Leanstral 1.5 vs. Grok 4.1 Fast

Leanstral 1.5 is a model under the Apache-2.0 license, crafted for proficient proof engineering within Lean 4, with the goal of improving both the functionality and accessibility of formal verification. This model features a remarkable total of 119 billion parameters, of which 6 billion are actively utilized, representing a major leap in efficiency for tasks related to theorem proving, agent-based proof engineering, and practical code verification. The evolution of Leanstral 1.5 included an extensive three-phase training regimen, which encompassed mid-training, supervised fine-tuning, and reinforcement learning through CISPO. In a multiturn environment, the model is designed to accept a theorem statement, propose a proof, and iteratively adjust its strategy based on input from the Lean compiler until the proof compiles successfully or resources run out. Operating in the code agent framework, Leanstral acts similarly to a developer navigating through a file system, enabling it to modify files, run bash commands, and engage with the Lean language server to track goals, errors, and type information in real time. This cutting-edge methodology not only simplifies the proof engineering workflow but also significantly enriches the overall user experience in formal verification tasks, making the process both more efficient and user-friendly. Furthermore, the integration of these capabilities positions Leanstral 1.5 as a transformative tool in the landscape of formal verification.

Grok 4.1 Fast is xAI’s state-of-the-art tool-calling model built to meet the needs of modern enterprise agents that require long-context reasoning, fast inference, and reliable real-world performance. It supports an expansive 2-million-token context, allowing it to maintain coherence during extended conversations, research tasks, or multi-step workflows without losing accuracy. xAI trained the model using real-world simulated environments and broad tool exposure, resulting in extremely strong benchmark performance across telecom, customer support, and autonomy-driven evaluations. When integrated with the Agent Tools API, Grok can combine web search, X search, document retrieval, and code execution to produce final answers grounded in real-time data. The model automatically determines when to call tools, how to plan tasks, and which steps to execute, making it capable of acting as a fully autonomous agent. Its tool-calling precision has been validated through multiple independent evaluations, including the Berkeley Function Calling v4 benchmark. Long-horizon reinforcement learning allows it to maintain performance even across millions of tokens, which is a major improvement over previous generations. These strengths make Grok 4.1 Fast especially valuable for enterprises that rely on automation, knowledge retrieval, or multi-step reasoning. Its low operational cost and strong factual correctness give developers a practical way to deploy high-performance agents at scale. With robust documentation, free introductory access, and native integration with the X ecosystem, Grok 4.1 Fast enables a new class of powerful AI-driven applications.