MetaStone-S1 makes “how long to think” a first-class dial—and it pays off

 Frontier models are learning to trade more inference compute for better answers. MetaStone-S1 turns that trend into a clean architecture: a Reflective Generative Form where the policy and a process reward model live in the same network, adding a light 53M-parameter scoring head instead of a separate, heavyweight judge. The scoring head is trained self-supervised from outcome rewards—no step-by-step human labels—so the system can generate multiple chains of thought and select the best one efficiently. 

Three “reasoning effort” modes, one model

Because the verifier is built-in, MetaStone-S1 exposes controllable thinking lengths—low, medium, high—implemented via different candidate counts (k = 2/8/32) at inference. That makes test-time scaling a product feature rather than a research trick. 

Benchmarks: o3-mini territory at 32B

Across AIME’24/’25 (math), LiveCodeBench (code), and C-Eval (Chinese reasoning), the 32B MetaStone-S1 variants lift accuracy over a strong 32B baseline and land comparable to OpenAI o3-mini (medium)—with the high mode leading math by a sizable margin. Example table slice (Pass@1): AIME’24 85.2, AIME’25 73.6, LiveCodeBench 64.2, C-Eval 89.7 for MetaStone-S1-32B-high vs. o3-mini-medium 79.6 / 74.8 / 67.4 / 75.9. 

At smaller scales, the 1.5B and 7B versions also beat peer open models (e.g., R1-Distill 7B/8B) on AIME and LiveCodeBench, showing the approach is not just a big-model hack. 

Why this matters

• Unified policy+PRM = cheaper selection. Sharing the backbone removes a second giant model from the loop and still delivers strong external TTS gains. 

• Label-free verifier training. The SPRM head learns step scoring from outcome signals, sidestepping costly, noisy process annotations. 

• Production-ready knob. Teams can ship speed/quality dials (k=2/8/32) instead of maintaining separate models for different latency tiers. 

• Open release. Code and checkpoints are public, inviting replication and adaptation. 

MetaStone-S1’s take-home: reasoning power isn’t only about bigger weights or longer chains—it’s about selecting the right trajectory at inference, with a verifier you can actually afford to run.

Paper link: arXiv 2507.01951 (PDF)

MetaStone-S1 shows how to scale ‘thinking time’ instead of parameter count

 For the past year, the mantra in large-language-model land has been simple: bigger weights, better brains. A new paper from the University of Science and Technology of China, Nanjing University and collaborators argues there’s another dial to turn—reasoning time at inference—and it introduces a purpose-built architecture called MetaStone-S1 to prove the point. 

A reflective twist on the policy-reward combo

Standard alignment pipelines bolt a separate process-reward model (PRM) onto a frozen policy network, adding hundreds of millions of parameters and latency. MetaStone-S1 bundles both roles into one backbone and sprinkles in two task-specific heads: one for next-token prediction, the other for step-level scoring. The resulting Self-supervised Process Reward Model (SPRM) weighs in at just 53 M parameters—99 % smaller than conventional PRMs. 

Dial-a-brain at test time

Because reward scoring lives inside the model, MetaStone-S1 can stretch or shrink its chain-of-thought on the fly:

Mode	Avg. reasoning steps	Typical use
Low	~8 steps	latency-sensitive chat
Medium	~24 steps	balanced Q&A
High	up to 64 steps	Olympiad math, code generation

The team coins this knob Test-Time Scaling (TTS) and backs it with an empirical scaling law linking “thinking FLOPs” to quality gains. 

Benchmark bump without parameter bloat

Running in high mode, the 32 B-parameter MetaStone-S1 matches or beats OpenAI o3-mini across AIME ’24/’25, LiveCodeBench and C-EVAL—despite using roughly half the weights. 

Why it matters

• Cheaper alignment. Folding the PRM inside the policy cuts training and inference costs.

• User-controllable latency. Products can trade speed for depth without model swaps.

• Open playground. All code, checkpoints (1.5 B→32 B) and the reasoning-length scheduler are on GitHub under an Apache-2 license. 

MetaStone-S1 won’t end the parameter-scaling race, but it offers a reminder that when and how long a model thinks can count as much as how big it is. Expect TTS dials and reflective reward heads to surface quickly in next-gen open-source stacks.

Paper link: arXiv 2507.01951 (PDF)