#s15: Agent Teams — One Agent Isn't Enough, Form a Team

s01 → ... → s13 → s14 → s15 → s16 → s17 → s18 → s19 → s20

"One agent isn't enough, form a team" — File-based inboxes + teammate threads.

Harness Layer: Teams — Multi-agent collaboration, message bus.

#The Problem

"Refactor the entire backend" touches auth, database layer, API routes, and tests. One agent working on API routes no longer has auth module details in context. The context window is limited, a single agent can't cover every module.

s06's sub-agents are temps, called in for one job, then gone. Some tasks need teammates that can communicate and collaborate.

#The Solution

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Teaching code carries forward S14's capabilities (prompt assembly, task system, background execution, cron scheduling). To stay focused on the team mechanism, it omits full error recovery, memory, and skill systems. Added: MessageBus (file-based inboxes), spawn_teammate_thread (launch teammate threads), inbox injection (Lead receives teammate messages and injects into history).

Sub-agent vs Teammate:

#How It Works

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#MessageBus: File-Based Inboxes

Each agent (including Lead and teammates) has a .jsonl inbox. Send = append a JSON line to the target's file. Read = read file + delete (consumption):

class MessageBus:
    def send(self, from_agent: str, to_agent: str,
             content: str, msg_type: str = "message"):
        msg = {"from": from_agent, "to": to_agent,
               "content": content, "type": msg_type,
               "ts": time.time()}
        inbox = MAILBOX_DIR / f"{to_agent}.jsonl"
        with open(inbox, "a") as f:
            f.write(json.dumps(msg) + "\n")

    def read_inbox(self, agent: str) -> list[dict]:
        inbox = MAILBOX_DIR / f"{agent}.jsonl"
        if not inbox.exists():
            return []
        msgs = [json.loads(line) for line in inbox.read_text().splitlines()]
        inbox.unlink()  # consume: read + delete
        return msgs

Why files instead of in-memory queues? Teaching code uses files because they're intuitive and observable across threads. Real CC also uses file inboxes (~/.claude/teams/{team}/inboxes/) but adds proper-lockfile for concurrent write safety. The teaching version's read_inbox has a read + unlink race, concurrent reads could lose messages, acceptable for teaching purposes.

#spawn_teammate_thread: Launching a Teammate

Lead calls the spawn_teammate tool to start a teammate. The teammate runs in its own daemon thread with its own system prompt, messages, and simplified tool set:

def spawn_teammate_thread(name: str, role: str, prompt: str) -> str:
    system = f"You are '{name}', a {role}. Use tools to complete tasks."

    def run():
        messages = [{"role": "user", "content": prompt}]
        sub_tools = [bash, read_file, write_file, send_message]
        for _ in range(10):           # max 10 rounds
            inbox = BUS.read_inbox(name)
            if inbox:
                messages.append({"role": "user",
                    "content": f"<inbox>{json.dumps(inbox)}</inbox>"})
            response = client.messages.create(
                model=MODEL, system=system, messages=messages[-20:],
                tools=sub_tools, max_tokens=8000)
            # ... execute tools, process results
        # Send final summary to Lead
        BUS.send(name, "lead", summary, "result")

    threading.Thread(target=run, daemon=True).start()

Key design:

• Simplified tool set: bash, read, write, send_message. Teaching code omits tasks and cron to focus on communication. Real CC teammates also have TaskCreate, TaskUpdate, etc., the task system is shared across the team
• Teaching: 10 rounds max: prevents infinite loops. Real CC uses idle loop: after each round, send idle_notification, wait for inbox messages, resume on arrival, exit only on shutdown_request
• Auto-report on completion: BUS.send(name, "lead", summary) sends the final result to Lead's inbox

#Lead's Inbox Injection

Lead checks inbox after each main loop iteration. Teammate messages are injected into history so the LLM can see and react to them:

# After main loop iteration
inbox = BUS.read_inbox("lead")
if inbox:
    inbox_text = "\n".join(
        f"From {m['from']}: {m['content'][:200]}" for m in inbox)
    history.append({"role": "user",
                    "content": f"[Inbox]\n{inbox_text}"})

Teaching code injects in the user input loop. Real CC is more refined, Lead's useInboxPoller checks every 1 second, submitting messages as new turns without waiting for user input.

#Permission Bubbling

Teaching code omits permission bubbling. Real CC's flow (permissionSync.ts, useSwarmPermissionPoller.ts):

1. Teammate encounters an operation needing approval → sends permission_request to Lead's inbox
2. Lead's useInboxPoller detects the request → routes to approval queue
3. User approves → Lead sends permission_response back to teammate
4. Teammate's useSwarmPermissionPoller (polls every 500ms) receives reply → continue or reject

#Putting It Together

1. Lead: "Build the backend: one agent isn't enough, form a team"
2. Lead → spawn_teammate("alice", "backend dev", "Create database schema")
3. Lead → spawn_teammate("bob", "frontend dev", "Write API client")
4. Alice thread starts → her own LLM call → bash "python manage.py migrate"
5. Bob thread starts → his own LLM call → write_file("client.ts", ...)
6. Alice done → BUS.send("alice", "lead", "Schema done: users, orders tables")
7. Bob done → BUS.send("bob", "lead", "Client written with types")
8. Lead next iteration → inbox injected into history → LLM sees both results

Two teammates work in parallel.

#Changes from s14

#Try It

cd learn-claude-code
python s15_agent_teams/code.py

Try these prompts:

1. Spawn alice as a backend developer. Ask her to create a file called schema.sql with a users table.
2. Check your inbox for alice's result.
3. Spawn bob as a tester. Ask him to check if schema.sql exists and list its contents.

What to observe: How does Lead spawn teammates? What do the .mailboxes/ JSONL files look like? After teammates finish, is Lead's inbox injected into history?

#What's Next

Teammates can work and communicate. But if Lead wants Alice to shut down, killing the thread outright could leave half-written files. A graceful shutdown protocol is needed: Lead sends shutdown_request, teammate wraps up and exits.

s16 Team Protocols → Shutdown handshake and message conventions.

{
  "name": "my-team",
  "leadAgentId": "lead@my-team",
  "members": [{
    "agentId": "researcher@my-team",
    "name": "researcher",
    "agentType": "general-purpose",
    "color": "blue",
    "isActive": true
  }]
}