RubricEM: Meta-RL with Rubric-guided Policy Decomposition beyond Verifiable Rewards

Deep research agents plan, search, evaluate evidence, and synthesize long-form reports. Training these agents with reinforcement learning (RL) is challenging because they operate over long horizons and their outputs lack exact answer verifiers. To this end, we introduce RubricEM, a unified RL framework for deep research that combines stagewise credit assignment with reflection-based meta-RL for experience reuse. Rubrics unify the framework by providing a shared interface for policy execution, judge feedback, and experience reuse. RubricEM first instills a rubric-guided scaffold through supervised fine-tuning, teaching the agent to plan with rubrics and organize its trajectory into semantic stages. Building on this structural prior, we optimize the task policy with Stage-Structured GRPO, using stagewise judge feedback for finer-grained credit assignment. In parallel, we asynchronously train a shared-backbone reflection meta-policy (with meta-RL) to distill completed trajectories into reusable guidance, contributing through both shared-parameter updates and textual memory. The resulting RubricEM-8B model achieves strong performance across four long-form research benchmarks, outperforming existing open models of larger sizes and approaching proprietary deep-research systems. Beyond final performance, we perform thorough controlled analyses to understand the key ingredients of RubricEM.