choosing-swarm-patterns

### Overview

10 orchestration patterns for multi-agent workflows. Pick the simplest pattern that solves the problem — add complexity only when the system proves it's insufficient.

### Quick Decision Framework

#### ```

```
Is the task independent per agent?
  YES → fan-out (parallel workers)

Does each step need the previous step's output?
  YES → Is it strictly linear?
    YES → pipeline
    NO  → dag (parallel where possible)

Does a coordinator need to stay alive and adapt?
  YES → Is there one level of management?
    YES → hub-spoke
    NO  → hierarchical (multi-level)

Is the task about making a decision?
  YES → Do agents need to argue opposing sides?
    YES → debate (adversarial)
    NO  → consensus (cooperative voting)

Does the right specialist emerge during processing?
  YES → handoff (dynamic routing)

Do all agents need to freely collaborate?
  YES → mesh (peer-to-peer)

Is cost the primary concern?
  YES → cascade (cheap model first, escalate if needed)
```


### Pattern Reference

| # | Pattern | Topology | Agents | Best For |
|---|---------|----------|--------|----------|
| 1 | **fan-out** | Star (SDK center) | N parallel | Independent subtasks (reviews, research, tests) |
| 2 | **pipeline** | Linear chain | Sequential | Ordered stages (design → implement → test) |
| 3 | **hub-spoke** | Star (live hub) | 1 lead + N workers | Dynamic coordination, lead reviews/adjusts |
| 4 | **consensus** | Broadcast + vote | N voters | Architecture decisions, approval gates |
| 5 | **mesh** | Fully connected | N peers | Brainstorming, collaborative debugging |
| 6 | **handoff** | Routing chain | 1 active at a time | Triage, specialist routing, support flows |
| 7 | **cascade** | Tiered escalation | Cheapest → most capable | Cost optimization, production workloads |
| 8 | **dag** | Dependency graph | Parallel + joins | Complex projects with mixed dependencies |
| 9 | **debate** | Adversarial rounds | 2+ debaters + judge | Rigorous evaluation, architecture trade-offs |
| 10 | **hierarchical** | Tree (multi-level) | Lead → coordinators → workers | Large teams, domain separation |

### Pattern Details

#### 1. fan-out — Parallel Workers

```ts
fanOut([
  { task: "Review auth.ts", name: "AuthReviewer" },
  { task: "Review db.ts", name: "DbReviewer" },
], { cli: "claude" });
```

#### 2. pipeline — Sequential Stages

```ts
pipeline([
  { task: "Design the API schema", name: "Designer" },
  { task: "Implement the endpoints", name: "Implementer" },
  { task: "Write integration tests", name: "Tester" },
]);
```

#### 3. hub-spoke — Persistent Coordinator

```ts
hubAndSpoke({
  hub: { task: "Coordinate building a REST API", name: "Lead" },
  workers: [
    { task: "Build database models", name: "DbWorker" },
    { task: "Build route handlers", name: "ApiWorker" },
  ],
});
```

#### 4. consensus — Cooperative Voting

```ts
consensus({
  proposal: "Should we migrate to Fastify?",
  voters: [
    { task: "Evaluate performance", name: "PerfExpert" },
    { task: "Evaluate DX", name: "DxExpert" },
  ],
  consensusType: "majority",
});
```

#### 5. mesh — Peer Collaboration

```ts
mesh({
  goal: "Debug the auth flow returning 500",
  agents: [
    { task: "Check server logs", name: "LogAnalyst" },
    { task: "Review auth code", name: "CodeReviewer" },
    { task: "Write repro test", name: "Tester" },
  ],
});
```

#### 6. handoff — Dynamic Routing

```ts
handoff({
  entryPoint: { task: "Triage the request", name: "Triage" },
  routes: [
    { agent: { task: "Handle billing", name: "Billing" }, condition: "billing, payment" },
    { agent: { task: "Handle tech issues", name: "TechSupport" }, condition: "error, bug" },
  ],
  maxHandoffs: 3,
});
```

#### 7. cascade — Cost-Aware Escalation

```ts
cascade({
  tiers: [
    { agent: { task: "Answer this", cli: "claude" }, confidenceThreshold: 0.7, costWeight: 1 },
    { agent: { task: "Answer this", cli: "claude" }, confidenceThreshold: 0.85, costWeight: 5 },
    { agent: { task: "Answer this", cli: "claude" }, costWeight: 20 },
  ],
});
```

#### 8. dag — Directed Acyclic Graph

```ts
dag({
  nodes: [
    { id: "scaffold", task: "Create project scaffold" },
    { id: "frontend", task: "Build React UI", dependsOn: ["scaffold"] },
    { id: "backend", task: "Build API", dependsOn: ["scaffold"] },
    { id: "integrate", task: "Wire together", dependsOn: ["frontend", "backend"] },
  ],
  maxConcurrency: 3,
});
```

#### 9. debate — Adversarial Refinement

```ts
debate({
  topic: "Monorepo vs polyrepo for the new platform?",
  debaters: [
    { task: "Argue for monorepo", position: "monorepo" },
    { task: "Argue for polyrepo", position: "polyrepo" },
  ],
  judge: { task: "Judge and decide", name: "ArchJudge" },
  maxRounds: 3,
});
```

#### 10. hierarchical — Multi-Level Delegation

```ts
hierarchical({
  agents: [
    { id: "lead", task: "Coordinate full-stack app", role: "lead" },
    { id: "fe-coord", task: "Manage frontend", role: "coordinator", reportsTo: "lead" },
    { id: "be-coord", task: "Manage backend", role: "coordinator", reportsTo: "lead" },
    { id: "fe-dev", task: "Build components", role: "worker", reportsTo: "fe-coord" },
    { id: "be-dev", task: "Build API", role: "worker", reportsTo: "be-coord" },
  ],
});
```


### Reflection Protocol

#### All patterns support reflection — periodic synthesis that enables course correction. Enabled via `reflectionThreshold` on WorkflowOptions.

```ts
{
  reflectionThreshold: 10, // trigger after 10 agent messages
  onReflect: async (ctx) => {
    // Examine ctx.recentMessages, ctx.agentStatuses
    // Return adjustments or null
  },
}
```


### Common Mistakes

| Mistake | Why It Fails | Fix |
|---------|-------------|-----|
| Using mesh for everything | O(n^2) communication, debugging nightmare | Use hub-spoke for most tasks |
| Pipeline for independent work | Sequential bottleneck | Use fan-out or dag |
| Hub-spoke for simple parallel tasks | Hub is unnecessary overhead | Use fan-out |
| Consensus for non-decisions | Voting on implementation tasks wastes time | Use hub-spoke, let lead decide |
| No circuit breaker on handoff | Infinite routing loops | Always set maxHandoffs |
| Cascade without confidence parsing | Agents don't report confidence | Convention injection handles this |
| Hierarchical for 3 agents | Management overhead exceeds benefit | Use hub-spoke for small teams |

### DAG Executor — Proven Pattern

#### Agent Completion: Detect → Release → Collect

```
Agent writes summary file → Orchestrator polls (5s) → Detects new mtime →
  Reads summary → Calls client.release(agent) → agent_exited fires → Node marked complete
```

#### State & Resume

```ts
saveState(completed, depsOutput, results, startTime);
// Restart with --resume to skip completed nodes
```


### YAML Workflow Definition

#### Any pattern can be defined in YAML for portability:

```yaml
version: "1.0"
name: feature-dev
pattern: hub-spoke
agents:
  - id: lead
    role: lead
    cli: claude
  - id: developer
    role: worker
    cli: codex
    reportsTo: lead
steps:
  - id: plan
    agent: lead
    prompt: "Create a development plan for: {{task}}"
    expects: "PLAN_COMPLETE"
  - id: implement
    agent: developer
    dependsOn: [plan]
    prompt: "Implement: {{steps.plan.output}}"
    expects: "DONE"
reflection:
  enabled: true
  threshold: 10
trajectory:
  enabled: true
```