diff --git a/Taskfile.yaml b/Taskfile.yaml
index 32f611e..9f3fefb 100644
--- a/Taskfile.yaml
+++ b/Taskfile.yaml
@@ -10,7 +10,7 @@ tasks:
     cmds:
       - go mod tidy
 
-  build:lib:
+  build:
     desc: Build the konfetty library
     cmds:
       - go build ./...
diff --git a/defaults.go b/defaults.go
new file mode 100644
index 0000000..bb32de6
--- /dev/null
+++ b/defaults.go
@@ -0,0 +1,118 @@
+package konfetty
+
+import "reflect"
+
+// DefaultProvider is an interface for types that can provide their own default values
+type DefaultProvider interface {
+	Defaults() any
+}
+
+// Defaulter is an interface for types that can set their own default values
+type Defaulter interface {
+	SetDefaults()
+}
+
+// fillDefaults recursively fills in default values for structs that implement DefaultProvider
+func fillDefaults(v any) error {
+	return fillDefaultsRecursive(reflect.ValueOf(v))
+}
+
+func fillDefaultsRecursive(v reflect.Value) error {
+	// Handle pointer types
+	if v.Kind() == reflect.Ptr {
+		if v.IsNil() {
+			// If the pointer is nil, create a new instance of the pointed-to type
+			v.Set(reflect.New(v.Type().Elem()))
+		}
+		v = v.Elem()
+	}
+
+	if v.Kind() != reflect.Struct {
+		return nil
+	}
+
+	t := v.Type()
+
+	// Iterate through all fields
+	for i := 0; i < v.NumField(); i++ {
+		field := v.Field(i)
+		fieldType := t.Field(i)
+
+		// Handle embedded fields
+		if fieldType.Anonymous {
+			if field.Kind() == reflect.Ptr && field.IsNil() {
+				// If the embedded field is a nil pointer, create a new instance
+				field.Set(reflect.New(field.Type().Elem()))
+			}
+			if err := fillDefaultsRecursive(field); err != nil {
+				return err
+			}
+
+			// Apply defaults for the embedded field if it implements DefaultProvider
+			if defaulter, ok := field.Addr().Interface().(DefaultProvider); ok {
+				defaults := reflect.ValueOf(defaulter.Defaults())
+				if defaults.Kind() == reflect.Ptr {
+					defaults = defaults.Elem()
+				}
+				fillFromDefaults(field, defaults)
+			}
+
+			continue
+		}
+
+		switch field.Kind() {
+		case reflect.Ptr:
+			if field.IsNil() {
+				// If the field is a nil pointer, create a new instance
+				field.Set(reflect.New(field.Type().Elem()))
+			}
+			if err := fillDefaultsRecursive(field.Elem()); err != nil {
+				return err
+			}
+		case reflect.Struct:
+			if err := fillDefaultsRecursive(field); err != nil {
+				return err
+			}
+		case reflect.Slice:
+			for j := 0; j < field.Len(); j++ {
+				if err := fillDefaultsRecursive(field.Index(j)); err != nil {
+					return err
+				}
+			}
+		}
+	}
+
+	// Apply defaults to the current struct if it implements DefaultProvider
+	if defaulter, ok := v.Addr().Interface().(DefaultProvider); ok {
+		defaults := reflect.ValueOf(defaulter.Defaults())
+		if defaults.Kind() == reflect.Ptr {
+			defaults = defaults.Elem()
+		}
+		fillFromDefaults(v, defaults)
+	}
+
+	return nil
+}
+
+func fillFromDefaults(dst, src reflect.Value) {
+	for i := 0; i < src.NumField(); i++ {
+		srcField := src.Field(i)
+		srcFieldName := src.Type().Field(i).Name
+
+		// Check if the destination has this field
+		dstField := dst.FieldByName(srcFieldName)
+		if !dstField.IsValid() {
+			continue // Skip fields that don't exist in the destination
+		}
+
+		if dstField.CanSet() && isZeroValue(dstField) {
+			// Only set the value if it's settable and currently zero
+			dstField.Set(srcField)
+		}
+	}
+}
+
+func isZeroValue(v reflect.Value) bool {
+	zero := reflect.Zero(v.Type()).Interface()
+	return reflect.DeepEqual(v.Interface(), zero)
+}
diff --git a/defaults_test.go b/defaults_test.go
new file mode 100644
index 0000000..44ea640
--- /dev/null
+++ b/defaults_test.go
@@ -0,0 +1,110 @@
+package konfetty
+
+import (
+	"testing"
+	"time"
+
+	"github.com/google/go-cmp/cmp"
+	"github.com/stretchr/testify/require"
+)
+
+// Structs provided by the user
+type Profile struct {
+	Checks Checks
+}
+
+type Checks struct {
+	Ping []PingCheck
+}
+
+type PingCheck struct {
+	*BaseCheck
+	Host string
+}
+
+type BaseCheck struct {
+	Name     string
+	Interval time.Duration
+	Timeout  time.Duration
+}
+
+// Implement DefaultProvider for BaseCheck
+func (b BaseCheck) Defaults() interface{} {
+	return BaseCheck{
+		Interval: 30 * time.Second,
+		Timeout:  5 * time.Second,
+	}
+}
+
+func TestFillDefaults(t *testing.T) {
+	tests := []struct {
+		name     string
+		input    *Profile
+		expected *Profile
+	}{
+		{
+			name: "Profile with partially filled checks",
+			input: &Profile{
+				Checks: Checks{
+					Ping: []PingCheck{
+						{
+							BaseCheck: &BaseCheck{Name: "Custom Ping"},
+							Host:      "example.com",
+						},
+					},
+				},
+			},
+			expected: &Profile{
+				Checks: Checks{
+					Ping: []PingCheck{
+						{
+							BaseCheck: &BaseCheck{
+								Name:     "Custom Ping",
+								Interval: 30 * time.Second,
+								Timeout:  5 * time.Second,
+							},
+							Host: "example.com",
+						},
+					},
+				},
+			},
+		},
+		{
+			name: "Profile with nil checks",
+			input: &Profile{
+				Checks: Checks{
+					Ping: []PingCheck{
+						{
+							Host: "example.com",
+						},
+					},
+				},
+			},
+			expected: &Profile{
+				Checks: Checks{
+					Ping: []PingCheck{
+						{
+							BaseCheck: &BaseCheck{
+								Interval: 30 * time.Second,
+								Timeout:  5 * time.Second,
+							},
+							Host: "example.com",
+						},
+					},
+				},
+			},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			err := fillDefaults(tt.input)
+			require.NoError(t, err)
+
+			diff := cmp.Diff(tt.expected, tt.input)
+			if diff != "" {
+				t.Errorf("FillDefaults() mismatch (-want +got):\n%s", diff)
+			}
+		})
+	}
+}
diff --git a/go.mod b/go.mod
index c6dd499..1152218 100644
--- a/go.mod
+++ b/go.mod
@@ -3,10 +3,18 @@ module github.com/nikoksr/konfetty
 go 1.22.5
 
 require (
+	github.com/google/go-cmp v0.6.0
 	github.com/knadh/koanf/parsers/yaml v0.1.0
 	github.com/knadh/koanf/providers/env v0.1.0
 	github.com/knadh/koanf/providers/file v1.0.0
 	github.com/knadh/koanf/v2 v2.1.1
+	github.com/stretchr/testify v1.9.0
+)
+
+require (
+	github.com/davecgh/go-spew v1.1.1 // indirect
+	github.com/pelletier/go-toml/v2 v2.2.2 // indirect
+	github.com/pmezard/go-difflib v1.0.0 // indirect
 )
 
 require (
@@ -22,6 +30,7 @@ require (
 )
 
 require (
-	dario.cat/mergo v1.0.0
+	github.com/knadh/koanf/parsers/json v0.1.0
+	github.com/knadh/koanf/parsers/toml/v2 v2.1.0
 	gopkg.in/yaml.v3 v3.0.1 // indirect
 )
diff --git a/go.sum b/go.sum
index 2da2443..d86c829 100644
--- a/go.sum
+++ b/go.sum
@@ -1,16 +1,19 @@
-dario.cat/mergo v1.0.0 h1:AGCNq9Evsj31mOgNPcLyXc+4PNABt905YmuqPYYpBWk=
-dario.cat/mergo v1.0.0/go.mod h1:uNxQE+84aUszobStD9th8a29P2fMDhsBdgRYvZOxGmk=
 github.com/creack/pty v1.1.9/go.mod h1:oKZEueFk5CKHvIhNR5MUki03XCEU+Q6VDXinZuGJ33E=
+github.com/davecgh/go-spew v1.1.0/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
 github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
 github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
 github.com/fsnotify/fsnotify v1.7.0 h1:8JEhPFa5W2WU7YfeZzPNqzMP6Lwt7L2715Ggo0nosvA=
 github.com/fsnotify/fsnotify v1.7.0/go.mod h1:40Bi/Hjc2AVfZrqy+aj+yEI+/bRxZnMJyTJwOpGvigM=
-github.com/go-viper/mapstructure/v2 v2.0.0-alpha.1 h1:TQcrn6Wq+sKGkpyPvppOz99zsMBaUOKXq6HSv655U1c=
-github.com/go-viper/mapstructure/v2 v2.0.0-alpha.1/go.mod h1:oJDH3BJKyqBA2TXFhDsKDGDTlndYOZ6rGS0BRZIxGhM=
 github.com/go-viper/mapstructure/v2 v2.0.0 h1:dhn8MZ1gZ0mzeodTG3jt5Vj/o87xZKuNAprG2mQfMfc=
 github.com/go-viper/mapstructure/v2 v2.0.0/go.mod h1:oJDH3BJKyqBA2TXFhDsKDGDTlndYOZ6rGS0BRZIxGhM=
+github.com/google/go-cmp v0.6.0 h1:ofyhxvXcZhMsU5ulbFiLKl/XBFqE1GSq7atu8tAmTRI=
+github.com/google/go-cmp v0.6.0/go.mod h1:17dUlkBOakJ0+DkrSSNjCkIjxS6bF9zb3elmeNGIjoY=
 github.com/knadh/koanf/maps v0.1.1 h1:G5TjmUh2D7G2YWf5SQQqSiHRJEjaicvU0KpypqB3NIs=
 github.com/knadh/koanf/maps v0.1.1/go.mod h1:npD/QZY3V6ghQDdcQzl1W4ICNVTkohC8E73eI2xW4yI=
+github.com/knadh/koanf/parsers/json v0.1.0 h1:dzSZl5pf5bBcW0Acnu20Djleto19T0CfHcvZ14NJ6fU=
+github.com/knadh/koanf/parsers/json v0.1.0/go.mod h1:ll2/MlXcZ2BfXD6YJcjVFzhG9P0TdJ207aIBKQhV2hY=
+github.com/knadh/koanf/parsers/toml/v2 v2.1.0 h1:EUdIKIeezfDj6e1ABDhIjhbURUpyrP1HToqW6tz8R0I=
+github.com/knadh/koanf/parsers/toml/v2 v2.1.0/go.mod h1:0KtwfsWJt4igUTQnsn0ZjFWVrP80Jv7edTBRbQFd2ho=
 github.com/knadh/koanf/parsers/yaml v0.1.0 h1:ZZ8/iGfRLvKSaMEECEBPM1HQslrZADk8fP1XFUxVI5w=
 github.com/knadh/koanf/parsers/yaml v0.1.0/go.mod h1:cvbUDC7AL23pImuQP0oRw/hPuccrNBS2bps8asS0CwY=
 github.com/knadh/koanf/providers/env v0.1.0 h1:LqKteXqfOWyx5Ab9VfGHmjY9BvRXi+clwyZozgVRiKg=
@@ -27,18 +30,28 @@ github.com/mitchellh/copystructure v1.2.0 h1:vpKXTN4ewci03Vljg/q9QvCGUDttBOGBIa1
 github.com/mitchellh/copystructure v1.2.0/go.mod h1:qLl+cE2AmVv+CoeAwDPye/v+N2HKCj9FbZEVFJRxO9s=
 github.com/mitchellh/reflectwalk v1.0.2 h1:G2LzWKi524PWgd3mLHV8Y5k7s6XUvT0Gef6zxSIeXaQ=
 github.com/mitchellh/reflectwalk v1.0.2/go.mod h1:mSTlrgnPZtwu0c4WaC2kGObEpuNDbx0jmZXqmk4esnw=
+github.com/pelletier/go-toml/v2 v2.2.2 h1:aYUidT7k73Pcl9nb2gScu7NSrKCSHIDE89b3+6Wq+LM=
+github.com/pelletier/go-toml/v2 v2.2.2/go.mod h1:1t835xjRzz80PqgE6HHgN2JOsmgYu/h4qDAS4n929Rs=
 github.com/pkg/diff v0.0.0-20210226163009-20ebb0f2a09e/go.mod h1:pJLUxLENpZxwdsKMEsNbx1VGcRFpLqf3715MtcvvzbA=
 github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
 github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
 github.com/rogpeppe/go-internal v1.9.0/go.mod h1:WtVeX8xhTBvf0smdhujwtBcq4Qrzq/fJaraNFVN+nFs=
 github.com/rogpeppe/go-internal v1.12.0 h1:exVL4IDcn6na9z1rAb56Vxr+CgyK3nn3O+epU5NdKM8=
 github.com/rogpeppe/go-internal v1.12.0/go.mod h1:E+RYuTGaKKdloAfM02xzb0FW3Paa99yedzYV+kq4uf4=
-github.com/stretchr/testify v1.8.1 h1:w7B6lhMri9wdJUVmEZPGGhZzrYTPvgJArz7wNPgYKsk=
-github.com/stretchr/testify v1.8.1/go.mod h1:w2LPCIKwWwSfY2zedu0+kehJoqGctiVI29o6fzry7u4=
+github.com/stretchr/objx v0.1.0/go.mod h1:HFkY916IF+rwdDfMAkV7OtwuqBVzrE8GR6GFx+wExME=
+github.com/stretchr/objx v0.4.0/go.mod h1:YvHI0jy2hoMjB+UWwv71VJQ9isScKT/TqJzVSSt89Yw=
+github.com/stretchr/objx v0.5.0/go.mod h1:Yh+to48EsGEfYuaHDzXPcE3xhTkx73EhmCGUpEOglKo=
+github.com/stretchr/objx v0.5.2/go.mod h1:FRsXN1f5AsAjCGJKqEizvkpNtU+EGNCLh3NxZ/8L+MA=
+github.com/stretchr/testify v1.7.1/go.mod h1:6Fq8oRcR53rry900zMqJjRRixrwX3KX962/h/Wwjteg=
+github.com/stretchr/testify v1.8.0/go.mod h1:yNjHg4UonilssWZ8iaSj1OCr/vHnekPRkoO+kdMU+MU=
+github.com/stretchr/testify v1.8.4/go.mod h1:sz/lmYIOXD/1dqDmKjjqLyZ2RngseejIcXlSw2iwfAo=
+github.com/stretchr/testify v1.9.0 h1:HtqpIVDClZ4nwg75+f6Lvsy/wHu+3BoSGCbBAcpTsTg=
+github.com/stretchr/testify v1.9.0/go.mod h1:r2ic/lqez/lEtzL7wO/rwa5dbSLXVDPFyf8C91i36aY=
 golang.org/x/sys v0.22.0 h1:RI27ohtqKCnwULzJLqkv897zojh5/DwS/ENaMzUOaWI=
 golang.org/x/sys v0.22.0/go.mod h1:/VUhepiaJMQUp4+oa/7Zr1D23ma6VTLIYjOOTFZPUcA=
 gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
 gopkg.in/check.v1 v1.0.0-20180628173108-788fd7840127 h1:qIbj1fsPNlZgppZ+VLlY7N33q108Sa+fhmuc+sWQYwY=
 gopkg.in/check.v1 v1.0.0-20180628173108-788fd7840127/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
+gopkg.in/yaml.v3 v3.0.0-20200313102051-9f266ea9e77c/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=
 gopkg.in/yaml.v3 v3.0.1 h1:fxVm/GzAzEWqLHuvctI91KS9hhNmmWOoWu0XTYJS7CA=
 gopkg.in/yaml.v3 v3.0.1/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=
diff --git a/konfetty.go b/konfetty.go
index 535d039..14d22d0 100644
--- a/konfetty.go
+++ b/konfetty.go
@@ -1,17 +1,29 @@
 package konfetty
 
 import (
+	"errors"
 	"fmt"
 	"strings"
 
-	"dario.cat/mergo"
 	"github.com/go-viper/mapstructure/v2"
+	"github.com/knadh/koanf/parsers/json"
+	"github.com/knadh/koanf/parsers/toml/v2"
 	"github.com/knadh/koanf/parsers/yaml"
 	"github.com/knadh/koanf/providers/env"
 	"github.com/knadh/koanf/providers/file"
 	"github.com/knadh/koanf/v2"
 )
 
+type FileFormat uint8
+
+const (
+	FileFormatYAML FileFormat = iota
+	FileFormatJSON
+	FileFormatTOML
+)
+
+const defaultStructTag = "konfetty"
+
 // Loader is the main interface for loading and validating configurations.
 type Loader[T any] interface {
 	Load(paths ...string) (*T, error)
@@ -25,16 +37,20 @@ type Option[T any] func(*loader[T])
 type loader[T any] struct {
 	k          *koanf.Koanf
 	envPrefix  string
-	fileFormat string
+	fileFormat FileFormat
+	structTag  string
 	validateFn func(*T) error
-	defaultsFn func() T
 }
 
 // NewLoader creates a new configuration loader.
 func NewLoader[T any](options ...Option[T]) Loader[T] {
 	l := &loader[T]{
-		k:          koanf.New("."),
-		fileFormat: "yaml", // default to YAML
+		k: koanf.NewWithConf(koanf.Conf{
+			Delim:       ".",
+			StrictMerge: true,
+		}),
+		fileFormat: FileFormatYAML, // default to YAML
+		structTag:  defaultStructTag,
 	}
 
 	for _, option := range options {
@@ -52,23 +68,23 @@ func WithEnvPrefix[T any](prefix string) Option[T] {
 }
 
 // WithFileFormat sets the format for configuration files.
-func WithFileFormat[T any](format string) Option[T] {
+func WithFileFormat[T any](format FileFormat) Option[T] {
 	return func(l *loader[T]) {
 		l.fileFormat = format
 	}
 }
 
-// WithValidator sets a custom validation function.
-func WithValidator[T any](fn func(*T) error) Option[T] {
+// WithStructTag sets the struct tag for configuration fields.
+func WithStructTag[T any](tag string) Option[T] {
 	return func(l *loader[T]) {
-		l.validateFn = fn
+		l.structTag = tag
 	}
 }
 
-// WithDefaults sets a function to provide default values.
-func WithDefaults[T any](fn func() T) Option[T] {
+// WithValidator sets a custom validation function.
+func WithValidator[T any](fn func(*T) error) Option[T] {
 	return func(l *loader[T]) {
-		l.defaultsFn = fn
+		l.validateFn = fn
 	}
 }
 
@@ -89,25 +105,27 @@ func (l *loader[T]) Load(paths ...string) (*T, error) {
 	}
 
 	// Unmarshal into the config struct
+	decodeHook := mapstructure.ComposeDecodeHookFunc(
+		mapstructure.StringToTimeDurationHookFunc(), // Convert strings to time.Duration
+		mapstructure.StringToSliceHookFunc(","),     // Convert comma-separated strings to slices
+	)
+
 	if err := l.k.UnmarshalWithConf("", &cfg, koanf.UnmarshalConf{
+		Tag: l.structTag,
 		DecoderConfig: &mapstructure.DecoderConfig{
 			Result:           &cfg,
 			WeaklyTypedInput: true,
-			DecodeHook: mapstructure.ComposeDecodeHookFunc(
-				mapstructure.StringToTimeDurationHookFunc(),
-				mapstructure.StringToSliceHookFunc(","),
-			),
+			Squash:           true,
+			TagName:          l.structTag,
+			DecodeHook:       decodeHook,
 		},
 	}); err != nil {
 		return nil, fmt.Errorf("unmarshal config: %w", err)
 	}
 
-	// Merge with defaults
-	if l.defaultsFn != nil {
-		defaults := l.defaultsFn()
-		if err := mergo.Merge(&cfg, defaults); err != nil {
-			return nil, fmt.Errorf("merge defaults: %w", err)
-		}
+	// Apply defaults
+	if err := fillDefaults(&cfg); err != nil {
+		return nil, fmt.Errorf("fill defaults: %w", err)
 	}
 
 	// Validate
@@ -123,11 +141,14 @@ func (l *loader[T]) loadFile(path string) error {
 	var parser koanf.Parser
 
 	switch l.fileFormat {
-	case "yaml":
+	case FileFormatYAML:
 		parser = yaml.Parser()
-	// Add more formats here as needed
+	case FileFormatJSON:
+		parser = json.Parser()
+	case FileFormatTOML:
+		parser = toml.Parser()
 	default:
-		return fmt.Errorf("unsupported file format: %s", l.fileFormat)
+		return errors.New("unsupported file format")
 	}
 
 	if err := l.k.Load(file.Provider(path), parser); err != nil {