diff --git a/internal/impl/legacy_message.go b/internal/impl/legacy_message.go
index 938c44c..dbc99ed 100644
--- a/internal/impl/legacy_message.go
+++ b/internal/impl/legacy_message.go
@@ -52,6 +52,12 @@
 	if _, ok := v.(legacyMarshaler); ok {
 		mi.methods.MarshalAppend = legacyMarshalAppend
 		mi.methods.Size = legacySize
+
+		// We have no way to tell whether the type's Marshal method
+		// supports deterministic serialization or not, but this
+		// preserves the v1 implementation's behavior of always
+		// calling Marshal methods when present.
+		mi.methods.Flags |= piface.SupportMarshalDeterministic
 	}
 	if _, ok := v.(legacyUnmarshaler); ok {
 		mi.methods.Unmarshal = legacyUnmarshal
@@ -331,6 +337,12 @@
 	Size:          legacySize,
 	MarshalAppend: legacyMarshalAppend,
 	Unmarshal:     legacyUnmarshal,
+
+	// We have no way to tell whether the type's Marshal method
+	// supports deterministic serialization or not, but this
+	// preserves the v1 implementation's behavior of always
+	// calling Marshal methods when present.
+	Flags: piface.SupportMarshalDeterministic,
 }
 
 func legacySize(m protoreflect.Message, opts piface.MarshalOptions) int {
diff --git a/proto/methods_test.go b/proto/methods_test.go
index 9b8535b..cc5dfce 100644
--- a/proto/methods_test.go
+++ b/proto/methods_test.go
@@ -58,6 +58,13 @@
 		if err != test.err || !bytes.Equal(b, want) {
 			t.Errorf("MarshalAppend(%v, %v) = %v, %v; want %v, %v", prefix, test, b, err, test.bytes, test.err)
 		}
+
+		b, err = proto.MarshalOptions{
+			Deterministic: true,
+		}.MarshalAppend(nil, m)
+		if err != test.err || !bytes.Equal(b, test.bytes) {
+			t.Errorf("MarshalOptions{Deterministic:true}.MarshalAppend(nil, %v) = %v, %v; want %v, %v", test, b, err, test.bytes, test.err)
+		}
 	}
 }