agent-builder

Agent Builder Skill

When to use this skill

Load this skill when the user wants to:

• Build a new agent from scratch
• Design a tool for an agent
• Structure a multi-agent system
• Debug an agent loop that isn't working
• Choose between agent architectures

Core principle

The agent is always the model. Your job is the harness.

Harness = Tools + Knowledge + Observation + Action + Permissions

Never try to encode intelligence in your harness code. Give the model clean tools, clear context, and get out of the way.

The minimal agent (always start here)

from anthropic import Anthropic
client = Anthropic()

def agent_loop(messages, tools, dispatch, system):
    while True:
        response = client.messages.create(
            model="claude-sonnet-4-20250514",
            system=system, messages=messages,
            tools=tools, max_tokens=8000,
        )
        messages.append({"role": "assistant", "content": response.content})
        if response.stop_reason != "tool_use":
            return
        results = []
        for block in response.content:
            if block.type == "tool_use":
                output = dispatch[block.name](block.input)
                results.append({"type": "tool_result",
                                 "tool_use_id": block.id, "content": output})
        messages.append({"role": "user", "content": results})

Do not add anything until you need it. Every mechanism should earn its place.

Tool design checklist

Before writing a tool, ask:

• Is the name a verb? (bash, read, write — not "file_manager")
• Does the description say WHEN to use it, not just what it does?
• Is the input schema minimal? No optional fields unless truly needed.
• Does it return plain text the model can reason about?
• Does it have a hard timeout?
• Is output truncated to a safe length (≤50k chars)?

Tool description formula

"[Action verb] [what it does]. Use when [specific situation].
[What it returns]. [Any important limits]."