Open Harness

Open Harness is best understood as a set of unopinionated primitives rather than a product. The vendor's pitch is explicit — "No magic, no lock-in — just clean TypeScript APIs built on top of Vercel AI SDK" — and the homepage code samples back it up. A working agent is roughly a dozen lines: import `Agent`, `createFsTools`, `createBashTool`, `NodeFsProvider`, `NodeShellProvider` from `@openharness/core`, construct the agent with a model, tools, optional `systemPrompt` and `maxSteps`, then iterate over `agent.run(messages, input)` and switch on event types. That event surface is narrow and typed, which is the point. The documented events are `text.delta` (with `ev.text`), `tool.done` (with `ev.toolName`), and `done` (with `ev.messages`). Because `done` returns the full message array, continuation is just reassignment — no session object, no opaque graph. For anyone who has debugged state locked inside an agent framework, this is the strongest argument in the SDK's favor, and it is the thing competitors are least able to copy without a rewrite. Strengths, concretely: stateless message arrays; opt-in middleware for turn tracking, retry, compaction, and persistence; hierarchical subagents with background execution and Promise-like combinators; two-phase compaction (prune old tool results, then LLM summarization); async tool-permission callbacks for CLI prompts and web modals; MCP over stdio, HTTP, and SSE; SKILL.md loading; AGENTS.md/CLAUDE.md injection; filesystem and bash tools for Node.js; React hooks and Vue composables. Weaknesses are equally concrete. It is code-first with no visual editor, so non-developers are out of scope and non-TypeScript shops would be maintaining internals they aren't set up to own. The built-in filesystem and bash tools target Node.js specifically. Model access is mediated by the Vercel AI SDK — that is a feature for provider swapping, but it also means your provider roadmap is partly someone else's. External capability arrives through MCP servers, so your integration depth is a function of which MCP servers exist and how well they behave. There is no built-in long-term memory or vector storage layer; if you need durable recall across sessions you bolt on your own persistence middleware (the middleware hook exists for exactly this) and your own store. And there is no managed control plane: deploying, scaling, and watching agents in production is your problem, which is fine for a platform team and less fine for a two-person startup that wanted an agent shipped this quarter. The honest framing: Open Harness is infrastructure for people who intend to own their agent loop. If that's you, the plain-array message model, the `approve` gate, and the subagent combinators are worth the setup. If you want an agent you configure rather than build, this SDK is deliberately not that.