Building a Tiny LangGraph Agent with a Tool: A Weather-Aware Trip Planner

Most agent demos are either too simple (toy calculators) or too complex (enterprise-scale pipelines).
Here’s a middle ground: a 5-node LangGraph agent that builds a half-day itinerary for a city, adapts to weather, respects user preferences, and calls a real tool along the way.

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Why this example?

• Small, visualizable graph: plan → weather → search → pick → draft → finalize

• Conditional routing: rainy → indoor picks, sunny → outdoor picks

• Real tool usage: random_weather is a LangChain tool, called explicitly from the graph

• Typed state with Pydantic v2

• Optional LLM for nicer itinerary text (but not required), this example used OpenAI

• Two runnable scenarios: London (rainy) and Barcelona (sunny)

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Architecture

• Nodes

  • plan: log initial context

  • weather: calls random_weather tool → Weather state

  • search: load city places

  • pick: filter by weather & prefs (budget, dislikes)

  • draft: generate itinerary text

  • fallback: relax prefs if nothing chosen

  • finalize: wrap up

• Edges

  • Normal path: plan → weather → search → pick → draft → finalize

  • Fallback path: pick → fallback → draft → finalize

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The Weather Tool

from langchain_core.tools import tool
import random

@tool
def random_weather(city: str) -> dict:
    """
    Return randomized weather for the city.
    """
    conditions = ["sunny", "cloudy", "rain", "storm"]
    cond = random.choice(conditions)
    low, high = (12, 30)
    return {
        "city": city,
        "condition": cond,
        "high_c": float(random.randint(20, high)),
        "low_c": float(random.randint(low, 19))
    }

And in the weather_node:

def weather_node(s: GraphState) -> GraphState:
    w = random_weather.invoke({"city": s.city})  # tool call
    s.weather = Weather(**w)
    s.audit.append(
        f"weather: {s.weather.city} {s.weather.condition} "
        f"{s.weather.high_c}/{s.weather.low_c}°C"
    )
    return s

Full code can be found here -> JordiCorbilla/langgraph-cookbook: langgraph-cookbook

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Example Run

Scenario A: London, dislikes museums, free-only budget

=== Scenario A: London + dislikes=['museum'], free_only ===
[Node] plan
[Node] weather
[Tool] random_weather(London) -> {'city': 'London', 'condition': 'sunny', 'high_c': 30.0, 'low_c': 28.0}
[Node] search
[Node] pick
[Node] draft
[Node] finalize
plan: city=London, half_day=morning, date=2025-08-31
weather: London sunny 30.0/28.0°C
search: 5 candidates
pick: Thames Riverside Walk, Hyde Park Loop, Sky Garden
draft: itinerary ready
finalize: done

Itinerary:
 - **Time:** 9:00 AM - 10:00 AM
 - **Tip:** Begin your day with a refreshing stroll along the river. Enjoy the views and the morning sun. Bring water and wear sunscreen!
 - **Transport:** Take the Tube from Embankment Station to Lancaster Gate (Circle or District Line).
 - **Tip:** This journey takes about 15 minutes. Consider grabbing a coffee from a nearby café before heading to the park.
 - **Time:** 10:15 AM - 11:30 AM
 - **Tip:** Enjoy the beautiful greenery and perhaps find a shaded spot to relax. Look out for the Serpentine Lake and the Diana Memorial Fountain.
 - **Transport:** Take the Tube from Lancaster Gate to Monument Station (Circle or District Line).
 - **Tip:** Allocate about 30 minutes for travel and check-in. Arrive by 11:45 AM to enjoy the stunning views of London.
 - **Time:** 12:00 PM - 12:45 PM
 - **Tip:** Enjoy a drink or snack at the café while taking in panoramic views of the city. Make sure to book your free entry in advance to avoid long waits!
Notes: Weather: sunny

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Scenario B: Barcelona, no dislikes, mixed budget

=== Scenario B: Barcelona + no dislikes, mixed budget ===
[Node] plan
[Node] weather
[Tool] random_weather(Barcelona) -> {'city': 'Barcelona', 'condition': 'sunny', 'high_c': 29.0, 'low_c': 26.0}
[Node] search
[Node] pick
[Node] draft
[Node] finalize
plan: city=Barcelona, half_day=afternoon, date=2025-08-31
weather: Barcelona sunny 29.0/26.0°C
search: 5 candidates
pick: La Sagrada Família (Exterior), Gothic Quarter Stroll, Park Güell
draft: itinerary ready
finalize: done

Itinerary:
 - **Start at La Sagrada Família (3:00 PM - 3:45 PM)**
 - Arrive early to admire the stunning exterior of this iconic basilica. Take photos and enjoy the intricate details of Gaudí's masterpiece.
 - **Gothic Quarter Stroll (4:00 PM - 5:00 PM)**
 - Take a short taxi or tram ride (about 10-15 minutes) to the Gothic Quarter. Wander through the narrow streets, explore hidden squares, and admire the medieval architecture.
 - **Park Güell (5:15 PM - 6:45 PM)**
 - Head to Park Güell via taxi or public transport (approximately 15-20 minutes). Enjoy a leisurely walk through the park, taking in Gaudí's colorful mosaics and panoramic views of the city.
 - **Transport:** Use taxis or rideshare apps for quick transfers between locations, especially in the heat.
 - **Hydration:** Bring water to stay hydrated, as temperatures can be high.
 - **Footwear:** Wear comfortable shoes for walking on cobblestone streets and park paths.
Notes: Weather: sunny

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Why this matters

• Graph, not guesses: deterministic orchestration

• Tools in action: explicit .invoke() calls with visible output

• Policy in code: dislikes, budget filters, weather routing

• Verbose logs: you can follow the agent step by step

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Pros & Cons

Pros

• Compact, easy-to-follow

• Demonstrates both nodes and tools

• Works with or without an LLM

• Great for teaching graph concepts

Cons

• Weather is mocked/randomized

• Places are static, not from real APIs

• No travel-time optimization

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How to run

python trip_agent.py

• Scenario A: London, free-only, dislikes museums → rainy picks

• Scenario B: Barcelona, mixed budget → sunny picks