Two moments every ML researcher knows. You get onto a new cluster, and week one goes to fitting the framework to your setup, not training. A new architecture lands, and trying it means hacking through a gigantic codebase to stay compatible with the pipeline. What you want to change is small. The code you wade through to change isn't.

This experience is likely not alone, and many researchers we’ve talked to run into similar issues. A year of this on CMU's FLAME cluster left us with one question: what if a framework were built for an agent to adapt and evolve, not just for humans to maintain?

So we introduce PithTrain: a compact, agent-native MoE training system, now ~11K lines of Python, on four principles:

- Compact: fits in one context window - Python-native: readable tracebacks, no compiled-extension rebuilds - No implicit indirection: direct calls, each model in its own file - Agent skills: in-repo playbooks for recurring tasks

Then we measured the thing nobody measures. Same agent, same tasks, only the framework underneath changes: on PithTrain it finishes with up to 62% fewer turns and 64% less GPU time than production frameworks, while training just as fast.

We call this second axis agent-task efficiency, and we believe it deserves to sit alongside training throughput as a metric worth optimizing. Excited to see what people build with it.

Built with amazing collaborators @haok1402, Haozhan Tang, Akaash Parthasarathy, @Zichun_Yu, @junrushao, Todd Mowry, @XiongChenyan and @tqchenml.

Blog: https://blog.mlc.ai/2026/06/01/pithtrain-compact-agent-native-moe-training-system Code: https://github.com/mlc-ai/pith-train Paper: https://arxiv.org/abs/2605.31463