Arcee Trinity-Large-Thinking vs OpenMythos

Arcee AI released Trinity-Large-Thinking on April 2, 2026 — a 398 billion parameter sparse Mixture-of-Experts reasoning model under the Apache 2.0 license. Built by a 35-person startup that committed $20 million (nearly half its total funding) to a 33-day training run on 2,048 NVIDIA B300 Blackwell GPUs, it's one of the most ambitious open-source bets from a US AI lab. The architecture is unusually sparse: 256 experts with only 4 active per token (a 1.56% routing fraction), which delivers 2–3× faster inference throughput compared to dense models of similar parameter count. At $0.90 per million output tokens via the Arcee API, it costs approximately 96% less than Claude Opus 4.6 at $25 per million — while scoring within two benchmark points on key agent tasks. For enterprises that need a powerful model they can download, fine-tune, and deploy on their own infrastructure without licensing restrictions, Trinity-Large-Thinking fills a real gap. Apache 2.0 means no restrictions on commercial use, and the US origin is an increasingly relevant compliance factor for government and defense customers.

OpenMythos is a community-driven theoretical reconstruction of Claude Mythos's suspected architecture, implementing a Recurrent-Depth Transformer (RDT) — a looped transformer that recycles layers multiple times per forward pass for deeper reasoning without massive parameter growth. The project drew 10,100 GitHub stars in its first week, reflecting intense developer curiosity about what's powering Anthropic's latest generation models. The architecture has three stages: a Prelude (initial layers), a Recurrent Block (looped up to 32 times with shared weights), and a Coda (final layers). Rather than stacking hundreds of unique layers, the recurrent block runs the same weights multiple times with learned injection parameters updating hidden states between loops — enabling implicit chain-of-thought reasoning in continuous latent space without generating intermediate tokens. The project supports Grouped Query Attention (GQA) with optional Flash Attention 2, Multi-Latent Attention (MLA), and sparse MoE with routed and shared experts. Model scales range from 1B to 1T parameters. The key claim is that RDT achieves reasoning depth comparable to fixed-depth models with far more parameters, since computational complexity scales with loop iterations rather than layer count. This would explain how Claude Mythos achieves strong reasoning performance without the extreme parameter counts of brute-force scaling — though Anthropic has neither confirmed nor denied the architecture.