The AI agent loop, in ~150 lines (no framework)

"AI agent" sounds like it needs a framework. It doesn't. Strip away the branding and an agent is one loop:

1. Send the conversation to the model.
2. If it asks to use a tool, run the tool.
3. Feed the result back.
4. Repeat until it answers.

Here's the whole thing with Claude:

for (let step = 0; step < MAX_STEPS; step++) {
  const runner = anthropic.messages.stream({ model, max_tokens: 1024, system, tools, messages });
  runner.on("text", (delta) => send(delta));            // stream tokens out
  const final = await runner.finalMessage();
  if (final.stop_reason !== "tool_use") break;          // model answered, we're done

  messages.push({ role: "assistant", content: final.content });
  const results = [];
  for (const block of final.content) {
    if (block.type === "tool_use") {
      const result = await runTool(block.name, block.input);   // your code
      results.push({ type: "tool_result", tool_use_id: block.id, content: result });
    }
  }
  messages.push({ role: "user", content: results });    // feed the results back, loop
}

That's it. Tool use is just: the model emits a tool_use block, you run the matching function, you hand back a tool_result. Streaming is just forwarding token deltas as they arrive.

I packaged this into a runnable starter — Next.js, a streaming UI, two example tools, one-command Vercel deploy. Clone it, add a key, ship:

https://github.com/mnifzied-create/agentloop

The free core is the loop above, MIT-licensed. If you want the production patterns next — running multiple tools in parallel, persistent memory on SQLite, retries, an eval harness, human-in-the-loop approval, sub-agents — I wrote those up as a small, equally-readable kit too (linked in the repo).

The point isn't the kit, though. It's that you can read every line of an agent in one sitting. Go look.