Building a Real Weather Agent with LangGraph

We have been training our assistant step by step. It began with toy tools, multiplying numbers, pretending to translate text, and always announcing sunny skies in Paris. Then we taught it to route intelligently, juggle tools in parallel, and recover gracefully when things went wrong.

Now it is time to graduate from sandbox mode. In this lesson, we will build a real weather agent powered by a live API. This will not be just a thought experiment. We will create a LangGraph workflow that can:

• Understand natural language queries about the weather.

• Geocode city names into latitude and longitude.

• Fetch real forecasts from Open-Meteo, a free public API.

• Translate cryptic weather codes into human-readable descriptions.

• Summarize multiple results in clear, conversational language.

• Handle missing data or rare codes gracefully without crashing.

At this point, our assistant stops being a polite role player and becomes a genuinely useful tool.

Why real APIs change the game?

Up until now, we have worked inside an idealized bubble. Our so-called weather expert was more like an actor in costume, reciting scripted lines with no real connection to the outside world. Every answer sounded polished but hollow because nothing was tied to actual data.

Bringing in a live API changes the rules in important ways:

• Truthfulness: The assistant is no longer free to invent conditions. It is anchored to external data feeds. If Paris is rainy, it will say so. If Berlin is windy, you will hear about it. The answers are tethered to reality instead of imagination.

• Unpredictability: The assistant’s output is now shaped by more than the model’s phrasing. It also reflects the real world’s ever-changing conditions. Testing becomes less about verifying canned responses and more about exploring dynamic scenarios.

• Complexity: Real APIs do not speak in plain English. They return JSON objects, numeric codes, and structured fields. Our assistant must act as translator, converting ...