Steer, Don't Solve: Training Small Critic Models for Large Code Agents

End-to-end code agent training is resource-intensive and plateaus on the strategy-level reasoning needed to resolve code issues, since jointly optimizing code-level execution and strategy-level reasoning leaves the latter underdeveloped. Instead, we freeze the agent and add a critic model to supply that signal. Prior code critics are post-hoc, scoring completed trajectories rather than steering the agent; we instead train a small critic that provides intra-trajectory feedback via Supervised Fine-Tuning. On SWE-bench Verified, a critic trained on CWM-32B trajectories transfers to two unseen agents (gains of +3.0 to +3.8 points), and adding target-agent trajectories to the corpus increases the gain to +3.8 on CWM-32B and +4.4 to +5.2 on two Qwen agents, at 30-92x lower critic cost than a strong teacher. On Qwen3-Next-80B-A3B, the critic-guided system is both more accurate (25.2% vs. 20.8%) and cheaper (\$0.04 vs. \$0.11) than the agent alone, because the critic also shortens trajectories. Our results show that a small, well-trained critic is a practical complement to scaling agent training. Code: https://github.com/shubhamrgandhi/critic-training. Data and models: https://huggingface.co/collections/shubhamrgandhi/critic-training-for-code-agents