Announcing Mastra Harness

Today we’re incredibly excited to announce Mastra Harness. We’ve been working on this for almost six months. The team built MastraCode, a TUI based coding agent we drive every day. Then, we extracted out the best parts into the Harness class for you to build your own harness.

Think about the Harness as the layer around the agent loop. It gives you a conversation you can watch, interrupt, and steer; memory suitable for long runs; storage that persists sessions; control over tool calls; task delegation to subagents; and modes you switch between with specialized agents.

The most fundamental concept of Mastra’s Harness is the Session that persists state across turns. You can switch between plan and build mode, with per-mode tools, models, and instructions, plus a plan-approval workflow.