analysis_claude_code/docs/en/s09-agent-teams.md
CrazyBoyM c6a27ef1d7 feat: build an AI agent from 0 to 1 -- 11 progressive sessions
- 11 sessions from basic agent loop to autonomous teams
- Python MVP implementations for each session
- Mental-model-first docs in en/zh/ja
- Interactive web platform with step-through visualizations
- Incremental architecture: each session adds one mechanism
2026-02-21 17:02:43 +08:00

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# s09: Agent Teams
> Persistent teammates with JSONL inboxes turn isolated agents into a communicating team -- spawn, message, broadcast, and drain.
## The Problem
Subagents (s04) are disposable: spawn, work, return summary, die. They
have no identity, no memory between invocations, and no way to receive
follow-up instructions. Background tasks (s08) run shell commands but
cannot make LLM-guided decisions or communicate findings.
For real teamwork you need three things: (1) persistent agents that
survive beyond a single prompt, (2) identity and lifecycle management,
and (3) a communication channel between agents. Without messaging, even
persistent teammates are deaf and mute -- they can work in parallel but
never coordinate.
The solution combines a TeammateManager for spawning persistent named
agents with a MessageBus using JSONL inbox files. Each teammate runs
its own agent loop in a thread, checks its inbox before every LLM call,
and can send messages to any other teammate or the lead.
Note on the s06-to-s07 bridge: TodoManager items from s03 die with
compression (s06). File-based tasks (s07) survive compression because
they live on disk. Teams build on this same principle -- config.json and
inbox files persist outside the context window.
## The Solution
```
Teammate lifecycle:
spawn -> WORKING -> IDLE -> WORKING -> ... -> SHUTDOWN
Communication:
.team/
config.json <- team roster + statuses
inbox/
alice.jsonl <- append-only, drain-on-read
bob.jsonl
lead.jsonl
+--------+ send("alice","bob","...") +--------+
| alice | -----------------------------> | bob |
| loop | bob.jsonl << {json_line} | loop |
+--------+ +--------+
^ |
| BUS.read_inbox("alice") |
+---- alice.jsonl -> read + drain ---------+
5 message types:
+-------------------------+------------------------------+
| message | Normal text between agents |
| broadcast | Sent to all teammates |
| shutdown_request | Request graceful shutdown |
| shutdown_response | Approve/reject shutdown |
| plan_approval_response | Approve/reject plan |
+-------------------------+------------------------------+
```
## How It Works
1. The TeammateManager maintains config.json with the team roster.
Each member has a name, role, and status.
```python
class TeammateManager:
def __init__(self, team_dir: Path):
self.dir = team_dir
self.dir.mkdir(exist_ok=True)
self.config_path = self.dir / "config.json"
self.config = self._load_config()
self.threads = {}
```
2. `spawn()` creates a teammate and starts its agent loop in a thread.
Re-spawning an idle teammate reactivates it.
```python
def spawn(self, name: str, role: str, prompt: str) -> str:
member = self._find_member(name)
if member:
if member["status"] not in ("idle", "shutdown"):
return f"Error: '{name}' is currently {member['status']}"
member["status"] = "working"
else:
member = {"name": name, "role": role, "status": "working"}
self.config["members"].append(member)
self._save_config()
thread = threading.Thread(
target=self._teammate_loop,
args=(name, role, prompt), daemon=True)
self.threads[name] = thread
thread.start()
return f"Spawned teammate '{name}' (role: {role})"
```
3. The MessageBus handles JSONL inbox files. `send()` appends a JSON
line; `read_inbox()` reads all lines and drains the file.
```python
class MessageBus:
def send(self, sender, to, content,
msg_type="message", extra=None):
msg = {"type": msg_type, "from": sender,
"content": content,
"timestamp": time.time()}
if extra:
msg.update(extra)
with open(self.dir / f"{to}.jsonl", "a") as f:
f.write(json.dumps(msg) + "\n")
return f"Sent {msg_type} to {to}"
def read_inbox(self, name):
path = self.dir / f"{name}.jsonl"
if not path.exists():
return "[]"
msgs = [json.loads(l)
for l in path.read_text().strip().splitlines()
if l]
path.write_text("") # drain
return json.dumps(msgs, indent=2)
```
4. Each teammate checks its inbox before every LLM call and injects
received messages into the conversation context.
```python
def _teammate_loop(self, name, role, prompt):
sys_prompt = f"You are '{name}', role: {role}, at {WORKDIR}."
messages = [{"role": "user", "content": prompt}]
for _ in range(50):
inbox = BUS.read_inbox(name)
if inbox != "[]":
messages.append({"role": "user",
"content": f"<inbox>{inbox}</inbox>"})
messages.append({"role": "assistant",
"content": "Noted inbox messages."})
response = client.messages.create(
model=MODEL, system=sys_prompt,
messages=messages, tools=TOOLS)
messages.append({"role": "assistant",
"content": response.content})
if response.stop_reason != "tool_use":
break
# execute tools, append results...
self._find_member(name)["status"] = "idle"
self._save_config()
```
5. `broadcast()` sends the same message to all teammates except the
sender.
```python
def broadcast(self, sender, content, teammates):
count = 0
for name in teammates:
if name != sender:
self.send(sender, name, content, "broadcast")
count += 1
return f"Broadcast to {count} teammates"
```
## Key Code
The TeammateManager + MessageBus core (from `agents/s09_agent_teams.py`):
```python
class TeammateManager:
def spawn(self, name, role, prompt):
member = self._find_member(name) or {
"name": name, "role": role, "status": "working"
}
member["status"] = "working"
self._save_config()
thread = threading.Thread(
target=self._teammate_loop,
args=(name, role, prompt), daemon=True)
thread.start()
return f"Spawned '{name}'"
class MessageBus:
def send(self, sender, to, content,
msg_type="message", extra=None):
msg = {"type": msg_type, "from": sender,
"content": content, "timestamp": time.time()}
if extra: msg.update(extra)
with open(self.dir / f"{to}.jsonl", "a") as f:
f.write(json.dumps(msg) + "\n")
def read_inbox(self, name):
path = self.dir / f"{name}.jsonl"
if not path.exists(): return "[]"
msgs = [json.loads(l)
for l in path.read_text().strip().splitlines()
if l]
path.write_text("")
return json.dumps(msgs, indent=2)
```
## What Changed From s08
| Component | Before (s08) | After (s09) |
|----------------|------------------|----------------------------|
| Tools | 6 | 9 (+spawn/send/read_inbox) |
| Agents | Single | Lead + N teammates |
| Persistence | None | config.json + JSONL inboxes|
| Threads | Background cmds | Full agent loops per thread|
| Lifecycle | Fire-and-forget | idle -> working -> idle |
| Communication | None | 5 message types + broadcast|
Teaching simplification: this implementation does not use lock files
for inbox access. In production, concurrent append from multiple writers
would need file locking or atomic rename. The single-writer-per-inbox
pattern used here is safe for the teaching scenario.
## Design Rationale
File-based mailboxes (append-only JSONL) provide concurrency-safe inter-agent communication. Append is atomic on most filesystems, avoiding lock contention. The "drain on read" pattern (read all, truncate) gives batch delivery. This is simpler and more robust than shared memory or socket-based IPC for agent coordination. The tradeoff is latency -- messages are only seen at the next poll -- but for LLM-driven agents where each turn takes seconds, polling latency is negligible compared to inference time.
## Try It
```sh
cd learn-claude-code
python agents/s09_agent_teams.py
```
Example prompts to try:
1. `Spawn alice (coder) and bob (tester). Have alice send bob a message.`
2. `Broadcast "status update: phase 1 complete" to all teammates`
3. `Check the lead inbox for any messages`
4. Type `/team` to see the team roster with statuses
5. Type `/inbox` to manually check the lead's inbox