Unhandled exceptions in main() (before runApp) are written to stdout, not stderr

[DanTup]

Steps to reproduce

It's possible this is intended, but the issue raised at Dart-Code/Dart-Code#5468 about errors not being shown as red error text in VS Code is because there's no distinction between error and non-error output (prior to runApp, after which point we usually get "structured errors" from Flutter that we can format better).

1. Create a Flutter app that throws in main (I think this would happen for Firebase config errors, for example):

   void main() {
     throw Exception("Firebase not initialized");
   }

2. Run in a way that lets you tell the difference between stdout and stderr
   For example on Windows,

   flutter run -d windows 2>&1 | % { $_ -is [System.Management.Automation.ErrorRecord] ? "stderr: $_" : "stdout: $_" }

Actual results

The exception is written to stdout, whereas a similar exception in Dart would be written to stderr. stderr output in VS Code would appear as red/error, whereas stdout does not:

Flutter output:

Flutter output in VS Code debug console:

Dart output:

Dart output in VS Code debug console:

Logs

No response

Flutter Doctor output

PS D:\Dev> flutter doctor -v
[!] Flutter (Channel master, 3.31.0-1.0.pre.227, on Microsoft Windows [Version 10.0.26100.3476], locale en-GB) [3.9s]
    • Flutter version 3.31.0-1.0.pre.227 on channel master at D:\Dev\Google\Flutter\Flutter main
    ! Warning: `dart` on your path resolves to D:\Tools\Dart\Stable\bin\dart.exe, which is not inside your current Flutter SDK checkout at
      D:\Dev\Google\Flutter\Flutter main. Consider adding D:\Dev\Google\Flutter\Flutter main\bin to the front of your path.
    ! Upstream repository git@github.com:DanTup/flutter.git is not a standard remote.
      Set environment variable "FLUTTER_GIT_URL" to git@github.com:DanTup/flutter.git to dismiss this error.
    • Framework revision 46969e531b (6 days ago), 2025-03-25 07:54:22 -0700
    • Engine revision 46969e531b
    • Dart version 3.8.0 (build 3.8.0-222.0.dev)
    • DevTools version 2.45.0-dev.0
    • If those were intentional, you can disregard the above warnings; however it is recommended to use "git" directly to perform update checks and
      upgrades.

[✓] Windows Version (11 Pro 64-bit, 24H2, 2009) [972ms]

[✓] Android toolchain - develop for Android devices (Android SDK version 35.0.0-rc3) [2.5s]
    • Android SDK at C:\Users\danny\AppData\Local\Android\sdk
    • Platform android-35, build-tools 35.0.0-rc3
    • Java binary at: C:\Program Files\Android\Android Studio\jbr\bin\java
      This is the JDK bundled with the latest Android Studio installation on this machine.
      To manually set the JDK path, use: `flutter config --jdk-dir="path/to/jdk"`.
    • Java version OpenJDK Runtime Environment (build 21.0.5+-12932927-b750.29)
    • All Android licenses accepted.

[✓] Chrome - develop for the web [54ms]
    • Chrome at C:\Program Files\Google\Chrome\Application\chrome.exe

[✓] Visual Studio - develop Windows apps (Visual Studio Community 2022 17.10.5) [54ms]
    • Visual Studio at C:\Program Files\Microsoft Visual Studio\2022\Community
    • Visual Studio Community 2022 version 17.10.35122.118
    • Windows 10 SDK version 10.0.22621.0

[✓] Android Studio (version 2024.2) [13ms]
    • Android Studio at C:\Program Files\Android\Android Studio
    • Flutter plugin can be installed from:
      🔨 https://plugins.jetbrains.com/plugin/9212-flutter
    • Dart plugin can be installed from:
      🔨 https://plugins.jetbrains.com/plugin/6351-dart
    • Java version OpenJDK Runtime Environment (build 21.0.5+-12932927-b750.29)

[✓] VS Code (version 1.98.2) [12ms]
    • VS Code at C:\Users\danny\AppData\Local\Programs\Microsoft VS Code
    • Flutter extension version 3.107.20250303

[✓] VS Code (version 1.93.0-insider) [11ms]
    • VS Code at C:\Users\danny\AppData\Local\Programs\Microsoft VS Code Insiders
    • Flutter extension version 3.105.20250203

[✓] Connected device (3 available) [108ms]
    • Windows (desktop) • windows • windows-x64    • Microsoft Windows [Version 10.0.26100.3476]
    • Chrome (web)      • chrome  • web-javascript • Google Chrome 134.0.6998.178
    • Edge (web)        • edge    • web-javascript • Microsoft Edge 133.0.3065.92

[✓] Network resources [574ms]
    • All expected network resources are available.

! Doctor found issues in 1 category.

[danagbemava-nc]

Reproducible using the code sample provided above. Tested on macOS. The macOS equivalent command is below.

flutter run -d macos 2>&1 | awk '{ if ($0 ~ /^\[ERROR\]/) { print "stderr: " $0 } else { print "stdout: " $0 } }'

stdout: Launching lib/main.dart on macOS in debug mode...
stdout: Building macOS application...
stdout: --- xcodebuild: WARNING: Using the first of multiple matching destinations:
stdout: { platform:macOS, arch:arm64, id:00008103-000329201AD1001E, name:My Mac }
stdout: { platform:macOS, arch:x86_64, id:00008103-000329201AD1001E, name:My Mac }
stdout: ✓ Built build/macos/Build/Products/Debug/mysample.app
stdout: Failed to foreground app; open returned 1
stdout: [ERROR:flutter/runtime/dart_vm_initializer.cc(40)] Unhandled Exception: Exception: Firebase not initialized
stdout: #0      main (package:mysample/main.dart:2:3)
stdout: #1      _runMain.<anonymous closure> (dart:ui/hooks.dart:320:23)
stdout: #2      _delayEntrypointInvocation.<anonymous closure> (dart:isolate-patch/isolate_patch.dart:315:19)
stdout: #3      _RawReceivePort._handleMessage (dart:isolate-patch/isolate_patch.dart:194:12)
stdout:
stdout: Syncing files to device macOS...                                    53ms
stdout:
stdout: Flutter run key commands.
stdout: r Hot reload. 🔥🔥🔥
stdout: R Hot restart.
stdout: h List all available interactive commands.
stdout: d Detach (terminate "flutter run" but leave application running).
stdout: c Clear the screen
stdout: q Quit (terminate the application on the device).
stdout:
stdout: A Dart VM Service on macOS is available at: http://127.0.0.1:59862/jI0SWa5Vkb8=/
stdout: 2025-03-31 12:38:48.881 mysample[48534:5956364] +[IMKClient subclass]: chose IMKClient_Modern
stdout: 2025-03-31 12:38:48.881 mysample[48534:5956364] +[IMKInputSession subclass]: chose IMKInputSession_Modern
stdout:
stdout: Application finished.

flutter doctor -v

[✓] Flutter (Channel stable, 3.29.2, on macOS 15.3.1 24D70 darwin-arm64, locale en-US) [2.6s]
    • Flutter version 3.29.2 on channel stable at /Users/deanli/dev/stable
    • Upstream repository https://github.com/flutter/flutter.git
    • Framework revision c236373904 (3 weeks ago), 2025-03-13 16:17:06 -0400
    • Engine revision 18b71d647a
    • Dart version 3.7.2
    • DevTools version 2.42.3

[✓] Android toolchain - develop for Android devices (Android SDK version 35.0.1) [3.2s]
    • Android SDK at /Users/deanli/Library/Android/sdk
    • Platform android-35, build-tools 35.0.1
    • Java binary at: /Users/deanli/Applications/Android Studio Meerkat 2024.3.1 RC 2.app/Contents/jbr/Contents/Home/bin/java
      This JDK is specified in your Flutter configuration.
      To change the current JDK, run: `flutter config --jdk-dir="path/to/jdk"`.
    • Java version OpenJDK Runtime Environment (build 21.0.5+-12932927-b750.29)
    • All Android licenses accepted.

[✓] Xcode - develop for iOS and macOS (Xcode 16.3) [1,849ms]
    • Xcode at /Applications/Xcode-16.3.0-Beta.2.app/Contents/Developer
    • Build 16E5121h
    • CocoaPods version 1.16.2

[✓] Chrome - develop for the web [12ms]
    • Chrome at /Applications/Google Chrome.app/Contents/MacOS/Google Chrome

[✓] Android Studio (version 2024.3) [11ms]
    • Android Studio at /Users/deanli/Applications/Android Studio Meerkat 2024.3.1 RC 2.app/Contents
    • Flutter plugin can be installed from:
      🔨 https://plugins.jetbrains.com/plugin/9212-flutter
    • Dart plugin can be installed from:
      🔨 https://plugins.jetbrains.com/plugin/6351-dart
    • android-studio-dir = /Users/deanli/Applications/Android Studio Meerkat 2024.3.1 RC 2.app
    • Java version OpenJDK Runtime Environment (build 21.0.5+-12932927-b750.29)

[✓] Android Studio (version 2024.2) [9ms]
    • Android Studio at /Users/deanli/Applications/Android Studio.app/Contents
    • Flutter plugin can be installed from:
      🔨 https://plugins.jetbrains.com/plugin/9212-flutter
    • Dart plugin can be installed from:
      🔨 https://plugins.jetbrains.com/plugin/6351-dart
    • Java version OpenJDK Runtime Environment (build 21.0.5+-12932927-b750.29)

[✓] VS Code (version 1.98.2) [8ms]
    • VS Code at /Applications/Visual Studio Code.app/Contents
    • Flutter extension version 3.106.0

[✓] Connected device (5 available) [8.2s]
    • Dean’s iPad (wireless) (mobile) • 00008103-000825C811E3401E            • ios            • iOS 18.3.1 22D72
    • iPhone 16 Pro (mobile)          • B0797AD6-E1FD-4B41-A022-2D7E2276C29B • ios            • com.apple.CoreSimulator.SimRuntime.iOS-18-4 (simulator)
    • macOS (desktop)                 • macos                                • darwin-arm64   • macOS 15.3.1 24D70 darwin-arm64
    • Mac Designed for iPad (desktop) • mac-designed-for-ipad                • darwin         • macOS 15.3.1 24D70 darwin-arm64
    • Chrome (web)                    • chrome                               • web-javascript • Google Chrome 134.0.6998.166
    ! Error: Browsing on the local area network for iPhone. Ensure the device is unlocked and attached with a cable or associated with the same local area network as this Mac.
      The device must be opted into Developer Mode to connect wirelessly. (code -27)
    ! Error: Browsing on the local area network for Nexus. Ensure the device is unlocked and attached with a cable or associated with the same local area network as this Mac.
      The device must be opted into Developer Mode to connect wirelessly. (code -27)

[✓] Network resources [1,439ms]
    • All expected network resources are available.

• No issues found!

[!] Flutter (Channel master, 3.31.0-1.0.pre.329, on macOS 15.3.1 24D70 darwin-arm64, locale en-US) [4.2s]
    • Flutter version 3.31.0-1.0.pre.329 on channel master at /Users/deanli/dev/master
    • Upstream repository https://github.com/flutter/flutter.git
    • Framework revision 948940e925 (9 hours ago), 2025-03-30 23:18:29 -0400
    • Engine revision 948940e925
    • Dart version 3.8.0 (build 3.8.0-243.0.dev)
    • DevTools version 2.45.0-dev.0

[✓] Android toolchain - develop for Android devices (Android SDK version 35.0.1) [1,491ms]
    • Android SDK at /Users/deanli/Library/Android/sdk
    • Platform android-35, build-tools 35.0.1
    • Java binary at: /Users/deanli/Applications/Android Studio Meerkat 2024.3.1 RC 2.app/Contents/jbr/Contents/Home/bin/java
      This JDK is specified in your Flutter configuration.
      To change the current JDK, run: `flutter config --jdk-dir="path/to/jdk"`.
    • Java version OpenJDK Runtime Environment (build 21.0.5+-12932927-b750.29)
    • All Android licenses accepted.

[✓] Xcode - develop for iOS and macOS (Xcode 16.3) [963ms]
    • Xcode at /Applications/Xcode-16.3.0-Beta.2.app/Contents/Developer
    • Build 16E5121h
    • CocoaPods version 1.16.2

[✓] Chrome - develop for the web [9ms]
    • Chrome at /Applications/Google Chrome.app/Contents/MacOS/Google Chrome

[✓] Android Studio (version 2024.3) [8ms]
    • Android Studio at /Users/deanli/Applications/Android Studio Meerkat 2024.3.1 RC 2.app/Contents
    • Flutter plugin can be installed from:
      🔨 https://plugins.jetbrains.com/plugin/9212-flutter
    • Dart plugin can be installed from:
      🔨 https://plugins.jetbrains.com/plugin/6351-dart
    • android-studio-dir = /Users/deanli/Applications/Android Studio Meerkat 2024.3.1 RC 2.app
    • Java version OpenJDK Runtime Environment (build 21.0.5+-12932927-b750.29)

[✓] Android Studio (version 2024.2) [7ms]
    • Android Studio at /Users/deanli/Applications/Android Studio.app/Contents
    • Flutter plugin can be installed from:
      🔨 https://plugins.jetbrains.com/plugin/9212-flutter
    • Dart plugin can be installed from:
      🔨 https://plugins.jetbrains.com/plugin/6351-dart
    • Java version OpenJDK Runtime Environment (build 21.0.5+-12932927-b750.29)

[✓] VS Code (version 1.98.2) [7ms]
    • VS Code at /Applications/Visual Studio Code.app/Contents
    • Flutter extension version 3.106.0

[✓] Connected device (4 available) [6.2s]
    • Dean’s iPad (wireless) (mobile) • 00008103-000825C811E3401E            • ios            • iOS 18.3.1 22D72
    • iPhone 16 Pro (mobile)          • B0797AD6-E1FD-4B41-A022-2D7E2276C29B • ios            • com.apple.CoreSimulator.SimRuntime.iOS-18-4 (simulator)
    • macOS (desktop)                 • macos                                • darwin-arm64   • macOS 15.3.1 24D70 darwin-arm64
    • Chrome (web)                    • chrome                               • web-javascript • Google Chrome 134.0.6998.166
    ! Error: Browsing on the local area network for iPhone. Ensure the device is unlocked and attached with a cable or associated with the same local area network as this Mac.
      The device must be opted into Developer Mode to connect wirelessly. (code -27)
    ! Error: Browsing on the local area network for Nexus. Ensure the device is unlocked and attached with a cable or associated with the same local area network as this Mac.
      The device must be opted into Developer Mode to connect wirelessly. (code -27)

[✓] Network resources [818ms]
    • All expected network resources are available.

! Doctor found issues in 1 category.

[bkonyi]

This logging is done in the engine, so I'm thinking we'll have to do some digging there.

[DanTup]

Interestingly, if I add dart:io and use stderr.writeln('my stderr'); it also comes through from stdout. Maybe this is related to how it's captured on the device and passed back to the tool?

[jonahwilliams]

The desktop log reader is redirecting stdout and stderr to stdout:

https://github.com/flutter/flutter/blob/master/packages/flutter_tools/lib/src/desktop_device.dart#L336-L357

This could be updated to send std:err through w/ logger/printError or similar.