# Built an agent fleet that helps farmer's for free

> Source: <https://dev.to/oj-iro/built-an-agent-fleet-that-helps-farmers-for-free-ebj>
> Published: 2026-08-31 23:30:32+00:00

*I built this for the All Things Agentic Hackathon. This post covers how it works and what broke along the way.*

A satellite can't see crop disease. It sees one signal — plants looking less healthy than they should — and that has a dozen causes: drought, harvest, haze, a field boundary drawn over a car park. Every satellite tool alerts on that drop, and because most alerts are nothing, farmers stop opening them.

So I built **Fieldhand**: a fleet of agents where most of the effort goes into trying to prove its own findings wrong.

Nobody logs in. Cloud Scheduler → Pub/Sub wakes it at 4am, a **Watchman** picks the fields worth looking at, and each runs through a Google ADK fleet: **Diagnostician** (reads the satellite evidence) → **Skeptic** (attacks the diagnosis) → **Agronomist** (plans treatment + a real spray window) → **Operations** (work orders). Anything that costs money stops at a human.

It's not told to "verify" — a model asked to verify goes and finds supporting evidence. It's told to find the reason the diagnosis is *wrong*. Its main instrument is arithmetic: compare this field to its neighbours, and to its own history in prior years. On a real Palouse field reading −5.5σ (catastrophic), the Skeptic pulled 3 years of history, saw it's harvested and bare every August, and refused to alarm. During testing it also caught a "dying 400-acre field" I'd drawn over downtown Pullman — it looked at the imagery, saw rooftops, and dropped confidence to 0.10.

`max_output_tokens`

covers thinking AND the answer.`"$6,044"`

), which crashed the policy layer until I coerced at the tool boundary.The policy layer is plain code with no model in it. Even a wrong or prompt-injected agent physically can't spend a grower's money. The interesting engineering wasn't making the agents smart — it was deciding where they *don't* get a vote.

Built with Google ADK · Gemini 3.5/3.7 Flash on Vertex AI · Gemma · Cloud Run · Firestore · Cloud Scheduler + Pub/Sub. Data from Sentinel-2 and Open-Meteo, both free.

Live: [https://fieldhand-web-37zk72cqha-uc.a.run.app](https://fieldhand-web-37zk72cqha-uc.a.run.app)

Code: [https://github.com/OJ-IRO/fieldhand](https://github.com/OJ-IRO/fieldhand)