OpenAI Agents SDK Vs Little Harness
When to choose OpenAI's agent runtime versus a model-neutral app harness.
Little Harness: published on npm as little-harness@0.2.0-alpha.0 (2026-10-05). Sections labeled UNRELEASED describe source-only changes. Release status
OpenAI Agents SDK is a batteries-included agent runtime with tools, handoffs, sessions, guardrails, MCP, hosted tools, tracing, and sandbox agents.
It is one of the strongest general-purpose alternatives because it solves more than just model calls.
Use OpenAI Agents SDK When
- the app is comfortable being OpenAI-first
- built-in sessions and tracing are a priority
- handoffs, guardrails, hosted tools, or MCP support are central
- the team wants a complete runner loop instead of designing one
- sandbox agents fit the execution model
Use Little Harness When
- the app wants model-neutral backend architecture
- the primary API should be AI SDK-style
UIMessage[]streaming - persistence should be app-backed harness directories, not only conversation/session stores
- artifacts and large tool outputs need product-specific handling
- the developer wants to own storage, auth, tenancy, and export side effects
Core Difference
| Question | OpenAI Agents SDK | Little Harness |
|---|---|---|
| Main abstraction | Agent runner with tools, handoffs, sessions, guardrails. | Harness session with files, artifacts, Persistent Dirs. |
| Provider posture | OpenAI-first. | Model-neutral by design. |
| Sessions | Built-in and custom session backends. | App session key plus managed workspace. |
| Files/sandbox | Sandbox agents provide a stronger file-capable story than most SDKs. | Filesystem and persistence are the core product abstraction. |
| Tracing | Built-in tracing by default. | Host-facing traces including model, tool, shell, file, artifact, and commit events. |
How They Could Work Together
Little Harness could expose an OpenAI Agents SDK runtime backend later, or tools inside Little Harness could call OpenAI hosted tools.
The boundary should stay clear: OpenAI Agents SDK can own the runner; Little Harness should own the product-facing workspace and persistence contract.