diff --git a/src/Bundle/ChillThirdPartyBundle/Tests/Action/ThirdpartyMergeManagerTest.php b/src/Bundle/ChillThirdPartyBundle/Tests/Action/ThirdpartyMergeManagerTest.php
new file mode 100644
index 000000000..a36a393bc
--- /dev/null
+++ b/src/Bundle/ChillThirdPartyBundle/Tests/Action/ThirdpartyMergeManagerTest.php
@@ -0,0 +1,100 @@
+<?php
+
+namespace Chill\ThirdPartyBundle\Tests\Action;
+
+use Chill\ActivityBundle\Entity\Activity;
+use Chill\PersonBundle\Entity\Person\PersonResource;
+use Chill\ThirdPartyBundle\Actions\MergeThirdparty\ThirdpartyMergeManager;
+use Chill\ThirdPartyBundle\Entity\ThirdParty;
+use Doctrine\ORM\EntityManagerInterface;
+use libphonenumber\PhoneNumber;
+use Symfony\Bundle\FrameworkBundle\Test\KernelTestCase;
+
+class ThirdpartyMergeManagerTest extends KernelTestCase
+{
+    private $mergeManager;
+    private $em;
+    private $connection;
+
+    protected function setUp(): void
+    {
+        self::bootKernel();
+
+        $this->mergeManager = $this->getContainer()->get(ThirdpartyMergeManager::class);
+        $this->em = $this->getContainer()->get(EntityManagerInterface::class);
+        $this->connection = $this->em->getConnection();
+
+        // Start a transaction before each test
+        $this->connection->beginTransaction();
+    }
+
+    protected function tearDown(): void
+    {
+        try {
+            // Rollback the transaction after each test to ensure no data is persisted
+            $this->connection->rollBack();
+        } catch (\Exception $e) {
+            $this->connection->close();
+        }
+
+        parent::tearDown();
+    }
+
+    public function testThirdpartyMerge(): void
+    {
+        // Arrange: Create Thirdparty entities
+        $toKeep = new ThirdParty();
+        $toKeep->setName('Thirdparty ToKeep');
+        $toKeep->setEmail('keep@example.com');
+
+        $toDelete = new Thirdparty();
+        $toDelete->setName('Thirdparty ToDelete'); // This should be ignored
+        $toDelete->setTelephone(new PhoneNumber('123456789'));
+
+        // Related entities
+        $activity = new Activity();
+        $activity->addThirdParty($toDelete); // This is a Many-to-Many relation
+
+        $personResource = new PersonResource();
+        $personResource->setThirdParty($toDelete); // This is a Many-to-One relation
+
+        $this->em->persist($toKeep);
+        $this->em->persist($toDelete);
+        $this->em->persist($activity);
+        $this->em->persist($personResource);
+        $this->em->flush();
+
+        // Merge
+        $this->mergeManager->merge($toKeep, $toDelete);
+        $this->em->clear();
+
+        // Verify data was merged correctly
+        $mergedThirdparty = $this->em->getRepository(Thirdparty::class)->find($toKeep->getId());
+        $this->assertNotNull($mergedThirdparty);
+        $this->assertEquals('Primary Name', $mergedThirdparty->getName(), 'Name should remain unchanged');
+        $this->assertEquals('keep@example.com', $mergedThirdparty->getEmail(), 'Email should remain unchanged');
+        $this->assertEquals('123456789', $mergedThirdparty->getPhone(), 'Phone should be transferred from Thirdparty ToDelete');
+
+        // Check that relationships are updated
+        $updatedActivity = $this->em->getRepository(Activity::class)->find($activity->getId());
+        $this->assertTrue(
+            $updatedActivity->getThirdParties()->contains($mergedThirdparty),
+            'Activity should be linked to the merged Thirdparty'
+        );
+        $this->assertFalse(
+            $updatedActivity->getThirdParties()->exists(fn($key, $tp) => $tp->getId() === $toDelete->getId()),
+            'Activity should no longer reference the deleted Thirdparty'
+        );
+
+        $updatedPersonResource = $this->em->getRepository(PersonResource::class)->find($personResource->getId());
+        $this->assertEquals(
+            $mergedThirdparty->getId(),
+            $updatedPersonResource->getThirdParty()->getId(),
+            'PersonResource should reference the merged Thirdparty'
+        );
+
+        // Ensure the 'toDelete' entity is removed
+        $deletedThirdparty = $this->em->getRepository(Thirdparty::class)->find($toDelete->getId());
+        $this->assertNull($deletedThirdparty, 'The deleted Thirdparty should no longer exist in the database');
+    }
+}