Arcee AI open-sources nac, the harness it built for itself

Arcee AI open-sourced nac on August 13, the Rust harness it uses internally to supervise training runs and prototype ideas, per Arcee's blog post. The code is Apache-2.0 licensed, free for any team to fork.

nac runs a central orchestrator that tracks threads and structured episodes, meant to keep a long AI task aligned with its original intent. As of August 16, it has 122 stars and 11 forks on GitHub.

nac ships an MCP server, so Claude Code or Codex can dispatch, monitor, and steer nac jobs like any other tool. Arcee describes this as a fast loop for a human to steer, paired with a slower asynchronous loop that does the actual work.

That's a small footprint, and it reads as a design document from a model lab rather than a tool other teams have tested in production.

Arcee argues harnesses are becoming a new kind of inference runtime. Nothing in the post says whether any team outside Arcee has run nac in production yet.