Dart Error: error: import of dart:mirrors is not supported in the current Dart runtime

[mdeleau]

Hello,

I tried to install Monarch today for my project on MacOS (Sequoia 15.5) and encountered the following problem when running Monarch on my project. I followed the instructions on the Monarch website.

> monarch run
Using flutter sdk at /Users/<user>/Dev/flutter/bin/flutter
Enabling Flutter for desktop

Downloading the Monarch UI for this project's flutter version...

We could not find a compatible Monarch UI for this version of Monarch and Flutter.
You could try switching Flutter versions. Or, create a GitHub issue in the Monarch repo 
and let us know which Flutter version your project is using: 
https://github.com/Dropsource/monarch/issues

My monarch install :

> monarch --version
Monarch version 2.3.1

Monarch modules:
- cli: 3.1.0
- controller: 1.5.1
- platform_app: 2.2.0
- preview_api: 2.5.1

Revision:
b30507e5d091ae77938e896ee8e292bbedcc7198

My flutter install :

> flutter --version
Flutter 3.32.4 • channel stable • https://github.com/flutter/flutter.git
Framework • revision 6fba2447e9 (3 weeks ago) • 2025-06-12 19:03:56 -0700
Engine • revision 8cd19e509d (3 weeks ago) • 2025-06-12 16:30:12 -0700
Tools • Dart 3.8.1 • DevTools 2.45.1

[fertrig]

@mdeleau Could you please try again? I just pushed the Monarch UI for that version of Flutter.

[mdeleau]

Thank you for your fast answer. The monarch run command managed to download the Monarch UI.
Unfortunately, there are no stories displayed in the window whereas the monarch init command previously generated sample stories in my project. I have the following errors in the logs of monarch run command that could explain the problem:

> monarch run                                                                                                                                            (22h36m) 15:52:53 
Using flutter sdk at /Users/<user>/Dev/flutter/bin/flutter
Enabling Flutter for desktop

Starting Monarch.

Preparing stories...
3.1sec elapsed, 1/2 tasks completed
Preparing stories completed, took 6.1sec.

Launching Monarch app...
[ERROR:flutter/shell/common/shell.cc(115)] Dart Error: error: import of dart:mirrors is not supported in the current Dart runtime
[ERROR:flutter/runtime/dart_isolate.cc(146)] Could not prepare isolate.

[ERROR:flutter/runtime/runtime_controller.cc(569)] Could not create root isolate.

[ERROR:flutter/shell/common/shell.cc(674)] Could not launch engine with configuration.

3.1sec elapsed, 0/1 tasks completed
6.2sec elapsed, 0/1 tasks completed
9.3sec elapsed, 0/1 tasks completed
12.4sec elapsed, 0/1 tasks completed
...

[fertrig]

build_runner 2.5.x is causing this error. Please set build_runner in your pubspec to version 2.4.15.

build_runner: 2.4.15

When Monarch uses build_runner 2.5.x to build the preview-bundle it fails as you documented. I need to isolate the issue so I can report it.

Setting build_runner to 2.4.15 is a workaround.

[mdeleau]

Thank you for your answer, this workaround allowed me to launch monarch as expected 👍