fernel

Track and compensate the climate impact of your AI usage.

Fernel tracks how much your Claude Code, Codex and Gemini CLI agents use, shows what it costs the planet, and helps you take that carbon back out of the air.

free · open methodology · private by design

You ask your agent one question.

It's brilliant at its job. To help you, it quietly re-reads everything it has seen so far: your code, the conversation, every file it opened.

~/code/acme · claude
>
tokens read
0
energy
0 Wh
CO₂e
0 g

It sends everything it has read to a data centre.

Not once, but on every step. GPUs read it all again before writing a single word back.

re-read context
98%
tokens for a 612-token answer
287K

Reading it all takes electricity.

Just a little per prompt, about 4.82 Wh: a LED bulb for half an hour. But agents work all day, and it adds up fast.

one prompt
0 Wh
≈ a 9 W LED bulb for 32 minutes
a heavy month · 12.5B tokens
0 Wh
≈ boiling a full kettle 2,079 times

Making that electricity releases CO₂.

Our grids are getting cleaner, but they still run partly on gas and coal. So the 4.82 Wh behind that one prompt put about 1.91 g of CO₂e into the air. Tiny on its own, but it adds up with every step your agent takes.

a heavy month
0 g
CO₂e, like driving 484 km in an average petrol car

Fernel shows you how much.

One command reads the usage logs already on your laptop: tokens, energy and CO₂e for every project. Your prompts and code never leave your machine.

{
  "date": "2026-11-04",
  "project": "acme-api",
  "model": "claude-opus-5-5",
  "tokens": { "input": 1204, "output": 38211,
              "cacheRead": 9120334 }
}
That's everything Fernel uploads: one line per day, project and model. See it for yourself with fernel sync --dry-run.
ferneloverviewmodels
your garden
80.6 kg CO₂e in 30 days
tokens
12.5B
energy
203 kWh
CO₂e
80.6 kg
  • acme-api
  • web
  • infra
  • docs

And helps you take it back out of the air.

For a few euros a month, fund durable carbon removal that matches your usage: projects that pull CO₂ out of the atmosphere and lock it away for good. Every tonne goes on a public ledger.

monthly contributionopening soon
  • Light
    under 5 kg CO₂e / month
    €3
  • Regular
    5 to 25 kg CO₂e / month
    €7
  • Heavy
    over 25 kg, top up to match
    €20+
Measuring is free, forever.