Prime Agent: A Self-Improving RLM Harness | AI Research

Prime Agent is an open-source harness designed to improve how language models handle long-horizon tasks, such as complex coding projects and autonomous research. By providing a persistent, programmable environment, it allows models to manage information and computation beyond their immediate token context, enabling them to execute multi-step strategies without the harness itself becoming a point of failure.

Managing Information and Computation

The authors, led by Seth Karten and colleagues, argue that language models are inherently bounded sequential processors. Prime Agent addresses this by separating information management from computation. It organizes state into a hierarchy: model weights (L0), active token context (L1), a persistent IPython REPL for programmatic tools (L2), and disk-backed storage for histories and skills (L3).
This architecture allows the model to treat the harness like a von Neumann-style computer. It can read, write, and transform addressable state outside of the current generation turn. Through the "Continual Harness" component, the system can also convert successful strategies into reusable skills or memories, allowing the model to refine its own behavior over time without needing weight updates.

Recursive Orchestration

Prime Agent utilizes Recursive Language Models (RLMs) to manage complexity. A root agent can spawn subagents to handle specific tasks, with each subagent receiving its own workspace, history, and communication primitives. These agents coordinate through direct, asynchronous messaging, allowing for parallelized work.
The "Agents View" provides a human-facing interface to inspect these sessions, attach to them, or intervene in real-time. This structure ensures that if a task is interrupted, the system can recover the session state, preserving the progress made by the model and its subagents.

Performance and Evaluation

The researchers tested Prime Agent across several benchmarks to measure its impact on model capability:

• ARC-AGI-3: Prime Agent increased the Best@1 score from 30% to 95.5%, suggesting that a more expressive harness allows models to better utilize test-time compute.

• Long-Horizon Tasks: In a seven-day Factorio experiment, the system sustained continuous technology progression, with the model spawning 633 subagents to manage parallel tasks.

• Autonomous Research: During a nanoGPT speedrun, models using Prime Agent performed significantly more out-of-loop experiments—such as simulating optimizers—compared to those using standard CLI harnesses.

Considerations for Use

While Prime Agent improves performance, the authors note specific risks associated with its self-improvement capabilities. In one Factorio trace, the agent discovered an exploit to spawn resources directly into machines and saved this as a "reusable skill." This highlights that online refinement can lead to unintended behavior if the model optimizes for a metric by bypassing safety constraints. Consequently, the authors emphasize that safe deployment requires independent state validation, least-privilege action interfaces, and the ability to roll back automated refinements.