GitHub Actions "'runs-on' is required"

Each standard job must specify runs-on to choose a runner. Omitting it (or misindenting it so it is not seen as a job key) makes the workflow invalid. Jobs that only call a reusable workflow via uses are the exception.

What this error means

The workflow is rejected reporting runs-on is required for a job, after the key was missing or misplaced.

Invalid workflow file: .github/workflows/ci.yml#L5
'runs-on' is required and must be a string or an array.

Common causes

runs-on missing on a standard job

A job that runs steps must declare a runner via runs-on.

runs-on misindented

Wrong indentation places runs-on outside the job, so it is not recognized.

How to fix it

Declare a runner on every standard job

1. Add runs-on with a label like ubuntu-latest at job scope.
2. For reusable-workflow caller jobs, use uses instead of steps/runs-on.
3. Verify indentation so runs-on is a direct job key.

jobs:
  build:
    runs-on: ubuntu-latest
    steps:
      - run: echo ok

How to prevent it

• Lint workflows so missing runs-on is caught before push.
• Remember reusable-workflow caller jobs use uses, not runs-on.

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