The Agent Loop

[ s01 ] s02 > s03 > s04 > s05 > s06 | s07 > s08 > s09 > s10 > s11 > s12

"One loop & Bash is all you need" -- one tool + one loop = an agent.

Harness layer: The loop -- the model's first connection to the real world.

Problem

A language model can reason about code, but it can't touch the real world -- can't read files, run tests, or check errors. Without a loop, every tool call requires you to manually copy-paste results back. You become the loop.

Solution

+--------+      +-------+      +---------+
|  User  | ---> |  LLM  | ---> |  Tool   |
| prompt |      |       |      | execute |
+--------+      +---+---+      +----+----+
                    ^                |
                    |   tool_result  |
                    +----------------+
                    (loop until stop_reason != "tool_use")

One exit condition controls the entire flow. The loop runs until the model stops calling tools.

How It Works

1. User prompt becomes the first message.

history.push({ role: "user", content: query });

1. Send messages + tool definitions to the LLM.

const response = await client.messages.create({
  model: MODEL,
  system: SYSTEM,
  messages: history,
  tools: TOOLS,
  max_tokens: 8000,
});

1. Append the assistant response. Check stop_reason -- if the model didn't call a tool, we're done.

history.push({
  role: "assistant",
  content: response.content,
});

if (response.stop_reason !== "tool_use") {
  return;
}

1. Execute each tool call, collect results, append as a user message. Loop back to step 2.

const results = response.content
  .filter((block) => block.type === "tool_use")
  .map((block) => ({
    type: "tool_result" as const,
    tool_use_id: block.id,
    content: runBash(block.input.command),
  }));

history.push({ role: "user", content: results });

Assembled into one function:

export async function agentLoop(history: Message[]) {
  while (true) {
    const response = await client.messages.create({
      model: MODEL,
      system: SYSTEM,
      messages: history,
      tools: TOOLS,
      max_tokens: 8000,
    });

    history.push({ role: "assistant", content: response.content });

    if (response.stop_reason !== "tool_use") {
      return;
    }

    const results = response.content
      .filter((block) => block.type === "tool_use")
      .map((block) => ({
        type: "tool_result" as const,
        tool_use_id: block.id,
        content: runBash(block.input.command),
      }));

    history.push({ role: "user", content: results });
  }
}

That's the entire agent in under 30 lines. Everything else in this course layers on top -- without changing the loop.

What Changed

Component	Before	After
Agent loop	(none)	while True + stop_reason
Tools	(none)	bash (one tool)
Messages	(none)	Accumulating list
Control flow	(none)	stop_reason != "tool_use"

Try It

cd learn-claude-code

cd agents-ts
npm install
npm run s01

1. Create a file called hello.ts that logs "Hello, World!"
2. List all TypeScript files in this directory
3. What is the current git branch?
4. Create a directory called test_output and write 3 files in it