diff --git a/packages/ack/lib/src/schemas/object_schema.dart b/packages/ack/lib/src/schemas/object_schema.dart
index 2578826a..ca4cff3f 100644
--- a/packages/ack/lib/src/schemas/object_schema.dart
+++ b/packages/ack/lib/src/schemas/object_schema.dart
@@ -24,10 +24,13 @@ final class ObjectSchema extends AckSchema<JsonMap, JsonMap>
     with FluentSchema<JsonMap, JsonMap, ObjectSchema> {
   final Map<String, AnyAckSchema> properties;
   final bool additionalProperties;
+  @internal
+  final Map<String, Object?> encodeOnlyDefaults;
 
   ObjectSchema(
     Map<String, AnyAckSchema>? properties, {
     this.additionalProperties = false,
+    this.encodeOnlyDefaults = const {},
     super.isNullable,
     super.isOptional,
     super.description,
@@ -184,7 +187,10 @@ final class ObjectSchema extends AckSchema<JsonMap, JsonMap>
       final hasValue = mapValue.containsKey(key);
 
       if (!hasValue) {
-        if (schema.isOptional || (isEncode && schema is DefaultSchema)) {
+        if (schema.isOptional ||
+            (isEncode &&
+                (schema is DefaultSchema ||
+                    encodeOnlyDefaults.containsKey(key)))) {
           continue;
         }
         final propertyCtx = context.createChild(
@@ -315,6 +321,8 @@ final class ObjectSchema extends AckSchema<JsonMap, JsonMap>
           continue;
         }
         propertyValue = defaultResult.getOrNull();
+      } else if (encodeOnlyDefaults.containsKey(key)) {
+        propertyValue = encodeOnlyDefaults[key];
       } else {
         continue;
       }
@@ -362,6 +370,7 @@ final class ObjectSchema extends AckSchema<JsonMap, JsonMap>
   ObjectSchema copyWith({
     Map<String, AnyAckSchema>? properties,
     bool? additionalProperties,
+    Map<String, Object?>? encodeOnlyDefaults,
     bool? isNullable,
     bool? isOptional,
     String? description,
@@ -371,6 +380,7 @@ final class ObjectSchema extends AckSchema<JsonMap, JsonMap>
     return ObjectSchema(
       properties ?? this.properties,
       additionalProperties: additionalProperties ?? this.additionalProperties,
+      encodeOnlyDefaults: encodeOnlyDefaults ?? this.encodeOnlyDefaults,
       isNullable: isNullable ?? this.isNullable,
       isOptional: isOptional ?? this.isOptional,
       description: description ?? this.description,
diff --git a/packages/ack/lib/src/utils/discriminated_branch_utils.dart b/packages/ack/lib/src/utils/discriminated_branch_utils.dart
index 157ee7fe..740f25d7 100644
--- a/packages/ack/lib/src/utils/discriminated_branch_utils.dart
+++ b/packages/ack/lib/src/utils/discriminated_branch_utils.dart
@@ -57,6 +57,7 @@ ObjectSchema effectiveDiscriminatedObjectBranch({
   required String discriminatorKey,
   required String discriminatorValue,
   required ObjectSchema objectSchema,
+  bool synthesizeOnEncode = false,
 }) {
   final existingDiscriminator = objectSchema.properties[discriminatorKey];
   if (existingDiscriminator != null &&
@@ -76,7 +77,15 @@ ObjectSchema effectiveDiscriminatedObjectBranch({
       if (entry.key != discriminatorKey) entry.key: entry.value,
   };
 
-  return objectSchema.copyWith(properties: properties);
+  return objectSchema.copyWith(
+    properties: properties,
+    encodeOnlyDefaults: synthesizeOnEncode
+        ? {
+            ...objectSchema.encodeOnlyDefaults,
+            discriminatorKey: discriminatorValue,
+          }
+        : null,
+  );
 }
 
 /// Builds the effective schema for a discriminated-union branch.
@@ -92,20 +101,26 @@ AnyAckSchema effectiveDiscriminatedBranch({
   required String discriminatorKey,
   required String discriminatorValue,
   required AnyAckSchema branchSchema,
+  bool underCodec = false,
 }) {
   if (branchSchema is ObjectSchema) {
     return effectiveDiscriminatedObjectBranch(
       discriminatorKey: discriminatorKey,
       discriminatorValue: discriminatorValue,
       objectSchema: branchSchema,
+      synthesizeOnEncode: underCodec,
     );
   }
 
   if (branchSchema is WrapperSchema) {
+    // Synthesize only for the branch-root object when a CodecSchema appears
+    // above it on the wrapper spine. This recursion follows `.inner` only, so
+    // nested property objects are unaffected.
     final effectiveInner = effectiveDiscriminatedBranch(
       discriminatorKey: discriminatorKey,
       discriminatorValue: discriminatorValue,
       branchSchema: branchSchema.inner,
+      underCodec: underCodec || branchSchema is CodecSchema,
     );
     return branchSchema.copyWithInner(effectiveInner);
   }
diff --git a/packages/ack/test/schemas/discriminated_object_schema_test.dart b/packages/ack/test/schemas/discriminated_object_schema_test.dart
index 693e1437..5cc64f0a 100644
--- a/packages/ack/test/schemas/discriminated_object_schema_test.dart
+++ b/packages/ack/test/schemas/discriminated_object_schema_test.dart
@@ -3,6 +3,18 @@ import 'package:ack/src/constraints/pattern_constraint.dart';
 import 'package:ack/src/constraints/validators.dart';
 import 'package:test/test.dart';
 
+final class _Cat {
+  const _Cat(this.name);
+
+  final String name;
+}
+
+final class _Dog {
+  const _Dog(this.name);
+
+  final String name;
+}
+
 void main() {
   group('DiscriminatedObjectSchema', () {
     late ObjectSchema catSchema;
@@ -165,6 +177,181 @@ void main() {
         },
       );
 
+      test(
+        'encodes a map-runtime branch codec that omits the discriminator',
+        () {
+          final codecSchema = Ack.discriminated<Map<String, Object?>>(
+            discriminatorKey: 'type',
+            schemas: {
+              'cat': Ack.object({'name': Ack.string()})
+                  .codec<Map<String, Object?>>(
+                    decode: (data) => {'name': data['name']!},
+                    encode: (cat) => {'name': cat['name']},
+                  ),
+            },
+          );
+
+          final result = codecSchema.safeEncode({'name': 'Mittens'});
+
+          expect(result.isOk, isTrue);
+          expect(result.getOrThrow(), {'type': 'cat', 'name': 'Mittens'});
+        },
+      );
+
+      test('round-trips through a map-runtime branch codec that omits the '
+          'discriminator', () {
+        final codecSchema = Ack.discriminated<Map<String, Object?>>(
+          discriminatorKey: 'type',
+          schemas: {
+            'cat': Ack.object({'name': Ack.string()})
+                .codec<Map<String, Object?>>(
+                  decode: (data) => {'name': data['name']!},
+                  encode: (cat) => {'name': cat['name']},
+                ),
+          },
+        );
+
+        final parsed = codecSchema.parse({'type': 'cat', 'name': 'Mittens'});
+        final encoded = codecSchema.safeEncode(parsed);
+
+        expect(encoded.isOk, isTrue);
+        expect(encoded.getOrThrow(), {'type': 'cat', 'name': 'Mittens'});
+      });
+
+      test('encodes passthrough extras once for a branch codec that omits the '
+          'discriminator', () {
+        final codecSchema = Ack.discriminated<Map<String, Object?>>(
+          discriminatorKey: 'type',
+          schemas: {
+            'cat': Ack.object({'name': Ack.string()})
+                .passthrough()
+                .codec<Map<String, Object?>>(
+                  decode: (data) => Map<String, Object?>.from(data),
+                  encode: (cat) => Map<String, Object?>.from(cat),
+                ),
+          },
+        );
+
+        final result = codecSchema.safeEncode({
+          'name': 'Mittens',
+          'color': 'tabby',
+        });
+
+        expect(result.isOk, isTrue);
+        expect(result.getOrThrow(), {
+          'type': 'cat',
+          'name': 'Mittens',
+          'color': 'tabby',
+        });
+      });
+
+      test('encodes the matching typed codec branch when codecs omit the '
+          'discriminator', () {
+        final codecSchema = Ack.discriminated<Object>(
+          discriminatorKey: 'type',
+          schemas: {
+            'cat': Ack.object({'name': Ack.string()}).codec<_Cat>(
+              decode: (data) => _Cat(data['name']! as String),
+              encode: (cat) => {'name': cat.name},
+            ),
+            'dog': Ack.object({'name': Ack.string()}).codec<_Dog>(
+              decode: (data) => _Dog(data['name']! as String),
+              encode: (dog) => {'name': dog.name},
+            ),
+          },
+        );
+
+        final result = codecSchema.safeEncode(const _Dog('Spot'));
+
+        expect(result.isOk, isTrue);
+        expect(result.getOrThrow(), {'type': 'dog', 'name': 'Spot'});
+      });
+
+      test(
+        'nested default and codec wrappers synthesize the discriminator only '
+        'under codecs',
+        () {
+          final defaultCodecSchema = Ack.discriminated<Map<String, Object?>>(
+            discriminatorKey: 'type',
+            schemas: {
+              'cat': Ack.object({'name': Ack.string()})
+                  .codec<Map<String, Object?>>(
+                    decode: (data) => {'name': data['name']!},
+                    encode: (cat) => {'name': cat['name']},
+                  )
+                  .withDefault(const {'name': 'Default'}),
+            },
+          );
+          final codecDefaultSchema = Ack.discriminated<Map<String, Object?>>(
+            discriminatorKey: 'type',
+            schemas: {
+              'cat': Ack.object({'name': Ack.string()})
+                  .withDefault(const {'name': 'Default'})
+                  .codec<Map<String, Object?>>(
+                    decode: (data) => {'name': data['name']!},
+                    encode: (cat) => {'name': cat['name']},
+                  ),
+            },
+          );
+          final defaultOnlySchema = Ack.discriminated<Map<String, Object?>>(
+            discriminatorKey: 'type',
+            schemas: {
+              'cat': Ack.object({
+                'name': Ack.string(),
+              }).withDefault(const {'name': 'Default'}),
+            },
+          );
+
+          final defaultCodecResult = defaultCodecSchema.safeEncode({
+            'name': 'Mittens',
+          });
+          final codecDefaultResult = codecDefaultSchema.safeEncode({
+            'name': 'Mittens',
+          });
+          final defaultOnlyResult = defaultOnlySchema.safeEncode({
+            'name': 'Mittens',
+          });
+
+          expect(defaultCodecResult.isOk, isTrue);
+          expect(defaultCodecResult.getOrThrow(), {
+            'type': 'cat',
+            'name': 'Mittens',
+          });
+          expect(codecDefaultResult.isOk, isTrue);
+          expect(codecDefaultResult.getOrThrow(), {
+            'type': 'cat',
+            'name': 'Mittens',
+          });
+          expect(defaultOnlyResult.isFail, isTrue);
+          final error = defaultOnlyResult.getError() as SchemaConstraintsError;
+          expect(
+            error.constraints.first.constraint,
+            isA<ObjectRequiredPropertiesConstraint>(),
+          );
+        },
+      );
+
+      test(
+        'plain branch still rejects encode input missing the discriminator',
+        () {
+          final plainSchema = Ack.discriminated<Map<String, Object?>>(
+            discriminatorKey: 'type',
+            schemas: {
+              'cat': Ack.object({'name': Ack.string()}),
+            },
+          );
+
+          final result = plainSchema.safeEncode({'name': 'Mittens'});
+
+          expect(result.isFail, isTrue);
+          final error = result.getError() as SchemaConstraintsError;
+          expect(
+            error.constraints.first.constraint,
+            isA<ObjectRequiredPropertiesConstraint>(),
+          );
+        },
+      );
+
       group('Encode error messages (union-owned)', () {
         test(
           'fails with a required-property constraint when type is missing',