In a composite action's action.yml, what additional field is REQUIRED for each 'run:' step that is NOT required in regular workflow run steps?
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Short Explanation and Infographic
In composite actions, you must explicitly declare 'shell:' for every run step — there's no inherited default like in regular workflow jobs.
Full explanation below image
Full Explanation
In composite actions, every 'run:' step under 'runs.steps' must specify 'shell:' explicitly (e.g., 'shell: bash', 'shell: pwsh', 'shell: python'). This is different from regular workflow steps where the shell defaults based on the runner OS (bash on Linux/macOS, pwsh on Windows). Composite actions are runner-agnostic at definition time, so they can't assume a default shell. 'working-directory:', 'name:', and 'env:' are optional in both composite action steps and regular workflow steps. Missing 'shell:' in a composite action run step causes a validation error.