Google open-sources Harness², a framework that improves an agent's code while the model stays frozen
Google Cloud AI Research open-sourced Harness², an Apache 2.0 framework that recursively improves an agent's tools and code around a frozen model.
Google Cloud AI Research has open-sourced Harness², a recursive framework that improves the code and files around a frozen model. The weights never change. What changes is the harness: prompts, tools, memory, skills and orchestration, the system that lets a model act as an agent.
It works on two levels. Task-level recursion refines the execution harness for the current task, without ground-truth feedback. Harnessing-level recursion updates a persistent improvement harness with adopted harnesses and lessons from each task. Eight components are exposed as files in the runtime: system prompt, guardrails, memory, tool guidance, subagents, skills, scripts and plugins.
The numbers, per the repo: a single Harness² step improved GLM-5 and Gemini 3.5 Flash over their base harnesses, with gains of up to 11.5 points on WorkBuddy-Bench, 4.5 on JobBench and 11.8 percentage points on LAB's partial pass rate. The paper says parallel recursion adds up to 3.9 points on WorkBuddy-Bench, 4.2 points on JobBench and 6.7 percentage points on LAB Partial. The default harness editor is Claude Opus 4.8 running under OpenCode.
The install is Linux, Python 3.12 and uv. The license is Apache 2.0, and the release supports OpenCode and Codex executors. Google says it is not an officially supported product and is not eligible for its Open Source Software Vulnerability Rewards Program.
My take: the release gives no API cost for multi-step recursion, and that is the number I want before pointing this at a billing account.
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A Google research team releases Harness² under Apache 2.0, and the default editor is a Claude model. Generous, glamorous and wonderfully awkward.
Three benchmarks, four configurations, and "up to" carrying every headline number. I will wait for the version where the editor is not Claude Opus 4.8.
