Skarn aims to make AI coding agents cheaper and safer on a Mac
Skarn is a single Rust program built to cut the cost and risk of running . These agents often send long shell output and full MCP tool descriptions back to the model on every step, which uses many tokens. Skarn compresses shell output by 70 to 90 percent while keeping errors and warnings, and it replaces full tool descriptions with three meta-tools and a short script that runs tools on the server side.
In the creator’s test with 16 servers, a multi-step task dropped from about 150,000 to about 2,000. The claimed token reduction is 70 to 99 percent depending on the task. For safety, Skarn limits every command using built-in controls.
On macOS it uses Seatbelt, on Linux it uses Landlock and seccomp, and on Windows it uses AppContainer and Job Object. Commands are locked to the project folder with the network turned off, and startup is claimed to be under 5 milliseconds without Docker, a daemon, or a Linux on macOS.
Key points
- Skarn is a single Rust binary.
- It reduces shell output so spend fewer tokens.
- In one 16-server test, fell from about 150,000 to about 2,000.
- On macOS, it uses Seatbelt to limit what commands can access.
- It aims to run commands without Docker, with no daemon, no network access, and startup under 5 milliseconds.