Arcee Trinity-Large-Thinking vs OpenMythos

Arcee AI, a 30-person startup, has released Trinity-Large-Thinking — a 399B sparse mixture-of-experts reasoning model under Apache 2.0. Only 13B parameters activate per token, giving it inference speed 2-3x faster than comparable dense models. In internal benchmarks and early community testing, it ranks #2 on PinchBench, trailing only Anthropic's Opus 4.6, at a list price of $0.90/M output tokens — roughly 96% cheaper than frontier closed models. The model was trained in a $20M, 33-day run on 2,048 NVIDIA Blackwell GPUs. Arcee trained it using a constitutional AI-style process with synthetic chain-of-thought data generated from multiple frontier models, then applied a reinforcement learning phase using outcome-based rewards on math, code, and logic benchmarks. Trinity-Large-Thinking is the strongest open-weight reasoning model released to date on a commercial-friendly license. For companies with privacy requirements or custom deployment needs, it represents a credible alternative to frontier closed APIs — especially for code generation, mathematical reasoning, and structured data tasks where the gap between open and closed models has historically been widest.

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.