jobs: runs-on does not accept group: object form — schema validator rejects it while top-level runs-on accepts it fine

[lindeberg]

Summary

Using the group: object form of runs-on inside a custom jobs: block fails at schema validation, even though the exact same syntax compiles correctly at the top-level runs-on field.

Reproduction

---
on:
  workflow_dispatch:

engine:
  id: copilot
  model: claude-sonnet-4.6

runs-on:
  group: "arc-custom"   # ✅ works fine at top level

jobs:
  my-prefetch:
    runs-on:
      group: "arc-custom"   # ❌ fails at schema validation
    steps:
      - run: echo "hello"

safe-outputs:
  noop:
---

Error

error: Unknown property: group. 'group' belongs under 'concurrency', 'safe-outputs/create-issue' or 'runs-on'

What works vs what doesn't

Form	Top-level runs-on	jobs: runs-on
String: ubuntu-latest	✅	✅
Array: [self-hosted, linux]	✅	✅ (fixed by #19407)
Object: {group: arc-custom}	✅	❌ schema error

Runtime behaviour (manually patched lock file)

We confirmed that manually editing the compiled lock file to use runs-on: {group: arc-custom} on a jobs: entry works correctly at runtime — GitHub Actions picks up the runner group and the job executes. The block is compiler-safe; only the schema validator rejects it.

Expected behaviour

All runs-on forms supported at the top level should be equally supported inside jobs:. The group: object form is the standard GitHub Actions syntax for targeting a runner group.

Context

PR #19407 fixed buildCustomJobs() in the compiler to correctly emit array and object runs-on forms (rather than silently dropping them). However, the schema validator for the jobs: frontmatter block was not updated — it still rejects the group: key. The compiler fix and the schema definition are in separate code paths.

Version

gh-aw v0.75.0 (latest)

[pelikhan]

@copilot add syntax support

• unify type in JSON schema
• refactor parser and data structure in Go compiler
• add tests