Discovering Hierarchical Software Engineering Agents via Bandit Optimization

Large language models (LLMs) are increasingly applied to software engineering (SWE), but they struggle on real-world tasks that are long-horizon and often out of distribution. Current systems typically adopt monolithic designs where a single model attempts to interpret ambiguous issues, navigate large codebases, and implement fixes in one extended reasoning chain. This design makes it difficult to generalize beyond training data. Inspired by how human engineers decompose problems into sub-tasks, we argue that SWE agents should be structured as orchestrators coordinating specialized sub-agents, each responsible for a specific sub-task such as bug reproduction, fault localization, code modification, or validation. The central challenge is how to design these hierarchies effectively. Manual decompositions follow human workflows but often mismatch LLM capabilities, while automated search methods such as evolutionary strategies require evaluating a very large number of candidates, making them prohibitively expensive for SWE. We show that formulating hierarchy discovery as a multi-armed bandit problem enables efficient exploration of sub-agent designs under limited budgets. On SWE-bench-Verified, this approach outperforms single-agent systems and manually designed multi-agent systems. On SWE-bench-Live, which features recent and out-of-distribution issues, our system ranks 2nd on the leaderboard with a 36B model, surpassing larger systems such as GPT-4 and Claude. This provides the first evidence that hierarchical multi-agent systems improves generalization on challenging long-horizon SWE tasks.