Agents could select coordinator/meta agents (Prometheus, Atlas,
Sisyphus/Ultraworker) as subagent targets via task() / delegation,
producing duplicate orchestration loops and conflicting team state.
This is the inverse of #3987 / #4065 — symmetric guard on the
delegation TARGET side, using the same AGENT_ELIGIBILITY_REGISTRY
classification.
Add a runtime guard at the delegation entry point that rejects
task() calls whose subagent_type resolves to an agent marked as
hard-reject / coordinator-only in the eligibility registry, with
an actionable error naming the agent. Regression test asserts a
prometheus-targeted delegation is rejected before any subagent
session spawns.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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