Google just open-sourced Harness².

(turn any harness into a self-improving system)

a harness is everything wrapped around the model, like the system prompt, tools, memory, skills, and subagents. today a person designs it once, and the agent runs every new task with that same fixed setup.

Harness² lets the agent refine that setup for each new task on its own, with the model weights left untouched.

on every task, the agent proposes edits to its harness, runs a few candidate versions, and composes a better harness by comparing how those runs behaved.

because it learns from the differences between runs, it needs no correct answer or grader for the task.

that covers a single task. Harness² also carries what it learns from one task into the next.

after each task, it saves the harness it settled on along with the lessons that held up across the runs. the next time it refines a harness, it starts from those notes instead of from scratch.

this means every finished task makes the next round of edits more informed.

Google tested this with Gemini 3.5 Flash and GLM-5 on legal, office, and professional work benchmarks. one round of edits was enough to beat their original harnesses, and giving the agent more rounds pushed scores higher.

those extra rounds can go wide or deep. the agent either tries several candidate harnesses at once or keeps refining a single one step by step, using OpenCode or Codex to carry out the work.

you can find the Harness² repo here: https://github.com/google-research/harness2…

I wrote about a similar idea for agents already running in production, where the harness should repair itself after a failure instead of waiting for someone to debug the trace by hand.

that piece walks through how @Cometml’s Opik (open-source) closes this loop, going from a failing trace to a proposed fix to a regression test that stops the same failure from coming back. it’s quoted below.

google-research/harness2

Source: https://github.com/google-research/harness2

Recursive Agent Harnessing for an Open World

A harness is the system around a model that enables it to act as an agent: prompts, tools, memory, skills, orchestration, and more. Harnessing is the process of optimizing that system.

Harness² is a plug-and-play framework for recursive agent harnessing in an open world, where agents encounter tasks unseen during harness design. It connects two levels of recursion while keeping model weights fixed:

• Task-level recursion refines the execution harness for the current task through a contrastive Propose–Probe–Compose step: propose edits, probe their effects on the same task, and compose a refined harness from behavioral contrasts between executions, without ground-truth feedback.
• Harnessing-level recursion updates a persistent improvement harness with adopted harnesses and evidence-supported lessons from each task to guide future refinement.

Across three professional benchmarks and four model–harness configurations, a single Harness² step improves GLM-5 and Gemini 3.5 Flash over their base harnesses, with gains of up to 11.5 points on WorkBuddy-Bench, 4.5 points on JobBench, and 11.8 percentage points on LAB’s partial pass rate. Increasing the task-level recursion budget brings further gains.

The release supports parallel and sequential recursion, OpenCode and Codex executors, and JobBench, LAB, and WorkBuddy-Bench.

Video overview

https://github.com/user-attachments/assets/76b10548-b14b-4c8f-9aa5-a93662df4d56

Contents

• Video overview
• Harness components
• Install
• Run a benchmark
• Experiments
• Web access
• Results and resume
• Citation
• Contributing
• License
• Disclaimer

Harness components

Harness² exposes eight editable components around the model: system prompt, guardrails, memory, tool guidance, subagents, skills, scripts, and plugins. They control context construction, the tool interface, delegation, and behavior inside the tool loop.

Install

Use Linux, Python 3.12, and uv. From the repository root:

uv venv --python 3.12 .venv
source .venv/bin/activate
uv pip install -e '.[render]'
harness2-demo --output runs/demo --k 2

The local demo runs both recursion modes with deterministic editors and real Python executions. It requires no model credentials.

For model-backed benchmarks, install Node.js 20+, the Google Cloud CLI, and the benchmark dependencies. LAB needs Pandoc and Poppler; WorkBuddy-Bench needs Docker. Then configure your models:

cp .env.example .env
# Edit .env with your Vertex AI project and model IDs.
gcloud auth application-default login

See the setup guide for model settings, authentication alternatives, and the Codex executor.

Run a benchmark

The following example installs LAB and runs one task, then evaluates and reports its result. Model-backed runs incur API costs; start with a small task selection.

harness2-setup lab
harness2-check --bench lab
harness2 --bench lab --k 1 --mode parallel \
  --only-domains antitrust-competition --max-tasks 1
harness2-judge --bench lab --k 1 --mode parallel
harness2-report --bench lab --k 1 --mode parallel

Sequential recursion runs the same commands with --mode sequential. --k is the task-level recursion budget; see Experiments. Judging and reporting must use the same benchmark, executor, subset, --k, --mode, and seed as the refinement run. Add --dry-run to harness2 to preview task selection without model calls.

Choose one benchmark per command. WorkBuddy also takes --subset office, --subset web, or --subset code on every command after setup. The benchmark guides cover each one’s data, domains, and grading.

Experiments

Scale task-level recursion in two ways: parallel recursion explores multiple candidate harnesses and composes refinements from their execution evidence; sequential recursion carries the refined harness and accumulated evidence through successive Propose–Probe–Compose steps.

Parallel recursion explores wider; sequential recursion refines deeper.

After each task, harnessing-level recursion retains the adopted harness and evidence-supported lessons to guide future refinement. See the experiment guide for task selection, concurrency, reproducibility, and baseline comparisons.

Web access

Executor network access is unrestricted, and Codex runs with its sandbox disabled. Public benchmark rubrics may be accessible online. Before evaluation, configure network restrictions to prevent access to benchmark answers and grading criteria.

Results and resume

Results are saved under runs/, organized by benchmark and subset where applicable. They include task summaries, deliverables, adopted harnesses, reflection notes, and evaluation results.

Repeat the same command to resume. Use a new HARNESS2_RUNS_DIR when changing models or task selection. See results and resume for details; each command also supports --help.

Citation

@article{xu2026harness2,
  title   = {Harness$^2$: Recursive Agent Harnessing for an Open World},
  author  = {Xu, Ruiyao and Chen, Yanfei and CuiZhu, Zhongying and Dalvi Mishra, Bhavana and Ming, Yifei and Yu, Han and Han, Rujun and Lee, Chen-Yu and Pfister, Tomas},
  journal = {arXiv preprint arXiv:2610.14224},
  year    = {2026},
  url     = {https://arxiv.org/abs/2610.14224}
}

Contributing

See CONTRIBUTING.md.

License

Harness² is licensed under Apache 2.0. Third-party code under third_party/, benchmarks, and datasets retain their upstream licenses; see third-party notices.

Disclaimer

This is not an officially supported Google product. This project is not eligible for the Google Open Source Software Vulnerability Rewards Program.