diff --git a/packages/pigeon/CHANGELOG.md b/packages/pigeon/CHANGELOG.md
index a1cf6d0f1e3..daab8da8bcb 100644
--- a/packages/pigeon/CHANGELOG.md
+++ b/packages/pigeon/CHANGELOG.md
@@ -1,3 +1,7 @@
+## 12.0.1
+
+* [swift] Adds protocol for Flutter APIs.
+
 ## 12.0.0
 
 * Adds error handling on Flutter API methods.
diff --git a/packages/pigeon/example/app/ios/Runner/Messages.g.swift b/packages/pigeon/example/app/ios/Runner/Messages.g.swift
index 3c0433a7425..ef38ee5849c 100644
--- a/packages/pigeon/example/app/ios/Runner/Messages.g.swift
+++ b/packages/pigeon/example/app/ios/Runner/Messages.g.swift
@@ -173,13 +173,16 @@ class ExampleHostApiSetup {
     }
   }
 }
-/// Generated class from Pigeon that represents Flutter messages that can be called from Swift.
-class MessageFlutterApi {
+/// Generated protocol from Pigeon that represents Flutter messages that can be called from Swift.
+protocol MessageFlutterApiProtocol {
+  func flutterMethod(aString aStringArg: String?, completion: @escaping (Result<String, FlutterError>) -> Void) 
+}
+class MessageFlutterApi: MessageFlutterApiProtocol {
   private let binaryMessenger: FlutterBinaryMessenger
   init(binaryMessenger: FlutterBinaryMessenger){
     self.binaryMessenger = binaryMessenger
   }
-  func flutterMethod(aString aStringArg: String?, completion: @escaping (Result<String, FlutterError>) -> Void) {
+  func flutterMethod(aString aStringArg: String?, completion: @escaping (Result<String, FlutterError>) -> Void)  {
     let channel = FlutterBasicMessageChannel(name: "dev.flutter.pigeon.pigeon_example_package.MessageFlutterApi.flutterMethod", binaryMessenger: binaryMessenger)
     channel.sendMessage([aStringArg] as [Any?]) { response in
       guard let listResponse = response as? [Any?] else {
diff --git a/packages/pigeon/lib/generator_tools.dart b/packages/pigeon/lib/generator_tools.dart
index 3dfd1141bc8..c2d2bfad607 100644
--- a/packages/pigeon/lib/generator_tools.dart
+++ b/packages/pigeon/lib/generator_tools.dart
@@ -13,7 +13,7 @@ import 'ast.dart';
 /// The current version of pigeon.
 ///
 /// This must match the version in pubspec.yaml.
-const String pigeonVersion = '12.0.0';
+const String pigeonVersion = '12.0.1';
 
 /// Read all the content from [stdin] to a String.
 String readStdin() {
diff --git a/packages/pigeon/lib/swift_generator.dart b/packages/pigeon/lib/swift_generator.dart
index d558c877b2b..af96efd0205 100644
--- a/packages/pigeon/lib/swift_generator.dart
+++ b/packages/pigeon/lib/swift_generator.dart
@@ -291,13 +291,22 @@ import FlutterMacOS
     if (isCustomCodec) {
       _writeCodec(indent, api, root);
     }
+
     const List<String> generatedComments = <String>[
-      ' Generated class from Pigeon that represents Flutter messages that can be called from Swift.'
+      ' Generated protocol from Pigeon that represents Flutter messages that can be called from Swift.'
     ];
     addDocumentationComments(indent, api.documentationComments, _docCommentSpec,
         generatorComments: generatedComments);
 
-    indent.write('class ${api.name} ');
+    indent.addScoped('protocol ${api.name}Protocol {', '}', () {
+      for (final Method func in api.methods) {
+        addDocumentationComments(
+            indent, func.documentationComments, _docCommentSpec);
+        indent.writeln(_getMethodSignature(func));
+      }
+    });
+
+    indent.write('class ${api.name}: ${api.name}Protocol ');
     indent.addScoped('{', '}', () {
       indent.writeln('private let binaryMessenger: FlutterBinaryMessenger');
       indent.write('init(binaryMessenger: FlutterBinaryMessenger)');
@@ -314,47 +323,19 @@ import FlutterMacOS
         });
       }
       for (final Method func in api.methods) {
-        final _SwiftFunctionComponents components =
-            _SwiftFunctionComponents.fromMethod(func);
-
         final String channelName = makeChannelName(api, func, dartPackageName);
-        final String returnType = func.returnType.isVoid
-            ? 'Void'
-            : _nullsafeSwiftTypeForDartType(func.returnType);
-        String sendArgument;
+
         addDocumentationComments(
             indent, func.documentationComments, _docCommentSpec);
-
-        if (func.arguments.isEmpty) {
-          indent.write(
-              'func ${func.name}(completion: @escaping (Result<$returnType, FlutterError>) -> Void) ');
-          sendArgument = 'nil';
-        } else {
-          final Iterable<String> argTypes = func.arguments
-              .map((NamedType e) => _nullsafeSwiftTypeForDartType(e.type));
-          final Iterable<String> argLabels = indexMap(components.arguments,
-              (int index, _SwiftFunctionArgument argument) {
-            return argument.label ??
-                _getArgumentName(index, argument.namedType);
-          });
-          final Iterable<String> argNames =
-              indexMap(func.arguments, _getSafeArgumentName);
+        indent.writeScoped('${_getMethodSignature(func)} {', '}', () {
           final Iterable<String> enumSafeArgNames = func.arguments
               .asMap()
               .entries
               .map((MapEntry<int, NamedType> e) =>
                   getEnumSafeArgumentExpression(root, e.key, e.value));
-          sendArgument = '[${enumSafeArgNames.join(', ')}] as [Any?]';
-          final String argsSignature = map3(
-              argTypes,
-              argLabels,
-              argNames,
-              (String type, String label, String name) =>
-                  '$label $name: $type').join(', ');
-          indent.write(
-              'func ${components.name}($argsSignature, completion: @escaping (Result<$returnType, FlutterError>) -> Void) ');
-        }
-        indent.addScoped('{', '}', () {
+          final String sendArgument = func.arguments.isEmpty
+              ? 'nil'
+              : '[${enumSafeArgNames.join(', ')}] as [Any?]';
           const String channel = 'channel';
           indent.writeln(
               'let $channel = FlutterBasicMessageChannel(name: "$channelName", binaryMessenger: binaryMessenger$codecArgumentString)');
@@ -893,6 +874,31 @@ String _nullsafeSwiftTypeForDartType(TypeDeclaration type) {
   return '${_swiftTypeForDartType(type)}$nullSafe';
 }
 
+String _getMethodSignature(Method func) {
+  final _SwiftFunctionComponents components =
+      _SwiftFunctionComponents.fromMethod(func);
+  final String returnType = func.returnType.isVoid
+      ? 'Void'
+      : _nullsafeSwiftTypeForDartType(func.returnType);
+
+  if (func.arguments.isEmpty) {
+    return 'func ${func.name}(completion: @escaping (Result<$returnType, FlutterError>) -> Void) ';
+  } else {
+    final Iterable<String> argTypes = func.arguments
+        .map((NamedType e) => _nullsafeSwiftTypeForDartType(e.type));
+    final Iterable<String> argLabels = indexMap(components.arguments,
+        (int index, _SwiftFunctionArgument argument) {
+      return argument.label ?? _getArgumentName(index, argument.namedType);
+    });
+    final Iterable<String> argNames =
+        indexMap(func.arguments, _getSafeArgumentName);
+    final String argsSignature = map3(argTypes, argLabels, argNames,
+            (String type, String label, String name) => '$label $name: $type')
+        .join(', ');
+    return 'func ${components.name}($argsSignature, completion: @escaping (Result<$returnType, FlutterError>) -> Void) ';
+  }
+}
+
 /// A class that represents a Swift function argument.
 ///
 /// The [name] is the name of the argument.
diff --git a/packages/pigeon/pigeons/core_tests.dart b/packages/pigeon/pigeons/core_tests.dart
index ffe94ace2d3..2f5ef13732b 100644
--- a/packages/pigeon/pigeons/core_tests.dart
+++ b/packages/pigeon/pigeons/core_tests.dart
@@ -661,6 +661,10 @@ abstract class FlutterSmallApi {
   @ObjCSelector('echoWrappedList:')
   @SwiftFunction('echo(_:)')
   TestMessage echoWrappedList(TestMessage msg);
+
+  @ObjCSelector('echoString:')
+  @SwiftFunction('echo(_:)')
+  String echoString(String aString);
 }
 
 /// A data class containing a List, used in unit tests.
diff --git a/packages/pigeon/platform_tests/alternate_language_test_plugin/android/src/main/java/com/example/alternate_language_test_plugin/CoreTests.java b/packages/pigeon/platform_tests/alternate_language_test_plugin/android/src/main/java/com/example/alternate_language_test_plugin/CoreTests.java
index 2eaaa7c3594..f23d543647d 100644
--- a/packages/pigeon/platform_tests/alternate_language_test_plugin/android/src/main/java/com/example/alternate_language_test_plugin/CoreTests.java
+++ b/packages/pigeon/platform_tests/alternate_language_test_plugin/android/src/main/java/com/example/alternate_language_test_plugin/CoreTests.java
@@ -4390,5 +4390,41 @@ public void echoWrappedList(@NonNull TestMessage msgArg, @NonNull Result<TestMes
             }
           });
     }
+
+    public void echoString(@NonNull String aStringArg, @NonNull Result<String> result) {
+      BasicMessageChannel<Object> channel =
+          new BasicMessageChannel<>(
+              binaryMessenger,
+              "dev.flutter.pigeon.pigeon_integration_tests.FlutterSmallApi.echoString",
+              getCodec());
+      channel.send(
+          new ArrayList<Object>(Collections.singletonList(aStringArg)),
+          channelReply -> {
+            if (channelReply instanceof List) {
+              List<Object> listReply = (List<Object>) channelReply;
+              if (listReply.size() > 1) {
+                result.error(
+                    new FlutterError(
+                        (String) listReply.get(0),
+                        (String) listReply.get(1),
+                        (String) listReply.get(2)));
+              } else if (listReply.get(0) == null) {
+                result.error(
+                    new FlutterError(
+                        "null-error",
+                        "Flutter api returned null value for non-null return value.",
+                        ""));
+              } else {
+                @SuppressWarnings("ConstantConditions")
+                String output = (String) listReply.get(0);
+                result.success(output);
+              }
+            } else {
+              result.error(
+                  new FlutterError(
+                      "channel-error", "Unable to establish connection on channel.", ""));
+            }
+          });
+    }
   }
 }
diff --git a/packages/pigeon/platform_tests/alternate_language_test_plugin/ios/Classes/CoreTests.gen.h b/packages/pigeon/platform_tests/alternate_language_test_plugin/ios/Classes/CoreTests.gen.h
index d2fac067fea..f330fc4c68f 100644
--- a/packages/pigeon/platform_tests/alternate_language_test_plugin/ios/Classes/CoreTests.gen.h
+++ b/packages/pigeon/platform_tests/alternate_language_test_plugin/ios/Classes/CoreTests.gen.h
@@ -494,6 +494,8 @@ NSObject<FlutterMessageCodec> *FlutterSmallApiGetCodec(void);
 - (instancetype)initWithBinaryMessenger:(id<FlutterBinaryMessenger>)binaryMessenger;
 - (void)echoWrappedList:(TestMessage *)msg
              completion:(void (^)(TestMessage *_Nullable, FlutterError *_Nullable))completion;
+- (void)echoString:(NSString *)aString
+        completion:(void (^)(NSString *_Nullable, FlutterError *_Nullable))completion;
 @end
 
 NS_ASSUME_NONNULL_END
diff --git a/packages/pigeon/platform_tests/alternate_language_test_plugin/ios/Classes/CoreTests.gen.m b/packages/pigeon/platform_tests/alternate_language_test_plugin/ios/Classes/CoreTests.gen.m
index 634c3e5503b..60bcbb34651 100644
--- a/packages/pigeon/platform_tests/alternate_language_test_plugin/ios/Classes/CoreTests.gen.m
+++ b/packages/pigeon/platform_tests/alternate_language_test_plugin/ios/Classes/CoreTests.gen.m
@@ -2971,4 +2971,30 @@ - (void)echoWrappedList:(TestMessage *)arg_msg
                    }
                  }];
 }
+- (void)echoString:(NSString *)arg_aString
+        completion:(void (^)(NSString *_Nullable, FlutterError *_Nullable))completion {
+  FlutterBasicMessageChannel *channel = [FlutterBasicMessageChannel
+      messageChannelWithName:
+          @"dev.flutter.pigeon.pigeon_integration_tests.FlutterSmallApi.echoString"
+             binaryMessenger:self.binaryMessenger
+                       codec:FlutterSmallApiGetCodec()];
+  [channel sendMessage:@[ arg_aString ?: [NSNull null] ]
+                 reply:^(NSArray<id> *reply) {
+                   if (reply != nil) {
+                     if (reply.count > 1) {
+                       completion(nil, [FlutterError errorWithCode:reply[0]
+                                                           message:reply[1]
+                                                           details:reply[2]]);
+                     } else {
+                       NSString *output = reply[0] == [NSNull null] ? nil : reply[0];
+                       completion(output, nil);
+                     }
+                   } else {
+                     completion(nil, [FlutterError
+                                         errorWithCode:@"channel-error"
+                                               message:@"Unable to establish connection on channel."
+                                               details:@""]);
+                   }
+                 }];
+}
 @end
diff --git a/packages/pigeon/platform_tests/alternate_language_test_plugin/macos/Classes/CoreTests.gen.h b/packages/pigeon/platform_tests/alternate_language_test_plugin/macos/Classes/CoreTests.gen.h
index d2fac067fea..f330fc4c68f 100644
--- a/packages/pigeon/platform_tests/alternate_language_test_plugin/macos/Classes/CoreTests.gen.h
+++ b/packages/pigeon/platform_tests/alternate_language_test_plugin/macos/Classes/CoreTests.gen.h
@@ -494,6 +494,8 @@ NSObject<FlutterMessageCodec> *FlutterSmallApiGetCodec(void);
 - (instancetype)initWithBinaryMessenger:(id<FlutterBinaryMessenger>)binaryMessenger;
 - (void)echoWrappedList:(TestMessage *)msg
              completion:(void (^)(TestMessage *_Nullable, FlutterError *_Nullable))completion;
+- (void)echoString:(NSString *)aString
+        completion:(void (^)(NSString *_Nullable, FlutterError *_Nullable))completion;
 @end
 
 NS_ASSUME_NONNULL_END
diff --git a/packages/pigeon/platform_tests/alternate_language_test_plugin/macos/Classes/CoreTests.gen.m b/packages/pigeon/platform_tests/alternate_language_test_plugin/macos/Classes/CoreTests.gen.m
index 634c3e5503b..60bcbb34651 100644
--- a/packages/pigeon/platform_tests/alternate_language_test_plugin/macos/Classes/CoreTests.gen.m
+++ b/packages/pigeon/platform_tests/alternate_language_test_plugin/macos/Classes/CoreTests.gen.m
@@ -2971,4 +2971,30 @@ - (void)echoWrappedList:(TestMessage *)arg_msg
                    }
                  }];
 }
+- (void)echoString:(NSString *)arg_aString
+        completion:(void (^)(NSString *_Nullable, FlutterError *_Nullable))completion {
+  FlutterBasicMessageChannel *channel = [FlutterBasicMessageChannel
+      messageChannelWithName:
+          @"dev.flutter.pigeon.pigeon_integration_tests.FlutterSmallApi.echoString"
+             binaryMessenger:self.binaryMessenger
+                       codec:FlutterSmallApiGetCodec()];
+  [channel sendMessage:@[ arg_aString ?: [NSNull null] ]
+                 reply:^(NSArray<id> *reply) {
+                   if (reply != nil) {
+                     if (reply.count > 1) {
+                       completion(nil, [FlutterError errorWithCode:reply[0]
+                                                           message:reply[1]
+                                                           details:reply[2]]);
+                     } else {
+                       NSString *output = reply[0] == [NSNull null] ? nil : reply[0];
+                       completion(output, nil);
+                     }
+                   } else {
+                     completion(nil, [FlutterError
+                                         errorWithCode:@"channel-error"
+                                               message:@"Unable to establish connection on channel."
+                                               details:@""]);
+                   }
+                 }];
+}
 @end
diff --git a/packages/pigeon/platform_tests/shared_test_plugin_code/lib/src/generated/core_tests.gen.dart b/packages/pigeon/platform_tests/shared_test_plugin_code/lib/src/generated/core_tests.gen.dart
index 0ec2bcd8a44..98a5b81a3f6 100644
--- a/packages/pigeon/platform_tests/shared_test_plugin_code/lib/src/generated/core_tests.gen.dart
+++ b/packages/pigeon/platform_tests/shared_test_plugin_code/lib/src/generated/core_tests.gen.dart
@@ -3120,6 +3120,8 @@ abstract class FlutterSmallApi {
 
   TestMessage echoWrappedList(TestMessage msg);
 
+  String echoString(String aString);
+
   static void setup(FlutterSmallApi? api, {BinaryMessenger? binaryMessenger}) {
     {
       final BasicMessageChannel<Object?> channel = BasicMessageChannel<Object?>(
@@ -3148,5 +3150,32 @@ abstract class FlutterSmallApi {
         });
       }
     }
+    {
+      final BasicMessageChannel<Object?> channel = BasicMessageChannel<Object?>(
+          'dev.flutter.pigeon.pigeon_integration_tests.FlutterSmallApi.echoString',
+          codec,
+          binaryMessenger: binaryMessenger);
+      if (api == null) {
+        channel.setMessageHandler(null);
+      } else {
+        channel.setMessageHandler((Object? message) async {
+          assert(message != null,
+              'Argument for dev.flutter.pigeon.pigeon_integration_tests.FlutterSmallApi.echoString was null.');
+          final List<Object?> args = (message as List<Object?>?)!;
+          final String? arg_aString = (args[0] as String?);
+          assert(arg_aString != null,
+              'Argument for dev.flutter.pigeon.pigeon_integration_tests.FlutterSmallApi.echoString was null, expected non-null String.');
+          try {
+            final String output = api.echoString(arg_aString!);
+            return wrapResponse(result: output);
+          } on PlatformException catch (e) {
+            return wrapResponse(error: e);
+          } catch (e) {
+            return wrapResponse(
+                error: PlatformException(code: 'error', message: e.toString()));
+          }
+        });
+      }
+    }
   }
 }
diff --git a/packages/pigeon/platform_tests/test_plugin/android/src/main/kotlin/com/example/test_plugin/CoreTests.gen.kt b/packages/pigeon/platform_tests/test_plugin/android/src/main/kotlin/com/example/test_plugin/CoreTests.gen.kt
index 07de3995f0a..86ec6755240 100644
--- a/packages/pigeon/platform_tests/test_plugin/android/src/main/kotlin/com/example/test_plugin/CoreTests.gen.kt
+++ b/packages/pigeon/platform_tests/test_plugin/android/src/main/kotlin/com/example/test_plugin/CoreTests.gen.kt
@@ -2479,4 +2479,21 @@ class FlutterSmallApi(private val binaryMessenger: BinaryMessenger) {
       } 
     }
   }
+  fun echoString(aStringArg: String, callback: (Result<String>) -> Unit) {
+    val channel = BasicMessageChannel<Any?>(binaryMessenger, "dev.flutter.pigeon.pigeon_integration_tests.FlutterSmallApi.echoString", codec)
+    channel.send(listOf(aStringArg)) {
+      if (it is List<*>) {
+        if (it.size > 1) {
+          callback(Result.failure(FlutterError(it[0] as String, it[1] as String, it[2] as String?)));
+        } else if (it[0] == null) {
+          callback(Result.failure(FlutterError("null-error", "Flutter api returned null value for non-null return value.", "")));
+        } else {
+          val output = it[0] as String
+          callback(Result.success(output));
+        }
+      } else {
+        callback(Result.failure(FlutterError("channel-error",  "Unable to establish connection on channel.", "")));
+      } 
+    }
+  }
 }
diff --git a/packages/pigeon/platform_tests/test_plugin/example/ios/RunnerTests/RunnerTests.swift b/packages/pigeon/platform_tests/test_plugin/example/ios/RunnerTests/RunnerTests.swift
index 4cbd6b0d1f2..9ad4dd95c0c 100644
--- a/packages/pigeon/platform_tests/test_plugin/example/ios/RunnerTests/RunnerTests.swift
+++ b/packages/pigeon/platform_tests/test_plugin/example/ios/RunnerTests/RunnerTests.swift
@@ -2,11 +2,14 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import Flutter
+import Foundation
 import XCTest
+
 @testable import test_plugin
 
 class RunnerTests: XCTestCase {
-  
+
   func testToListAndBack() throws {
     let reply = MessageSearchReply(result: "foobar")
     let dict = reply.toList()
@@ -27,4 +30,32 @@ class RunnerTests: XCTestCase {
     let copy = MessageSearchReply.fromList(dict)
     XCTAssertEqual(reply.error, copy?.error)
   }
+
+  /// This validates that pigeon clients can easily write tests that mock out Flutter API
+  /// calls using a pigeon-generated protocol.
+  func testEchoStringFromProtocol() throws {
+    let api: FlutterApiFromProtocol = FlutterApiFromProtocol()
+    let aString = "aString"
+    api.echo(aString) { response in
+      switch response {
+      case .success(let res):
+        XCTAssertEqual(aString, res)
+      case .failure(let error):
+        XCTFail(error.code)
+      }
+    }
+  }
+}
+
+class FlutterApiFromProtocol: FlutterSmallApiProtocol {
+  func echo(_ aStringArg: String, completion: @escaping (Result<String, FlutterError>) -> Void) {
+    completion(.success(aStringArg))
+  }
+
+  func echo(
+    _ msgArg: test_plugin.TestMessage,
+    completion: @escaping (Result<test_plugin.TestMessage, FlutterError>) -> Void
+  ) {
+    completion(.success(msgArg))
+  }
 }
diff --git a/packages/pigeon/platform_tests/test_plugin/ios/Classes/CoreTests.gen.swift b/packages/pigeon/platform_tests/test_plugin/ios/Classes/CoreTests.gen.swift
index 601136c458d..8b83e3d1771 100644
--- a/packages/pigeon/platform_tests/test_plugin/ios/Classes/CoreTests.gen.swift
+++ b/packages/pigeon/platform_tests/test_plugin/ios/Classes/CoreTests.gen.swift
@@ -1739,8 +1739,62 @@ class FlutterIntegrationCoreApiCodec: FlutterStandardMessageCodec {
 /// The core interface that the Dart platform_test code implements for host
 /// integration tests to call into.
 ///
-/// Generated class from Pigeon that represents Flutter messages that can be called from Swift.
-class FlutterIntegrationCoreApi {
+/// Generated protocol from Pigeon that represents Flutter messages that can be called from Swift.
+protocol FlutterIntegrationCoreApiProtocol {
+  /// A no-op function taking no arguments and returning no value, to sanity
+  /// test basic calling.
+  func noop(completion: @escaping (Result<Void, FlutterError>) -> Void) 
+  /// Responds with an error from an async function returning a value.
+  func throwError(completion: @escaping (Result<Any?, FlutterError>) -> Void) 
+  /// Responds with an error from an async void function.
+  func throwErrorFromVoid(completion: @escaping (Result<Void, FlutterError>) -> Void) 
+  /// Returns the passed object, to test serialization and deserialization.
+  func echo(_ everythingArg: AllTypes, completion: @escaping (Result<AllTypes, FlutterError>) -> Void) 
+  /// Returns the passed object, to test serialization and deserialization.
+  func echoNullable(_ everythingArg: AllNullableTypes?, completion: @escaping (Result<AllNullableTypes?, FlutterError>) -> Void) 
+  /// Returns passed in arguments of multiple types.
+  ///
+  /// Tests multiple-arity FlutterApi handling.
+  func sendMultipleNullableTypes(aBool aNullableBoolArg: Bool?, anInt aNullableIntArg: Int64?, aString aNullableStringArg: String?, completion: @escaping (Result<AllNullableTypes, FlutterError>) -> Void) 
+  /// Returns the passed boolean, to test serialization and deserialization.
+  func echo(_ aBoolArg: Bool, completion: @escaping (Result<Bool, FlutterError>) -> Void) 
+  /// Returns the passed int, to test serialization and deserialization.
+  func echo(_ anIntArg: Int64, completion: @escaping (Result<Int64, FlutterError>) -> Void) 
+  /// Returns the passed double, to test serialization and deserialization.
+  func echo(_ aDoubleArg: Double, completion: @escaping (Result<Double, FlutterError>) -> Void) 
+  /// Returns the passed string, to test serialization and deserialization.
+  func echo(_ aStringArg: String, completion: @escaping (Result<String, FlutterError>) -> Void) 
+  /// Returns the passed byte list, to test serialization and deserialization.
+  func echo(_ aListArg: FlutterStandardTypedData, completion: @escaping (Result<FlutterStandardTypedData, FlutterError>) -> Void) 
+  /// Returns the passed list, to test serialization and deserialization.
+  func echo(_ aListArg: [Any?], completion: @escaping (Result<[Any?], FlutterError>) -> Void) 
+  /// Returns the passed map, to test serialization and deserialization.
+  func echo(_ aMapArg: [String?: Any?], completion: @escaping (Result<[String?: Any?], FlutterError>) -> Void) 
+  /// Returns the passed enum to test serialization and deserialization.
+  func echo(_ anEnumArg: AnEnum, completion: @escaping (Result<AnEnum, FlutterError>) -> Void) 
+  /// Returns the passed boolean, to test serialization and deserialization.
+  func echoNullable(_ aBoolArg: Bool?, completion: @escaping (Result<Bool?, FlutterError>) -> Void) 
+  /// Returns the passed int, to test serialization and deserialization.
+  func echoNullable(_ anIntArg: Int64?, completion: @escaping (Result<Int64?, FlutterError>) -> Void) 
+  /// Returns the passed double, to test serialization and deserialization.
+  func echoNullable(_ aDoubleArg: Double?, completion: @escaping (Result<Double?, FlutterError>) -> Void) 
+  /// Returns the passed string, to test serialization and deserialization.
+  func echoNullable(_ aStringArg: String?, completion: @escaping (Result<String?, FlutterError>) -> Void) 
+  /// Returns the passed byte list, to test serialization and deserialization.
+  func echoNullable(_ aListArg: FlutterStandardTypedData?, completion: @escaping (Result<FlutterStandardTypedData?, FlutterError>) -> Void) 
+  /// Returns the passed list, to test serialization and deserialization.
+  func echoNullable(_ aListArg: [Any?]?, completion: @escaping (Result<[Any?]?, FlutterError>) -> Void) 
+  /// Returns the passed map, to test serialization and deserialization.
+  func echoNullable(_ aMapArg: [String?: Any?]?, completion: @escaping (Result<[String?: Any?]?, FlutterError>) -> Void) 
+  /// Returns the passed enum to test serialization and deserialization.
+  func echoNullable(_ anEnumArg: AnEnum?, completion: @escaping (Result<AnEnum?, FlutterError>) -> Void) 
+  /// A no-op function taking no arguments and returning no value, to sanity
+  /// test basic asynchronous calling.
+  func noopAsync(completion: @escaping (Result<Void, FlutterError>) -> Void) 
+  /// Returns the passed in generic Object asynchronously.
+  func echoAsync(_ aStringArg: String, completion: @escaping (Result<String, FlutterError>) -> Void) 
+}
+class FlutterIntegrationCoreApi: FlutterIntegrationCoreApiProtocol {
   private let binaryMessenger: FlutterBinaryMessenger
   init(binaryMessenger: FlutterBinaryMessenger){
     self.binaryMessenger = binaryMessenger
@@ -1750,14 +1804,14 @@ class FlutterIntegrationCoreApi {
   }
   /// A no-op function taking no arguments and returning no value, to sanity
   /// test basic calling.
-  func noop(completion: @escaping (Result<Void, FlutterError>) -> Void) {
+  func noop(completion: @escaping (Result<Void, FlutterError>) -> Void)  {
     let channel = FlutterBasicMessageChannel(name: "dev.flutter.pigeon.pigeon_integration_tests.FlutterIntegrationCoreApi.noop", binaryMessenger: binaryMessenger, codec: codec)
     channel.sendMessage(nil) { _ in
       completion(.success(Void()))
     }
   }
   /// Responds with an error from an async function returning a value.
-  func throwError(completion: @escaping (Result<Any?, FlutterError>) -> Void) {
+  func throwError(completion: @escaping (Result<Any?, FlutterError>) -> Void)  {
     let channel = FlutterBasicMessageChannel(name: "dev.flutter.pigeon.pigeon_integration_tests.FlutterIntegrationCoreApi.throwError", binaryMessenger: binaryMessenger, codec: codec)
     channel.sendMessage(nil) { response in
       guard let listResponse = response as? [Any?] else {
@@ -1776,14 +1830,14 @@ class FlutterIntegrationCoreApi {
     }
   }
   /// Responds with an error from an async void function.
-  func throwErrorFromVoid(completion: @escaping (Result<Void, FlutterError>) -> Void) {
+  func throwErrorFromVoid(completion: @escaping (Result<Void, FlutterError>) -> Void)  {
     let channel = FlutterBasicMessageChannel(name: "dev.flutter.pigeon.pigeon_integration_tests.FlutterIntegrationCoreApi.throwErrorFromVoid", binaryMessenger: binaryMessenger, codec: codec)
     channel.sendMessage(nil) { _ in
       completion(.success(Void()))
     }
   }
   /// Returns the passed object, to test serialization and deserialization.
-  func echo(_ everythingArg: AllTypes, completion: @escaping (Result<AllTypes, FlutterError>) -> Void) {
+  func echo(_ everythingArg: AllTypes, completion: @escaping (Result<AllTypes, FlutterError>) -> Void)  {
     let channel = FlutterBasicMessageChannel(name: "dev.flutter.pigeon.pigeon_integration_tests.FlutterIntegrationCoreApi.echoAllTypes", binaryMessenger: binaryMessenger, codec: codec)
     channel.sendMessage([everythingArg] as [Any?]) { response in
       guard let listResponse = response as? [Any?] else {
@@ -1804,7 +1858,7 @@ class FlutterIntegrationCoreApi {
     }
   }
   /// Returns the passed object, to test serialization and deserialization.
-  func echoNullable(_ everythingArg: AllNullableTypes?, completion: @escaping (Result<AllNullableTypes?, FlutterError>) -> Void) {
+  func echoNullable(_ everythingArg: AllNullableTypes?, completion: @escaping (Result<AllNullableTypes?, FlutterError>) -> Void)  {
     let channel = FlutterBasicMessageChannel(name: "dev.flutter.pigeon.pigeon_integration_tests.FlutterIntegrationCoreApi.echoAllNullableTypes", binaryMessenger: binaryMessenger, codec: codec)
     channel.sendMessage([everythingArg] as [Any?]) { response in
       guard let listResponse = response as? [Any?] else {
@@ -1825,7 +1879,7 @@ class FlutterIntegrationCoreApi {
   /// Returns passed in arguments of multiple types.
   ///
   /// Tests multiple-arity FlutterApi handling.
-  func sendMultipleNullableTypes(aBool aNullableBoolArg: Bool?, anInt aNullableIntArg: Int64?, aString aNullableStringArg: String?, completion: @escaping (Result<AllNullableTypes, FlutterError>) -> Void) {
+  func sendMultipleNullableTypes(aBool aNullableBoolArg: Bool?, anInt aNullableIntArg: Int64?, aString aNullableStringArg: String?, completion: @escaping (Result<AllNullableTypes, FlutterError>) -> Void)  {
     let channel = FlutterBasicMessageChannel(name: "dev.flutter.pigeon.pigeon_integration_tests.FlutterIntegrationCoreApi.sendMultipleNullableTypes", binaryMessenger: binaryMessenger, codec: codec)
     channel.sendMessage([aNullableBoolArg, aNullableIntArg, aNullableStringArg] as [Any?]) { response in
       guard let listResponse = response as? [Any?] else {
@@ -1846,7 +1900,7 @@ class FlutterIntegrationCoreApi {
     }
   }
   /// Returns the passed boolean, to test serialization and deserialization.
-  func echo(_ aBoolArg: Bool, completion: @escaping (Result<Bool, FlutterError>) -> Void) {
+  func echo(_ aBoolArg: Bool, completion: @escaping (Result<Bool, FlutterError>) -> Void)  {
     let channel = FlutterBasicMessageChannel(name: "dev.flutter.pigeon.pigeon_integration_tests.FlutterIntegrationCoreApi.echoBool", binaryMessenger: binaryMessenger, codec: codec)
     channel.sendMessage([aBoolArg] as [Any?]) { response in
       guard let listResponse = response as? [Any?] else {
@@ -1867,7 +1921,7 @@ class FlutterIntegrationCoreApi {
     }
   }
   /// Returns the passed int, to test serialization and deserialization.
-  func echo(_ anIntArg: Int64, completion: @escaping (Result<Int64, FlutterError>) -> Void) {
+  func echo(_ anIntArg: Int64, completion: @escaping (Result<Int64, FlutterError>) -> Void)  {
     let channel = FlutterBasicMessageChannel(name: "dev.flutter.pigeon.pigeon_integration_tests.FlutterIntegrationCoreApi.echoInt", binaryMessenger: binaryMessenger, codec: codec)
     channel.sendMessage([anIntArg] as [Any?]) { response in
       guard let listResponse = response as? [Any?] else {
@@ -1888,7 +1942,7 @@ class FlutterIntegrationCoreApi {
     }
   }
   /// Returns the passed double, to test serialization and deserialization.
-  func echo(_ aDoubleArg: Double, completion: @escaping (Result<Double, FlutterError>) -> Void) {
+  func echo(_ aDoubleArg: Double, completion: @escaping (Result<Double, FlutterError>) -> Void)  {
     let channel = FlutterBasicMessageChannel(name: "dev.flutter.pigeon.pigeon_integration_tests.FlutterIntegrationCoreApi.echoDouble", binaryMessenger: binaryMessenger, codec: codec)
     channel.sendMessage([aDoubleArg] as [Any?]) { response in
       guard let listResponse = response as? [Any?] else {
@@ -1909,7 +1963,7 @@ class FlutterIntegrationCoreApi {
     }
   }
   /// Returns the passed string, to test serialization and deserialization.
-  func echo(_ aStringArg: String, completion: @escaping (Result<String, FlutterError>) -> Void) {
+  func echo(_ aStringArg: String, completion: @escaping (Result<String, FlutterError>) -> Void)  {
     let channel = FlutterBasicMessageChannel(name: "dev.flutter.pigeon.pigeon_integration_tests.FlutterIntegrationCoreApi.echoString", binaryMessenger: binaryMessenger, codec: codec)
     channel.sendMessage([aStringArg] as [Any?]) { response in
       guard let listResponse = response as? [Any?] else {
@@ -1930,7 +1984,7 @@ class FlutterIntegrationCoreApi {
     }
   }
   /// Returns the passed byte list, to test serialization and deserialization.
-  func echo(_ aListArg: FlutterStandardTypedData, completion: @escaping (Result<FlutterStandardTypedData, FlutterError>) -> Void) {
+  func echo(_ aListArg: FlutterStandardTypedData, completion: @escaping (Result<FlutterStandardTypedData, FlutterError>) -> Void)  {
     let channel = FlutterBasicMessageChannel(name: "dev.flutter.pigeon.pigeon_integration_tests.FlutterIntegrationCoreApi.echoUint8List", binaryMessenger: binaryMessenger, codec: codec)
     channel.sendMessage([aListArg] as [Any?]) { response in
       guard let listResponse = response as? [Any?] else {
@@ -1951,7 +2005,7 @@ class FlutterIntegrationCoreApi {
     }
   }
   /// Returns the passed list, to test serialization and deserialization.
-  func echo(_ aListArg: [Any?], completion: @escaping (Result<[Any?], FlutterError>) -> Void) {
+  func echo(_ aListArg: [Any?], completion: @escaping (Result<[Any?], FlutterError>) -> Void)  {
     let channel = FlutterBasicMessageChannel(name: "dev.flutter.pigeon.pigeon_integration_tests.FlutterIntegrationCoreApi.echoList", binaryMessenger: binaryMessenger, codec: codec)
     channel.sendMessage([aListArg] as [Any?]) { response in
       guard let listResponse = response as? [Any?] else {
@@ -1972,7 +2026,7 @@ class FlutterIntegrationCoreApi {
     }
   }
   /// Returns the passed map, to test serialization and deserialization.
-  func echo(_ aMapArg: [String?: Any?], completion: @escaping (Result<[String?: Any?], FlutterError>) -> Void) {
+  func echo(_ aMapArg: [String?: Any?], completion: @escaping (Result<[String?: Any?], FlutterError>) -> Void)  {
     let channel = FlutterBasicMessageChannel(name: "dev.flutter.pigeon.pigeon_integration_tests.FlutterIntegrationCoreApi.echoMap", binaryMessenger: binaryMessenger, codec: codec)
     channel.sendMessage([aMapArg] as [Any?]) { response in
       guard let listResponse = response as? [Any?] else {
@@ -1993,7 +2047,7 @@ class FlutterIntegrationCoreApi {
     }
   }
   /// Returns the passed enum to test serialization and deserialization.
-  func echo(_ anEnumArg: AnEnum, completion: @escaping (Result<AnEnum, FlutterError>) -> Void) {
+  func echo(_ anEnumArg: AnEnum, completion: @escaping (Result<AnEnum, FlutterError>) -> Void)  {
     let channel = FlutterBasicMessageChannel(name: "dev.flutter.pigeon.pigeon_integration_tests.FlutterIntegrationCoreApi.echoEnum", binaryMessenger: binaryMessenger, codec: codec)
     channel.sendMessage([anEnumArg.rawValue] as [Any?]) { response in
       guard let listResponse = response as? [Any?] else {
@@ -2014,7 +2068,7 @@ class FlutterIntegrationCoreApi {
     }
   }
   /// Returns the passed boolean, to test serialization and deserialization.
-  func echoNullable(_ aBoolArg: Bool?, completion: @escaping (Result<Bool?, FlutterError>) -> Void) {
+  func echoNullable(_ aBoolArg: Bool?, completion: @escaping (Result<Bool?, FlutterError>) -> Void)  {
     let channel = FlutterBasicMessageChannel(name: "dev.flutter.pigeon.pigeon_integration_tests.FlutterIntegrationCoreApi.echoNullableBool", binaryMessenger: binaryMessenger, codec: codec)
     channel.sendMessage([aBoolArg] as [Any?]) { response in
       guard let listResponse = response as? [Any?] else {
@@ -2033,7 +2087,7 @@ class FlutterIntegrationCoreApi {
     }
   }
   /// Returns the passed int, to test serialization and deserialization.
-  func echoNullable(_ anIntArg: Int64?, completion: @escaping (Result<Int64?, FlutterError>) -> Void) {
+  func echoNullable(_ anIntArg: Int64?, completion: @escaping (Result<Int64?, FlutterError>) -> Void)  {
     let channel = FlutterBasicMessageChannel(name: "dev.flutter.pigeon.pigeon_integration_tests.FlutterIntegrationCoreApi.echoNullableInt", binaryMessenger: binaryMessenger, codec: codec)
     channel.sendMessage([anIntArg] as [Any?]) { response in
       guard let listResponse = response as? [Any?] else {
@@ -2052,7 +2106,7 @@ class FlutterIntegrationCoreApi {
     }
   }
   /// Returns the passed double, to test serialization and deserialization.
-  func echoNullable(_ aDoubleArg: Double?, completion: @escaping (Result<Double?, FlutterError>) -> Void) {
+  func echoNullable(_ aDoubleArg: Double?, completion: @escaping (Result<Double?, FlutterError>) -> Void)  {
     let channel = FlutterBasicMessageChannel(name: "dev.flutter.pigeon.pigeon_integration_tests.FlutterIntegrationCoreApi.echoNullableDouble", binaryMessenger: binaryMessenger, codec: codec)
     channel.sendMessage([aDoubleArg] as [Any?]) { response in
       guard let listResponse = response as? [Any?] else {
@@ -2071,7 +2125,7 @@ class FlutterIntegrationCoreApi {
     }
   }
   /// Returns the passed string, to test serialization and deserialization.
-  func echoNullable(_ aStringArg: String?, completion: @escaping (Result<String?, FlutterError>) -> Void) {
+  func echoNullable(_ aStringArg: String?, completion: @escaping (Result<String?, FlutterError>) -> Void)  {
     let channel = FlutterBasicMessageChannel(name: "dev.flutter.pigeon.pigeon_integration_tests.FlutterIntegrationCoreApi.echoNullableString", binaryMessenger: binaryMessenger, codec: codec)
     channel.sendMessage([aStringArg] as [Any?]) { response in
       guard let listResponse = response as? [Any?] else {
@@ -2090,7 +2144,7 @@ class FlutterIntegrationCoreApi {
     }
   }
   /// Returns the passed byte list, to test serialization and deserialization.
-  func echoNullable(_ aListArg: FlutterStandardTypedData?, completion: @escaping (Result<FlutterStandardTypedData?, FlutterError>) -> Void) {
+  func echoNullable(_ aListArg: FlutterStandardTypedData?, completion: @escaping (Result<FlutterStandardTypedData?, FlutterError>) -> Void)  {
     let channel = FlutterBasicMessageChannel(name: "dev.flutter.pigeon.pigeon_integration_tests.FlutterIntegrationCoreApi.echoNullableUint8List", binaryMessenger: binaryMessenger, codec: codec)
     channel.sendMessage([aListArg] as [Any?]) { response in
       guard let listResponse = response as? [Any?] else {
@@ -2109,7 +2163,7 @@ class FlutterIntegrationCoreApi {
     }
   }
   /// Returns the passed list, to test serialization and deserialization.
-  func echoNullable(_ aListArg: [Any?]?, completion: @escaping (Result<[Any?]?, FlutterError>) -> Void) {
+  func echoNullable(_ aListArg: [Any?]?, completion: @escaping (Result<[Any?]?, FlutterError>) -> Void)  {
     let channel = FlutterBasicMessageChannel(name: "dev.flutter.pigeon.pigeon_integration_tests.FlutterIntegrationCoreApi.echoNullableList", binaryMessenger: binaryMessenger, codec: codec)
     channel.sendMessage([aListArg] as [Any?]) { response in
       guard let listResponse = response as? [Any?] else {
@@ -2128,7 +2182,7 @@ class FlutterIntegrationCoreApi {
     }
   }
   /// Returns the passed map, to test serialization and deserialization.
-  func echoNullable(_ aMapArg: [String?: Any?]?, completion: @escaping (Result<[String?: Any?]?, FlutterError>) -> Void) {
+  func echoNullable(_ aMapArg: [String?: Any?]?, completion: @escaping (Result<[String?: Any?]?, FlutterError>) -> Void)  {
     let channel = FlutterBasicMessageChannel(name: "dev.flutter.pigeon.pigeon_integration_tests.FlutterIntegrationCoreApi.echoNullableMap", binaryMessenger: binaryMessenger, codec: codec)
     channel.sendMessage([aMapArg] as [Any?]) { response in
       guard let listResponse = response as? [Any?] else {
@@ -2147,7 +2201,7 @@ class FlutterIntegrationCoreApi {
     }
   }
   /// Returns the passed enum to test serialization and deserialization.
-  func echoNullable(_ anEnumArg: AnEnum?, completion: @escaping (Result<AnEnum?, FlutterError>) -> Void) {
+  func echoNullable(_ anEnumArg: AnEnum?, completion: @escaping (Result<AnEnum?, FlutterError>) -> Void)  {
     let channel = FlutterBasicMessageChannel(name: "dev.flutter.pigeon.pigeon_integration_tests.FlutterIntegrationCoreApi.echoNullableEnum", binaryMessenger: binaryMessenger, codec: codec)
     channel.sendMessage([anEnumArg?.rawValue] as [Any?]) { response in
       guard let listResponse = response as? [Any?] else {
@@ -2167,14 +2221,14 @@ class FlutterIntegrationCoreApi {
   }
   /// A no-op function taking no arguments and returning no value, to sanity
   /// test basic asynchronous calling.
-  func noopAsync(completion: @escaping (Result<Void, FlutterError>) -> Void) {
+  func noopAsync(completion: @escaping (Result<Void, FlutterError>) -> Void)  {
     let channel = FlutterBasicMessageChannel(name: "dev.flutter.pigeon.pigeon_integration_tests.FlutterIntegrationCoreApi.noopAsync", binaryMessenger: binaryMessenger, codec: codec)
     channel.sendMessage(nil) { _ in
       completion(.success(Void()))
     }
   }
   /// Returns the passed in generic Object asynchronously.
-  func echoAsync(_ aStringArg: String, completion: @escaping (Result<String, FlutterError>) -> Void) {
+  func echoAsync(_ aStringArg: String, completion: @escaping (Result<String, FlutterError>) -> Void)  {
     let channel = FlutterBasicMessageChannel(name: "dev.flutter.pigeon.pigeon_integration_tests.FlutterIntegrationCoreApi.echoAsyncString", binaryMessenger: binaryMessenger, codec: codec)
     channel.sendMessage([aStringArg] as [Any?]) { response in
       guard let listResponse = response as? [Any?] else {
@@ -2307,8 +2361,12 @@ class FlutterSmallApiCodec: FlutterStandardMessageCodec {
 
 /// A simple API called in some unit tests.
 ///
-/// Generated class from Pigeon that represents Flutter messages that can be called from Swift.
-class FlutterSmallApi {
+/// Generated protocol from Pigeon that represents Flutter messages that can be called from Swift.
+protocol FlutterSmallApiProtocol {
+  func echo(_ msgArg: TestMessage, completion: @escaping (Result<TestMessage, FlutterError>) -> Void) 
+  func echo(_ aStringArg: String, completion: @escaping (Result<String, FlutterError>) -> Void) 
+}
+class FlutterSmallApi: FlutterSmallApiProtocol {
   private let binaryMessenger: FlutterBinaryMessenger
   init(binaryMessenger: FlutterBinaryMessenger){
     self.binaryMessenger = binaryMessenger
@@ -2316,7 +2374,7 @@ class FlutterSmallApi {
   var codec: FlutterStandardMessageCodec {
     return FlutterSmallApiCodec.shared
   }
-  func echo(_ msgArg: TestMessage, completion: @escaping (Result<TestMessage, FlutterError>) -> Void) {
+  func echo(_ msgArg: TestMessage, completion: @escaping (Result<TestMessage, FlutterError>) -> Void)  {
     let channel = FlutterBasicMessageChannel(name: "dev.flutter.pigeon.pigeon_integration_tests.FlutterSmallApi.echoWrappedList", binaryMessenger: binaryMessenger, codec: codec)
     channel.sendMessage([msgArg] as [Any?]) { response in
       guard let listResponse = response as? [Any?] else {
@@ -2336,4 +2394,24 @@ class FlutterSmallApi {
       }
     }
   }
+  func echo(_ aStringArg: String, completion: @escaping (Result<String, FlutterError>) -> Void)  {
+    let channel = FlutterBasicMessageChannel(name: "dev.flutter.pigeon.pigeon_integration_tests.FlutterSmallApi.echoString", binaryMessenger: binaryMessenger, codec: codec)
+    channel.sendMessage([aStringArg] as [Any?]) { response in
+      guard let listResponse = response as? [Any?] else {
+        completion(.failure(FlutterError(code: "channel-error", message: "Unable to establish connection on channel.", details: "")))
+        return
+      }
+      if (listResponse.count > 1) {
+        let code: String = listResponse[0] as! String
+        let message: String? = nilOrValue(listResponse[1])
+        let details: String? = nilOrValue(listResponse[2])
+        completion(.failure(FlutterError(code: code, message: message, details: details)));
+      } else if (listResponse[0] == nil) {
+        completion(.failure(FlutterError(code: "null-error", message: "Flutter api returned null value for non-null return value.", details: "")))
+      } else {
+        let result = listResponse[0] as! String
+        completion(.success(result))
+      }
+    }
+  }
 }
diff --git a/packages/pigeon/platform_tests/test_plugin/macos/Classes/CoreTests.gen.swift b/packages/pigeon/platform_tests/test_plugin/macos/Classes/CoreTests.gen.swift
index 601136c458d..8b83e3d1771 100644
--- a/packages/pigeon/platform_tests/test_plugin/macos/Classes/CoreTests.gen.swift
+++ b/packages/pigeon/platform_tests/test_plugin/macos/Classes/CoreTests.gen.swift
@@ -1739,8 +1739,62 @@ class FlutterIntegrationCoreApiCodec: FlutterStandardMessageCodec {
 /// The core interface that the Dart platform_test code implements for host
 /// integration tests to call into.
 ///
-/// Generated class from Pigeon that represents Flutter messages that can be called from Swift.
-class FlutterIntegrationCoreApi {
+/// Generated protocol from Pigeon that represents Flutter messages that can be called from Swift.
+protocol FlutterIntegrationCoreApiProtocol {
+  /// A no-op function taking no arguments and returning no value, to sanity
+  /// test basic calling.
+  func noop(completion: @escaping (Result<Void, FlutterError>) -> Void) 
+  /// Responds with an error from an async function returning a value.
+  func throwError(completion: @escaping (Result<Any?, FlutterError>) -> Void) 
+  /// Responds with an error from an async void function.
+  func throwErrorFromVoid(completion: @escaping (Result<Void, FlutterError>) -> Void) 
+  /// Returns the passed object, to test serialization and deserialization.
+  func echo(_ everythingArg: AllTypes, completion: @escaping (Result<AllTypes, FlutterError>) -> Void) 
+  /// Returns the passed object, to test serialization and deserialization.
+  func echoNullable(_ everythingArg: AllNullableTypes?, completion: @escaping (Result<AllNullableTypes?, FlutterError>) -> Void) 
+  /// Returns passed in arguments of multiple types.
+  ///
+  /// Tests multiple-arity FlutterApi handling.
+  func sendMultipleNullableTypes(aBool aNullableBoolArg: Bool?, anInt aNullableIntArg: Int64?, aString aNullableStringArg: String?, completion: @escaping (Result<AllNullableTypes, FlutterError>) -> Void) 
+  /// Returns the passed boolean, to test serialization and deserialization.
+  func echo(_ aBoolArg: Bool, completion: @escaping (Result<Bool, FlutterError>) -> Void) 
+  /// Returns the passed int, to test serialization and deserialization.
+  func echo(_ anIntArg: Int64, completion: @escaping (Result<Int64, FlutterError>) -> Void) 
+  /// Returns the passed double, to test serialization and deserialization.
+  func echo(_ aDoubleArg: Double, completion: @escaping (Result<Double, FlutterError>) -> Void) 
+  /// Returns the passed string, to test serialization and deserialization.
+  func echo(_ aStringArg: String, completion: @escaping (Result<String, FlutterError>) -> Void) 
+  /// Returns the passed byte list, to test serialization and deserialization.
+  func echo(_ aListArg: FlutterStandardTypedData, completion: @escaping (Result<FlutterStandardTypedData, FlutterError>) -> Void) 
+  /// Returns the passed list, to test serialization and deserialization.
+  func echo(_ aListArg: [Any?], completion: @escaping (Result<[Any?], FlutterError>) -> Void) 
+  /// Returns the passed map, to test serialization and deserialization.
+  func echo(_ aMapArg: [String?: Any?], completion: @escaping (Result<[String?: Any?], FlutterError>) -> Void) 
+  /// Returns the passed enum to test serialization and deserialization.
+  func echo(_ anEnumArg: AnEnum, completion: @escaping (Result<AnEnum, FlutterError>) -> Void) 
+  /// Returns the passed boolean, to test serialization and deserialization.
+  func echoNullable(_ aBoolArg: Bool?, completion: @escaping (Result<Bool?, FlutterError>) -> Void) 
+  /// Returns the passed int, to test serialization and deserialization.
+  func echoNullable(_ anIntArg: Int64?, completion: @escaping (Result<Int64?, FlutterError>) -> Void) 
+  /// Returns the passed double, to test serialization and deserialization.
+  func echoNullable(_ aDoubleArg: Double?, completion: @escaping (Result<Double?, FlutterError>) -> Void) 
+  /// Returns the passed string, to test serialization and deserialization.
+  func echoNullable(_ aStringArg: String?, completion: @escaping (Result<String?, FlutterError>) -> Void) 
+  /// Returns the passed byte list, to test serialization and deserialization.
+  func echoNullable(_ aListArg: FlutterStandardTypedData?, completion: @escaping (Result<FlutterStandardTypedData?, FlutterError>) -> Void) 
+  /// Returns the passed list, to test serialization and deserialization.
+  func echoNullable(_ aListArg: [Any?]?, completion: @escaping (Result<[Any?]?, FlutterError>) -> Void) 
+  /// Returns the passed map, to test serialization and deserialization.
+  func echoNullable(_ aMapArg: [String?: Any?]?, completion: @escaping (Result<[String?: Any?]?, FlutterError>) -> Void) 
+  /// Returns the passed enum to test serialization and deserialization.
+  func echoNullable(_ anEnumArg: AnEnum?, completion: @escaping (Result<AnEnum?, FlutterError>) -> Void) 
+  /// A no-op function taking no arguments and returning no value, to sanity
+  /// test basic asynchronous calling.
+  func noopAsync(completion: @escaping (Result<Void, FlutterError>) -> Void) 
+  /// Returns the passed in generic Object asynchronously.
+  func echoAsync(_ aStringArg: String, completion: @escaping (Result<String, FlutterError>) -> Void) 
+}
+class FlutterIntegrationCoreApi: FlutterIntegrationCoreApiProtocol {
   private let binaryMessenger: FlutterBinaryMessenger
   init(binaryMessenger: FlutterBinaryMessenger){
     self.binaryMessenger = binaryMessenger
@@ -1750,14 +1804,14 @@ class FlutterIntegrationCoreApi {
   }
   /// A no-op function taking no arguments and returning no value, to sanity
   /// test basic calling.
-  func noop(completion: @escaping (Result<Void, FlutterError>) -> Void) {
+  func noop(completion: @escaping (Result<Void, FlutterError>) -> Void)  {
     let channel = FlutterBasicMessageChannel(name: "dev.flutter.pigeon.pigeon_integration_tests.FlutterIntegrationCoreApi.noop", binaryMessenger: binaryMessenger, codec: codec)
     channel.sendMessage(nil) { _ in
       completion(.success(Void()))
     }
   }
   /// Responds with an error from an async function returning a value.
-  func throwError(completion: @escaping (Result<Any?, FlutterError>) -> Void) {
+  func throwError(completion: @escaping (Result<Any?, FlutterError>) -> Void)  {
     let channel = FlutterBasicMessageChannel(name: "dev.flutter.pigeon.pigeon_integration_tests.FlutterIntegrationCoreApi.throwError", binaryMessenger: binaryMessenger, codec: codec)
     channel.sendMessage(nil) { response in
       guard let listResponse = response as? [Any?] else {
@@ -1776,14 +1830,14 @@ class FlutterIntegrationCoreApi {
     }
   }
   /// Responds with an error from an async void function.
-  func throwErrorFromVoid(completion: @escaping (Result<Void, FlutterError>) -> Void) {
+  func throwErrorFromVoid(completion: @escaping (Result<Void, FlutterError>) -> Void)  {
     let channel = FlutterBasicMessageChannel(name: "dev.flutter.pigeon.pigeon_integration_tests.FlutterIntegrationCoreApi.throwErrorFromVoid", binaryMessenger: binaryMessenger, codec: codec)
     channel.sendMessage(nil) { _ in
       completion(.success(Void()))
     }
   }
   /// Returns the passed object, to test serialization and deserialization.
-  func echo(_ everythingArg: AllTypes, completion: @escaping (Result<AllTypes, FlutterError>) -> Void) {
+  func echo(_ everythingArg: AllTypes, completion: @escaping (Result<AllTypes, FlutterError>) -> Void)  {
     let channel = FlutterBasicMessageChannel(name: "dev.flutter.pigeon.pigeon_integration_tests.FlutterIntegrationCoreApi.echoAllTypes", binaryMessenger: binaryMessenger, codec: codec)
     channel.sendMessage([everythingArg] as [Any?]) { response in
       guard let listResponse = response as? [Any?] else {
@@ -1804,7 +1858,7 @@ class FlutterIntegrationCoreApi {
     }
   }
   /// Returns the passed object, to test serialization and deserialization.
-  func echoNullable(_ everythingArg: AllNullableTypes?, completion: @escaping (Result<AllNullableTypes?, FlutterError>) -> Void) {
+  func echoNullable(_ everythingArg: AllNullableTypes?, completion: @escaping (Result<AllNullableTypes?, FlutterError>) -> Void)  {
     let channel = FlutterBasicMessageChannel(name: "dev.flutter.pigeon.pigeon_integration_tests.FlutterIntegrationCoreApi.echoAllNullableTypes", binaryMessenger: binaryMessenger, codec: codec)
     channel.sendMessage([everythingArg] as [Any?]) { response in
       guard let listResponse = response as? [Any?] else {
@@ -1825,7 +1879,7 @@ class FlutterIntegrationCoreApi {
   /// Returns passed in arguments of multiple types.
   ///
   /// Tests multiple-arity FlutterApi handling.
-  func sendMultipleNullableTypes(aBool aNullableBoolArg: Bool?, anInt aNullableIntArg: Int64?, aString aNullableStringArg: String?, completion: @escaping (Result<AllNullableTypes, FlutterError>) -> Void) {
+  func sendMultipleNullableTypes(aBool aNullableBoolArg: Bool?, anInt aNullableIntArg: Int64?, aString aNullableStringArg: String?, completion: @escaping (Result<AllNullableTypes, FlutterError>) -> Void)  {
     let channel = FlutterBasicMessageChannel(name: "dev.flutter.pigeon.pigeon_integration_tests.FlutterIntegrationCoreApi.sendMultipleNullableTypes", binaryMessenger: binaryMessenger, codec: codec)
     channel.sendMessage([aNullableBoolArg, aNullableIntArg, aNullableStringArg] as [Any?]) { response in
       guard let listResponse = response as? [Any?] else {
@@ -1846,7 +1900,7 @@ class FlutterIntegrationCoreApi {
     }
   }
   /// Returns the passed boolean, to test serialization and deserialization.
-  func echo(_ aBoolArg: Bool, completion: @escaping (Result<Bool, FlutterError>) -> Void) {
+  func echo(_ aBoolArg: Bool, completion: @escaping (Result<Bool, FlutterError>) -> Void)  {
     let channel = FlutterBasicMessageChannel(name: "dev.flutter.pigeon.pigeon_integration_tests.FlutterIntegrationCoreApi.echoBool", binaryMessenger: binaryMessenger, codec: codec)
     channel.sendMessage([aBoolArg] as [Any?]) { response in
       guard let listResponse = response as? [Any?] else {
@@ -1867,7 +1921,7 @@ class FlutterIntegrationCoreApi {
     }
   }
   /// Returns the passed int, to test serialization and deserialization.
-  func echo(_ anIntArg: Int64, completion: @escaping (Result<Int64, FlutterError>) -> Void) {
+  func echo(_ anIntArg: Int64, completion: @escaping (Result<Int64, FlutterError>) -> Void)  {
     let channel = FlutterBasicMessageChannel(name: "dev.flutter.pigeon.pigeon_integration_tests.FlutterIntegrationCoreApi.echoInt", binaryMessenger: binaryMessenger, codec: codec)
     channel.sendMessage([anIntArg] as [Any?]) { response in
       guard let listResponse = response as? [Any?] else {
@@ -1888,7 +1942,7 @@ class FlutterIntegrationCoreApi {
     }
   }
   /// Returns the passed double, to test serialization and deserialization.
-  func echo(_ aDoubleArg: Double, completion: @escaping (Result<Double, FlutterError>) -> Void) {
+  func echo(_ aDoubleArg: Double, completion: @escaping (Result<Double, FlutterError>) -> Void)  {
     let channel = FlutterBasicMessageChannel(name: "dev.flutter.pigeon.pigeon_integration_tests.FlutterIntegrationCoreApi.echoDouble", binaryMessenger: binaryMessenger, codec: codec)
     channel.sendMessage([aDoubleArg] as [Any?]) { response in
       guard let listResponse = response as? [Any?] else {
@@ -1909,7 +1963,7 @@ class FlutterIntegrationCoreApi {
     }
   }
   /// Returns the passed string, to test serialization and deserialization.
-  func echo(_ aStringArg: String, completion: @escaping (Result<String, FlutterError>) -> Void) {
+  func echo(_ aStringArg: String, completion: @escaping (Result<String, FlutterError>) -> Void)  {
     let channel = FlutterBasicMessageChannel(name: "dev.flutter.pigeon.pigeon_integration_tests.FlutterIntegrationCoreApi.echoString", binaryMessenger: binaryMessenger, codec: codec)
     channel.sendMessage([aStringArg] as [Any?]) { response in
       guard let listResponse = response as? [Any?] else {
@@ -1930,7 +1984,7 @@ class FlutterIntegrationCoreApi {
     }
   }
   /// Returns the passed byte list, to test serialization and deserialization.
-  func echo(_ aListArg: FlutterStandardTypedData, completion: @escaping (Result<FlutterStandardTypedData, FlutterError>) -> Void) {
+  func echo(_ aListArg: FlutterStandardTypedData, completion: @escaping (Result<FlutterStandardTypedData, FlutterError>) -> Void)  {
     let channel = FlutterBasicMessageChannel(name: "dev.flutter.pigeon.pigeon_integration_tests.FlutterIntegrationCoreApi.echoUint8List", binaryMessenger: binaryMessenger, codec: codec)
     channel.sendMessage([aListArg] as [Any?]) { response in
       guard let listResponse = response as? [Any?] else {
@@ -1951,7 +2005,7 @@ class FlutterIntegrationCoreApi {
     }
   }
   /// Returns the passed list, to test serialization and deserialization.
-  func echo(_ aListArg: [Any?], completion: @escaping (Result<[Any?], FlutterError>) -> Void) {
+  func echo(_ aListArg: [Any?], completion: @escaping (Result<[Any?], FlutterError>) -> Void)  {
     let channel = FlutterBasicMessageChannel(name: "dev.flutter.pigeon.pigeon_integration_tests.FlutterIntegrationCoreApi.echoList", binaryMessenger: binaryMessenger, codec: codec)
     channel.sendMessage([aListArg] as [Any?]) { response in
       guard let listResponse = response as? [Any?] else {
@@ -1972,7 +2026,7 @@ class FlutterIntegrationCoreApi {
     }
   }
   /// Returns the passed map, to test serialization and deserialization.
-  func echo(_ aMapArg: [String?: Any?], completion: @escaping (Result<[String?: Any?], FlutterError>) -> Void) {
+  func echo(_ aMapArg: [String?: Any?], completion: @escaping (Result<[String?: Any?], FlutterError>) -> Void)  {
     let channel = FlutterBasicMessageChannel(name: "dev.flutter.pigeon.pigeon_integration_tests.FlutterIntegrationCoreApi.echoMap", binaryMessenger: binaryMessenger, codec: codec)
     channel.sendMessage([aMapArg] as [Any?]) { response in
       guard let listResponse = response as? [Any?] else {
@@ -1993,7 +2047,7 @@ class FlutterIntegrationCoreApi {
     }
   }
   /// Returns the passed enum to test serialization and deserialization.
-  func echo(_ anEnumArg: AnEnum, completion: @escaping (Result<AnEnum, FlutterError>) -> Void) {
+  func echo(_ anEnumArg: AnEnum, completion: @escaping (Result<AnEnum, FlutterError>) -> Void)  {
     let channel = FlutterBasicMessageChannel(name: "dev.flutter.pigeon.pigeon_integration_tests.FlutterIntegrationCoreApi.echoEnum", binaryMessenger: binaryMessenger, codec: codec)
     channel.sendMessage([anEnumArg.rawValue] as [Any?]) { response in
       guard let listResponse = response as? [Any?] else {
@@ -2014,7 +2068,7 @@ class FlutterIntegrationCoreApi {
     }
   }
   /// Returns the passed boolean, to test serialization and deserialization.
-  func echoNullable(_ aBoolArg: Bool?, completion: @escaping (Result<Bool?, FlutterError>) -> Void) {
+  func echoNullable(_ aBoolArg: Bool?, completion: @escaping (Result<Bool?, FlutterError>) -> Void)  {
     let channel = FlutterBasicMessageChannel(name: "dev.flutter.pigeon.pigeon_integration_tests.FlutterIntegrationCoreApi.echoNullableBool", binaryMessenger: binaryMessenger, codec: codec)
     channel.sendMessage([aBoolArg] as [Any?]) { response in
       guard let listResponse = response as? [Any?] else {
@@ -2033,7 +2087,7 @@ class FlutterIntegrationCoreApi {
     }
   }
   /// Returns the passed int, to test serialization and deserialization.
-  func echoNullable(_ anIntArg: Int64?, completion: @escaping (Result<Int64?, FlutterError>) -> Void) {
+  func echoNullable(_ anIntArg: Int64?, completion: @escaping (Result<Int64?, FlutterError>) -> Void)  {
     let channel = FlutterBasicMessageChannel(name: "dev.flutter.pigeon.pigeon_integration_tests.FlutterIntegrationCoreApi.echoNullableInt", binaryMessenger: binaryMessenger, codec: codec)
     channel.sendMessage([anIntArg] as [Any?]) { response in
       guard let listResponse = response as? [Any?] else {
@@ -2052,7 +2106,7 @@ class FlutterIntegrationCoreApi {
     }
   }
   /// Returns the passed double, to test serialization and deserialization.
-  func echoNullable(_ aDoubleArg: Double?, completion: @escaping (Result<Double?, FlutterError>) -> Void) {
+  func echoNullable(_ aDoubleArg: Double?, completion: @escaping (Result<Double?, FlutterError>) -> Void)  {
     let channel = FlutterBasicMessageChannel(name: "dev.flutter.pigeon.pigeon_integration_tests.FlutterIntegrationCoreApi.echoNullableDouble", binaryMessenger: binaryMessenger, codec: codec)
     channel.sendMessage([aDoubleArg] as [Any?]) { response in
       guard let listResponse = response as? [Any?] else {
@@ -2071,7 +2125,7 @@ class FlutterIntegrationCoreApi {
     }
   }
   /// Returns the passed string, to test serialization and deserialization.
-  func echoNullable(_ aStringArg: String?, completion: @escaping (Result<String?, FlutterError>) -> Void) {
+  func echoNullable(_ aStringArg: String?, completion: @escaping (Result<String?, FlutterError>) -> Void)  {
     let channel = FlutterBasicMessageChannel(name: "dev.flutter.pigeon.pigeon_integration_tests.FlutterIntegrationCoreApi.echoNullableString", binaryMessenger: binaryMessenger, codec: codec)
     channel.sendMessage([aStringArg] as [Any?]) { response in
       guard let listResponse = response as? [Any?] else {
@@ -2090,7 +2144,7 @@ class FlutterIntegrationCoreApi {
     }
   }
   /// Returns the passed byte list, to test serialization and deserialization.
-  func echoNullable(_ aListArg: FlutterStandardTypedData?, completion: @escaping (Result<FlutterStandardTypedData?, FlutterError>) -> Void) {
+  func echoNullable(_ aListArg: FlutterStandardTypedData?, completion: @escaping (Result<FlutterStandardTypedData?, FlutterError>) -> Void)  {
     let channel = FlutterBasicMessageChannel(name: "dev.flutter.pigeon.pigeon_integration_tests.FlutterIntegrationCoreApi.echoNullableUint8List", binaryMessenger: binaryMessenger, codec: codec)
     channel.sendMessage([aListArg] as [Any?]) { response in
       guard let listResponse = response as? [Any?] else {
@@ -2109,7 +2163,7 @@ class FlutterIntegrationCoreApi {
     }
   }
   /// Returns the passed list, to test serialization and deserialization.
-  func echoNullable(_ aListArg: [Any?]?, completion: @escaping (Result<[Any?]?, FlutterError>) -> Void) {
+  func echoNullable(_ aListArg: [Any?]?, completion: @escaping (Result<[Any?]?, FlutterError>) -> Void)  {
     let channel = FlutterBasicMessageChannel(name: "dev.flutter.pigeon.pigeon_integration_tests.FlutterIntegrationCoreApi.echoNullableList", binaryMessenger: binaryMessenger, codec: codec)
     channel.sendMessage([aListArg] as [Any?]) { response in
       guard let listResponse = response as? [Any?] else {
@@ -2128,7 +2182,7 @@ class FlutterIntegrationCoreApi {
     }
   }
   /// Returns the passed map, to test serialization and deserialization.
-  func echoNullable(_ aMapArg: [String?: Any?]?, completion: @escaping (Result<[String?: Any?]?, FlutterError>) -> Void) {
+  func echoNullable(_ aMapArg: [String?: Any?]?, completion: @escaping (Result<[String?: Any?]?, FlutterError>) -> Void)  {
     let channel = FlutterBasicMessageChannel(name: "dev.flutter.pigeon.pigeon_integration_tests.FlutterIntegrationCoreApi.echoNullableMap", binaryMessenger: binaryMessenger, codec: codec)
     channel.sendMessage([aMapArg] as [Any?]) { response in
       guard let listResponse = response as? [Any?] else {
@@ -2147,7 +2201,7 @@ class FlutterIntegrationCoreApi {
     }
   }
   /// Returns the passed enum to test serialization and deserialization.
-  func echoNullable(_ anEnumArg: AnEnum?, completion: @escaping (Result<AnEnum?, FlutterError>) -> Void) {
+  func echoNullable(_ anEnumArg: AnEnum?, completion: @escaping (Result<AnEnum?, FlutterError>) -> Void)  {
     let channel = FlutterBasicMessageChannel(name: "dev.flutter.pigeon.pigeon_integration_tests.FlutterIntegrationCoreApi.echoNullableEnum", binaryMessenger: binaryMessenger, codec: codec)
     channel.sendMessage([anEnumArg?.rawValue] as [Any?]) { response in
       guard let listResponse = response as? [Any?] else {
@@ -2167,14 +2221,14 @@ class FlutterIntegrationCoreApi {
   }
   /// A no-op function taking no arguments and returning no value, to sanity
   /// test basic asynchronous calling.
-  func noopAsync(completion: @escaping (Result<Void, FlutterError>) -> Void) {
+  func noopAsync(completion: @escaping (Result<Void, FlutterError>) -> Void)  {
     let channel = FlutterBasicMessageChannel(name: "dev.flutter.pigeon.pigeon_integration_tests.FlutterIntegrationCoreApi.noopAsync", binaryMessenger: binaryMessenger, codec: codec)
     channel.sendMessage(nil) { _ in
       completion(.success(Void()))
     }
   }
   /// Returns the passed in generic Object asynchronously.
-  func echoAsync(_ aStringArg: String, completion: @escaping (Result<String, FlutterError>) -> Void) {
+  func echoAsync(_ aStringArg: String, completion: @escaping (Result<String, FlutterError>) -> Void)  {
     let channel = FlutterBasicMessageChannel(name: "dev.flutter.pigeon.pigeon_integration_tests.FlutterIntegrationCoreApi.echoAsyncString", binaryMessenger: binaryMessenger, codec: codec)
     channel.sendMessage([aStringArg] as [Any?]) { response in
       guard let listResponse = response as? [Any?] else {
@@ -2307,8 +2361,12 @@ class FlutterSmallApiCodec: FlutterStandardMessageCodec {
 
 /// A simple API called in some unit tests.
 ///
-/// Generated class from Pigeon that represents Flutter messages that can be called from Swift.
-class FlutterSmallApi {
+/// Generated protocol from Pigeon that represents Flutter messages that can be called from Swift.
+protocol FlutterSmallApiProtocol {
+  func echo(_ msgArg: TestMessage, completion: @escaping (Result<TestMessage, FlutterError>) -> Void) 
+  func echo(_ aStringArg: String, completion: @escaping (Result<String, FlutterError>) -> Void) 
+}
+class FlutterSmallApi: FlutterSmallApiProtocol {
   private let binaryMessenger: FlutterBinaryMessenger
   init(binaryMessenger: FlutterBinaryMessenger){
     self.binaryMessenger = binaryMessenger
@@ -2316,7 +2374,7 @@ class FlutterSmallApi {
   var codec: FlutterStandardMessageCodec {
     return FlutterSmallApiCodec.shared
   }
-  func echo(_ msgArg: TestMessage, completion: @escaping (Result<TestMessage, FlutterError>) -> Void) {
+  func echo(_ msgArg: TestMessage, completion: @escaping (Result<TestMessage, FlutterError>) -> Void)  {
     let channel = FlutterBasicMessageChannel(name: "dev.flutter.pigeon.pigeon_integration_tests.FlutterSmallApi.echoWrappedList", binaryMessenger: binaryMessenger, codec: codec)
     channel.sendMessage([msgArg] as [Any?]) { response in
       guard let listResponse = response as? [Any?] else {
@@ -2336,4 +2394,24 @@ class FlutterSmallApi {
       }
     }
   }
+  func echo(_ aStringArg: String, completion: @escaping (Result<String, FlutterError>) -> Void)  {
+    let channel = FlutterBasicMessageChannel(name: "dev.flutter.pigeon.pigeon_integration_tests.FlutterSmallApi.echoString", binaryMessenger: binaryMessenger, codec: codec)
+    channel.sendMessage([aStringArg] as [Any?]) { response in
+      guard let listResponse = response as? [Any?] else {
+        completion(.failure(FlutterError(code: "channel-error", message: "Unable to establish connection on channel.", details: "")))
+        return
+      }
+      if (listResponse.count > 1) {
+        let code: String = listResponse[0] as! String
+        let message: String? = nilOrValue(listResponse[1])
+        let details: String? = nilOrValue(listResponse[2])
+        completion(.failure(FlutterError(code: code, message: message, details: details)));
+      } else if (listResponse[0] == nil) {
+        completion(.failure(FlutterError(code: "null-error", message: "Flutter api returned null value for non-null return value.", details: "")))
+      } else {
+        let result = listResponse[0] as! String
+        completion(.success(result))
+      }
+    }
+  }
 }
diff --git a/packages/pigeon/platform_tests/test_plugin/windows/pigeon/core_tests.gen.cpp b/packages/pigeon/platform_tests/test_plugin/windows/pigeon/core_tests.gen.cpp
index f4f557a664f..445911f7b30 100644
--- a/packages/pigeon/platform_tests/test_plugin/windows/pigeon/core_tests.gen.cpp
+++ b/packages/pigeon/platform_tests/test_plugin/windows/pigeon/core_tests.gen.cpp
@@ -4749,4 +4749,42 @@ void FlutterSmallApi::EchoWrappedList(
   });
 }
 
+void FlutterSmallApi::EchoString(
+    const std::string& a_string_arg,
+    std::function<void(const std::string&)>&& on_success,
+    std::function<void(const FlutterError&)>&& on_error) {
+  auto channel = std::make_unique<BasicMessageChannel<>>(
+      binary_messenger_,
+      "dev.flutter.pigeon.pigeon_integration_tests.FlutterSmallApi.echoString",
+      &GetCodec());
+  EncodableValue encoded_api_arguments = EncodableValue(EncodableList{
+      EncodableValue(a_string_arg),
+  });
+  channel->Send(encoded_api_arguments, [on_success = std::move(on_success),
+                                        on_error = std::move(on_error)](
+                                           const uint8_t* reply,
+                                           size_t reply_size) {
+    std::unique_ptr<EncodableValue> response =
+        GetCodec().DecodeMessage(reply, reply_size);
+    const auto& encodable_return_value = *response;
+    const auto* list_return_value =
+        std::get_if<EncodableList>(&encodable_return_value);
+    if (list_return_value) {
+      if (list_return_value->size() > 1) {
+        on_error(FlutterError(std::get<std::string>(list_return_value->at(0)),
+                              std::get<std::string>(list_return_value->at(1)),
+                              list_return_value->at(2)));
+      } else {
+        const auto& return_value =
+            std::get<std::string>(list_return_value->at(0));
+        on_success(return_value);
+      }
+    } else {
+      on_error(FlutterError("channel-error",
+                            "Unable to establish connection on channel.",
+                            EncodableValue("")));
+    }
+  });
+}
+
 }  // namespace core_tests_pigeontest
diff --git a/packages/pigeon/platform_tests/test_plugin/windows/pigeon/core_tests.gen.h b/packages/pigeon/platform_tests/test_plugin/windows/pigeon/core_tests.gen.h
index 1076ae465c1..b579f9fa87e 100644
--- a/packages/pigeon/platform_tests/test_plugin/windows/pigeon/core_tests.gen.h
+++ b/packages/pigeon/platform_tests/test_plugin/windows/pigeon/core_tests.gen.h
@@ -847,6 +847,9 @@ class FlutterSmallApi {
   void EchoWrappedList(const TestMessage& msg,
                        std::function<void(const TestMessage&)>&& on_success,
                        std::function<void(const FlutterError&)>&& on_error);
+  void EchoString(const std::string& a_string,
+                  std::function<void(const std::string&)>&& on_success,
+                  std::function<void(const FlutterError&)>&& on_error);
 
  private:
   flutter::BinaryMessenger* binary_messenger_;
diff --git a/packages/pigeon/pubspec.yaml b/packages/pigeon/pubspec.yaml
index 98b0fa577a5..867537f4be2 100644
--- a/packages/pigeon/pubspec.yaml
+++ b/packages/pigeon/pubspec.yaml
@@ -2,7 +2,7 @@ name: pigeon
 description: Code generator tool to make communication between Flutter and the host platform type-safe and easier.
 repository: https://github.com/flutter/packages/tree/main/packages/pigeon
 issue_tracker: https://github.com/flutter/flutter/issues?q=is%3Aissue+is%3Aopen+label%3Apigeon
-version: 12.0.0 # This must match the version in lib/generator_tools.dart
+version: 12.0.1 # This must match the version in lib/generator_tools.dart
 
 environment:
   sdk: ">=2.19.0 <4.0.0"