An LLM API call, in 4 GIFs

This is the first post of series Building TinyAgent where we are going to build a small agent from scratch in Node.js with no frameworks just the API calls.

But before we write an agent, we need to understand what actually happens when you call an LLM. If you've only ever used a SDK, you've probably never seen the raw request and understand how it works. Six lines of code, an API key, and it just works but you have no idea what happened when request was dispatched and response was printed on the screen.

1. The request

Here is the sample API call with each and every section explained in detail.

A few things worth noticing in the API call.

The API is stateless: Every new API call does not remember previous call context. If you want a chatbot that "remembers" earlier messages, you hold the messages array and resend the whole thing every time.

max_tokens is a hard stop, not a target. If you hit the target the response stops mid sentence.

The API call pattern is universal. Different URL, Authorization: Bearer instead of x-api-key, the system prompt lives inside messages rather than at the top level. But it's the same POST, the same JSON, the same {model, messages, max_tokens}. Once you understand the shape, switching providers is just a find-and-replace.

2. The response

The API answers with a JSON blob. There are ~10 fields in it, but only four actually matter:

The one which is mostly skipped is: stop_reason.

It tells you why the model stopped, and in real systems and there could be possible reasons behind it:

end_turn      → finished naturally, you're done
max_tokens    → hit your ceiling, response is truncated
tool_use      → model wants to call a tool (next post!)
stop_sequence → matched one of your stop strings

If you only check the text and ignore stop_reason, you will ship a bug at some point. The response looks fine right up until it doesn't.

The other field worth burning in: usage. It shows you how many tokens went in and came out. You want this number in your logs from day one not after you get a surprise bill. 🤯

3. Tokens

I keep saying "24 input tokens." Here's what that means:

Things that surprise people and is worth noting:

Words don't equal tokens. "Unbelievable" is one word but four tokens. The tokenizer splits on common substrings, not spaces.

Code costs more than it looks def add(a, b): is 8 tokens. Every bracket and comma is its own token.

JSON is expensive. {"a":1} is 7 tokens. If your tool schemas are bloated, they're quietly eating into your budget on every single request.

Non-English costs more Japanese, Hindi, Arabic tend to run 2–4× the token count of the same content in English. If you're building for a global audience, this changes your cost math a lot.

Rule of thumb for English prose: ~1 token ≈ 4 characters ≈ 0.75 words. For everything else, run it through the tokenizer yourself before assuming.

4. The bill

Two meters run on every call. They are priced differently

Output tokens cost roughly 3–5× more than input tokens. That's the one number to internalize about LLM pricing.

cost = (input_tokens  / 1,000,000) × input_price
     + (output_tokens / 1,000,000) × output_price

Three things that follow from the asymmetry:

Long prompts are cheap. Long responses are expensive. Stuffing 50 KB of context into a system prompt is fine. Asking for 50 KB of output is roughly 5× more expensive.
"Thinking" tokens count as output. Reasoning models bill their internal thought at the output rate, even though you don't see it.
Tool schemas eat input on every call. They get resent with every request, just like the system prompt.

At $0.006 per call, 100k calls a day is $600/month from one small feature. Add usage logging now, not when you get the alert. 🚨

5. The whole thing in 20 lines

Here is the complete code of the API call we have discussed above:

Jasmin2895 / TinyAgent

No dependencies and no install setup it is just Node file with API key.

Three things to try before the next post

Run it and watch the numbers Make ten calls, change the prompt length, see how usage moves. You'll build a real instinct for cost faster this way than reading any doc.
Set max_tokens: 20 and ask for something long. Watch it cut off. Check stop_reason. This is a bug you'll hit in production eventually better to meet it on purpose right now
Build a multi-turn chat by hand. Keep a messages array, push each user message and each model reply onto it, and resend the whole thing every turn. Once you do this, you'll immediately understand why long conversations get expensive you're paying for the full history on every call.