diff --git a/src/costy/application/category/create_category.py b/src/costy/application/category/create_category.py
index 38f0c91..961cc9a 100644
--- a/src/costy/application/category/create_category.py
+++ b/src/costy/application/category/create_category.py
@@ -1,3 +1,5 @@
+from dataclasses import dataclass
+
 from costy.domain.models.category import CategoryId, CategoryType
 from costy.domain.services.category import CategoryService
 
@@ -7,6 +9,7 @@
 from ..common.uow import UoW
 
 
+@dataclass
 class NewCategoryDTO:
     name: str
 
diff --git a/src/costy/application/common/category_gateway.py b/src/costy/application/common/category_gateway.py
index a82479f..00b298c 100644
--- a/src/costy/application/common/category_gateway.py
+++ b/src/costy/application/common/category_gateway.py
@@ -1,7 +1,7 @@
 from abc import abstractmethod
 from typing import Protocol, runtime_checkable
 
-from costy.application.category.read_available_categories import CategoryDTO
+# from costy.application.category.read_available_categories import CategoryDTO
 from costy.domain.models.category import Category, CategoryId
 from costy.domain.models.user import UserId
 
@@ -23,7 +23,7 @@ async def get_category(self, category_id: CategoryId) -> Category | None:
 @runtime_checkable
 class CategoriesReader(Protocol):
     @abstractmethod
-    async def find_categories(self, user_id: UserId) -> list[CategoryDTO]:
+    async def find_categories(self, user_id: UserId) -> list[Category]:
         raise NotImplementedError
 
 
diff --git a/src/costy/domain/models/operation.py b/src/costy/domain/models/operation.py
index b30a636..4f138bb 100644
--- a/src/costy/domain/models/operation.py
+++ b/src/costy/domain/models/operation.py
@@ -7,7 +7,7 @@
 OperationId = NewType("OperationId", int)
 
 
-@dataclass(kw_only=True)
+@dataclass
 class Operation:
     id: OperationId | None
     amount: int
diff --git a/tests/application/__init__.py b/tests/application/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/tests/application/category/__init__.py b/tests/application/category/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/tests/application/category/test_create_category.py b/tests/application/category/test_create_category.py
new file mode 100644
index 0000000..ff65235
--- /dev/null
+++ b/tests/application/category/test_create_category.py
@@ -0,0 +1,41 @@
+from unittest.mock import Mock
+
+import pytest
+from pytest import fixture
+
+from costy.application.category.create_category import (
+    CreateCategory,
+    NewCategoryDTO,
+)
+from costy.application.common.category_gateway import CategorySaver
+from costy.application.common.uow import UoW
+from costy.domain.models.category import Category, CategoryType
+
+
+@fixture
+def category_info() -> NewCategoryDTO:
+    return NewCategoryDTO("test")
+
+
+@fixture
+def interactor(id_provider, category_id, user_id, category_info) -> CreateCategory:
+    category_service = Mock()
+    category_service.create.return_value = Category(
+        id=None,
+        name=category_info.name,
+        kind=CategoryType.PERSONAL.value,
+        user_id=user_id,
+    )
+
+    async def save_category_mock(category: Category) -> None:
+        category.id = category_id
+
+    category_gateway = Mock(spec=CategorySaver)
+    category_gateway.save_category = save_category_mock
+    uow = Mock(spec=UoW)
+    return CreateCategory(category_service, category_gateway, id_provider, uow)
+
+
+@pytest.mark.asyncio
+async def test_create_operation(interactor: CreateCategory, category_info: NewCategoryDTO, category_id) -> None:
+    assert await interactor(category_info) == category_id
diff --git a/tests/application/operation/__init__.py b/tests/application/operation/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/tests/application/operation/test_create_operation.py b/tests/application/operation/test_create_operation.py
new file mode 100644
index 0000000..aa33506
--- /dev/null
+++ b/tests/application/operation/test_create_operation.py
@@ -0,0 +1,49 @@
+from unittest.mock import Mock
+
+import pytest
+from pytest import fixture
+
+from costy.application.common.operation_gateway import OperationSaver
+from costy.application.common.uow import UoW
+from costy.application.operation.create_operation import (
+    CreateOperation,
+    NewOperationDTO,
+)
+from costy.domain.models.category import CategoryId
+from costy.domain.models.operation import Operation
+
+
+@fixture
+def operation_info() -> NewOperationDTO:
+    return NewOperationDTO(
+        100,
+        "description",
+        10000,
+        CategoryId(999)
+    )
+
+
+@fixture
+def interactor(id_provider, operation_id, user_id, operation_info) -> CreateOperation:
+    operation_service = Mock()
+    operation_service.create.return_value = Operation(
+        id=None,
+        amount=operation_info.amount,
+        description=operation_info.description,
+        time=operation_info.time,
+        user_id=user_id,
+        category_id=operation_info.category_id,
+    )
+
+    async def save_operation_mock(operation: Operation) -> None:
+        operation.id = operation_id
+
+    operation_gateway = Mock(spec=OperationSaver)
+    operation_gateway.save_operation = save_operation_mock
+    uow = Mock(spec=UoW)
+    return CreateOperation(operation_service, operation_gateway, id_provider, uow)
+
+
+@pytest.mark.asyncio
+async def test_create_operation(interactor: CreateOperation, operation_info: NewOperationDTO, operation_id) -> None:
+    assert await interactor(operation_info) == operation_id
diff --git a/tests/application/test_authenticate.py b/tests/application/test_authenticate.py
new file mode 100644
index 0000000..3ee43c6
--- /dev/null
+++ b/tests/application/test_authenticate.py
@@ -0,0 +1,30 @@
+from unittest.mock import Mock
+
+import pytest
+from pytest import fixture
+
+from costy.application.authenticate import Authenticate, LoginInputDTO
+from costy.application.common.uow import UoW
+from costy.application.common.user_gateway import UserReader
+from costy.domain.models.user import User, UserId
+
+
+@fixture
+def login_info() -> LoginInputDTO:
+    return LoginInputDTO(email="test@email.com", password="password")
+
+
+@fixture
+def interactor(user_id, login_info):
+    user_gateway = Mock(spec=UserReader)
+    user_gateway.get_user_by_email.return_value = User(
+        id=user_id, email=login_info.email,
+        hashed_password=login_info.password,
+    )
+    uow = Mock(spec=UoW)
+    return Authenticate(user_gateway, uow)
+
+
+@pytest.mark.asyncio
+async def test_authenticate(interactor: Authenticate, user_id: UserId, login_info: LoginInputDTO):
+    assert await interactor(login_info) == user_id
diff --git a/tests/application/user/__init__.py b/tests/application/user/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/tests/application/user/test_create_user.py b/tests/application/user/test_create_user.py
new file mode 100644
index 0000000..3f63aa5
--- /dev/null
+++ b/tests/application/user/test_create_user.py
@@ -0,0 +1,38 @@
+from unittest.mock import Mock
+
+import pytest
+from pytest import fixture
+
+from costy.application.common.uow import UoW
+from costy.application.common.user_gateway import UserSaver
+from costy.application.user.create_user import CreateUser, NewUserDTO
+from costy.domain.models.user import User, UserId
+from costy.domain.services.user import UserService
+
+
+@fixture
+def user_info() -> NewUserDTO:
+    return NewUserDTO(email="test@email.com", password="password")
+
+
+@fixture
+def interactor(user_id, user_info):
+    user_service = Mock(spec=UserService)
+    user_service.create.return_value = User(
+        id=None,
+        email=user_info.email,
+        hashed_password=user_info.password
+    )
+
+    async def mock_save_user(user: User):
+        user.id = user_id
+
+    user_gateway = Mock(spec=UserSaver)
+    user_gateway.save_user = mock_save_user
+    uow = Mock(spec=UoW)
+    return CreateUser(user_service, user_gateway, uow)
+
+
+@pytest.mark.asyncio
+async def test_create_user(interactor: CreateUser, user_info: NewUserDTO, user_id: UserId):
+    assert await interactor(user_info) == user_id
diff --git a/tests/conftest.py b/tests/conftest.py
new file mode 100644
index 0000000..754af96
--- /dev/null
+++ b/tests/conftest.py
@@ -0,0 +1,30 @@
+from unittest.mock import Mock
+
+from pytest import fixture
+
+from costy.application.common.id_provider import IdProvider
+from costy.domain.models.category import CategoryId
+from costy.domain.models.operation import OperationId
+from costy.domain.models.user import UserId
+
+
+@fixture
+def user_id() -> UserId:
+    return UserId(999)
+
+
+@fixture
+def operation_id() -> OperationId:
+    return OperationId(999)
+
+
+@fixture
+def category_id() -> CategoryId:
+    return CategoryId(999)
+
+
+@fixture
+def id_provider(user_id: UserId) -> IdProvider:
+    provider = Mock(spec=IdProvider)
+    provider.get_current_user_id.return_value = user_id
+    return provider
diff --git a/tests/domain/__init__.py b/tests/domain/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/tests/domain/test_create.py b/tests/domain/test_create.py
new file mode 100644
index 0000000..d7ff151
--- /dev/null
+++ b/tests/domain/test_create.py
@@ -0,0 +1,29 @@
+import pytest
+
+from costy.domain.models.category import Category, CategoryId, CategoryType
+from costy.domain.models.operation import Operation
+from costy.domain.models.user import User, UserId
+from costy.domain.services.category import CategoryService
+from costy.domain.services.operation import OperationService
+from costy.domain.services.user import UserService
+
+
+@pytest.mark.parametrize("domain_service, data, expected_model", [
+    (
+        UserService(),
+        ("email@test.com", "password"),
+        User(None, "email@test.com", "password")
+    ),
+    (
+        OperationService(),
+        (100, "desc", 10000, UserId(9999), CategoryId(9999)),
+        Operation(None, 100, "desc", 10000, UserId(9999), CategoryId(9999))
+    ),
+    (
+        CategoryService(),
+        ("test", CategoryType.GENERAL, UserId(9999)),
+        Category(None, "test", CategoryType.GENERAL.value, UserId(9999))
+    ),
+])
+def test_create_domain_service(domain_service, data, expected_model):
+    assert domain_service.create(*data) == expected_model
diff --git a/tests/test_stable.py b/tests/test_stable.py
deleted file mode 100644
index 1af438d..0000000
--- a/tests/test_stable.py
+++ /dev/null
@@ -1,2 +0,0 @@
-def test_stable():
-    assert True