4. Subagents

The Problem

As the agent works, its messages array grows. Every file read, every bash output stays in context permanently. “What testing framework does this project use?” might require reading 5 files, but the parent only needs the answer: “pytest.”

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 gets a task tool. The child gets all base tools except task (no recursive spawning).

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

2. The subagent starts with messages=[] and runs its own loop. Only the final text returns to the parent.

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
        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)"

The child’s entire message history (possibly 30+ tool calls) is discarded. The parent receives a one-paragraph summary as a normal tool_result.

What Changed From TodoWrite

Key Takeaway

Context isolation is the key insight. The subagent pattern lets the parent delegate messy, exploratory work without polluting its own context. The parent thinks in high-level goals; the child handles the details. Side effects persist on disk, but conversation noise is discarded.