There are many agent harnesses.
traaviis builds yours▋

Every sub-agent writes to the same pipe.

What the orchestrator hands each sub-agent is a command line, and what comes back is a structured value — not text. So results compose with | exactly like a shell: generic combinators (where, map, each, count) work with any command without re-parsing. Build a pipeline and watch it stream — this is the real engine.

Teach the engineer a new move.

There is no plugin API beyond "register a command." Drop a .mjs in ~/.traaviis/extensions/ and it joins the same registry the built-ins use — instantly available to you, to a pipe, and to the orchestrator's sub-agents. Declare a capability and the built-ins it needs, and it composes with everything else. Register the example and run it:

// ~/.traaviis/extensions/deploy.mjs
export default function (h) {
  h.command({
    name: 'deploy',
    capability: 'ops.deploy',        // resolvable by capability
    needs: ['build'],              // composes with a built-in
    async run(h, [name], input) {
      await h.invoke('build', [name]);  // reuse another command
      return { deployed: name };        // pipe-able output
    },
  });
}

Swap an implementation, not the kernel.

Sub-agents, providers, and governance backends are reached by capability, never by hard wiring. Pick a provider for each capability below and watch the harness re-resolve — and flag any unmet need. /plugins reports exactly this at runtime.

Every action clears a verdict, not a popup.

Before the orchestrator lets a sub-agent act, the action is piped through the box-and-box kernel, which returns a certificate: floor-then-gradient over an un-weakenable safety floor, feasible ▸ permitted ▸ best. The verdict is a value you can pipe, attach to the session tree, and gate CI on — so autonomy never outruns governance.

A kernel you can hold in your head.

The engineer is small on purpose: a command registry, a pipe engine, capability manifests, a session tree, output modes — plus the orchestration loop that drives them. Everything else is an extension. Here's what's deliberately left as a seam, not baked in.