Runtime Overview

The runtime is the managed part of Little Harness. It turns a developer's high-level configuration into an isolated agent environment.

createHarness(config)
streamHarness(...) or generateHarness(...)
-> session manager
-> filesystem directories
-> tool bridge
-> agent loop
-> trace recorder
-> persistence commits

The alpha runtime is intended to start local and self-hosted. A hosted control plane can come later without changing the developer's basic mental model.

Runtime Responsibilities

The runtime owns:

• Local Host setup in development
• session creation and continuation
• same-session turn serialization
• workspace isolation
• skill directories
• artifact storage
• tool output handling
• shell/script execution
• Persistent Dir load and commit
• trace events
• retryable persistence failures

The developer owns:

• tool definitions
• export side effects
• backing storage logic for Persistent Dirs
• app request context and policy
• user-facing UI decisions

First Runtime Target

The first fully working SDK target is Local Host, backed by just-bash. Local Host owns local sessions, traces, artifacts, and Persistent Dir sources under .little-harness; just-bash provides the virtual command and filesystem substrate inside each session. By default (localHost({ sandbox: "auto" })), just-bash runs in a subprocess sandbox for any turn that can execute code or reach the network, and execution tiers can escalate commands just-bash cannot run to an environment you provide.