diff --git a/packages/ack/CHANGELOG.md b/packages/ack/CHANGELOG.md
index c23f6fd2..69bdfd7a 100644
--- a/packages/ack/CHANGELOG.md
+++ b/packages/ack/CHANGELOG.md
@@ -5,6 +5,9 @@
 * Honor explicit boundary-wrapper nullability consistently during parse and
   encode. Preserve non-null restrictions on wrappers and lazy references when
   exporting nullable primitives, unions, and imported JSON Schema resources.
+* Validate discriminated codec output against its selected map branch or a
+  matching typed runtime branch, including required fields and refinements,
+  without decoding runtime values again or adding encode callback passes.
 * Expose the shared `deepUnmodifiableCopy` helper for generated immutable
   constructors. It snapshots nested collections without validating data or
   changing opaque noncollection objects.
diff --git a/packages/ack/lib/src/schemas/discriminated_object_schema.dart b/packages/ack/lib/src/schemas/discriminated_object_schema.dart
index 862250ec..91cb0aa7 100644
--- a/packages/ack/lib/src/schemas/discriminated_object_schema.dart
+++ b/packages/ack/lib/src/schemas/discriminated_object_schema.dart
@@ -334,6 +334,55 @@ final class DiscriminatedObjectSchema<T extends Object>
     Object? value,
     SchemaContext context,
   ) {
+    final checked = _checkRuntimeType(value, context);
+    if (checked.isFail || value == null) return checked;
+    final runtime = checked.getOrNull()!;
+
+    final mapRuntime = jsonMapOrNull(runtime);
+    if (mapRuntime != null) {
+      final discResult = _validateDiscriminatorKey(mapRuntime, context);
+      if (discResult.isFail) return SchemaResult.fail(discResult.getError());
+      final discValue = discResult.getOrNull()!;
+      final effective = effectiveBranch(discValue);
+      final branchContext = context.createChild(
+        name: 'when $discriminatorKey="$discValue"',
+        schema: effective,
+        value: runtime,
+        pathSegment: '',
+      );
+      final result = effective.validateRuntimeWithContext(
+        runtime,
+        branchContext,
+      );
+      if (result.isFail) return SchemaResult.fail(result.getError());
+      return applyConstraintsAndRefinements(result.getOrNull()!, context);
+    }
+
+    final errors = <SchemaError>[];
+    for (final discValue in schemas.keys) {
+      final effective = effectiveBranch(discValue);
+      final branchContext = context.createChild(
+        name: 'when $discriminatorKey="$discValue"',
+        schema: effective,
+        value: runtime,
+        pathSegment: '',
+      );
+      final result = effective.validateRuntimeWithContext(
+        runtime,
+        branchContext,
+      );
+      if (result.isOk) {
+        return applyConstraintsAndRefinements(result.getOrNull()!, context);
+      }
+      errors.add(result.getError());
+    }
+
+    return SchemaResult.fail(
+      SchemaNestedError(errors: errors, context: context),
+    );
+  }
+
+  SchemaResult<T> _checkRuntimeType(Object? value, SchemaContext context) {
     final nullResult = handleNullInput(value, context);
     if (nullResult != null) return nullResult;
     if (value is! T) {
@@ -346,13 +395,20 @@ final class DiscriminatedObjectSchema<T extends Object>
       );
     }
 
-    return applyConstraintsAndRefinements(value, context);
+    return SchemaResult.ok(value);
   }
 
   @override
   @protected
   SchemaResult<JsonMap> encodeWithContext(T value, SchemaContext context) {
-    final validated = validateRuntimeWithContext(value, context);
+    final checked = _checkRuntimeType(value, context);
+    if (checked.isFail) return SchemaResult.fail(checked.getError());
+    final typed = checked.getOrNull();
+    if (typed == null) return SchemaResult.ok(null);
+
+    // Encoding validates each branch below. Repeating the full runtime
+    // membership check here would execute branch callbacks an extra time.
+    final validated = applyConstraintsAndRefinements(typed, context);
     if (validated.isFail) {
       return SchemaResult.fail(validated.getError());
     }
diff --git a/packages/ack/test/schemas/discriminated_runtime_validation_test.dart b/packages/ack/test/schemas/discriminated_runtime_validation_test.dart
new file mode 100644
index 00000000..6a3b3a36
--- /dev/null
+++ b/packages/ack/test/schemas/discriminated_runtime_validation_test.dart
@@ -0,0 +1,229 @@
+import 'package:ack/ack.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;
+}
+
+enum _Role { member }
+
+CodecSchema<String, T> _codecWithOutput<T extends Object>(
+  AckSchema<dynamic, T> output,
+  T decoded,
+) => Ack.codec<String, String, T>(
+  input: Ack.string(),
+  decode: (_) => decoded,
+  encode: (_) => 'wire',
+  output: output,
+);
+
+Iterable<String> _errorPaths(SchemaError error) sync* {
+  yield error.path;
+  if (error is SchemaNestedError) {
+    for (final child in error.errors) {
+      yield* _errorPaths(child);
+    }
+  }
+}
+
+void main() {
+  group('discriminated codec output validation', () {
+    final output = Ack.discriminated<JsonMap>(
+      discriminatorKey: 'kind',
+      schemas: {
+        'a': Ack.object({'count': Ack.integer()}),
+      },
+    );
+    final invalidCases = <({String name, JsonMap value, String errorPath})>[
+      (
+        name: 'wrong property runtime type',
+        value: {'kind': 'a', 'count': 'bad'},
+        errorPath: '#/count',
+      ),
+      (
+        name: 'unknown discriminator',
+        value: {'kind': 'unknown', 'count': 1},
+        errorPath: '#/kind',
+      ),
+      (name: 'missing discriminator', value: {'count': 1}, errorPath: '#/kind'),
+      (
+        name: 'missing required property',
+        value: {'kind': 'a'},
+        errorPath: '#/count',
+      ),
+    ];
+    for (final testCase in invalidCases) {
+      test('rejects ${testCase.name} on decode and before encode', () {
+        var encodeCalls = 0;
+        final codec = Ack.codec<String, String, JsonMap>(
+          input: Ack.string(),
+          decode: (_) => testCase.value,
+          encode: (_) {
+            encodeCalls++;
+            return 'wire';
+          },
+          output: output,
+        );
+        for (final result in [
+          codec.safeParse('wire'),
+          codec.safeEncode(testCase.value),
+        ]) {
+          expect(result.isFail, isTrue);
+          expect(_errorPaths(result.getError()), contains(testCase.errorPath));
+        }
+        expect(encodeCalls, 0);
+      });
+    }
+
+    test('validates already-decoded nested codec and enum runtimes', () {
+      final output = Ack.discriminated<JsonMap>(
+        discriminatorKey: 'kind',
+        schemas: {
+          'a': Ack.object({
+            'createdAt': Ack.datetime(),
+            'role': Ack.enumValues(_Role.values),
+          }),
+        },
+      );
+      final runtime = <String, Object?>{
+        'kind': 'a',
+        'createdAt': DateTime.utc(2026, 1, 1),
+        'role': _Role.member,
+      };
+      final codec = _codecWithOutput(output, runtime);
+      expect(codec.parse('wire'), runtime);
+      expect(codec.encode(runtime), 'wire');
+    });
+
+    test('typed runtimes must satisfy at least one branch runtime schema', () {
+      final output = Ack.discriminated<Object>(
+        discriminatorKey: 'kind',
+        schemas: {
+          'cat': Ack.object({'name': Ack.string()}).codec<_Cat>(
+            decode: (map) => _Cat(map['name']! as String),
+            encode: (cat) => {'name': cat.name},
+            output: Ack.instance<_Cat>().refine((cat) => cat.name.isNotEmpty),
+          ),
+          'dog': Ack.object({'name': Ack.string()}).codec<_Dog>(
+            decode: (map) => _Dog(map['name']! as String),
+            encode: (dog) => {'name': dog.name},
+          ),
+        },
+      );
+      for (final invalid in <Object>[Object(), const _Cat('')]) {
+        final codec = _codecWithOutput(output, invalid);
+        expect(codec.safeParse('wire').isFail, isTrue);
+        expect(codec.safeEncode(invalid).isFail, isTrue);
+      }
+      for (final valid in <Object>[const _Cat('Mittens'), const _Dog('Spot')]) {
+        final codec = _codecWithOutput(output, valid);
+        expect(codec.parse('wire'), same(valid));
+        expect(codec.encode(valid), 'wire');
+      }
+    });
+
+    test('branch membership checks never execute branch codecs', () {
+      var decodeCalls = 0;
+      var encodeCalls = 0;
+      final output = Ack.discriminated<_Cat>(
+        discriminatorKey: 'kind',
+        schemas: {
+          'cat': Ack.object({'name': Ack.string()}).codec<_Cat>(
+            decode: (map) {
+              decodeCalls++;
+              return _Cat(map['name']! as String);
+            },
+            encode: (cat) {
+              encodeCalls++;
+              return {'name': cat.name};
+            },
+          ),
+        },
+      );
+      const value = _Cat('Mittens');
+      final codec = _codecWithOutput(output, value);
+      expect(codec.parse('wire'), same(value));
+      expect(codec.encode(value), 'wire');
+      expect(decodeCalls, 0);
+      expect(encodeCalls, 0);
+    });
+
+    test('ambiguous runtime membership remains an encode-only failure', () {
+      final branch = Ack.object({'name': Ack.string()}).codec<_Cat>(
+        decode: (map) => _Cat(map['name']! as String),
+        encode: (cat) => {'name': cat.name},
+      );
+      final union = Ack.discriminated<_Cat>(
+        discriminatorKey: 'kind',
+        schemas: {'a': branch, 'b': branch},
+      );
+      final parsed = union.parse({'kind': 'a', 'name': 'Mittens'})!;
+      final codec = _codecWithOutput(union, parsed);
+      expect(codec.parse('wire'), same(parsed));
+      expect(codec.encode(parsed), 'wire');
+      expect(union.safeEncode(parsed).isFail, isTrue);
+    });
+  });
+
+  group('direct discriminated encoding callback counts', () {
+    test('map branches do not acquire an extra runtime refinement pass', () {
+      var refinements = 0;
+      var encoders = 0;
+      final union = Ack.discriminated<JsonMap>(
+        discriminatorKey: 'kind',
+        schemas: {
+          'a': Ack.object({'count': Ack.integer()}).codec<JsonMap>(
+            decode: (map) => map,
+            encode: (map) {
+              encoders++;
+              return {'count': map['count']};
+            },
+            output: Ack.instance<JsonMap>().refine((_) {
+              refinements++;
+              return true;
+            }),
+          ),
+        },
+      );
+      expect(union.encode({'kind': 'a', 'count': 1}), {
+        'kind': 'a',
+        'count': 1,
+      });
+      expect(refinements, 1);
+      expect(encoders, 1);
+    });
+
+    test('typed branches retain one membership probe and one encode pass', () {
+      var refinements = 0;
+      var encoders = 0;
+      final union = Ack.discriminated<_Cat>(
+        discriminatorKey: 'kind',
+        schemas: {
+          'cat': Ack.object({'name': Ack.string()}).codec<_Cat>(
+            decode: (map) => _Cat(map['name']! as String),
+            encode: (cat) {
+              encoders++;
+              return {'name': cat.name};
+            },
+            output: Ack.instance<_Cat>().refine((_) {
+              refinements++;
+              return true;
+            }),
+          ),
+        },
+      );
+      expect(union.encode(const _Cat('Mittens')), {
+        'kind': 'cat',
+        'name': 'Mittens',
+      });
+      expect(refinements, 2);
+      expect(encoders, 1);
+    });
+  });
+}