Day 55: Swarm Failure Modes

Swarms fail in boring ways: two planners emit incompatible plans, two implementers open duplicate PRs, and nobody notices until review. Treat split-brain and duplication as first-class failure modes with detectors, not postmortems.

⚡ TL;DR: One active plan per run_id. Lease the implementer slot. Fingerprint PR titles/branches. Alert on divergent plan hashes for the same ticket.

Split-brain plans

# detect/split_brain.py
def assert_single_plan(store, ticket_id: str, run_id: str):
    plans = store.list_plans(ticket_id)
    active = [p for p in plans if p.status == "active"]
    if len(active) > 1:
        raise RuntimeError(f"split_brain:{ticket_id}:{[p.hash for p in active]}")
    if active and active[0].run_id != run_id:
        raise RuntimeError("stale_run_holds_plan")

Duplicate PR detector

// detect/dupPr.ts
export function prFingerprint(title: string, files: string[]): string {
  const norm = title.toLowerCase().replace(/\s+/g, " ").trim();
  const f = [...files].sort().join("|");
  return `${norm}::${f}`;
}

export function findDupes(open: { title: string; files: string[]; url: string }[]) {
  const map = new Map<string, string[]>();
  for (const p of open) {
    const k = prFingerprint(p.title, p.files);
    map.set(k, [...(map.get(k) || []), p.url]);
  }
  return [...map.values()].filter((v) => v.length > 1);
}

✅ Close or mark duplicates automatically; never merge both.

Leases for implementers

# lease.py
def acquire_implementer(redis, run_id: str, ttl=600) -> bool:
    # ✅ SET NX EX — only one implementer mutates the branch
    return bool(redis.set(f"lease:impl:{run_id}", "1", nx=True, ex=ttl))

Closing checklist

• [ ] Single active plan per ticket/run
• [ ] Implementer leases
• [ ] Duplicate PR fingerprints in CI
• [ ] Alarm on multiple active plans
• [ ] Cancel stale runs when a newer plan activates

Series navigation

Day 54: Specialist Tools per Agent · Day 56: Supervisor Patterns on Bedrock Agents

Last updated September 11, 2026