Fable CLI fails on Github CI

[MangelMaxime]

Description

Hello,

A Fable user reported that Fable fails on Github CI.

I first though that we broke something in Fable since we are working on a new version, but today after working on a project of mine which use a stable version I have the same error.

https://github.com/thoth-org/Thoth.Json/actions/runs/12873451418/job/35891008876#step:8:85

After investigating it seems like the main things that changed on Thoth.Json between the current failure CI and previous working one is the CI runner image Image: macos-14-arm64 - Version: 20241216.651 to Image: macos-14-arm64 - Version: 20250120.774 (failing).

Platforms affected

• Azure DevOps
• GitHub Actions - Standard Runners
• GitHub Actions - Larger Runners

Runner images affected

• Ubuntu 20.04
• Ubuntu 22.04
• Ubuntu 24.04
• macOS 12
• macOS 13
• macOS 13 Arm64
• macOS 14
• macOS 14 Arm64
• macOS 15
• macOS 15 Arm64
• Windows Server 2019
• Windows Server 2022
• Windows Server 2025

Image version and build link

20250120.774

Failing build: https://github.com/thoth-org/Thoth.Json/actions/runs/12873451418/job/35891008876#step:8:85

Is it regression?

https://github.com/thoth-org/Thoth.Json/actions/runs/12539540940/job/34965770804

Expected behavior

The CI should not fails.

Actual behavior

There is an error on the CI which didn't happen in the past and does not occurs on a local Mac ARM machine.

Repro steps

I am not sure how you want to reproduce it but I suppose forking https://github.com/thoth-org/Thoth.Json/ and making the CI run on it?

I am not sure how to debug this issue, because I can't reproduce it locally and it seems to happens only on Github Mac CI as Ubuntu and Windows CI works.

[pravinade]

Hi @MangelMaxime
We will look into the issue and keep you posted with updates.

Thank You.

[pravinade]

After further investigation, switching the runner to macos-14 explicitly resolved the issue. The macos-latest runner was picking up newer versions of macOS, which caused the build to fail. By using macos-14, we explicitly specify the desired macOS version (14), which is more stable for the current CI setup.

for your reference below workaround:
`name: CI
on:
pull_request:
branches: [ main, develop ]
push:
branches:
- main
workflow_dispatch:
jobs:
build:
strategy:
matrix:
platform: [ ubuntu-latest, macos-14, macos-latest, windows-latest ]

runs-on: ${{ matrix.platform }}

steps:
  - uses: actions/checkout@v3
  - uses: actions/setup-dotnet@v3
    with:
      dotnet-version: '8.0.100'
  - uses: actions/setup-node@v3
    with:
      node-version: '20'
  - uses: pnpm/action-setup@v2
    with:
      version: 9
  - name: Install Python (Windows)
    if: matrix.platform == 'windows-latest'
    uses: actions/setup-python@v4
    with:
      python-version: '3.12'
  - run: python --version
  - name: Test (Unix)
    if: matrix.platform == 'ubuntu-latest' || matrix.platform == 'macOS-latest'
    run: ./build.sh test
  - name: Test (Windows)
    if: matrix.platform == 'windows-latest'
    run: .\build.bat test

`

Let me know if you have any further information.

[MangelMaxime]

Thank you for looking into it.

By using macos-14, we explicitly specify the desired macOS version (14), which is more stable for the current CI setup

I don't understand why setting macos-latest use another version of the image than macos-14.

My supposition is using macos-latest tells the CI to use the latest stable version of Mac OS which is 14 at the moment. So from this point of view macos-latest and macos-14 should point to the same image no ?

I am asking, because I fear that if we switch to macos-14 it will solve the issue temporary and later it will appears again because at some point it will start using the version 20250113.751.

[MangelMaxime]

Thank you for the help, we believe we have some idea on what could be causing the issue.

It is possible that .NET for some reason emit a warning which mess up the JSON formatting and cause the parser to throw. We are trying to investigate in this direction.

[MangelMaxime]

I am closing, it seems like the issue is because there is now .NET8 and .NET9 installed on the CI.

We need to handle that scenario in Fable itself

Thank you for the help