# s04: Subagents

> A subagent runs with a fresh messages list, shares the filesystem with the parent, and returns only a summary -- keeping the parent context clean.

## The Problem

As the agent works, its messages array grows. Every tool call, every file
read, every bash output accumulates. After 20-30 tool calls, the context
window is crowded with irrelevant history. Reading a 500-line file to
answer a quick question permanently adds 500 lines to the context.

This is particularly bad for exploratory tasks. "What testing framework
does this project use?" might require reading 5 files, but the parent
agent does not need all 5 file contents in its history -- it just needs
the answer: "pytest with conftest.py configuration."

The solution is process isolation: spawn a child agent with `messages=[]`.
The child explores, reads files, runs commands. When it finishes, only its
final text response returns to the parent. The child's entire message
history is discarded.

## The Solution

```
Parent agent                     Subagent
+------------------+             +------------------+
| messages=[...]   |             | messages=[]      | <-- fresh
|                  |  dispatch   |                  |
| tool: task       | ---------->| while tool_use:  |
|   prompt="..."   |            |   call tools     |
|                  |  summary   |   append results |
|   result = "..." | <--------- | return last text |
+------------------+             +------------------+
          |
Parent context stays clean.
Subagent context is discarded.
```

## How It Works

1. The parent agent gets a `task` tool that triggers subagent spawning.
   The child gets all base tools except `task` (no recursive spawning).

```python
PARENT_TOOLS = CHILD_TOOLS + [
    {"name": "task",
     "description": "Spawn a subagent with fresh context.",
     "input_schema": {
         "type": "object",
         "properties": {
             "prompt": {"type": "string"},
             "description": {"type": "string"},
         },
         "required": ["prompt"],
     }},
]
```

2. The subagent starts with a fresh messages list containing only
   the delegated prompt. It shares the same filesystem.

```python
def run_subagent(prompt: str) -> str:
    sub_messages = [{"role": "user", "content": prompt}]
    for _ in range(30):  # safety limit
        response = client.messages.create(
            model=MODEL, system=SUBAGENT_SYSTEM,
            messages=sub_messages,
            tools=CHILD_TOOLS, max_tokens=8000,
        )
        sub_messages.append({
            "role": "assistant", "content": response.content
        })
        if response.stop_reason != "tool_use":
            break
        # execute tools, append results...
```

3. Only the final text returns to the parent. The child's 30+ tool
   call history is discarded.

```python
    return "".join(
        b.text for b in response.content if hasattr(b, "text")
    ) or "(no summary)"
```

4. The parent receives this summary as a normal tool_result.

```python
if block.name == "task":
    output = run_subagent(block.input["prompt"])
results.append({
    "type": "tool_result",
    "tool_use_id": block.id,
    "content": str(output),
})
```

## Key Code

The subagent function (from `agents/s04_subagent.py`,
lines 110-128):

```python
def run_subagent(prompt: str) -> str:
    sub_messages = [{"role": "user", "content": prompt}]
    for _ in range(30):
        response = client.messages.create(
            model=MODEL, system=SUBAGENT_SYSTEM,
            messages=sub_messages,
            tools=CHILD_TOOLS, max_tokens=8000,
        )
        sub_messages.append({"role": "assistant",
                             "content": response.content})
        if response.stop_reason != "tool_use":
            break
        results = []
        for block in response.content:
            if block.type == "tool_use":
                handler = TOOL_HANDLERS.get(block.name)
                output = handler(**block.input)
                results.append({"type": "tool_result",
                    "tool_use_id": block.id,
                    "content": str(output)[:50000]})
        sub_messages.append({"role": "user", "content": results})
    return "".join(
        b.text for b in response.content if hasattr(b, "text")
    ) or "(no summary)"
```

## What Changed From s03

| Component      | Before (s03)     | After (s04)               |
|----------------|------------------|---------------------------|
| Tools          | 5                | 5 (base) + task (parent)  |
| Context        | Single shared    | Parent + child isolation  |
| Subagent       | None             | `run_subagent()` function |
| Return value   | N/A              | Summary text only         |
| Todo system    | TodoManager      | Removed (not needed here) |

## Design Rationale

Process isolation gives context isolation for free. A fresh `messages[]` means the subagent cannot be confused by the parent's conversation history. The tradeoff is communication overhead -- results must be compressed back to the parent, losing detail. This is the same tradeoff as OS process isolation: safety and cleanliness in exchange for serialization cost. Limiting subagent depth (no recursive spawning) prevents unbounded resource consumption, and a max iteration count ensures runaway children terminate.

## Try It

```sh
cd learn-claude-code
python agents/s04_subagent.py
```

Example prompts to try:

1. `Use a subtask to find what testing framework this project uses`
2. `Delegate: read all .py files and summarize what each one does`
3. `Use a task to create a new module, then verify it from here`