jsonrpc2: avoid streaming decoder allocation for method strings

internal/jsonrpc2/messages.go

@@ -198,7 +198,9 @@ func DecodeMessage(data []byte) (Message, error) {
 	if len(msg.Method) > 0 {
 		// The "method" key was present. Decode its value (including "").
 		var method string
-		if err := internaljson.Unmarshal(msg.Method, &method); err != nil {
+		// The envelope is already depth-checked and field names matched exactly.
+		// A scalar needs no streaming decoder and its 32 KiB buffer.
+		if err := json.Unmarshal(msg.Method, &method); err != nil {
 			return nil, fmt.Errorf("unmarshaling jsonrpc message: %w", err)
 		}
 		return &Request{

internal/jsonrpc2/method_decode_test.go

@@ -0,0 +1,80 @@
+// Copyright 2025 The Go MCP SDK Authors. All rights reserved.
+// Use of this source code is governed by the license
+// that can be found in the LICENSE file.
+
+package jsonrpc2
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestDecodeMethodValue(t *testing.T) {
+	for _, tt := range []struct {
+		name, value, want string
+		invalid           bool
+	}{
+		{"plain", `"tools/call"`, "tools/call", false},
+		{"empty", `""`, "", false},
+		{"escaped", `"tools\u002fcall"`, "tools/call", false},
+		{"unicode", `"\ud83d\ude00"`, "😀", false},
+		{"null", `null`, "", false},
+		{"number", `42`, "", true},
+		{"boolean", `true`, "", true},
+		{"object", `{}`, "", true},
+		{"array", `[]`, "", true},
+	} {
+		t.Run(tt.name, func(t *testing.T) {
+			wire := []byte(`{"jsonrpc":"2.0","id":1,"method":` + tt.value + `}`)
+			msg, err := DecodeMessage(wire)
+			if tt.invalid {
+				if err == nil {
+					t.Fatal("expected invalid method error")
+				}
+				return
+			}
+			if err != nil {
+				t.Fatal(err)
+			}
+			req, ok := msg.(*Request)
+			if !ok || req.Method != tt.want {
+				t.Fatalf("decoded message = %#v, want request method %q", msg, tt.want)
+			}
+		})
+	}
+}
+
+func TestDecodeMethodEnvelopeGuards(t *testing.T) {
+	t.Run("case sensitive field", func(t *testing.T) {
+		wire := []byte(`{"jsonrpc":"2.0","id":1,"Method":"ping","result":{}}`)
+		msg, err := DecodeMessage(wire)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if _, ok := msg.(*Response); !ok {
+			t.Fatalf("decoded %T, want response", msg)
+		}
+	})
+	t.Run("depth limit", func(t *testing.T) {
+		wire := []byte(`{"jsonrpc":"2.0","id":1,"method":` + strings.Repeat("[", 1001) + `0` + strings.Repeat("]", 1001) + `}`)
+		_, err := DecodeMessage(wire)
+		if err == nil || !strings.Contains(err.Error(), "maximum nesting depth") {
+			t.Fatalf("error = %v, want depth rejection", err)
+		}
+	})
+}
+
+func BenchmarkDecodeMethodRequest(b *testing.B) {
+	for _, method := range []string{`"tools/call"`, `"tools\u002fcall"`} {
+		b.Run(method, func(b *testing.B) {
+			wire := []byte(`{"jsonrpc":"2.0","id":1,"method":` + method + `,"params":{}}`)
+			b.ReportAllocs()
+			b.ResetTimer()
+			for i := 0; i < b.N; i++ {
+				if _, err := DecodeMessage(wire); err != nil {
+					b.Fatal(err)
+				}
+			}
+		})
+	}
+}