Firebase Crashlytics

---
name: firebase-crashlytics
description: "Use when implementing crash reporting, capturing fatal/non-fatal errors, recording isolate/async exceptions, customizing reports, or uploading obfuscated symbols."
license: MIT
---

# Firebase Crashlytics Skill

This skill defines how to correctly use Firebase Crashlytics in Flutter applications.

## When to Use

Use this skill when:

* Implementing crash reporting in a Flutter project.
* Capturing fatal errors, non-fatal exceptions, and async/isolate errors.
* Customizing crash reports with keys, logs, and user identifiers.
* Configuring opt-in data collection or disabling reporting in debug builds.
* Uploading symbol files for obfuscated builds.

---

## 1. Setup and Configuration

```
flutter pub add firebase_crashlytics
flutter pub add firebase_analytics  # enables breadcrumb logs for better crash context
```

Run `flutterfire configure` to update the Firebase configuration and add the required Crashlytics Gradle plugin for Android.

```dart
import 'package:firebase_crashlytics/firebase_crashlytics.dart';
```

**Obfuscated code:**
- For apps built with `--split-debug-info` and/or `--obfuscate`, upload symbol files for readable stack traces.
- **iOS:** Flutter 3.12.0+ and Crashlytics Flutter plugin 3.3.4+ handle symbol upload automatically.
- **Android:** Use Firebase CLI (v11.9.0+) to upload Flutter debug symbols:

```bash
firebase crashlytics:symbols:upload --app=FIREBASE_APP_ID PATH/TO/symbols
```

---

## 2. Error Handling

Configure comprehensive error capture in `main()` to catch errors from all sources:

**Fatal Flutter errors:**

```dart
void main() async {
  WidgetsFlutterBinding.ensureInitialized();
  await Firebase.initializeApp();

  // Pass all uncaught fatal errors from the framework to Crashlytics
  FlutterError.onError = FirebaseCrashlytics.instance.recordFlutterFatalError;

  // Catch async errors not handled by the Flutter framework
  PlatformDispatcher.instance.onError = (error, stack) {
    FirebaseCrashlytics.instance.recordError(error, stack, fatal: true);
    return true;
  };

  runApp(MyApp());
}
```

**Non-fatal Flutter errors:** use `recordFlutterError` instead of `recordFlutterFatalError`.

**Isolate errors:**

```dart
Isolate.current.addErrorListener(RawReceivePort((pair) async {
  final List<dynamic> errorAndStacktrace = pair;
  await FirebaseCrashlytics.instance.recordError(
    errorAndStacktrace.first,
    errorAndStacktrace.last,
    fatal: true,
  );
}).sendPort);
```

**Caught exceptions (non-fatal):**

```dart
await FirebaseCrashlytics.instance.recordError(
  error,
  stackTrace,
  reason: 'a non-fatal error',
  information: ['further diagnostic information about the error', 'version 2.0'],
);
```

- Crashlytics only stores the **most recent 8 non-fatal exceptions** per session — older ones are discarded.

---

## 3. Crash Report Customization

**Custom keys** (max 64 key/value pairs, up to 1 kB each):

```dart
FirebaseCrashlytics.instance.setCustomKey('str_key', 'hello');
FirebaseCrashlytics.instance.setCustomKey('bool_key', true);
FirebaseCrashlytics.instance.setCustomKey('int_key', 1);
```

**Custom log messages** (limit: 64 kB per session):

```dart
FirebaseCrashlytics.instance.log("User tapped on payment button");
```

**User identifier:**

```dart
FirebaseCrashlytics.instance.setUserIdentifier("user-123");
// Clear by setting to blank string
FirebaseCrashlytics.instance.setUserIdentifier("");
```

- Avoid putting unique values (user IDs, timestamps) directly in exception messages — use custom keys instead.

---

## 4. Performance and Optimization

- Crashlytics processes exceptions on a **dedicated background thread** to minimize performance impact.
- **Fatal** reports are sent in real-time without requiring an app restart.
- **Non-fatal** reports are written to disk and sent with the next fatal report or on app restart.
- Use breadcrumb logs (requires Firebase Analytics) to understand user actions leading up to a crash.

**Disable Crashlytics in debug builds:**

```dart
if (kReleaseMode) {
  await FirebaseCrashlytics.instance.setCrashlyticsCollectionEnabled(true);
} else {
  await FirebaseCrashlytics.instance.setCrashlyticsCollectionEnabled(false);
}
```

---

## 5. Testing and Debugging

Force a test crash to verify the setup:

```dart
FirebaseCrashlytics.instance.crash();
```

**Verification workflow:**
1. Build and run the app in release mode.
2. Trigger the test crash.
3. Reopen the app so the crash report is uploaded.
4. Check the Firebase Console Crashlytics dashboard within 5 minutes.
5. Verify that custom keys, logs, and user identifiers appear on the crash report.

- Verify stack traces are properly symbolicated when using code obfuscation.

---

## 6. Opt-in Reporting

By default, Crashlytics automatically collects crash reports for all users.

To give users control over data collection:
- Disable automatic reporting and enable it only via `setCrashlyticsCollectionEnabled(true)` when users opt in.
- The override value **persists** across all subsequent app launches.
- To opt a user out, pass `false` — this applies from the next app launch.
- When disabled, crash info is **stored locally**; if later enabled, locally stored crashes are sent to Crashlytics.

---

## References

- [Firebase Crashlytics Flutter documentation](https://firebase.google.com/docs/crashlytics/get-started?platform=flutter)
- [Customize crash reports](https://firebase.google.com/docs/crashlytics/customize-crash-reports?platform=flutter)