diff --git a/CHANGELOG.md b/CHANGELOG.md
index 9a94781d..83001afc 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -4,6 +4,12 @@ that can be found in the LICENSE file. -->
 
 # Changelog
 
+## 1.0.0-dev.33
+
+- Use an explicit serialization method rather than `toJson`.
+- Revert #88.
+- Revert part of #87.
+
 ## 1.0.0-dev.32
 
 - Make parsable big number string as covariant of Fixed classes.
diff --git a/packages/agent_dart/pubspec.yaml b/packages/agent_dart/pubspec.yaml
index 7058f19e..6c7ff0f7 100644
--- a/packages/agent_dart/pubspec.yaml
+++ b/packages/agent_dart/pubspec.yaml
@@ -1,5 +1,5 @@
 name: agent_dart
-version: 1.0.0-dev.32
+version: 1.0.0-dev.33
 
 description: |
   An agent library built for Internet Computer,
diff --git a/packages/agent_dart_base/lib/agent/utils/leb128.dart b/packages/agent_dart_base/lib/agent/utils/leb128.dart
index b0183146..5f86dc85 100644
--- a/packages/agent_dart_base/lib/agent/utils/leb128.dart
+++ b/packages/agent_dart_base/lib/agent/utils/leb128.dart
@@ -22,12 +22,7 @@ List<T> safeRead<T>(BufferPipe<T> pipe, int ref) {
 /// nearest integer.
 /// @param value The number to encode.
 Uint8List lebEncode(dynamic value) {
-  BigInt bn = switch (value) {
-    BigInt() => value,
-    num() => BigInt.from(value),
-    String() => BigInt.parse(value),
-    _ => throw ArgumentError('Invalid big number: $value', 'lebEncode'),
-  };
+  BigInt bn = value is BigInt ? value : BigInt.from(value);
   if (bn < BigInt.zero) {
     throw StateError('Cannot leb-encode negative values.');
   }
@@ -63,13 +58,9 @@ BigInt lebDecode<T>(BufferPipe<T> pipe) {
 /// Encode a number (or bigint) into a Buffer, with support for negative numbers.
 /// The number will be floored to the nearest integer.
 /// @param value The number to encode.
-Uint8List slebEncode(dynamic value) {
-  BigInt bn = switch (value) {
-    BigInt() => value,
-    num() => BigInt.from(value),
-    String() => BigInt.parse(value),
-    _ => throw ArgumentError('Invalid big number: $value', 'slebEncode'),
-  };
+Uint8List slebEncode(Comparable value) {
+  BigInt bn = value is BigInt ? value : BigInt.from(value as num);
+
   final isNeg = bn < BigInt.zero;
   if (isNeg) {
     bn = -bn - BigInt.one;
diff --git a/packages/agent_dart_base/lib/candid/idl.dart b/packages/agent_dart_base/lib/candid/idl.dart
index 779ce55e..5238ee20 100644
--- a/packages/agent_dart_base/lib/candid/idl.dart
+++ b/packages/agent_dart_base/lib/candid/idl.dart
@@ -55,7 +55,12 @@ Uint8List? tryToJson(CType type, dynamic value) {
       // obj may be a map, must be ignore.
       value is! Map) {
     try {
-      return type.encodeValue(value.toJson());
+      try {
+        value = value.toIDLSerializable();
+      } on NoSuchMethodError {
+        value = value.toJson();
+      }
+      return type.encodeValue(value);
     } catch (e) {
       return null;
     }
@@ -528,9 +533,7 @@ class NatClass extends PrimitiveType {
 
   @override
   bool covariant(x) {
-    return (x is BigInt && x >= BigInt.zero) ||
-        (x is int && x >= 0) ||
-        (x is String && BigInt.parse(x) >= BigInt.zero);
+    return (x is BigInt && x >= BigInt.zero) || (x is int && x >= 0);
   }
 
   @override
@@ -636,9 +639,6 @@ class FixedIntClass extends PrimitiveType {
     } else if (x is int) {
       final v = BigInt.from(x);
       return v >= min && v <= max;
-    } else if (x is String && BigInt.tryParse(x) != null) {
-      final v = BigInt.parse(x);
-      return v >= min && v <= max;
     } else {
       return false;
     }
@@ -699,11 +699,6 @@ class FixedNatClass extends PrimitiveType<dynamic> {
     } else if (x is int && x >= 0) {
       final v = BigInt.from(x);
       return v < max;
-    } else if (x is String &&
-        BigInt.tryParse(x) != null &&
-        BigInt.parse(x) >= BigInt.zero) {
-      final v = BigInt.parse(x);
-      return v < max;
     } else {
       return false;
     }
@@ -904,7 +899,11 @@ class RecordClass extends ConstructType<Map> {
   bool covariant(dynamic x) {
     if (x is! Map) {
       try {
-        x = x.toJson();
+        try {
+          x = x.toIDLSerializable();
+        } on NoSuchMethodError {
+          x = x.toJson();
+        }
       } catch (e) {
         return false;
       }
diff --git a/packages/agent_dart_base/pubspec.yaml b/packages/agent_dart_base/pubspec.yaml
index 7efc1494..fc11eaca 100644
--- a/packages/agent_dart_base/pubspec.yaml
+++ b/packages/agent_dart_base/pubspec.yaml
@@ -1,5 +1,5 @@
 name: agent_dart_base
-version: 1.0.0-dev.32
+version: 1.0.0-dev.33
 
 description: The Dart plugin that bridges Rust implementation for agent_dart.
 repository: https://github.com/AstroxNetwork/agent_dart
diff --git a/packages/agent_dart_base/test/agent/utils/leb128.dart b/packages/agent_dart_base/test/agent/utils/leb128.dart
index 831e949a..072a97aa 100644
--- a/packages/agent_dart_base/test/agent/utils/leb128.dart
+++ b/packages/agent_dart_base/test/agent/utils/leb128.dart
@@ -24,7 +24,6 @@ void leb128Test() {
       lebEncode(BigInt.from(60000000000000000)).toHex(),
       '808098f4e9b5ca6a',
     );
-    expect(lebEncode('1').toHex(), '01');
 
     expect(lebDecode(BufferPipe<int>(Uint8List.fromList([0]))), BigInt.zero);
     expect(lebDecode(BufferPipe<int>(Uint8List.fromList([1]))), BigInt.one);
@@ -60,7 +59,6 @@ void leb128Test() {
       slebEncode(BigInt.parse('60000000000000000')).toHex(),
       '808098f4e9b5caea00',
     );
-    expect(slebEncode('1').toHex(), '01');
 
     expect(
       slebDecode(BufferPipe<int>(Uint8List.fromList([0x7f]))),
diff --git a/packages/agent_dart_ffi/pubspec.yaml b/packages/agent_dart_ffi/pubspec.yaml
index 5401f0af..af9f0b01 100644
--- a/packages/agent_dart_ffi/pubspec.yaml
+++ b/packages/agent_dart_ffi/pubspec.yaml
@@ -1,5 +1,5 @@
 name: agent_dart_ffi
-version: 1.0.0-dev.32
+version: 1.0.0-dev.33
 
 description: The FFI plugin that bridges Rust implementation for agent_dart.
 repository: https://github.com/AstroxNetwork/agent_dart