diff --git a/internal/json/json.go b/internal/json/json.go
index 1148770..b4c897f 100644
--- a/internal/json/json.go
+++ b/internal/json/json.go
@@ -8,12 +8,25 @@ package json
 
 import (
 	"bytes"
+	"io"
 
 	"github.com/segmentio/encoding/json"
 )
 
-func Unmarshal(data []byte, v any) error {
-	dec := json.NewDecoder(bytes.NewReader(data))
-	dec.DontMatchCaseInsensitiveStructFields()
-	return dec.Decode(v)
+type Decoder struct {
+	dec *json.Decoder
+}
+
+func NewDecoder(r io.Reader) *Decoder {
+	dec := json.NewDecoder(r)
+	dec.DontMatchCaseInsensitiveStructFields()
+	return &Decoder{dec: dec}
+}
+
+func (d *Decoder) Decode(v any) error {
+	return d.dec.Decode(v)
+}
+
+func Unmarshal(data []byte, v any) error {
+	return NewDecoder(bytes.NewReader(data)).Decode(v)
 }
diff --git a/internal/json/json_test.go b/internal/json/json_test.go
index 34d0226..d037e88 100644
--- a/internal/json/json_test.go
+++ b/internal/json/json_test.go
@@ -8,6 +8,7 @@ import (
 	"bytes"
 	"encoding/json"
 	"fmt"
+	"strings"
 	"testing"
 
 	"github.com/google/go-cmp/cmp"
@@ -65,6 +66,43 @@ func TestUnmarshalCaseSensitivity(t *testing.T) {
 	}
 }
 
+func TestNewDecoderCaseSensitivity(t *testing.T) {
+	type Target struct {
+		Field       string `json:"field"`
+		TaggedField string `json:"custom_tag"`
+	}
+
+	tests := []struct {
+		name string
+		json string
+		want Target
+	}{
+		{
+			name: "exact match",
+			json: `{"field": "value", "custom_tag": "tagged"}`,
+			want: Target{Field: "value", TaggedField: "tagged"},
+		},
+		{
+			name: "case mismatch",
+			json: `{"Field": "value", "Custom_tag": "tagged"}`,
+			want: Target{},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			var got Target
+			dec := NewDecoder(strings.NewReader(tt.json))
+			if err := dec.Decode(&got); err != nil {
+				t.Fatalf("Decode failed: %v", err)
+			}
+			if diff := cmp.Diff(tt.want, got); diff != "" {
+				t.Errorf("Decode mismatch (-want +got):\n%s", diff)
+			}
+		})
+	}
+}
+
 func TestUnmarshalNullCharacter(t *testing.T) {
 	type Target struct {
 		Field string `json:"field"`
diff --git a/oauthex/oauth2.go b/oauthex/oauth2.go
index d8aeb3c..82c1c28 100644
--- a/oauthex/oauth2.go
+++ b/oauthex/oauth2.go
@@ -10,7 +10,6 @@ package oauthex
 
 import (
 	"context"
-	"encoding/json"
 	"fmt"
 	"io"
 	"mime"
@@ -18,6 +17,7 @@ import (
 	"net/url"
 	"strings"
 
+	"github.com/modelcontextprotocol/go-sdk/internal/json"
 	"github.com/modelcontextprotocol/go-sdk/internal/util"
 )