ -0,0 +1,116 @@
+<?php
+
+namespace App\Tests\Validator;
+
+use App\Entity\Bucket;
+use App\Entity\Scenario;
+use App\Enum\BucketAllocationType;
+use App\Enum\BucketType;
+use App\Validator\BufferOnlyForFixedLimit;
+use Symfony\Bundle\FrameworkBundle\Test\KernelTestCase;
+use Symfony\Component\Validator\ConstraintViolationListInterface;
+use Symfony\Component\Validator\Validator\ValidatorInterface;
+
+final class BufferOnlyForFixedLimitTest extends KernelTestCase
+{
+    private ValidatorInterface $validator;
+
+    protected function setUp(): void
+    {
+        self::bootKernel();
+        $this->validator = self::getContainer()->get(ValidatorInterface::class);
+    }
+
+    public function testFixedLimitBucketWithNonZeroBufferIsValid(): void
+    {
+        $bucket = $this->makeBucket(BucketType::NEED, BucketAllocationType::FIXED_LIMIT, 50000, '1.50');
+
+        $violations = $this->validator->validate($bucket);
+
+        $matching = $this->violationsAt($violations, 'bufferMultiplier', BufferOnlyForFixedLimit::class);
+
+        self::assertCount(
+            0,
+            $matching,
+            'FIXED_LIMIT bucket with a non-zero buffer must not raise BufferOnlyForFixedLimit.',
+        );
+    }
+
+    public function testUnlimitedBucketWithNonZeroBufferIsInvalid(): void
+    {
+        $bucket = $this->makeBucket(BucketType::OVERFLOW, BucketAllocationType::UNLIMITED, null, '2.00');
+
+        $violations = $this->validator->validate($bucket);
+
+        $matching = $this->violationsAt($violations, 'bufferMultiplier', BufferOnlyForFixedLimit::class);
+
+        self::assertNotCount(
+            0,
+            $matching,
+            'UNLIMITED bucket with a non-zero buffer must raise a violation on bufferMultiplier.',
+        );
+    }
+
+    public function testNegativeBufferIsInvalid(): void
+    {
+        $bucket = $this->makeBucket(BucketType::NEED, BucketAllocationType::FIXED_LIMIT, 50000, '-1.00');
+
+        $violations = $this->validator->validate($bucket);
+
+        $matching = $this->violationsAt($violations, 'bufferMultiplier');
+
+        self::assertNotCount(
+            0,
+            $matching,
+            'A negative bufferMultiplier must raise a violation on bufferMultiplier (GreaterThanOrEqual(0)).',
+        );
+    }
+
+    private function makeBucket(
+        BucketType $type,
+        BucketAllocationType $allocationType,
+        ?int $allocationValue,
+        string $bufferMultiplier,
+    ): Bucket {
+        $scenario = new Scenario();
+        $scenario->setName('Household Budget');
+
+        $bucket = new Bucket();
+        $bucket->setScenario($scenario);
+        $bucket->setType($type);
+        $bucket->setName('Test Bucket');
+        $bucket->setPriority(1);
+        $bucket->setSortOrder(0);
+        $bucket->setAllocationType($allocationType);
+        $bucket->setAllocationValue($allocationValue);
+        $bucket->setStartingAmount(0);
+        $bucket->setBufferMultiplier($bufferMultiplier);
+
+        return $bucket;
+    }
+
+    /**
+     * @return list<\Symfony\Component\Validator\ConstraintViolationInterface>
+     */
+    private function violationsAt(
+        ConstraintViolationListInterface $violations,
+        string $propertyPath,
+        ?string $constraintClass = null,
+    ): array {
+        $matching = [];
+
+        foreach ($violations as $violation) {
+            if ($violation->getPropertyPath() !== $propertyPath) {
+                continue;
+            }
+
+            if (null !== $constraintClass && !$violation->getConstraint() instanceof $constraintClass) {
+                continue;
+            }
+
+            $matching[] = $violation;
+        }
+
+        return $matching;
+    }
+}
diff --git a/tests/Validator/BufferOnlyForFixedLimitValidatorTest.php b/tests/Validator/BufferOnlyForFixedLimitValidatorTest.php
new file mode 100644
index 0000000..0519b96
--- /dev/null
+++ b/tests/Validator/BufferOnlyForFixedLimitValidatorTest.php
@@ -0,0 +1,102 @@
+<?php
+
+namespace App\Tests\Validator;
+
+use App\Entity\Bucket;
+use App\Entity\Scenario;
+use App\Enum\BucketAllocationType;
+use App\Enum\BucketType;
+use App\Validator\BufferOnlyForFixedLimit;
+use App\Validator\BufferOnlyForFixedLimitValidator;
+use Symfony\Component\Validator\ConstraintValidatorInterface;
+use Symfony\Component\Validator\Test\ConstraintValidatorTestCase;
+
+/**
+ * @extends ConstraintValidatorTestCase<BufferOnlyForFixedLimitValidator>
+ */
+final class BufferOnlyForFixedLimitValidatorTest extends ConstraintValidatorTestCase
+{
+    protected function createValidator(): ConstraintValidatorInterface
+    {
+        return new BufferOnlyForFixedLimitValidator();
+    }
+
+    public function testFixedLimitWithNonZeroBufferIsValid(): void
+    {
+        $bucket = $this->makeBucket(BucketAllocationType::FIXED_LIMIT, '1.50');
+
+        $this->validator->validate($bucket, new BufferOnlyForFixedLimit());
+
+        $this->assertNoViolation();
+    }
+
+    public function testFixedLimitWithZeroBufferIsValid(): void
+    {
+        $bucket = $this->makeBucket(BucketAllocationType::FIXED_LIMIT, '0.00');
+
+        $this->validator->validate($bucket, new BufferOnlyForFixedLimit());
+
+        $this->assertNoViolation();
+    }
+
+    public function testPercentageWithNonZeroBufferIsInvalid(): void
+    {
+        $bucket = $this->makeBucket(BucketAllocationType::PERCENTAGE, '1.50');
+
+        $this->validator->validate($bucket, new BufferOnlyForFixedLimit());
+
+        $this->buildViolation('The buffer multiplier can only be set for fixed-limit buckets.')
+            ->atPath('property.path.bufferMultiplier')
+            ->setCode(BufferOnlyForFixedLimit::BUFFER_NOT_ALLOWED_ERROR)
+            ->assertRaised();
+    }
+
+    public function testUnlimitedWithNonZeroBufferIsInvalid(): void
+    {
+        $bucket = $this->makeBucket(BucketAllocationType::UNLIMITED, '2.00');
+
+        $this->validator->validate($bucket, new BufferOnlyForFixedLimit());
+
+        $this->buildViolation('The buffer multiplier can only be set for fixed-limit buckets.')
+            ->atPath('property.path.bufferMultiplier')
+            ->setCode(BufferOnlyForFixedLimit::BUFFER_NOT_ALLOWED_ERROR)
+            ->assertRaised();
+    }
+
+    public function testPercentageWithZeroBufferIsValid(): void
+    {
+        $bucket = $this->makeBucket(BucketAllocationType::PERCENTAGE, '0.00');
+
+        $this->validator->validate($bucket, new BufferOnlyForFixedLimit());
+
+        $this->assertNoViolation();
+    }
+
+    public function testPercentageWithZeroBufferWithoutDecimalsIsValid(): void
+    {
+        $bucket = $this->makeBucket(BucketAllocationType::PERCENTAGE, '0');
+
+        $this->validator->validate($bucket, new BufferOnlyForFixedLimit());
+
+        $this->assertNoViolation();
+    }
+
+    private function makeBucket(BucketAllocationType $allocationType, string $bufferMultiplier): Bucket
+    {
+        $scenario = new Scenario();
+        $scenario->setName('Household Budget');
+
+        $bucket = new Bucket();
+        $bucket->setScenario($scenario);
+        $bucket->setType(BucketType::NEED);
+        $bucket->setName('Test Bucket');
+        $bucket->setPriority(1);
+        $bucket->setSortOrder(0);
+        $bucket->setAllocationType($allocationType);
+        $bucket->setAllocationValue(BucketAllocationType::FIXED_LIMIT === $allocationType ? 50000 : null);
+        $bucket->setStartingAmount(0);
+        $bucket->setBufferMultiplier($bufferMultiplier);
+
+        return $bucket;
+    }
+}