Flutter synthetic-package Removed: How to Fix Your l10n.yaml (2026)

Quick answer: Newer Flutter versions deprecate and remove the synthetic package that used to host generated localizations (the package:flutter_gen/gen_l10n/app_localizations.dart import). To fix the breaking change, set synthetic-package: false in l10n.yaml, add an output-dir (e.g. lib/l10n), and update your imports to the real generated file path. You can build a correct config in seconds with the free l10n.yaml generator.

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🔧 Generate a correct l10n.yaml instantly

✅ Sets synthetic-package: false for you	✅ Adds the right output-dir
✅ Presets for app, web & multi-locale	✅ Copy or download l10n.yaml

→ Open the l10n.yaml Generator

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What changed, and why your build broke

For years, flutter gen-l10n wrote your AppLocalizations class into a hidden synthetic package named flutter_gen. You imported it like this:

import 'package:flutter_gen/gen_l10n/app_localizations.dart';

This was convenient but fragile — IDEs couldn't always resolve it, it broke with some build setups, and it hid generated code you couldn't inspect. The Flutter team deprecated the synthetic package and is removing it. If you upgraded Flutter and now see errors like:

Target of URI doesn't exist: 'package:flutter_gen/gen_l10n/app_localizations.dart'

The synthetic package output feature is deprecated / has been removed.

…this migration is what you need.

The fix in 3 steps

Step 1 — Update l10n.yaml

Turn off the synthetic package and tell gen-l10n where to write the real file:

arb-dir: lib/l10n
output-dir: lib/l10n
template-arb-file: app_en.arb
output-localization-file: app_localizations.dart
synthetic-package: false
nullable-getter: false

Not sure which options you need? The l10n.yaml Generator builds this for you with sensible defaults and explains every line.

Step 2 — Keep generate: true in pubspec.yaml

dependencies:
  flutter:
    sdk: flutter
  flutter_localizations:
    sdk: flutter
  intl: any

flutter:
  generate: true

Step 3 — Fix your imports

Replace the old synthetic import with the real path (relative to your project). If you generated into lib/l10n:

// ❌ Old — synthetic package
import 'package:flutter_gen/gen_l10n/app_localizations.dart';

// ✅ New — real generated file
import 'package:your_app/l10n/app_localizations.dart';
// or a relative import:
import 'l10n/app_localizations.dart';

Then regenerate:

flutter clean
flutter pub get
flutter gen-l10n

Your AppLocalizations class now lives in lib/l10n/app_localizations.dart where you can actually open it.

Before vs after

	Synthetic package (old)	Real output-dir (new)
l10n.yaml	synthetic-package: true (default)	synthetic-package: false + output-dir
Import	package:flutter_gen/gen_l10n/...	package:your_app/l10n/...
File visible in repo?	No	Yes
IDE resolution	Sometimes flaky	Reliable
Future-proof	Removed	Supported

Common pitfalls

• Forgot output-dir — gen-l10n falls back to arb-dir, which is fine, but be explicit so imports are predictable.
• Stale build — always run flutter clean after the switch; old synthetic artifacts can linger.
• Wrong package name in the import — it must match the name: in your pubspec.yaml.
• Committing generated code or not — either is valid; just be consistent across the team.

Frequently asked questions

Why was synthetic-package removed in Flutter?

The synthetic flutter_gen package hid generated localization code and caused IDE resolution and build issues. Flutter now generates into a real directory you control, which is more reliable and inspectable. Set synthetic-package: false and an output-dir.

What do I replace the flutter_gen import with?

Import the real generated file. If you set output-dir: lib/l10n, use import 'package:your_app/l10n/app_localizations.dart'; (replace your_app with your pubspec name), or a relative import.

Do I still need flutter_localizations and intl?

Yes. Keep flutter_localizations (SDK) and intl in dependencies, and generate: true under the flutter: section so gen-l10n runs on each build.

How do I generate a correct l10n.yaml?

Use the l10n.yaml Generator — toggle the options you need and copy a ready-to-use config that already sets synthetic-package: false.

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Related tools & guides:

• l10n.yaml Generator
• l10n.yaml: The Complete Configuration Guide
• Flutter Localization Complete Guide 2026
• ARB Editor & Validator