microsoft/agent-lightning

3,500-Line Lightweight Agentic RL Framework for Training Agents with Real Harnesses!

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Agent Lightning was completely refactored in v1.0. For legacy releases earlier than v1.0, see this branch.

• [2026/09] We release a new coding agent example based on an MoE model. Pure RL improves Qwen3.5-35B-A3B on SWE-bench Verified from 47.8% to 61.6% after training it on only 1.8K training examples!
• [2026/08] Agent Lightning Skill is released. It helps a coding agent improve another AI agent against a benchmark.
• [2026/08] Agent Lightning v1.0 technical report is released!
• [2026/08] We open source Agent Lightning v1.0!

• 🪶 ~3,500 lines of code: We treat simplicity as the first principle.
• 🧩 Train with real agent harnesses: Agents interact with the model through the Agent Lightning v1.0 proxy with ZERO changes, while keeping tools, context, control flow, and environments in the loop.
• ☸️ Native Kubernetes support: Run agents directly as Kubernetes Jobs without relying on external sandbox services.
• 💻 Full coding agent training example: Using only 6K training samples, an end-to-end Qwen3.5-9B workflow improves SWE-bench Verified from 41.8% to 56.4%, a gain of 14.6 percentage points. We release the full pipeline, including data cleaning, reward-hacking prevention, and training scripts. Update: We release a new coding agent training example based on Qwen3.5-35B-A3B. Pure RL improves Qwen3.5-35B-A3B on SWE-bench Verified from 47.8% to 61.6% after only 1.8K training examples, a gain of 13.8 percentage points.

The following is an example installation on a CUDA 13.0 machine:

cd <this-repo>
uv sync
bash scripts/setup_verl.sh 0.8.0 cu130

See the Installation Guide for details.

Agent Lightning v1.0 keeps the training architecture simple with three lightweight components:

• Trainer: Runs verl and vLLM, builds training samples, and updates the policy.
• API Gateway: Proxies model requests and captures training data.
• Rollout Controller: Runs agents locally or as Kubernetes Jobs.

The Trainer creates rollouts, the Controller launches agents, and the Gateway turns interactions into training data, while agents continue to run with their real harnesses.

We evaluate Agent Lightning v1.0 across several practical training domains, including Search R1, LLM-in-Sandbox, and Coding Agent. Pure RL delivers substantial improvements across all three domains, as shown below.

• 8/19/2026 Agent Lightning v1.0: Towards Harnessed Agentic RL technical report.
• 12/17/2025 Adopting the Trajectory Level Aggregation for Faster Training Agent-lightning blog.
• 11/4/2025 Tuning ANY AI agent with Tinker ✕ Agent-lightning Medium. See also Part 2.
• 10/22/2025 No More Retokenization Drift: Returning Token IDs via the OpenAI Compatible API Matters in Agent RL vLLM blog. See also Zhihu writeup.
• 8/11/2025 Training AI Agents to Write and Self-correct SQL with Reinforcement Learning Medium.
• 8/5/2025 Agent Lightning: Train ANY AI Agents with Reinforcement Learning arXiv paper.
• 7/26/2025 We discovered an approach to train any AI agent with RL, with (almost) zero code changes. Reddit.
• 6/6/2025 Agent Lightning - Microsoft Research Project page.

• DeepWerewolf — A case study of agent RL training for the Chinese Werewolf game built with AgentScope and Agent Lightning.
• AgentFlow — A modular multi-agent framework that combines planner, executor, verifier, and generator agents with the Flow-GRPO algorithm to tackle long-horizon, sparse-reward tasks.
• Youtu-Agent — Youtu-Agent lets you build and train your agent with ease. Built with a modified branch of Agent Lightning, Youtu-Agent has verified up to 128 GPUs RL training on maths/code and search capabilities with steady convergence. Also check the recipe and their blog Stop Wrestling with Your Agent RL: How Youtu-Agent Achieved Stable, 128-GPU Scaling Without Breaking a Sweat.

If you use Agent Lightning v1.0 in your research or projects, please cite the technical report:

@misc{he2026agentlightningv10harnessed,
  title={Agent Lightning v1.0: Towards Harnessed Agentic RL},
  author={Zhiyuan He and Siwei Zhang and Zhiwen Zhou and Yuqing Yang and Yu Kang and Yuge Zhang and Luna K. Qiu and Tin Yan Tsui and Jiahang Xu and Chong Luo},
  year={2026},
  eprint={2608.17528},
  archivePrefix={arXiv},
  primaryClass={cs.AI},
  url={https://arxiv.org/abs/2608.17528},
}

For the original Agent Lightning paper, please use:

@misc{luo2025agentlightningtrainai,
      title={Agent Lightning: Train ANY AI Agents with Reinforcement Learning},
      author={Xufang Luo and Yuge Zhang and Zhiyuan He and Zilong Wang and Siyun Zhao and Dongsheng Li and Luna K. Qiu and Yuqing Yang},
      year={2025},
      eprint={2508.03680},
      archivePrefix={arXiv},
      primaryClass={cs.AI},
      url={https://arxiv.org/abs/2508.03680},
}

This project welcomes contributions and suggestions. Start by reading the Contributing Guide for recommended contribution points, environment setup, branching conventions, and pull request expectations. Most contributions require you to agree to a Contributor License Agreement (CLA) declaring that you have the right to, and actually do, grant us the rights to use your contribution. For details, visit https://cla.opensource.microsoft.com.

When you submit a pull request, a CLA bot will automatically determine whether you need to provide a CLA and decorate the PR appropriately (e.g., status check, comment). Simply follow the instructions provided by the bot. You will only need to do this once across all repos using our CLA.

This project has adopted the Microsoft Open Source Code of Conduct. For more information see the Code of Conduct FAQ or contact opencode@microsoft.com with any additional questions or comments.

This project may contain trademarks or logos for projects, products, or services. Authorized use of Microsoft trademarks or logos is subject to and must follow Microsoft's Trademark & Brand Guidelines. Use of Microsoft trademarks or logos in modified versions of this project must not cause confusion or imply Microsoft sponsorship. Any use of third-party trademarks or logos are subject to those third-party's policies.

This project has been evaluated and certified to comply with the Microsoft Responsible AI Standard. The team will continue to monitor and maintain the repository, addressing any severe issues, including potential harms, if they arise.

Agent Lightning v1.0 is released under the MIT License.