diff --git a/core/core.go b/core/core.go
index c6ce81b..6f9abf6 100644
--- a/core/core.go
+++ b/core/core.go
@@ -236,7 +236,7 @@ func reserveCoderDirective(r *owl.Resolver, name string) error {
 
 	namedAdaptor := namedStringableAdaptors[d.Argv[0]]
 	if namedAdaptor == nil {
-		return fmt.Errorf("directive %s: unregistered coder: %q", name, d.Argv[0])
+		return fmt.Errorf("directive %s: %w: %q", name, ErrUnregisteredCoder, d.Argv[0])
 	}
 
 	r.Context = context.WithValue(r.Context, CtxCustomCoder, namedAdaptor)
@@ -248,7 +248,7 @@ func reserveCoderDirective(r *owl.Resolver, name string) error {
 func ensureDirectiveExecutorsRegistered(r *owl.Resolver) error {
 	for _, d := range r.Directives {
 		if decoderNamespace.LookupExecutor(d.Name) == nil {
-			return fmt.Errorf("unregistered directive: %q", d.Name)
+			return fmt.Errorf("%w: %q", ErrUnregisteredDirective, d.Name)
 		}
 		// NOTE: don't need to check encoderNamespace because a directive
 		// will always be registered in both namespaces. See RegisterDirective().
diff --git a/core/core_test.go b/core/core_test.go
index fe2ebfd..9897b4d 100644
--- a/core/core_test.go
+++ b/core/core_test.go
@@ -6,6 +6,7 @@ import (
 	"io"
 	"net/http"
 	"net/url"
+	"strconv"
 	"strings"
 	"testing"
 	"time"
@@ -320,6 +321,52 @@ func TestCore_Decode_PointerTypes(t *testing.T) {
 	assert.ErrorContains(err, "invalid place")
 }
 
+type CommaSeparatedIntegerArray struct {
+	Value []int
+}
+
+func (a CommaSeparatedIntegerArray) ToString() (string, error) {
+	var res = make([]string, len(a.Value))
+	for i := range a.Value {
+		res[i] = strconv.Itoa(a.Value[i])
+	}
+	return strings.Join(res, ","), nil
+}
+
+func (pa *CommaSeparatedIntegerArray) FromString(value string) error {
+	a := CommaSeparatedIntegerArray{}
+	values := strings.Split(value, ",")
+	a.Value = make([]int, len(values))
+	for i := range values {
+		if value, err := strconv.Atoi(values[i]); err != nil {
+			return err
+		} else {
+			a.Value[i] = value
+		}
+	}
+	*pa = a
+	return nil
+}
+
+func TestCore_Decode_CustomTypeSliceValueWrapper(t *testing.T) {
+	assert := assert.New(t)
+
+	type Input struct {
+		Ids CommaSeparatedIntegerArray `in:"form=ids"`
+	}
+	co, err := New(Input{})
+	assert.NoError(err)
+
+	// Missing fields.
+	r := newMultipartFormRequestFromMap(map[string]any{
+		"ids": "1,2,3",
+	})
+	gotValue, err := co.Decode(r)
+	assert.NoError(err)
+	got := gotValue.(*Input)
+	assert.Equal([]int{1, 2, 3}, got.Ids.Value)
+}
+
 // Test: register named coders and use them in the "coder" directive,
 // i.e. customizing the encoding/decoding for a specific struct field.
 
diff --git a/core/directiveruntime.go b/core/directiveruntime.go
index 0d60c31..b31df82 100644
--- a/core/directiveruntime.go
+++ b/core/directiveruntime.go
@@ -6,7 +6,6 @@ import (
 	"net/http"
 	"reflect"
 
-	"github.com/ggicci/httpin/internal"
 	"github.com/ggicci/owl"
 )
 
@@ -88,7 +87,7 @@ func (rtm *DirectiveRuntime) SetValue(value any) error {
 
 	if !newValue.Type().AssignableTo(targetType) {
 		return fmt.Errorf("%w: value of type %q is not assignable to type %q",
-			internal.ErrTypeMismatch, reflect.TypeOf(value), targetType)
+			ErrTypeMismatch, reflect.TypeOf(value), targetType)
 	}
 
 	rtm.Value.Elem().Set(newValue)
diff --git a/core/error.go b/core/error.go
index a804ecb..d555b9f 100644
--- a/core/error.go
+++ b/core/error.go
@@ -5,9 +5,16 @@ import (
 	"fmt"
 	"strings"
 
+	"github.com/ggicci/httpin/internal"
 	"github.com/ggicci/owl"
 )
 
+var (
+	ErrUnregisteredDirective = errors.New("unregistered directive")
+	ErrUnregisteredCoder     = errors.New("unregistered coder")
+	ErrTypeMismatch          = internal.ErrTypeMismatch
+)
+
 type InvalidFieldError struct {
 	// err is the underlying error thrown by the directive executor.
 	err error
diff --git a/httpin.go b/httpin.go
index 02e27f9..efabee4 100644
--- a/httpin.go
+++ b/httpin.go
@@ -6,6 +6,7 @@ package httpin
 import (
 	"context"
 	"fmt"
+	"io"
 	"net/http"
 	"reflect"
 
@@ -22,6 +23,13 @@ const (
 	Input contextKey = iota
 )
 
+// Option is a collection of options for creating a Core instance.
+var Option coreOptions = coreOptions{
+	WithErrorHandler:            core.WithErrorHandler,
+	WithMaxMemory:               core.WithMaxMemory,
+	WithNestedDirectivesEnabled: core.WithNestedDirectivesEnabled,
+}
+
 // New calls core.New to create a new Core instance. Which is responsible for both:
 //
 //   - decoding an HTTP request to an instance of the inputStruct;
@@ -39,7 +47,9 @@ const (
 // API, chained with other middlewares, and also reused in other APIs. You even
 // don't need to call the Deocde() method explicitly, the middleware will do it
 // for you and put the decoded instance to the request's context.
-var New = core.New
+func New(inputStruct any, opts ...core.Option) (*core.Core, error) {
+	return core.New(inputStruct, opts...)
+}
 
 // File is the builtin type of httpin to manupulate file uploads. On the server
 // side, it is used to represent a file in a multipart/form-data request. On the
@@ -48,11 +58,15 @@ type File = core.File
 
 // UploadFile is a helper function to create a File instance from a file path.
 // It is useful when you want to upload a file from the local file system.
-var UploadFile = core.UploadFile
+func UploadFile(path string) *File {
+	return core.UploadFile(path)
+}
 
 // UploadStream is a helper function to create a File instance from a io.Reader. It
 // is useful when you want to upload a file from a stream.
-var UploadStream = core.UploadStream
+func UploadStream(r io.ReadCloser) *File {
+	return core.UploadStream(r)
+}
 
 // Decode decodes an HTTP request to the given input struct. The input must be a
 // pointer to a struct instance. For example:
@@ -61,12 +75,12 @@ var UploadStream = core.UploadStream
 //	if err := Decode(req, &input); err != nil { ... }
 //
 // input is now populated with data from the request.
-func Decode(req *http.Request, input any) error {
+func Decode(req *http.Request, input any, opts ...core.Option) error {
 	originalType := reflect.TypeOf(input)
 	if originalType.Kind() != reflect.Ptr {
 		return fmt.Errorf("httpin: input must be a pointer")
 	}
-	co, err := New(originalType.Elem())
+	co, err := New(originalType.Elem(), opts...)
 	if err != nil {
 		return err
 	}
@@ -83,15 +97,15 @@ func Decode(req *http.Request, input any) error {
 }
 
 // NewRequest wraps NewRequestWithContext using context.Background.
-func NewRequest(method, url string, input any) (*http.Request, error) {
+func NewRequest(method, url string, input any, opts ...core.Option) (*http.Request, error) {
 	return NewRequestWithContext(context.Background(), method, url, input)
 }
 
 // NewRequestWithContext returns a new http.Request given a method, url and an
 // input struct instance. The fields of the input struct will be encoded to the
 // request by resolving the "in" tags and executing the directives.
-func NewRequestWithContext(ctx context.Context, method, url string, input any) (*http.Request, error) {
-	co, err := New(input)
+func NewRequestWithContext(ctx context.Context, method, url string, input any, opts ...core.Option) (*http.Request, error) {
+	co, err := New(input, opts...)
 	if err != nil {
 		return nil, err
 	}
@@ -145,3 +159,16 @@ func NewInput(inputStruct any, opts ...core.Option) func(http.Handler) http.Hand
 		})
 	}
 }
+
+type coreOptions struct {
+	// WithErrorHandler overrides the default error handler.
+	WithErrorHandler func(core.ErrorHandler) core.Option
+
+	// WithMaxMemory overrides the default maximum memory size (32MB) when reading
+	// the request body. See https://pkg.go.dev/net/http#Request.ParseMultipartForm
+	// for more details.
+	WithMaxMemory func(int64) core.Option
+
+	// WithNestedDirectivesEnabled enables/disables nested directives.
+	WithNestedDirectivesEnabled func(bool) core.Option
+}