diff --git a/packages/ack/CHANGELOG.md b/packages/ack/CHANGELOG.md
index b7374aaf..c91e42ef 100644
--- a/packages/ack/CHANGELOG.md
+++ b/packages/ack/CHANGELOG.md
@@ -1,5 +1,14 @@
 ## Unreleased
 
+### Breaking
+
+* Preserve programmer `Error` objects and their original stack traces when
+  codec decoders or one-way transforms throw. Nested schemas and `anyOf`
+  no longer disguise those defects as validation failures or try a fallback
+  branch. Recoverable `Exception`s still become `SchemaTransformError`
+  failures. Decoder callbacks should throw an `Exception`, such as
+  `FormatException`, to report invalid input rather than a programmer defect.
+
 ### Added
 
 * `Ack.fromJsonSchema()` now supports `multipleOf` for `number` and `integer`
diff --git a/packages/ack/lib/src/schemas/codec_schema.dart b/packages/ack/lib/src/schemas/codec_schema.dart
index 29e548d1..4a83b31d 100644
--- a/packages/ack/lib/src/schemas/codec_schema.dart
+++ b/packages/ack/lib/src/schemas/codec_schema.dart
@@ -109,6 +109,9 @@ final class CodecSchema<Boundary extends Object, Runtime extends Object>
     try {
       runtime = _decoder(intermediate);
     } catch (e, st) {
+      if (e is Error) {
+        Error.throwWithStackTrace(e, st);
+      }
       return SchemaResult.fail(
         SchemaTransformError(
           message: 'Codec decode failed: ${e.toString()}',
diff --git a/packages/ack/test/codecs/codec_error_contract_test.dart b/packages/ack/test/codecs/codec_error_contract_test.dart
new file mode 100644
index 00000000..8b561ebc
--- /dev/null
+++ b/packages/ack/test/codecs/codec_error_contract_test.dart
@@ -0,0 +1,111 @@
+import 'package:ack/ack.dart';
+import 'package:test/test.dart';
+
+void main() {
+  group('codec decoder error contract', () {
+    test('parse and safeParse propagate the original programmer error', () {
+      final error = StateError('decoder defect');
+      final schema = Ack.string().codec<int>(
+        decode: (_) => throw error,
+        encode: (value) => value.toString(),
+      );
+
+      expect(() => schema.safeParse('7'), throwsA(same(error)));
+      expect(() => schema.parse('7'), throwsA(same(error)));
+    });
+
+    test('nested objects, lists, and maps preserve decoder errors', () {
+      final error = StateError('nested decoder defect');
+      final schema = Ack.string().codec<int>(
+        decode: (_) => throw error,
+        encode: (value) => value.toString(),
+      );
+
+      expect(
+        () => Ack.object({'value': schema}).safeParse({'value': '7'}),
+        throwsA(same(error)),
+      );
+      expect(() => Ack.list(schema).safeParse(['7']), throwsA(same(error)));
+      expect(
+        () => Ack.map(schema).safeParse({'value': '7'}),
+        throwsA(same(error)),
+      );
+    });
+
+    test('one-way transforms preserve programmer errors too', () {
+      final error = StateError('transform defect');
+      final schema = Ack.string().transform<int>((_) => throw error);
+
+      expect(() => schema.safeParse('7'), throwsA(same(error)));
+    });
+
+    test('anyOf does not hide a decoder error with a later branch', () {
+      final error = StateError('first branch defect');
+      var fallbackCalled = false;
+      final broken = Ack.string().codec<int>(
+        decode: (_) => throw error,
+        encode: (value) => value.toString(),
+      );
+      final fallback = Ack.string().refine((_) {
+        fallbackCalled = true;
+        return true;
+      });
+
+      expect(
+        () => Ack.anyOf([broken, fallback]).safeParse('7'),
+        throwsA(same(error)),
+      );
+      expect(fallbackCalled, isFalse);
+    });
+
+    test('recoverable decoder exceptions remain transform failures', () {
+      const exception = FormatException('not an integer');
+      final schema = Ack.string().codec<int>(
+        decode: (_) => throw exception,
+        encode: (value) => value.toString(),
+      );
+
+      final result = schema.safeParse('bad');
+
+      expect(result.isFail, isTrue);
+      expect(
+        result.getError(),
+        isA<SchemaTransformError>().having(
+          (error) => error.cause,
+          'cause',
+          same(exception),
+        ),
+      );
+    });
+
+    test('anyOf can still recover from a decoder exception', () {
+      final schema = Ack.string().codec<int>(
+        decode: (_) => throw const FormatException('not an integer'),
+        encode: (value) => value.toString(),
+      );
+
+      expect(Ack.anyOf([schema, Ack.string()]).parse('text'), 'text');
+    });
+
+    test('decoder errors retain their original stack trace', () {
+      final error = StateError('decoder defect');
+      final originalStack = StackTrace.fromString('original decoder stack');
+      final schema = Ack.string().codec<int>(
+        decode: (_) => Error.throwWithStackTrace(error, originalStack),
+        encode: (value) => value.toString(),
+      );
+      Object? caughtError;
+      StackTrace? caughtStack;
+
+      try {
+        schema.safeParse('7');
+      } catch (error, stackTrace) {
+        caughtError = error;
+        caughtStack = stackTrace;
+      }
+
+      expect(caughtError, same(error));
+      expect(caughtStack.toString(), originalStack.toString());
+    });
+  });
+}
diff --git a/packages/ack/test/schemas/encode_error_propagation_test.dart b/packages/ack/test/schemas/encode_error_propagation_test.dart
index bda62fbe..280a5fca 100644
--- a/packages/ack/test/schemas/encode_error_propagation_test.dart
+++ b/packages/ack/test/schemas/encode_error_propagation_test.dart
@@ -283,17 +283,16 @@ void main() {
     });
   });
 
-  group('decoder policy is unchanged', () {
-    test('a decoder Error still becomes a SchemaTransformError', () {
+  group('decoder policy matches encoder policy', () {
+    test('a decoder Error propagates unchanged', () {
+      final error = StateError('decode boom');
       final schema = Ack.string().codec<int>(
-        decode: (_) => throw StateError('decode boom'),
+        decode: (_) => throw error,
         encode: (value) => value.toString(),
       );
 
-      final result = schema.safeParse('1');
-
-      expect(result.isFail, isTrue);
-      expect(result.getError(), isA<SchemaTransformError>());
+      expect(() => schema.safeParse('1'), throwsA(same(error)));
+      expect(() => schema.parse('1'), throwsA(same(error)));
     });
   });
 }
diff --git a/packages/ack/test/schemas/extensions/transform_extension_test.dart b/packages/ack/test/schemas/extensions/transform_extension_test.dart
index 0371b6d3..926951df 100644
--- a/packages/ack/test/schemas/extensions/transform_extension_test.dart
+++ b/packages/ack/test/schemas/extensions/transform_extension_test.dart
@@ -49,16 +49,14 @@ void main() {
     );
 
     test(
-      'should error when transformer returns null for non-nullable output',
+      'should propagate TypeError when a non-nullable transform returns null',
       () {
         final schema = Ack.string().transform<int>((val) {
           return (null as dynamic);
         });
 
-        final result = schema.safeParse('value');
-
-        expect(result.isFail, isTrue);
-        expect(result.getError(), isA<SchemaTransformError>());
+        expect(() => schema.safeParse('value'), throwsA(isA<TypeError>()));
+        expect(() => schema.parse('value'), throwsA(isA<TypeError>()));
       },
     );
 
diff --git a/packages/ack/test/typed_codecs_characterization_test.dart b/packages/ack/test/typed_codecs_characterization_test.dart
index cf45c85f..61a18792 100644
--- a/packages/ack/test/typed_codecs_characterization_test.dart
+++ b/packages/ack/test/typed_codecs_characterization_test.dart
@@ -281,7 +281,7 @@ void main() {
 
     test('decoder exceptions use codec decode wording', () {
       final transformSchema = Ack.string().transform<int>(
-        (_) => throw StateError('transform decoder failed'),
+        (_) => throw const FormatException('transform decoder failed'),
       );
       final transformResult = transformSchema.safeParse('value');
 
@@ -291,7 +291,7 @@ void main() {
       expect(transformError.message, startsWith('Codec decode failed:'));
 
       final codecSchema = Ack.string().codec<int>(
-        decode: (_) => throw StateError('codec decoder failed'),
+        decode: (_) => throw const FormatException('codec decoder failed'),
         encode: (value) => value.toString(),
       );
       final codecResult = codecSchema.safeParse('value');