Copilot cloud agent fails to launch when copilot-setup-steps.yml uses a parallel: step group

[harupy]

🏷️ Discussion Type

Bug

💬 Feature/Topic Area

Copilot in GitHub

Body

Since parallel steps shipped, a copilot-setup-steps.yml that uses a parallel: step group makes every Copilot cloud agent session fail immediately with Failed to launch agent.

The failure is total and produces nothing to debug with: ~2 seconds, 0 premium requests, no session logs, and no Running Copilot cloud agent workflow run created at all.

Evidence

In mlflow/mlflow, the same task was launched three times across a change to the default branch:

time (UTC)	model	result	session
18:57:01	auto	Failed to launch agent	7eec130c
19:03:51	gpt-5.6-terra	Failed to launch agent	d93e8d65
19:17:07	—	the parallel: group removed from the default branch	—
19:18:27	gpt-5.6-luna	launched normally	1968111c

All three sessions are on the same task, 0f36cc5c, in the public repository mlflow/mlflow, against pull request #25544. The group was added in #25542 and removed in #25545.

Seven consecutive sessions succeeded before the group was introduced, and the first session after it merged failed. Because the agent reads this file from the default branch, nothing failed while the pull request adding it was open.

The construct is valid

The same file runs green as an ordinary Actions workflow, including on the pull request that introduced the group, so the runner executes it correctly. Only the agent launch path fails.

The shape that broke it:

jobs:
  copilot-setup-steps:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v7
      - parallel:
          - uses: ./.github/actions/setup-node
          - uses: ./.github/actions/setup-python
          - name: Restore pre-commit hooks
            uses: actions/cache/restore@v6
            with:
              path: ~/.cache/pre-commit
              key: pre-commit-...
      - name: Install dependencies
        run: uv sync --only-group lint

Flattening that group back into sequential steps, changing nothing else, restored agent launches.

Questions

1. Are parallel: groups expected to work in copilot-setup-steps.yml? If they are not supported, could the file be rejected at configuration time rather than failing every session at launch?
2. Could Failed to launch agent carry a reason? With no logs and no workflow run, there is nothing to correlate against, and the only way I found the cause was bisecting the default branch.

This is closely related to #170315, where container: in the same file also breaks the agent while being valid Actions syntax.

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