#s01: The Agent Loop — One Loop Is All You Need

s01 → s02 → s03 → s04 → ... → s20

"One loop & Bash is all you need" — One tool + one loop = one Agent.

Harness Layer: The Loop — the first bridge between the model and the real world.

#The Problem

You ask the model: "List the files in my directory and run XXX.py."

The model can output a bash command, but once it's done outputting, it stops — it won't execute the command on its own, and it won't keep reasoning based on the result.

You could run it manually, paste the output back into the chat, and let it continue. Next command comes out, you run it again, paste it back.

Every round-trip, you're the middle layer. Automating that is what this chapter is about.

#The Solution

)

A while True loop: keep going when the model calls a tool, stop when it doesn't. The entire process hinges on two signals:

#How It Works

Let's translate this process into code. Step by step:

Step 1: Start with the user's question as the first message.

messages = [{"role": "user", "content": query}]

Step 2: Send the messages and tool definitions to the LLM.

response = client.messages.create(
    model=MODEL, system=SYSTEM, messages=messages,
    tools=TOOLS, max_tokens=8000,
)

Step 3: Append the model's response and check whether it called a tool. No tool call → done.

messages.append({"role": "assistant", "content": response.content})
if response.stop_reason != "tool_use":
    return

Step 4: Execute the tool the model requested and collect the results.

results = []
for block in response.content:
    if block.type == "tool_use":
        output = run_bash(block.input["command"])
        results.append({
            "type": "tool_result",
            "tool_use_id": block.id,
            "content": output,
        })

Step 5: Append the tool results as a new message and go back to Step 2.

messages.append({"role": "user", "content": results})

Assembled into a complete function:

def agent_loop(messages):
    while True:
        response = client.messages.create(
            model=MODEL, 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 = run_bash(block.input["command"])
                results.append({
                    "type": "tool_result",
                    "tool_use_id": block.id,
                    "content": output,
                })
        messages.append({"role": "user", "content": results})

Under 30 lines — that's the minimal runnable agent harness kernel. It's not intelligence itself, but the smallest runtime framework that lets the model keep acting. The model decides (whether to call a tool, which one), the harness executes (if called, run it, feed the result back). The next 18 chapters all add mechanisms on top of this loop. The loop itself never changes.

#Try It

Teaching demo notice: The code executes shell commands generated by the model. Run it in a temporary test directory to avoid affecting your project files. s03 covers the real permission system.

Setup (first run):

pip install -r requirements.txt
cp .env.example .env
# Edit .env, fill in ANTHROPIC_API_KEY and MODEL_ID

Run:

python s01_agent_loop/code.py

Try these prompts:

1. Create a file called hello.py that prints "Hello, World!"
2. List all Python files in this directory
3. What is the current git branch?

What to watch for: When does the model call a tool (loop continues), and when does it not (loop ends)?

#What's Next

Right now the model only has bash — reading files requires cat, writing files requires echo ... >, finding files requires find. Ugly and error-prone.

→ s02 Tool Use: What happens when we give it 5 proper tools? Will the model call multiple tools at once? Will parallel tool executions step on each other?

// query.ts:554-558
// stop_reason === 'tool_use' is unreliable.
// Set during streaming whenever a tool_use block arrives.
let needsFollowUp = false