diff --git a/backend/tests/Unit/Auth/UseCases/AuthenticateUserTest.php b/backend/tests/Unit/Auth/UseCases/AuthenticateUserTest.php
index 6d00862..1fc1d17 100644
--- a/backend/tests/Unit/Auth/UseCases/AuthenticateUserTest.php
+++ b/backend/tests/Unit/Auth/UseCases/AuthenticateUserTest.php
@@ -33,13 +33,16 @@ class AuthenticateUserTest extends TestCase
 
     private function seedUser(
         string $email,
+        string $displayName,
         string $password,
         bool $isAdmin,
     ): User {
         return $this->userRepo->create(new CreateUserDto(
             email: new EmailAddress($email),
+            displayName: $displayName,
             passwordHash: $this->hasher->hash($password),
             isAdmin: $isAdmin,
+            emailConfirmedAt: null,
         ));
     }
 
@@ -101,6 +104,7 @@ class AuthenticateUserTest extends TestCase
     {
         $this->seedUser(
             email: 'user@example.com',
+            displayName: 'user',
             password: 'correctpassword',
             isAdmin: false,
         );
@@ -116,6 +120,7 @@ class AuthenticateUserTest extends TestCase
     {
         $seeded = $this->seedUser(
             email: 'user@example.com',
+            displayName: 'user',
             password: 'correctpassword',
             isAdmin: false,
         );
@@ -140,6 +145,7 @@ class AuthenticateUserTest extends TestCase
     {
         $this->seedUser(
             email: 'admin@example.com',
+            displayName: 'admin',
             password: 'adminpassword',
             isAdmin: true,
         );
diff --git a/backend/tests/Unit/User/UseCases/SignupUserTest.php b/backend/tests/Unit/User/UseCases/SignupUserTest.php
index 1ae841e..9babb7e 100644
--- a/backend/tests/Unit/User/UseCases/SignupUserTest.php
+++ b/backend/tests/Unit/User/UseCases/SignupUserTest.php
@@ -36,6 +36,7 @@ class SignupUserTest extends TestCase
         $this->expectException(BadRequestException::class);
         $this->useCase->execute(new SignupUserRequest(
             email: null,
+            displayName: 'alice',
             password: 'longenoughpassword',
         ));
     }
@@ -45,6 +46,7 @@ class SignupUserTest extends TestCase
         $this->expectException(BadRequestException::class);
         $this->useCase->execute(new SignupUserRequest(
             email: '',
+            displayName: 'alice',
             password: 'longenoughpassword',
         ));
     }
@@ -54,6 +56,37 @@ class SignupUserTest extends TestCase
         $this->expectException(BadRequestException::class);
         $this->useCase->execute(new SignupUserRequest(
             email: 'not-an-email',
+            displayName: 'alice',
+            password: 'longenoughpassword',
+        ));
+    }
+
+    public function test_null_display_name_throws_bad_request(): void
+    {
+        $this->expectException(BadRequestException::class);
+        $this->useCase->execute(new SignupUserRequest(
+            email: 'user@example.com',
+            displayName: null,
+            password: 'longenoughpassword',
+        ));
+    }
+
+    public function test_short_display_name_throws_bad_request(): void
+    {
+        $this->expectException(BadRequestException::class);
+        $this->useCase->execute(new SignupUserRequest(
+            email: 'user@example.com',
+            displayName: 'ab',
+            password: 'longenoughpassword',
+        ));
+    }
+
+    public function test_display_name_with_invalid_chars_throws(): void
+    {
+        $this->expectException(BadRequestException::class);
+        $this->useCase->execute(new SignupUserRequest(
+            email: 'user@example.com',
+            displayName: 'Has Spaces',
             password: 'longenoughpassword',
         ));
     }
@@ -63,6 +96,7 @@ class SignupUserTest extends TestCase
         $this->expectException(BadRequestException::class);
         $this->useCase->execute(new SignupUserRequest(
             email: 'user@example.com',
+            displayName: 'alice',
             password: null,
         ));
     }
@@ -72,6 +106,7 @@ class SignupUserTest extends TestCase
         $this->expectException(BadRequestException::class);
         $this->useCase->execute(new SignupUserRequest(
             email: 'user@example.com',
+            displayName: 'alice',
             password: 'short',
         ));
     }
@@ -80,13 +115,34 @@ class SignupUserTest extends TestCase
     {
         $this->userRepo->create(new CreateUserDto(
             email: new EmailAddress('user@example.com'),
+            displayName: 'first',
             passwordHash: $this->hasher->hash('original-password'),
             isAdmin: false,
+            emailConfirmedAt: null,
         ));
 
         $this->expectException(DomainException::class);
         $this->useCase->execute(new SignupUserRequest(
             email: 'user@example.com',
+            displayName: 'second',
+            password: 'second-attempt-password',
+        ));
+    }
+
+    public function test_duplicate_display_name_throws_domain_exception(): void
+    {
+        $this->userRepo->create(new CreateUserDto(
+            email: new EmailAddress('first@example.com'),
+            displayName: 'taken',
+            passwordHash: $this->hasher->hash('original-password'),
+            isAdmin: false,
+            emailConfirmedAt: null,
+        ));
+
+        $this->expectException(DomainException::class);
+        $this->useCase->execute(new SignupUserRequest(
+            email: 'second@example.com',
+            displayName: 'taken',
             password: 'second-attempt-password',
         ));
     }
@@ -95,22 +151,26 @@ class SignupUserTest extends TestCase
     {
         $created = $this->useCase->execute(new SignupUserRequest(
             email: 'new@example.com',
+            displayName: 'newuser',
             password: 'longenoughpassword',
         ));
 
         $this->assertInstanceOf(User::class, $created);
         $this->assertSame('new@example.com', $created->getEmail()->value());
+        $this->assertSame('newuser', $created->getDisplayName());
         $this->assertSame(
             $this->hasher->hash('longenoughpassword'),
             $created->getPasswordHash(),
         );
         $this->assertFalse($created->isAdmin());
+        $this->assertFalse($created->isEmailConfirmed());
     }
 
     public function test_created_user_is_findable_by_email(): void
     {
         $created = $this->useCase->execute(new SignupUserRequest(
             email: 'lookup@example.com',
+            displayName: 'lookup',
             password: 'longenoughpassword',
         ));
 
@@ -121,10 +181,24 @@ class SignupUserTest extends TestCase
         $this->assertSame($created->getId(), $found->getId());
     }
 
+    public function test_created_user_is_findable_by_display_name(): void
+    {
+        $created = $this->useCase->execute(new SignupUserRequest(
+            email: 'lookup@example.com',
+            displayName: 'lookupbyname',
+            password: 'longenoughpassword',
+        ));
+
+        $found = $this->userRepo->findByDisplayName('lookupbyname');
+        $this->assertNotNull($found);
+        $this->assertSame($created->getId(), $found->getId());
+    }
+
     public function test_signup_normalizes_email_domain(): void
     {
         $created = $this->useCase->execute(new SignupUserRequest(
             email: 'Mixed@CASE.com',
+            displayName: 'mixed',
             password: 'longenoughpassword',
         ));