Harness Engineering for Self-Improvement | Lil'Log
Table of Contents
Harness Design Patterns
Pattern 1: Workflow Automation
Pattern 2: File System as Persistent Memory
Pattern 3: Sub-agent and Backend Jobs
Case study: Coding Agent Harness
Harness Layer vs Core Intelligence?
Harness Optimization
Context Engineering
Workflow Design
Self-Improving Harness
Evolutionary Search
Joint Optimization with Model Weights
Future Challenges
Citation
Appendix: Some useful benchmarks
References
The concept of recursive self-improvement (RSI) dates back to I. J. Good (1965), where he defined an “ultraintelligent machine” as a system that can surpass humans in all intellectual activities and design better machines to improve itself. Yudkowsky (2008) used the phrase “recursive self-improvement” for a specific feedback loop: an AI uses its current intelligence to improve the cognitive machinery that produces its intelligence.
This feedback loop in modern AI may indicate the model rewriting its own weights directly, or more broadly the model improves the training pipeline and the deployment system, which in turn enables a better successor model with improved performance across economically valuable tasks. The speed of research development in AI has been shown to drastically accelerated in frontier labs (Anthropic; OpenAI).
I explicitly mention “deployment system” because the layer between the raw model and the real-world context seems to be as important as the model’s raw intelligence (i.e. the evals right after pretraining). Harnesses are important components of AI deployment, as shown by successful coding agent products such as Claude Code and Codex. A harness is the system surrounding a base model that orchestrates execution and decides how the model thinks and plans, calls tools and acts, perceives and manages context, stores artifacts, and evaluates results.
This one post will focus on research around harness engineering and how it contributes to RSI. Much recent work on auto-research, self-improving agents, and evolutionary program search can be organized around this question. Other work on model self-play, synthetic data, test-time training and a broader theme of continual learning also matches the RSI vision (e.g. Yuan et al. 2024, Chen et al. 2024), Zhao et al. 2025, Choi et al. 2026)) but they will not be the focus of this post.
Harness Design Patterns#
Compared with early agent frameworks, “agent = LLM + memory + tools + planning + action”, harnesses engineering additionally include workflow design (e.g. loop engineering), evaluation, permission controls, and persistent state management. It is no longer only prompt templates, but closer to runtime and software system design: how the model observes, acts, memorizes, checks itself, and improves.
The design should be deliberately simple and generic to enable generalization, likely with reference to existing software engineering practices to benefit from prertaining knowlege. There is also a strong analogy between operating systems and harnesses. Similar to an OS, a harness should encapsulate complicated logic while keeping the interface simple. Meanwhile, configs, tool interfaces and other protocols may gradually become standardized across the industry.
Pattern 1: Workflow Automation#
Defining a workflow in which the model can operate, test, and iterate is a key design for automation. Karpathy’s autoresearch repo (https://github.com/karpathy/autoresearch) is a clean example of how such a workflow can be constructed. A common workflow follows a goal-oriented loop of plan, execute, observe/test, improve, and execute again until the goal is achieved. The process may trigger proactive requests to users for clarity in task specification or execution preference.
A simplified Codex agent loop: the agent calls tools and tool responses affect the model's next generation.<br>(Image source: OpenAI codex agent post)
The workflow graph also emphasizes the model analyzing its own trajectories and failure cases and then iterating on its progress through an “agent runtime” rather than a static prompt template.
Pattern 2: File System as Persistent Memory#
A recurring pattern in long-horizon agent systems is simple control over rich states and artifacts. A harness should not carry the entire workflow and all logs in context; instead, it should keep durable state in files. In long-horizon agentic rollout, artifacts such as experiment logs, code diffs, paper summaries, error traces, and past rollout trajectories often grow much longer than the context window that the model has trained for.
Learning how to read, write, and edit the file system (commonly via bash commands) is a foundation skill for LLMs, and thus managing persistent memory in the simple form of files naturally benefits from improvements in core model capability.
Pattern 3: Sub-agent and Backend Jobs#
A harness can spawn...