Widget Preview terminates with "is not a valid Flutter project" for valid Flutter project #178052
Description
Steps to reproduce
I cloned the flutter/packages repo and opened it in VS Code, and got a notification that the widget preview server terminated. In the logs I found:
[4:04:03 PM] [FlutterWidgetPreview] [Info] <== [{"event":"widget_preview.logMessage","params":{"level":"error","message":"D:\\Dev\\Google\\Flutter\\packages\\packages\\animations is not a valid Flutter project."}}]
[4:04:03 PM] [FlutterWidgetPreview] [Info] Process D:\Dev\Google\Flutter\Flutter main\bin\flutter.bat terminated! 1, null
It certainly looks like a valid Flutter project to me. My guess is that this might be because I haven't yet run pub get for this package, and therefore there are bits missing. However, since this is not an uncommon case, I think it needs to be handled (ideally in the tool here, to avoid IDEs having to do this kind of detailed tracking of the state of the Flutter projects).
cc @bkonyi
Expected results
The preview server should start up and just provide no widget previews (and pick up the previews once the project becomes in a state where this is possible).
Actual results
The server terminates with an error.
Code sample
Code sample
N/A
Screenshots or Video
Screenshots / Video demonstration
N/A
Logs
Logs
Wed Nov 05 2025 [16:04:01 GMT+0000 (Greenwich Mean Time)] Log file started
[4:04:02 PM] [FlutterWidgetPreview] [Info] Spawning D:\Dev\Google\Flutter\Flutter main\bin\flutter.bat with args ["widget-preview","start","--machine","--web-server","--dtd-url","ws://127.0.0.1:52532/CsKolHD-nZg=","--devtools-server-address","http://127.0.0.1:9100/"]
[4:04:02 PM] [FlutterWidgetPreview] [Info] .. in D:\Dev\Google\Flutter\packages\packages\animations
[4:04:02 PM] [FlutterWidgetPreview] [Info] .. with {"toolEnv":{"FLUTTER_HOST":"VSCode","PUB_ENVIRONMENT":"vscode.dart-code","FLUTTER_ROOT":"D:\\Dev\\Google\\Flutter\\Flutter main"}}
[4:04:02 PM] [FlutterWidgetPreview] [Info] PID: 22732
[4:04:03 PM] [FlutterWidgetPreview] [Info] <== [{"event":"widget_preview.initializing","params":{"pid":13596}}]
[4:04:03 PM] [FlutterWidgetPreview] [Info] <== [{"event":"widget_preview.logMessage","params":{"level":"error","message":"D:\\Dev\\Google\\Flutter\\packages\\packages\\animations is not a valid Flutter project."}}]
[4:04:03 PM] [FlutterWidgetPreview] [Info] Process D:\Dev\Google\Flutter\Flutter main\bin\flutter.bat terminated! 1, null
[4:04:03 PM] [General] [Error] Failed to set up Widget Preview: Widget Preview server process terminatedFlutter Doctor output
Doctor output
PS D:\Dev\Google\Flutter\packages\packages\animations> flutter doctor -v
[!] Flutter (Channel master, 3.38.0-1.0.pre-38, on Microsoft Windows [Version 10.0.26100.6899], locale en-GB) [3.3s]
• Flutter version 3.38.0-1.0.pre-38 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 498545a341 (4 weeks ago), 2025-10-09 17:34:03 -0500
• Engine revision d64282028b
• Dart version 3.11.0 (build 3.11.0-8.0.dev)
• DevTools version 2.51.0
• Feature flags: enable-web, enable-linux-desktop, enable-macos-desktop, enable-windows-desktop, enable-android, enable-ios, cli-animations,
enable-native-assets, omit-legacy-version-file, enable-lldb-debugging
• 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) [652ms]
[!] Android toolchain - develop for Android devices (Android SDK version 35.0.0) [2.1s]
• Android SDK at C:\Users\danny\AppData\Local\Android\sdk
• Emulator version 35.4.9.0 (build_id 13025442) (CL:N/A)
• Platform android-36, build-tools 35.0.0
• 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 21.0.6+-13355223-b631.42)
! Some Android licenses not accepted. To resolve this, run: flutter doctor --android-licenses
[✓] 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) [53ms]
• Visual Studio at C:\Program Files\Microsoft Visual Studio\2022\Community
• Visual Studio Community 2022 version 17.10.35122.118
• Windows 10 SDK version 10.0.26100.0
[✓] Connected device (3 available) [105ms]
• Windows (desktop) • windows • windows-x64 • Microsoft Windows [Version 10.0.26100.6899]
• Chrome (web) • chrome • web-javascript • Google Chrome 142.0.7444.60
• Edge (web) • edge • web-javascript • Microsoft Edge 141.0.3537.92
[✓] Network resources [269ms]
• All expected network resources are available.
! Doctor found issues in 2 categories.
PS D:\Dev\Google\Flutter\packages\packages\animations>Metadata
Metadata
Assignees
Labels
High-priority issues at the top of the work listIssues related to the Widget Previewer toolingStack traces logged to the consoleFound to occur in 3.35Found to occur in 3.38The issue has been confirmed reproducible and is ready to work onIssue is closed as already fixed in a newer versionOwned by Flutter Tool teamAffects the "flutter" command-line tool. See also t: labels.Triaged by Flutter Tool team