diff --git a/cmd/main.go b/cmd/main.go
index 73302f7..211d5a5 100644
--- a/cmd/main.go
+++ b/cmd/main.go
@@ -20,14 +20,11 @@ import (
 	"cruder/internal/middleware"
 	"cruder/internal/repository"
 	"cruder/internal/service"
-	"fmt"
 	"log/slog"
 	"os"
-	"strings"
 
 	"github.com/gin-gonic/gin"
 	"github.com/joho/godotenv"
-	"github.com/spf13/viper"
 )
 
 func main() {
@@ -48,7 +45,7 @@ func main() {
 		os.Exit(1)
 	}
 
-	cfg, err := loadConfig()
+	cfg, err := config.LoadConfig()
 	if err != nil {
 		logger.Error("failed to load config", slog.Any("err", err))
 		os.Exit(1)
@@ -79,27 +76,8 @@ func main() {
 	r.Use(apiKeyMiddleware.Handler())
 	_ = r.SetTrustedProxies(nil)
 
-	handler.New(r, controllers.Users)
+	handler.New(r, controllers.Users, controllers.Health)
 	if err := r.Run(); err != nil {
 		logger.Error("failed to run server", slog.Any("err", err))
 	}
 }
-
-func loadConfig() (*config.Config, error) {
-	viper.SetEnvKeyReplacer(strings.NewReplacer(".", "_"))
-	viper.AutomaticEnv()
-	viper.AddConfigPath("config")
-	viper.SetConfigName("config")
-	viper.SetConfigType("yaml")
-	viper.AddConfigPath(".")
-
-	if err := viper.ReadInConfig(); err != nil {
-		return nil, fmt.Errorf("error reading config file: %w", err)
-	}
-
-	var cfg config.Config
-	if err := viper.Unmarshal(&cfg); err != nil {
-		return nil, fmt.Errorf("unable to decode config into struct: %w", err)
-	}
-	return &cfg, nil
-}
diff --git a/docs/docs.go b/docs/docs.go
index 9e52716..6282bc1 100644
--- a/docs/docs.go
+++ b/docs/docs.go
@@ -15,7 +15,7 @@ const docTemplate = `{
     "host": "{{.Host}}",
     "basePath": "{{.BasePath}}",
     "paths": {
-        "/users": {
+        "/users/": {
             "get": {
                 "security": [
                     {
diff --git a/docs/swagger.json b/docs/swagger.json
index 11d6457..937f6dc 100644
--- a/docs/swagger.json
+++ b/docs/swagger.json
@@ -8,7 +8,7 @@
     },
     "basePath": "/api/v1",
     "paths": {
-        "/users": {
+        "/users/": {
             "get": {
                 "security": [
                     {
diff --git a/docs/swagger.yaml b/docs/swagger.yaml
index 7c4bc48..292e58d 100644
--- a/docs/swagger.yaml
+++ b/docs/swagger.yaml
@@ -25,7 +25,7 @@ info:
   title: Users API
   version: "1.0"
 paths:
-  /users:
+  /users/:
     get:
       produces:
       - application/json
diff --git a/internal/config/config.go b/internal/config/config.go
index 35a4a8f..7573305 100644
--- a/internal/config/config.go
+++ b/internal/config/config.go
@@ -2,7 +2,11 @@ package config
 
 import (
 	"errors"
+	"fmt"
 	"os"
+	"strings"
+
+	"github.com/spf13/viper"
 )
 
 type Config struct {
@@ -29,3 +33,22 @@ func (c *Config) GetDSN() (string, error) {
 
 	return dsn, nil
 }
+
+func LoadConfig() (*Config, error) {
+	viper.SetEnvKeyReplacer(strings.NewReplacer(".", "_"))
+	viper.AutomaticEnv()
+	viper.AddConfigPath("config")
+	viper.SetConfigName("config")
+	viper.SetConfigType("yaml")
+	viper.AddConfigPath(".")
+
+	if err := viper.ReadInConfig(); err != nil {
+		return nil, fmt.Errorf("error reading config file: %w", err)
+	}
+
+	var cfg Config
+	if err := viper.Unmarshal(&cfg); err != nil {
+		return nil, fmt.Errorf("unable to decode config into struct: %w", err)
+	}
+	return &cfg, nil
+}
diff --git a/internal/controller/controllers.go b/internal/controller/controllers.go
index 2ea7d0d..aaf5e09 100644
--- a/internal/controller/controllers.go
+++ b/internal/controller/controllers.go
@@ -3,11 +3,13 @@ package controller
 import "cruder/internal/service"
 
 type Controller struct {
-	Users *UserController
+	Users  *UserController
+	Health *HealthController
 }
 
 func NewController(services *service.Service) *Controller {
 	return &Controller{
-		Users: NewUserController(services.Users),
+		Users:  NewUserController(services.Users),
+		Health: NewHealthController(),
 	}
 }
diff --git a/internal/controller/health.go b/internal/controller/health.go
new file mode 100644
index 0000000..de66234
--- /dev/null
+++ b/internal/controller/health.go
@@ -0,0 +1,13 @@
+package controller
+
+import "github.com/gin-gonic/gin"
+
+type HealthController struct{}
+
+func NewHealthController() *HealthController {
+	return &HealthController{}
+}
+
+func (c *HealthController) HealthCheck(ctx *gin.Context) {
+	ctx.JSON(200, gin.H{"status": "ok"})
+}
diff --git a/internal/controller/health_test.go b/internal/controller/health_test.go
new file mode 100644
index 0000000..79414ce
--- /dev/null
+++ b/internal/controller/health_test.go
@@ -0,0 +1,29 @@
+package controller
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/gin-gonic/gin"
+	"github.com/stretchr/testify/assert"
+)
+
+func setupHealthRouter(c *HealthController) *gin.Engine {
+	gin.SetMode(gin.TestMode)
+	r := gin.Default()
+	r.GET("/healthz", c.HealthCheck)
+	return r
+}
+
+func TestHealthCheck_Success(t *testing.T) {
+	controller := NewHealthController()
+	router := setupHealthRouter(controller)
+
+	req, err := http.NewRequest("GET", "/healthz", nil)
+	w := httptest.NewRecorder()
+	router.ServeHTTP(w, req)
+
+	assert.NoError(t, err)
+	assert.Equal(t, http.StatusOK, w.Code)
+}
diff --git a/internal/controller/users.go b/internal/controller/users.go
index e39c680..a43c52e 100644
--- a/internal/controller/users.go
+++ b/internal/controller/users.go
@@ -5,7 +5,6 @@ import (
 	"strconv"
 
 	"cruder/internal/model"
-	"cruder/internal/repository"
 	"cruder/internal/service"
 
 	"github.com/gin-gonic/gin"
@@ -26,7 +25,7 @@ func NewUserController(service service.UserService) *UserController {
 // @Success 200 {array} model.User
 // @Failure 500 {object} model.ErrorResponse "internal server error"
 // @Security ApiKeyAuth
-// @Router /users [get]
+// @Router /users/ [get]
 func (c *UserController) GetAllUsers(ctx *gin.Context) {
 	users, err := c.service.GetAll()
 	if handleError(ctx, err) {
@@ -96,7 +95,7 @@ func (c *UserController) GetUserByID(ctx *gin.Context) {
 // @Failure 409 {object} model.ErrorResponse "user with username/email already exists"
 // @Failure 500 {object} model.ErrorResponse "internal server error"
 // @Security ApiKeyAuth
-// @Router /users [post]
+// @Router /users/ [post]
 func (c *UserController) CreateUser(ctx *gin.Context) {
 	var user model.User
 
@@ -191,11 +190,11 @@ func handleError(ctx *gin.Context, err error) bool {
 }
 
 var errToStatus = map[error]int{
-	repository.ErrUserNotFound:          http.StatusNotFound,
-	repository.ErrUserAlreadyExists:     http.StatusConflict,
-	repository.ErrUsernameAlreadyExists: http.StatusConflict,
-	repository.ErrEmailAlreadyExists:    http.StatusConflict,
-	service.ErrInvalidEmail:             http.StatusBadRequest,
-	service.ErrInvalidUsername:          http.StatusBadRequest,
-	service.ErrInvalidFullName:          http.StatusBadRequest,
+	service.ErrUserNotFound:          http.StatusNotFound,
+	service.ErrUserAlreadyExists:     http.StatusConflict,
+	service.ErrUsernameAlreadyExists: http.StatusConflict,
+	service.ErrEmailAlreadyExists:    http.StatusConflict,
+	service.ErrInvalidEmail:          http.StatusBadRequest,
+	service.ErrInvalidUsername:       http.StatusBadRequest,
+	service.ErrInvalidFullName:       http.StatusBadRequest,
 }
diff --git a/internal/controller/users_test.go b/internal/controller/users_test.go
index 1cdf251..33c7e48 100644
--- a/internal/controller/users_test.go
+++ b/internal/controller/users_test.go
@@ -4,7 +4,6 @@ import (
 	"bytes"
 	mock_service "cruder/internal/mocks/service"
 	"cruder/internal/model"
-	"cruder/internal/repository"
 	"cruder/internal/service"
 	"encoding/json"
 	"net/http"
@@ -16,7 +15,7 @@ import (
 	"go.uber.org/mock/gomock"
 )
 
-func setupRouter(c *UserController) *gin.Engine {
+func setupUserRouter(c *UserController) *gin.Engine {
 	gin.SetMode(gin.TestMode)
 	r := gin.Default()
 	r.GET("/users", c.GetAllUsers)
@@ -37,7 +36,7 @@ func TestGetAllUsers_Success(t *testing.T) {
 	mockSvc.EXPECT().GetAll().Return(users, nil)
 
 	controller := NewUserController(mockSvc)
-	router := setupRouter(controller)
+	router := setupUserRouter(controller)
 
 	// When: GET /users is called
 	req, _ := http.NewRequest("GET", "/users", nil)
@@ -61,7 +60,7 @@ func TestGetUserByUsername_Success(t *testing.T) {
 	mockSvc.EXPECT().GetByUsername("john_doe").Return(expected, nil)
 
 	controller := NewUserController(mockSvc)
-	router := setupRouter(controller)
+	router := setupUserRouter(controller)
 
 	// When: GET /users/username/john_doe is called
 	req, _ := http.NewRequest("GET", "/users/username/john_doe", nil)
@@ -81,10 +80,10 @@ func TestGetUserByUsername_NotFound(t *testing.T) {
 	ctrl := gomock.NewController(t)
 	defer ctrl.Finish()
 	mockSvc := mock_service.NewMockUserService(ctrl)
-	mockSvc.EXPECT().GetByUsername("missing").Return(nil, repository.ErrUserNotFound)
+	mockSvc.EXPECT().GetByUsername("missing").Return(nil, service.ErrUserNotFound)
 
 	controller := NewUserController(mockSvc)
-	router := setupRouter(controller)
+	router := setupUserRouter(controller)
 
 	// When: GET /users/username/missing is called
 	req, _ := http.NewRequest("GET", "/users/username/missing", nil)
@@ -107,7 +106,7 @@ func TestGetUserByID_Success(t *testing.T) {
 	mockSvc.EXPECT().GetByID(int64(1)).Return(expected, nil)
 
 	controller := NewUserController(mockSvc)
-	router := setupRouter(controller)
+	router := setupUserRouter(controller)
 
 	// When: GET /users/id/1 is called
 	req, _ := http.NewRequest("GET", "/users/id/1", nil)
@@ -129,7 +128,7 @@ func TestGetUserByID_InvalidID(t *testing.T) {
 	mockSvc := mock_service.NewMockUserService(ctrl)
 
 	controller := NewUserController(mockSvc)
-	router := setupRouter(controller)
+	router := setupUserRouter(controller)
 
 	// When: GET /users/id/abc is called
 	req, _ := http.NewRequest("GET", "/users/id/abc", nil)
@@ -153,7 +152,7 @@ func TestCreateUser_Success(t *testing.T) {
 	mockSvc.EXPECT().Create(input).Return(created, nil)
 
 	controller := NewUserController(mockSvc)
-	router := setupRouter(controller)
+	router := setupUserRouter(controller)
 
 	body, _ := json.Marshal(input)
 
@@ -177,7 +176,7 @@ func TestCreateUser_InvalidBody(t *testing.T) {
 	mockSvc := mock_service.NewMockUserService(ctrl)
 
 	controller := NewUserController(mockSvc)
-	router := setupRouter(controller)
+	router := setupUserRouter(controller)
 
 	// When: POST /users with invalid JSON
 	req, _ := http.NewRequest("POST", "/users", bytes.NewBuffer([]byte("{bad json")))
@@ -197,10 +196,10 @@ func TestDeleteUser_NotFound(t *testing.T) {
 	ctrl := gomock.NewController(t)
 	defer ctrl.Finish()
 	mockSvc := mock_service.NewMockUserService(ctrl)
-	mockSvc.EXPECT().Delete(int64(99)).Return(repository.ErrUserNotFound)
+	mockSvc.EXPECT().Delete(int64(99)).Return(service.ErrUserNotFound)
 
 	controller := NewUserController(mockSvc)
-	router := setupRouter(controller)
+	router := setupUserRouter(controller)
 
 	// When: DELETE /users/99 is called
 	req, _ := http.NewRequest("DELETE", "/users/99", nil)
@@ -221,7 +220,7 @@ func TestUpdateUser_IDMismatch(t *testing.T) {
 	mockSvc := mock_service.NewMockUserService(ctrl)
 
 	controller := NewUserController(mockSvc)
-	router := setupRouter(controller)
+	router := setupUserRouter(controller)
 
 	user := model.User{ID: 2, Username: "john", Email: "john@doe.ee", FullName: "John Doe"}
 	body, _ := json.Marshal(user)
@@ -249,7 +248,7 @@ func TestUpdateUser_Success(t *testing.T) {
 	mockSvc.EXPECT().Update(input).Return(updated, nil)
 
 	controller := NewUserController(mockSvc)
-	router := setupRouter(controller)
+	router := setupUserRouter(controller)
 
 	body, _ := json.Marshal(input)
 
@@ -272,7 +271,7 @@ func TestHandleError_UserNotFound(t *testing.T) {
 	c, _ := gin.CreateTestContext(w)
 
 	// When: calling handleError with ErrUserNotFound
-	result := handleError(c, repository.ErrUserNotFound)
+	result := handleError(c, service.ErrUserNotFound)
 
 	// Then: should return true and response with 404 + correct error message
 	assert.True(t, result)
@@ -289,7 +288,7 @@ func TestHandleError_UserAlreadyExists(t *testing.T) {
 	c, _ := gin.CreateTestContext(w)
 
 	// When: calling handleError with ErrUserAlreadyExists
-	result := handleError(c, repository.ErrUserAlreadyExists)
+	result := handleError(c, service.ErrUserAlreadyExists)
 
 	// Then: should return true and response with 409 + correct error message
 	assert.True(t, result)
@@ -306,7 +305,7 @@ func TestHandleError_UsernameAlreadyExists(t *testing.T) {
 	c, _ := gin.CreateTestContext(w)
 
 	// When: calling handleError with ErrUsernameAlreadyExists
-	result := handleError(c, repository.ErrUsernameAlreadyExists)
+	result := handleError(c, service.ErrUsernameAlreadyExists)
 
 	// Then: should return true and response with 409 + correct error message
 	assert.True(t, result)
@@ -323,7 +322,7 @@ func TestHandleError_EmailAlreadyExists(t *testing.T) {
 	c, _ := gin.CreateTestContext(w)
 
 	// When: calling handleError with ErrEmailAlreadyExists
-	result := handleError(c, repository.ErrEmailAlreadyExists)
+	result := handleError(c, service.ErrEmailAlreadyExists)
 
 	// Then: should return true and response with 409 + correct error message
 	assert.True(t, result)
diff --git a/internal/handler/router.go b/internal/handler/router.go
index eda1695..b0a6c0b 100644
--- a/internal/handler/router.go
+++ b/internal/handler/router.go
@@ -10,10 +10,8 @@ import (
 	ginSwagger "github.com/swaggo/gin-swagger"
 )
 
-func New(router *gin.Engine, userController *controller.UserController) *gin.Engine {
-	router.GET("/healthz", func(c *gin.Context) {
-		c.JSON(200, gin.H{"status": "ok"})
-	})
+func New(router *gin.Engine, userController *controller.UserController, healthController *controller.HealthController) *gin.Engine {
+	router.GET("/healthz", healthController.HealthCheck)
 	v1 := router.Group("/api/v1")
 	{
 		userGroup := v1.Group("/users")
diff --git a/internal/middleware/apikey.go b/internal/middleware/apikey.go
index 14e021f..d33e3bc 100644
--- a/internal/middleware/apikey.go
+++ b/internal/middleware/apikey.go
@@ -1,6 +1,10 @@
 package middleware
 
-import "github.com/gin-gonic/gin"
+import (
+	"slices"
+
+	"github.com/gin-gonic/gin"
+)
 
 type ApiKeyMiddleware struct {
 	apiKey  string
@@ -13,11 +17,9 @@ func NewApiKeyMiddleware(apiKey string, ignored []string) *ApiKeyMiddleware {
 
 func (am *ApiKeyMiddleware) Handler() gin.HandlerFunc {
 	return func(c *gin.Context) {
-		for _, path := range am.ignored {
-			if c.FullPath() == path {
-				c.Next()
-				return
-			}
+		if slices.Contains(am.ignored, c.FullPath()) {
+			c.Next()
+			return
 		}
 
 		providedKey := c.GetHeader("X-Api-Key")
diff --git a/internal/repository/users.go b/internal/repository/users.go
index 97e0e57..fd310f3 100644
--- a/internal/repository/users.go
+++ b/internal/repository/users.go
@@ -5,6 +5,7 @@ import (
 	"cruder/internal/model"
 	"database/sql"
 	"errors"
+	"fmt"
 
 	"github.com/lib/pq"
 )
@@ -57,7 +58,7 @@ func (r *userRepository) GetByUsername(username string) (*model.User, error) {
 	if err := r.db.QueryRowContext(context.Background(), `SELECT id, username, email, full_name FROM users WHERE username = $1`, username).
 		Scan(&u.ID, &u.Username, &u.Email, &u.FullName); err != nil {
 		if err == sql.ErrNoRows {
-			return nil, ErrUserNotFound
+			return nil, ErrRowNotFound
 		}
 		return nil, err
 	}
@@ -69,7 +70,7 @@ func (r *userRepository) GetByID(id int64) (*model.User, error) {
 	if err := r.db.QueryRowContext(context.Background(), `SELECT id, username, email, full_name FROM users WHERE id = $1`, id).
 		Scan(&u.ID, &u.Username, &u.Email, &u.FullName); err != nil {
 		if err == sql.ErrNoRows {
-			return nil, ErrUserNotFound
+			return nil, ErrRowNotFound
 		}
 		return nil, err
 	}
@@ -80,14 +81,7 @@ func (r *userRepository) Create(user *model.User) (*model.User, error) {
 	if err := r.db.QueryRowContext(context.Background(), `INSERT INTO users (username, email, full_name) VALUES ($1, $2, $3) RETURNING id, username, email, full_name`, user.Username, user.Email, user.FullName).
 		Scan(&user.ID, &user.Username, &user.Email, &user.FullName); err != nil {
 		if pqErr, ok := err.(*pq.Error); ok && pqErr.Code == "23505" {
-			switch pqErr.Constraint {
-			case "users_username_key":
-				return nil, ErrUsernameAlreadyExists
-			case "users_email_key":
-				return nil, ErrEmailAlreadyExists
-			default:
-				return nil, ErrUserAlreadyExists
-			}
+			return nil, handleUniqueConstraintError(pqErr.Constraint)
 		}
 		return nil, err
 	}
@@ -99,13 +93,13 @@ func (r *userRepository) Delete(id int64) error {
 	if err := r.db.QueryRowContext(context.Background(), `DELETE FROM users WHERE id = $1 RETURNING id`, id).
 		Scan(&idCheck); err != nil {
 		if err == sql.ErrNoRows {
-			return ErrUserNotFound
+			return ErrRowNotFound
 		} else {
 			return err
 		}
 	}
 	if idCheck == 0 {
-		return ErrUserNotFound
+		return ErrRowNotFound
 	}
 	return nil
 }
@@ -114,15 +108,32 @@ func (r *userRepository) Update(user *model.User) (*model.User, error) {
 	if err := r.db.QueryRowContext(context.Background(), `UPDATE users SET username = $1, email = $2, full_name = $3 WHERE id = $4 RETURNING id, username, email, full_name`, user.Username, user.Email, user.FullName, user.ID).
 		Scan(&user.ID, &user.Username, &user.Email, &user.FullName); err != nil {
 		if err == sql.ErrNoRows {
-			return nil, ErrUserNotFound
-		} else {
-			return nil, err
+			return nil, ErrRowNotFound
+		} else if pqErr, ok := err.(*pq.Error); ok && pqErr.Code == "23505" {
+			return nil, handleUniqueConstraintError(pqErr.Constraint)
 		}
+		return nil, err
 	}
 	return user, nil
 }
 
-var ErrUserNotFound = errors.New("user not found")
-var ErrUserAlreadyExists = errors.New("user already exists")
-var ErrUsernameAlreadyExists = errors.New("username already exists")
-var ErrEmailAlreadyExists = errors.New("email already exists")
+func handleUniqueConstraintError(constraint string) error {
+	switch constraint {
+	case "users_username_key":
+		return &UniqueConstraintError{Field: "username"}
+	case "users_email_key":
+		return &UniqueConstraintError{Field: "email"}
+	default:
+		return &UniqueConstraintError{}
+	}
+}
+
+var ErrRowNotFound = errors.New("user not found")
+
+type UniqueConstraintError struct {
+	Field string
+}
+
+func (e *UniqueConstraintError) Error() string {
+	return fmt.Sprintf("%s unique constraint violation", e.Field)
+}
diff --git a/internal/repository/users_test.go b/internal/repository/users_test.go
index bf55ba7..708a07c 100644
--- a/internal/repository/users_test.go
+++ b/internal/repository/users_test.go
@@ -70,7 +70,7 @@ func TestGetByUsername_NotFound(t *testing.T) {
 	user, err := repo.GetByUsername("missing")
 
 	// Then: ErrUserNotFound should be returned and user should be nil
-	assert.ErrorIs(t, err, ErrUserNotFound)
+	assert.ErrorIs(t, err, ErrRowNotFound)
 	assert.Nil(t, user)
 	assert.NoError(t, mock.ExpectationsWereMet())
 }
@@ -106,7 +106,7 @@ func TestGetByID_NotFound(t *testing.T) {
 	user, err := repo.GetByID(99)
 
 	// Then: ErrUserNotFound should be returned and user should be nil
-	assert.ErrorIs(t, err, ErrUserNotFound)
+	assert.ErrorIs(t, err, ErrRowNotFound)
 	assert.Nil(t, user)
 	assert.NoError(t, mock.ExpectationsWereMet())
 }
@@ -144,8 +144,11 @@ func TestCreateUser_DuplicateUsername(t *testing.T) {
 	// When: calling Create with duplicate username
 	created, err := repo.Create(newUser)
 
-	// Then: ErrUsernameAlreadyExists should be returned
-	assert.ErrorIs(t, err, ErrUsernameAlreadyExists)
+	// Then: UniqueConstraintError with field username should be returned
+	var ce *UniqueConstraintError
+	if assert.ErrorAs(t, err, &ce) {
+		assert.Equal(t, "username", ce.Field)
+	}
 	assert.Nil(t, created)
 	assert.NoError(t, mock.ExpectationsWereMet())
 }
@@ -163,8 +166,11 @@ func TestCreateUser_DuplicateEmail(t *testing.T) {
 	// When: calling Create with duplicate email
 	created, err := repo.Create(newUser)
 
-	// Then: ErrEmailAlreadyExists should be returned
-	assert.ErrorIs(t, err, ErrEmailAlreadyExists)
+	// Then: UniqueConstraintError with field email should be returned
+	var ce *UniqueConstraintError
+	if assert.ErrorAs(t, err, &ce) {
+		assert.Equal(t, "email", ce.Field)
+	}
 	assert.Nil(t, created)
 	assert.NoError(t, mock.ExpectationsWereMet())
 }
@@ -182,8 +188,11 @@ func TestCreateUser_DuplicateOther(t *testing.T) {
 	// When: calling Create with some other duplicate
 	created, err := repo.Create(newUser)
 
-	// Then: ErrUserAlreadyExists should be returned
-	assert.ErrorIs(t, err, ErrUserAlreadyExists)
+	// Then: UniqueConstraintError should be returned
+	var ce *UniqueConstraintError
+	if assert.ErrorAs(t, err, &ce) {
+		assert.Equal(t, "", ce.Field)
+	}
 	assert.Nil(t, created)
 	assert.NoError(t, mock.ExpectationsWereMet())
 }
@@ -217,7 +226,7 @@ func TestDeleteUser_NotFound(t *testing.T) {
 	err := repo.Delete(99)
 
 	// Then: ErrUserNotFound should be returned
-	assert.ErrorIs(t, err, ErrUserNotFound)
+	assert.ErrorIs(t, err, ErrRowNotFound)
 	assert.NoError(t, mock.ExpectationsWereMet())
 }
 
@@ -254,7 +263,7 @@ func TestUpdateUser_NotFound(t *testing.T) {
 	updated, err := repo.Update(user)
 
 	// Then: ErrUserNotFound should be returned
-	assert.ErrorIs(t, err, ErrUserNotFound)
+	assert.ErrorIs(t, err, ErrRowNotFound)
 	assert.Nil(t, updated)
 	assert.NoError(t, mock.ExpectationsWereMet())
 }
diff --git a/internal/service/users.go b/internal/service/users.go
index c1deecf..858e56b 100644
--- a/internal/service/users.go
+++ b/internal/service/users.go
@@ -29,29 +29,79 @@ func (s *userService) GetAll() ([]model.User, error) {
 }
 
 func (s *userService) GetByUsername(username string) (*model.User, error) {
-	return s.repo.GetByUsername(username)
+	user, err := s.repo.GetByUsername(username)
+
+	if err != nil {
+		if errors.Is(err, repository.ErrRowNotFound) {
+			return nil, ErrUserNotFound
+		}
+		return nil, err
+	}
+
+	return user, nil
 }
 
 func (s *userService) GetByID(id int64) (*model.User, error) {
-	return s.repo.GetByID(id)
+	user, err := s.repo.GetByID(id)
+
+	if err != nil {
+		if errors.Is(err, repository.ErrRowNotFound) {
+			return nil, ErrUserNotFound
+		}
+		return nil, err
+	}
+
+	return user, nil
 }
 
 func (s *userService) Create(user *model.User) (*model.User, error) {
 	if err := ValidateUser(*user); err != nil {
 		return nil, err
 	}
-	return s.repo.Create(user)
+	user, err := s.repo.Create(user)
+
+	if ce, ok := err.(*repository.UniqueConstraintError); ok {
+		return nil, handleUniqueConstraintError(ce)
+	}
+
+	return user, err
+}
+
+func handleUniqueConstraintError(err *repository.UniqueConstraintError) error {
+	switch err.Field {
+	case "username":
+		return ErrUsernameAlreadyExists
+	case "email":
+		return ErrEmailAlreadyExists
+	default:
+		return ErrUserAlreadyExists
+	}
 }
 
 func (s *userService) Delete(id int64) error {
-	return s.repo.Delete(id)
+	err := s.repo.Delete(id)
+
+	if errors.Is(err, repository.ErrRowNotFound) {
+		return ErrUserNotFound
+	}
+	return err
 }
 
 func (s *userService) Update(user *model.User) (*model.User, error) {
 	if err := ValidateUser(*user); err != nil {
 		return nil, err
 	}
-	return s.repo.Update(user)
+
+	user, err := s.repo.Update(user)
+
+	if errors.Is(err, repository.ErrRowNotFound) {
+		return nil, ErrUserNotFound
+	}
+
+	if ce, ok := err.(*repository.UniqueConstraintError); ok {
+		return nil, handleUniqueConstraintError(ce)
+	}
+	return user, err
 }
 
 func ValidateUser(user model.User) error {
@@ -71,6 +121,12 @@ var emailRegex = regexp.MustCompile(`^[\w-\.]+@([\w-]+\.)+[\w-]{2,4}$`)
 var usernameRegex = regexp.MustCompile(`^[a-z][a-z0-9_]{2,49}$`)
 var fullNameRegex = regexp.MustCompile(`^[A-Za-z][A-Za-z' -]{0,98}[A-Za-z]$`)
 
-var ErrInvalidEmail = errors.New("invalid email format")
-var ErrInvalidUsername = errors.New("invalid username format (3-50 chars, lowercase letters, numbers, underscores, starts with letter)")
-var ErrInvalidFullName = errors.New("invalid full name format (2-100 chars, letters, spaces, apostrophes, hyphens, starts/ends with letter)")
+var (
+	ErrUserNotFound          = errors.New("user not found")
+	ErrUserAlreadyExists     = errors.New("user already exists")
+	ErrUsernameAlreadyExists = errors.New("username already exists")
+	ErrEmailAlreadyExists    = errors.New("email already exists")
+	ErrInvalidEmail          = errors.New("invalid email format")
+	ErrInvalidUsername       = errors.New("invalid username format (3-50 chars, lowercase letters, numbers, underscores, starts with letter)")
+	ErrInvalidFullName       = errors.New("invalid full name format (2-100 chars, letters, spaces, apostrophes, hyphens, starts/ends with letter)")
+)