From b5d33f067839b8203883fc081dbdea976ae3c913 Mon Sep 17 00:00:00 2001
From: wuyangfan <yangfan.wu@succaiss.com>
Date: Mon, 25 May 2026 10:41:01 +0800
Subject: [PATCH] fix(bind): restore []string values for map[string]interface{}
 duplicates

When binding multipart/form-data to map[string]interface{}, store a single
value as string and multiple values as []string. This matches the behavior
before v4.13.0 and the intent of #2656.

Fixes #2731

Co-authored-by: Cursor <cursoragent@cursor.com>
---
 bind.go      |  8 +++++---
 bind_test.go | 22 ++++++++++++++++++++--
 2 files changed, 25 insertions(+), 5 deletions(-)

diff --git a/bind.go b/bind.go
index 1d4fe6f0a..1e8393735 100644
--- a/bind.go
+++ b/bind.go
@@ -181,9 +181,11 @@ func (b *DefaultBinder) bindData(destination interface{}, data map[string][]stri
 			if isElemString {
 				val.SetMapIndex(reflect.ValueOf(k), reflect.ValueOf(v[0]))
 			} else if isElemInterface {
-				// To maintain backward compatibility, we always bind to the first string value
-				// and not the slice of strings when dealing with map[string]interface{}{}
-				val.SetMapIndex(reflect.ValueOf(k), reflect.ValueOf(v[0]))
+				if len(v) == 1 {
+					val.SetMapIndex(reflect.ValueOf(k), reflect.ValueOf(v[0]))
+				} else {
+					val.SetMapIndex(reflect.ValueOf(k), reflect.ValueOf(v))
+				}
 			} else {
 				val.SetMapIndex(reflect.ValueOf(k), reflect.ValueOf(v))
 			}
diff --git a/bind_test.go b/bind_test.go
index 3e387ba19..9c6e1048a 100644
--- a/bind_test.go
+++ b/bind_test.go
@@ -498,7 +498,7 @@ func TestDefaultBinder_bindDataToMap(t *testing.T) {
 		assert.NoError(t, new(DefaultBinder).bindData(&dest, exampleData, "param", nil))
 		assert.Equal(t,
 			map[string]interface{}{
-				"multiple": "1",
+				"multiple": []string{"1", "2"},
 				"single":   "3",
 			},
 			dest,
@@ -510,7 +510,7 @@ func TestDefaultBinder_bindDataToMap(t *testing.T) {
 		assert.NoError(t, new(DefaultBinder).bindData(&dest, exampleData, "param", nil))
 		assert.Equal(t,
 			map[string]interface{}{
-				"multiple": "1",
+				"multiple": []string{"1", "2"},
 				"single":   "3",
 			},
 			dest,
@@ -542,6 +542,24 @@ func TestDefaultBinder_bindDataToMap(t *testing.T) {
 	})
 }
 
+func TestBindMultipartFormToMapInterface(t *testing.T) {
+	bodyBuffer := new(bytes.Buffer)
+	mw := multipart.NewWriter(bodyBuffer)
+	assert.NoError(t, mw.WriteField("ima_slice", "WEBHOOK"))
+	assert.NoError(t, mw.WriteField("ima_slice", "OTHER"))
+	assert.NoError(t, mw.Close())
+
+	e := New()
+	req := httptest.NewRequest(http.MethodPost, "/", bodyBuffer)
+	req.Header.Set(HeaderContentType, mw.FormDataContentType())
+	c := e.NewContext(req, nil)
+
+	data := map[string]interface{}{}
+	err := c.Bind(&data)
+	assert.NoError(t, err)
+	assert.Equal(t, []string{"WEBHOOK", "OTHER"}, data["ima_slice"])
+}
+
 func TestBindbindData(t *testing.T) {
 	ts := new(bindTestStruct)
 	b := new(DefaultBinder)