diff --git a/migrations/Version20260619212401.php b/migrations/Version20260619212401.php
new file mode 100644
index 0000000..dec3a7d
--- /dev/null
+++ b/migrations/Version20260619212401.php
@@ -0,0 +1,29 @@
+<?php
+
+namespace DoctrineMigrations;
+
+use Doctrine\DBAL\Schema\Schema;
+use Doctrine\Migrations\AbstractMigration;
+
+/**
+ * Auto-generated Migration: Please modify to your needs!
+ */
+final class Version20260619212401 extends AbstractMigration
+{
+    public function getDescription(): string
+    {
+        return '';
+    }
+
+    public function up(Schema $schema): void
+    {
+        // this up() migration is auto-generated, please modify it to your needs
+        $this->addSql('ALTER TABLE bucket ALTER allocation_type SET DEFAULT \'fixed_limit\'');
+    }
+
+    public function down(Schema $schema): void
+    {
+        // this down() migration is auto-generated, please modify it to your needs
+        $this->addSql('ALTER TABLE bucket ALTER allocation_type DROP DEFAULT');
+    }
+}
diff --git a/src/Entity/Bucket.php b/src/Entity/Bucket.php
index 5084446..6d6811d 100644
--- a/src/Entity/Bucket.php
+++ b/src/Entity/Bucket.php
@@ -5,12 +5,14 @@ namespace App\Entity;
 use App\Enum\BucketAllocationType;
 use App\Enum\BucketType;
 use App\Repository\BucketRepository;
+use App\Validator\CompatibleAllocationType;
 use Doctrine\ORM\Mapping as ORM;
 use Symfony\Component\Uid\UuidV7;
 
 #[ORM\Entity(repositoryClass: BucketRepository::class)]
 #[ORM\Table(name: 'bucket')]
 #[ORM\UniqueConstraint(name: 'unique_scenario_priority', columns: ['scenario_id', 'priority'])]
+#[CompatibleAllocationType]
 class Bucket
 {
     #[ORM\Id]
@@ -33,8 +35,8 @@ class Bucket
     #[ORM\Column(options: ['default' => 0])]
     private int $sortOrder = 0;
 
-    #[ORM\Column(enumType: BucketAllocationType::class)]
-    private BucketAllocationType $allocationType;
+    #[ORM\Column(enumType: BucketAllocationType::class, options: ['default' => BucketAllocationType::FIXED_LIMIT])]
+    private BucketAllocationType $allocationType = BucketAllocationType::FIXED_LIMIT;
 
     #[ORM\Column(nullable: true)]
     private ?int $allocationValue = null;
diff --git a/src/Validator/CompatibleAllocationType.php b/src/Validator/CompatibleAllocationType.php
new file mode 100644
index 0000000..3a2a08c
--- /dev/null
+++ b/src/Validator/CompatibleAllocationType.php
@@ -0,0 +1,18 @@
+<?php
+
+namespace App\Validator;
+
+use Symfony\Component\Validator\Constraint;
+
+#[\Attribute(\Attribute::TARGET_CLASS)]
+final class CompatibleAllocationType extends Constraint
+{
+    public string $message = 'This allocation type is not compatible with the bucket type.';
+
+    public const string NOT_COMPATIBLE_ERROR = 'NOT_COMPATIBLE_ERROR';
+
+    public function getTargets(): string
+    {
+        return self::CLASS_CONSTRAINT;
+    }
+}
diff --git a/src/Validator/CompatibleAllocationTypeValidator.php b/src/Validator/CompatibleAllocationTypeValidator.php
new file mode 100644
index 0000000..03f1bb2
--- /dev/null
+++ b/src/Validator/CompatibleAllocationTypeValidator.php
@@ -0,0 +1,32 @@
+<?php
+
+namespace App\Validator;
+
+use App\Entity\Bucket;
+use Symfony\Component\Validator\Constraint;
+use Symfony\Component\Validator\ConstraintValidator;
+use Symfony\Component\Validator\Exception\UnexpectedTypeException;
+
+class CompatibleAllocationTypeValidator extends ConstraintValidator
+{
+    public function validate(mixed $value, Constraint $constraint): void
+    {
+        if (!$constraint instanceof CompatibleAllocationType) {
+            throw new UnexpectedTypeException($constraint, CompatibleAllocationType::class);
+        }
+
+        if (!$value instanceof Bucket) {
+            return;
+        }
+
+        $type = $value->getType();
+        $allocationType = $value->getAllocationType();
+
+        if (!\in_array($allocationType, $type->getAllowedAllocationTypes(), true)) {
+            $this->context->buildViolation($constraint->message)
+                ->atPath('allocationType')
+                ->setCode(CompatibleAllocationType::NOT_COMPATIBLE_ERROR)
+                ->addViolation();
+        }
+    }
+}
diff --git a/tests/Entity/BucketPersistenceTest.php b/tests/Entity/BucketPersistenceTest.php
index b93b0b6..da4cf94 100644
--- a/tests/Entity/BucketPersistenceTest.php
+++ b/tests/Entity/BucketPersistenceTest.php
@@ -9,6 +9,7 @@ use App\Enum\BucketType;
 use Doctrine\DBAL\Exception\UniqueConstraintViolationException;
 use Doctrine\ORM\EntityManagerInterface;
 use Symfony\Bundle\FrameworkBundle\Test\KernelTestCase;
+use Symfony\Component\Uid\UuidV7;
 
 final class BucketPersistenceTest extends KernelTestCase
 {
@@ -90,6 +91,85 @@ final class BucketPersistenceTest extends KernelTestCase
         );
     }
 
+    public function testAllocationTypeDefaultsToFixedLimitWhenNotSet(): void
+    {
+        $scenario = new Scenario();
+        $scenario->setName('Household Budget');
+        $this->em->persist($scenario);
+        $this->em->flush();
+
+        $bucket = new Bucket();
+        $bucket->setScenario($scenario);
+        $bucket->setType(BucketType::NEED);
+        $bucket->setName('Rent');
+        $bucket->setPriority(1);
+        $bucket->setSortOrder(0);
+        // setAllocationType() deliberately NOT called — pins the default.
+        $bucket->setAllocationValue(150000);
+        $bucket->setStartingAmount(5000);
+        $bucket->setBufferMultiplier('0.00');
+
+        $this->em->persist($bucket);
+        $this->em->flush();
+
+        $bucketId = $bucket->getId();
+
+        $this->em->clear();
+
+        $reloaded = $this->em->find(Bucket::class, $bucketId);
+
+        self::assertNotNull($reloaded, 'Bucket must be retrievable from the database after an identity-map clear.');
+
+        self::assertSame(
+            BucketAllocationType::FIXED_LIMIT,
+            $reloaded->getAllocationType(),
+            'allocation_type must default to FIXED_LIMIT at the entity (PHP) level when not explicitly set. '
+            .'This does not exercise the DB column default, since Doctrine always includes allocation_type '
+            .'in its INSERT once the property holds a value — see testAllocationTypeDbColumnHasFixedLimitDefaultWhenOmittedFromInsert '
+            .'for the DB-level proof.',
+        );
+    }
+
+    public function testAllocationTypeDbColumnHasFixedLimitDefaultWhenOmittedFromInsert(): void
+    {
+        $scenario = new Scenario();
+        $scenario->setName('Household Budget');
+        $this->em->persist($scenario);
+        $this->em->flush();
+
+        $scenarioId = $scenario->getId();
+
+        $bucketId = new UuidV7();
+        $connection = $this->em->getConnection();
+
+        // Raw INSERT deliberately omits allocation_type — the DB column default must supply it.
+        // All other NOT NULL columns without a DB default (id, name, type, priority, scenario_id)
+        // are provided explicitly.
+        $connection->executeStatement(
+            'INSERT INTO bucket (id, name, type, priority, scenario_id) VALUES (:id, :name, :type, :priority, :scenarioId)',
+            [
+                'id' => $bucketId->toRfc4122(),
+                'name' => 'Rent',
+                'type' => BucketType::NEED->value,
+                'priority' => 1,
+                'scenarioId' => $scenarioId->toRfc4122(),
+            ],
+        );
+
+        $allocationType = $connection->executeQuery(
+            'SELECT allocation_type FROM bucket WHERE id = :id',
+            ['id' => $bucketId->toRfc4122()],
+        )->fetchOne();
+
+        self::assertSame(
+            'fixed_limit',
+            $allocationType,
+            'When allocation_type is omitted from a raw INSERT, the DB column default must supply '
+            .'\'fixed_limit\'. If this fails, the migration setting the column DEFAULT has regressed '
+            .'(the insert would fail outright with a not-null violation instead of defaulting).',
+        );
+    }
+
     public function testScenarioPriorityUniquenessIsEnforced(): void
     {
         $scenario = new Scenario();
diff --git a/tests/Validator/CompatibleAllocationTypeTest.php b/tests/Validator/CompatibleAllocationTypeTest.php
new file mode 100644
index 0000000..b4829e6
--- /dev/null
+++ b/tests/Validator/CompatibleAllocationTypeTest.php
@@ -0,0 +1,124 @@
+<?php
+
+namespace App\Tests\Validator;
+
+use App\Entity\Bucket;
+use App\Entity\Scenario;
+use App\Enum\BucketAllocationType;
+use App\Enum\BucketType;
+use App\Validator\CompatibleAllocationType;
+use Symfony\Bundle\FrameworkBundle\Test\KernelTestCase;
+use Symfony\Component\Validator\Validator\ValidatorInterface;
+
+final class CompatibleAllocationTypeTest extends KernelTestCase
+{
+    private ValidatorInterface $validator;
+
+    protected function setUp(): void
+    {
+        self::bootKernel();
+        $this->validator = self::getContainer()->get(ValidatorInterface::class);
+    }
+
+    public function testOverflowBucketWithFixedLimitAllocationIsInvalid(): void
+    {
+        $bucket = $this->makeBucket(BucketType::OVERFLOW, BucketAllocationType::FIXED_LIMIT, 50000);
+
+        $violations = $this->validator->validate($bucket);
+
+        $matching = $this->violationsAt($violations, 'allocationType');
+
+        self::assertNotCount(
+            0,
+            $matching,
+            'OVERFLOW bucket with FIXED_LIMIT allocation must raise a violation on allocationType.',
+        );
+    }
+
+    public function testNeedBucketWithUnlimitedAllocationIsInvalid(): void
+    {
+        $bucket = $this->makeBucket(BucketType::NEED, BucketAllocationType::UNLIMITED, null);
+
+        $violations = $this->validator->validate($bucket);
+
+        $matching = $this->violationsAt($violations, 'allocationType');
+
+        self::assertNotCount(
+            0,
+            $matching,
+            'NEED bucket with UNLIMITED allocation must raise a violation on allocationType.',
+        );
+    }
+
+    public function testOverflowBucketWithUnlimitedAllocationIsValid(): void
+    {
+        $bucket = $this->makeBucket(BucketType::OVERFLOW, BucketAllocationType::UNLIMITED, null);
+
+        $violations = $this->validator->validate($bucket);
+
+        $matching = $this->violationsAt($violations, 'allocationType');
+
+        self::assertCount(
+            0,
+            $matching,
+            'OVERFLOW bucket with UNLIMITED allocation is the compatible pairing and must not raise this constraint.',
+        );
+    }
+
+    public function testNeedBucketWithFixedLimitAllocationIsValid(): void
+    {
+        $bucket = $this->makeBucket(BucketType::NEED, BucketAllocationType::FIXED_LIMIT, 50000);
+
+        $violations = $this->validator->validate($bucket);
+
+        $matching = $this->violationsAt($violations, 'allocationType');
+
+        self::assertCount(
+            0,
+            $matching,
+            'NEED bucket with FIXED_LIMIT allocation is a compatible pairing and must not raise this constraint.',
+        );
+    }
+
+    private function makeBucket(
+        BucketType $type,
+        BucketAllocationType $allocationType,
+        ?int $allocationValue,
+    ): 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('0.00');
+
+        return $bucket;
+    }
+
+    /**
+     * @return list<\Symfony\Component\Validator\ConstraintViolationInterface>
+     */
+    private function violationsAt(
+        \Symfony\Component\Validator\ConstraintViolationListInterface $violations,
+        string $propertyPath,
+    ): array {
+        $matching = [];
+
+        foreach ($violations as $violation) {
+            if ($violation->getPropertyPath() === $propertyPath
+                && $violation->getConstraint() instanceof CompatibleAllocationType
+            ) {
+                $matching[] = $violation;
+            }
+        }
+
+        return $matching;
+    }
+}
diff --git a/tests/Validator/CompatibleAllocationTypeValidatorTest.php b/tests/Validator/CompatibleAllocationTypeValidatorTest.php
new file mode 100644
index 0000000..6593775
--- /dev/null
+++ b/tests/Validator/CompatibleAllocationTypeValidatorTest.php
@@ -0,0 +1,135 @@
+<?php
+
+namespace App\Tests\Validator;
+
+use App\Entity\Bucket;
+use App\Entity\Scenario;
+use App\Enum\BucketAllocationType;
+use App\Enum\BucketType;
+use App\Validator\CompatibleAllocationType;
+use App\Validator\CompatibleAllocationTypeValidator;
+use Symfony\Component\Validator\ConstraintValidatorInterface;
+use Symfony\Component\Validator\Test\ConstraintValidatorTestCase;
+
+/**
+ * @extends ConstraintValidatorTestCase<CompatibleAllocationTypeValidator>
+ */
+final class CompatibleAllocationTypeValidatorTest extends ConstraintValidatorTestCase
+{
+    protected function createValidator(): ConstraintValidatorInterface
+    {
+        return new CompatibleAllocationTypeValidator();
+    }
+
+    public function testOverflowWithUnlimitedIsValid(): void
+    {
+        $bucket = $this->makeBucket(BucketType::OVERFLOW, BucketAllocationType::UNLIMITED);
+
+        $this->validator->validate($bucket, new CompatibleAllocationType());
+
+        $this->assertNoViolation();
+    }
+
+    public function testOverflowWithFixedLimitIsInvalid(): void
+    {
+        $bucket = $this->makeBucket(BucketType::OVERFLOW, BucketAllocationType::FIXED_LIMIT);
+
+        $this->validator->validate($bucket, new CompatibleAllocationType());
+
+        $this->buildViolation('This allocation type is not compatible with the bucket type.')
+            ->atPath('property.path.allocationType')
+            ->setCode(CompatibleAllocationType::NOT_COMPATIBLE_ERROR)
+            ->assertRaised();
+    }
+
+    public function testOverflowWithPercentageIsInvalid(): void
+    {
+        $bucket = $this->makeBucket(BucketType::OVERFLOW, BucketAllocationType::PERCENTAGE);
+
+        $this->validator->validate($bucket, new CompatibleAllocationType());
+
+        $this->buildViolation('This allocation type is not compatible with the bucket type.')
+            ->atPath('property.path.allocationType')
+            ->setCode(CompatibleAllocationType::NOT_COMPATIBLE_ERROR)
+            ->assertRaised();
+    }
+
+    public function testNeedWithFixedLimitIsValid(): void
+    {
+        $bucket = $this->makeBucket(BucketType::NEED, BucketAllocationType::FIXED_LIMIT);
+
+        $this->validator->validate($bucket, new CompatibleAllocationType());
+
+        $this->assertNoViolation();
+    }
+
+    public function testNeedWithPercentageIsValid(): void
+    {
+        $bucket = $this->makeBucket(BucketType::NEED, BucketAllocationType::PERCENTAGE);
+
+        $this->validator->validate($bucket, new CompatibleAllocationType());
+
+        $this->assertNoViolation();
+    }
+
+    public function testNeedWithUnlimitedIsInvalid(): void
+    {
+        $bucket = $this->makeBucket(BucketType::NEED, BucketAllocationType::UNLIMITED);
+
+        $this->validator->validate($bucket, new CompatibleAllocationType());
+
+        $this->buildViolation('This allocation type is not compatible with the bucket type.')
+            ->atPath('property.path.allocationType')
+            ->setCode(CompatibleAllocationType::NOT_COMPATIBLE_ERROR)
+            ->assertRaised();
+    }
+
+    public function testWantWithFixedLimitIsValid(): void
+    {
+        $bucket = $this->makeBucket(BucketType::WANT, BucketAllocationType::FIXED_LIMIT);
+
+        $this->validator->validate($bucket, new CompatibleAllocationType());
+
+        $this->assertNoViolation();
+    }
+
+    public function testWantWithPercentageIsValid(): void
+    {
+        $bucket = $this->makeBucket(BucketType::WANT, BucketAllocationType::PERCENTAGE);
+
+        $this->validator->validate($bucket, new CompatibleAllocationType());
+
+        $this->assertNoViolation();
+    }
+
+    public function testWantWithUnlimitedIsInvalid(): void
+    {
+        $bucket = $this->makeBucket(BucketType::WANT, BucketAllocationType::UNLIMITED);
+
+        $this->validator->validate($bucket, new CompatibleAllocationType());
+
+        $this->buildViolation('This allocation type is not compatible with the bucket type.')
+            ->atPath('property.path.allocationType')
+            ->setCode(CompatibleAllocationType::NOT_COMPATIBLE_ERROR)
+            ->assertRaised();
+    }
+
+    private function makeBucket(BucketType $type, BucketAllocationType $allocationType): 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(null);
+        $bucket->setStartingAmount(0);
+        $bucket->setBufferMultiplier('0.00');
+
+        return $bucket;
+    }
+}