In a reusable workflow's on.workflow_call.inputs block, what happens if a caller doesn't provide a 'required: true' input?
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Short Explanation and Infographic
Required inputs without a default cause a validation failure — the workflow refuses to start until the caller provides the value.
Full explanation below image
Full Explanation
When a reusable workflow defines an input with 'required: true' and the calling workflow does not provide a value for that input in its 'with:' block, the workflow run fails at validation — the job is not created and an error is shown. If the input has 'required: true' AND 'default:', the default is used even if required (though marking required with a default is somewhat contradictory — the required check means 'caller must explicitly pass this' in some documentation contexts). The safest design: use 'required: false' with 'default:' for optional inputs, and 'required: true' without 'default:' for truly mandatory inputs.