Next Week in The Sequence:

Our series about evaluations dives into coding benchmarks. In research we discuss DeepSeek’s new model Prover-v2. The opinion section discusses the fall of vector DBs. Engineering covers another interesting framework.

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📝 Editorial: The Leaderboard Illusion

This week "The Leaderboard Illusion," researchers from Cohere Labs, Stanford, Princeton, and several other top institutions conduct a sweeping audit of Chatbot Arena, the most visible human preference leaderboard for LLMs. Through analysis of 2 million battles across 243 models and 42 providers, the paper uncovers significant systemic bias in Arena's evaluation pipeline. A small cohort of proprietary model providers have gained a structural advantage via undisclosed private testing, score retraction privileges, preferential sampling, and asymmetrical model deprecation. These mechanisms introduce artifacts that distort leaderboard rankings and encourage overfitting to Arena-specific dynamics, rather than meaningful generalization. For systems like LMArena that aim to elevate transparent, community-driven benchmarking, the implications are immediate and existential.

A core finding is the strategic use of private variant testing by providers like Meta, OpenAI, and Google. These providers routinely evaluate dozens of model variants behind the scenes before choosing which checkpoint to publish. This best-of-N strategy, when combined with Arena's willingness to let providers selectively retract scores, effectively inflates reported performance. The authors simulate this scenario and show that submitting ten private variants can boost Arena Scores by ~100 points—a dramatic shift in a system where single-digit differences influence perceived leadership. Real-world experiments reinforce this: two identical checkpoints for Aya-Vision-8B received Arena Scores differing by 17 points solely due to randomness in sampling. Such dynamics violate the assumptions of the Bradley-Terry model underlying Arena's ranking scheme, compromising its statistical reliability.

This leaderboard gaming is exacerbated by data access asymmetries. Proprietary models from a handful of labs receive the lion's share of Arena battle data, with OpenAI and Google capturing over 20% each. In contrast, 83 open-weight models together account for less than 30%. These disparities are further amplified by aggressive deprecation practices: 205 models were silently removed from the Arena—many open-weight—without public disclosure. Sampling policies also skew in favor of proprietary labs; Meta and Google models reached sampling rates exceeding 30%, while academic or non-profit models often remained in single digits. These feedback loops reward incumbents, turning Arena into an optimization surface for those with privileged access.

Perhaps most alarmingly, performance on Arena is demonstrably tunable. By fine-tuning models on increasing proportions of Arena-style data, the authors observe performance gains of over 112% on ArenaHard, a test set highly correlated with Arena win-rates. These improvements, however, come at the cost of generalization: the same models perform worse on MMLU. This suggests Arena encourages brittle optimization, and that models are being trained to win Arena battles rather than solve real tasks. As the paper shows, prompts on Arena tend to be short, sometimes duplicative, and skew heavily toward developer-centric topics like code and logic puzzles. In effect, access to Arena data is becoming a form of leaderboard insider trading.

The fragility of Arena’s rankings is further exposed through an elegant transitivity breakdown. Deprecating high-signal models under shifting prompt distributions (e.g., increased multilinguality or math tasks) severs important links in the Arena comparison graph. Simulations show that removing a single model mid-evaluation can invert relative rankings between others. Such violations of the Bradley-Terry model's assumptions create latent instabilities that accumulate over time—an especially dangerous dynamic for systems like Arena that are increasingly treated as scientific gold standards.

To its credit, the paper does not stop at critique. It offers a slate of precise, actionable reforms: prohibit score retraction, cap the number of private variants per provider, implement stratified and auditable model deprecation, adopt variance-based sampling, and release quarterly transparency reports. These are not abstract ideals but engineering guidelines that can restore statistical integrity to Arena and help ensure that leaderboards remain tools for science, not marketing.

For initiatives like LMArena that aspire to open and principled AI evaluation, The Leaderboard Illusion reads like both a warning and a blueprint. It challenges us to think beyond gamified rankings and focus instead on fairness, representativeness, and the long-term credibility of our benchmarks. In an era where LLM progress is often measured by leaderboard deltas, this paper reminds us that the scoreboard can be hacked—and that fixing it requires more than better models. It requires better rules.

🔎 AI Research

The Leaderboard Illusion

In the paper The Leaderboard Illusion, researchers from Cohere Labs and academic collaborators audit the Chatbot Arena leaderboard and identify systemic biases. They uncover that proprietary models from major labs enjoy disproportionate data access, private testing privileges, and selective score disclosures, resulting in distorted rankings that favor large providers over open-weight alternatives.

Prover v2

In the paper DeepSeek-Prover-V2: Advancing Formal Mathematical Reasoning via Reinforcement Learning for Subgoal Decomposition, researchers from DeepSeek-AI present a state-of-the-art large language model trained to perform formal theorem proving in Lean 4 by leveraging reinforcement learning and recursive subgoal decomposition. The model, DeepSeek-Prover-V2-671B, achieves 88.9% accuracy on the MiniF2F-test and introduces a new benchmark, ProverBench, while demonstrating a narrowing performance gap between formal proof generation and informal reasoning through models like DeepSeek-V3.

Multimodal Math Reasoning

In the paper Benchmarking Multimodal Mathematical Reasoning with Explicit Visual Dependency, researchers from DAMO Academy, Alibaba Group introduce VCBench to evaluate elementary-level math with visual dependencies. The benchmark contains 1,720 problems with 6,697 images and tests reasoning across spatial, geometric, temporal, and pattern recognition tasks, exposing that even top vision-language models fail to exceed 50% accuracy.

ReasonIR

In the paper ReasonIR: Training Retrievers for Reasoning Tasks, researchers from Meta FAIR and partner institutions propose ReasonIR-8B, a bi-encoder retriever designed for reasoning-intensive retrieval. Using synthetic training data from their ReasonIR-Synthesizer pipeline, ReasonIR-8B achieves state-of-the-art results on BRIGHT and improves RAG performance on MMLU and GPQA while being over 200× cheaper than reranker-based methods.

Phi-4-Mini-Reasoning

In the paper Phi-4-Mini-Reasoning: Exploring the Limits of Small Reasoning Language Models in Math, researchers from Microsoft present a four-stage training pipeline for reasoning-augmented small language models. Their Phi-4-Mini-Reasoning (3.8B) model combines large-scale CoT distillation, preference learning, and RL with verifiable reward to outperform models twice its size on math benchmarks like AIME24 and MATH-500.

🤖 AI Tech Releases

LlamaCon

Meta announced several releases at its LlamaCon conference.

Qwen 3

Alibaba released Qwen3, its latest model specialized in reasoning.

Phi 4 Reasoning

Microsoft released a new version of its Phi models focused on reasoning.

Claude Integrations

Anthropic release Claude Research Integrations with public and private tools.

🛠 AI in Production

📡AI Radar

• Apple and Anthropic are partnering to build a vibe-coding platform.

• FutureHouse just launched its AI platform for scientific discovery.

• Sam Altman’s Worldcoin unveiled a vision for a human-verified identity in the era of AI.

• Google expanded NotebookLM audio summaries to over 50 languages.

• Cast AI raised $108 million for its container automation platform heavily used in AI workflows.

• Data movement platform Fivetran, acquired Census.

• Meta is forecasting $1.4 trillion in AI revenues by 2035.

• Duolingo announced 148 new language courses created with AI.

• Gruve announced $37.5 million in two rounds of finance to use AI to challenge the IT services industry.

• JetBrains open sourced Mellum, a new developer-focused LLM.

• AI recruiting platform Dex raised a $3.1 million pre-seed round.