diff --git a/internal/client/repository.go b/internal/client/repository.go
index 0389498..53d294b 100644
--- a/internal/client/repository.go
+++ b/internal/client/repository.go
@@ -6,13 +6,53 @@ import (
 	"source.toby3d.me/toby3d/auth/internal/domain"
 )
 
-type Repository interface {
-	Create(ctx context.Context, client domain.Client) error
-	Get(ctx context.Context, cid domain.ClientID) (*domain.Client, error)
-}
+type (
+	Repository interface {
+		Create(ctx context.Context, client domain.Client) error
+		Get(ctx context.Context, cid domain.ClientID) (*domain.Client, error)
+	}
+
+	dummyClientRepository struct{}
+
+	stubClientRepository struct {
+		client *domain.Client
+		error  error
+	}
+)
 
 var ErrNotExist error = domain.NewError(
 	domain.ErrorCodeInvalidClient,
 	"client with the specified ID does not exist",
 	"",
 )
+
+// NewDummyClientRepository creates a new dummy [Repository] implementation what
+// do nothing.
+func NewDummyClientRepository() Repository {
+	return dummyClientRepository{}
+}
+
+func (dummyClientRepository) Create(_ context.Context, _ domain.Client) error {
+	return nil
+}
+
+func (dummyClientRepository) Get(_ context.Context, _ domain.ClientID) (*domain.Client, error) {
+	return nil, nil
+}
+
+// NewStubClientRepository creates a new stub [Repository] implementation what
+// always return data provided here.
+func NewStubClientRepository(client *domain.Client, err error) Repository {
+	return &stubClientRepository{
+		client: client,
+		error:  err,
+	}
+}
+
+func (repo *stubClientRepository) Create(_ context.Context, _ domain.Client) error {
+	return repo.error
+}
+
+func (repo *stubClientRepository) Get(_ context.Context, _ domain.ClientID) (*domain.Client, error) {
+	return repo.client, repo.error
+}
diff --git a/internal/client/usecase.go b/internal/client/usecase.go
index 7a2a706..828d8cd 100644
--- a/internal/client/usecase.go
+++ b/internal/client/usecase.go
@@ -6,13 +6,44 @@ import (
 	"source.toby3d.me/toby3d/auth/internal/domain"
 )
 
-type UseCase interface {
-	// Discovery returns client public information bu ClientID URL.
-	Discovery(ctx context.Context, id domain.ClientID) (*domain.Client, error)
-}
+type (
+	UseCase interface {
+		// Discovery returns client public information bu ClientID URL.
+		Discovery(ctx context.Context, id domain.ClientID) (*domain.Client, error)
+	}
+
+	dummyClientUseCase struct{}
+
+	stubClientUseCase struct {
+		client *domain.Client
+		error  error
+	}
+)
 
 var ErrInvalidMe error = domain.NewError(
 	domain.ErrorCodeInvalidRequest,
 	"cannot fetch client endpoints on provided me",
 	"",
 )
+
+// NewDummyClientUseCase returns dummy [UseCase] what do nothing.
+func NewDummyClientUseCase() UseCase {
+	return dummyClientUseCase{}
+}
+
+func (dummyClientUseCase) Discovery(_ context.Context, _ domain.ClientID) (*domain.Client, error) {
+	return nil, nil
+}
+
+// NewDummyClientUseCase returns stub [UseCase] what always returns data
+// provided here.
+func NewStubClientUseCase(client *domain.Client, err error) UseCase {
+	return &stubClientUseCase{
+		client: client,
+		error:  err,
+	}
+}
+
+func (ucase *stubClientUseCase) Discovery(_ context.Context, _ domain.ClientID) (*domain.Client, error) {
+	return ucase.client, ucase.error
+}