diff --git a/packages/ack/CHANGELOG.md b/packages/ack/CHANGELOG.md
index 69bdfd7a..ad83adde 100644
--- a/packages/ack/CHANGELOG.md
+++ b/packages/ack/CHANGELOG.md
@@ -8,6 +8,8 @@
 * 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.
+* Reject duration millisecond inputs that overflow the platform's `Duration`
+  representation instead of silently decoding to a different duration.
 * 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/ack.dart b/packages/ack/lib/src/ack.dart
index 1f0fc756..15774f87 100644
--- a/packages/ack/lib/src/ack.dart
+++ b/packages/ack/lib/src/ack.dart
@@ -232,6 +232,9 @@ final class Ack {
   /// Creates a bidirectional duration codec between integer milliseconds and
   /// `Duration` values.
   ///
+  /// Inputs that cannot be represented by `Duration` on the current platform
+  /// without changing their millisecond value are rejected.
+  ///
   /// Runtime invariant: the `Duration` must be a whole number of
   /// milliseconds (sub-millisecond precision is rejected to avoid silent
   /// truncation on encode).
@@ -271,7 +274,13 @@ bool _isWholeMillisecondDuration(Duration value) {
 }
 
 Duration _decodeDuration(int milliseconds) {
-  return Duration(milliseconds: milliseconds);
+  final duration = Duration(milliseconds: milliseconds);
+  if (duration.inMilliseconds != milliseconds) {
+    throw const FormatException(
+      'Milliseconds cannot be represented by Duration without loss.',
+    );
+  }
+  return duration;
 }
 
 int _encodeDuration(Duration value) => value.inMilliseconds;
diff --git a/packages/ack/test/schemas/duration_range_test.dart b/packages/ack/test/schemas/duration_range_test.dart
new file mode 100644
index 00000000..6fabb4a8
--- /dev/null
+++ b/packages/ack/test/schemas/duration_range_test.dart
@@ -0,0 +1,78 @@
+import 'package:ack/ack.dart';
+import 'package:test/test.dart';
+
+final _usesJavaScriptNumberRepresentation = <num>[1e20].single is int;
+
+void main() {
+  test('preserves milliseconds within the exact range on VM and web', () {
+    final schema = Ack.duration();
+    for (final milliseconds in <int>[0, 1, -1, 9007199254740, -9007199254740]) {
+      final parsed = schema.parse(milliseconds)!;
+      expect(parsed.inMilliseconds, milliseconds);
+      expect(schema.encode(parsed), milliseconds);
+    }
+  });
+
+  test('retains lossless JSON integer normalization', () {
+    final schema = Ack.duration();
+    expect(schema.encode(schema.parse(1500.0)), 1500);
+    expect(schema.encode(schema.parse(-1500.0)), -1500);
+    expect(schema.safeParse(1.5).isFail, isTrue);
+  });
+
+  test('any accepted large input must round-trip without corruption', () {
+    final schema = Ack.duration();
+    for (final text in [
+      '9007199254740991',
+      '-9007199254740991',
+      '9223372036854775',
+      '-9223372036854775',
+      '9223372036854776',
+      '-9223372036854776',
+      '9223372036854775807',
+      '-9223372036854775808',
+    ]) {
+      final milliseconds = int.parse(text);
+      final result = schema.safeParse(milliseconds);
+      if (result.isOk) {
+        final parsed = result.getOrNull()!;
+        expect(parsed.inMilliseconds, milliseconds, reason: text);
+        expect(schema.encode(parsed), milliseconds, reason: text);
+      }
+    }
+  });
+
+  group(
+    'native signed 64-bit Duration bounds',
+    () {
+      test('accepts both whole-millisecond supported edges', () {
+        final schema = Ack.duration();
+        final edge = (BigInt.parse('9223372036854775807') ~/ BigInt.from(1000))
+            .toInt();
+        for (final milliseconds in [edge, -edge]) {
+          final parsed = schema.parse(milliseconds)!;
+          expect(parsed.inMilliseconds, milliseconds);
+          expect(schema.encode(parsed), milliseconds);
+        }
+      });
+
+      test('rejects values outside the representable microsecond range', () {
+        final schema = Ack.duration();
+        for (final text in [
+          '9223372036854776',
+          '-9223372036854776',
+          '9223372036854775807',
+          '-9223372036854775808',
+        ]) {
+          final result = schema.safeParse(int.parse(text));
+          expect(result.isFail, isTrue, reason: text);
+          expect(result.getError(), isA<SchemaTransformError>(), reason: text);
+          expect(result.getError().cause, isA<FormatException>(), reason: text);
+        }
+      });
+    },
+    skip: _usesJavaScriptNumberRepresentation
+        ? 'JavaScript does not use the native signed 64-bit representation.'
+        : false,
+  );
+}