Kimina Lean Server: Technical Report

Abstract: We introduce the Kimina Lean Server, an open-source project that enables fast and scalable interaction with Lean 4 via a unified REST API, designed as a simple verifier for reinforcement learning pipelines. Built on top of the Lean FRO's LeanREPL, it combines server-side parallelization by managing multiple Lean REPL processes in parallel, with an LRU caching strategy that reuses Lean imports across multiple requests. These features help reduce initialization overhead and allow large-scale batch processing of Lean code. The client-side interface allows users to submit batches of proofs and receive Lean feedback, including extracted tactics and tactic states via infotree processing. These features enable a high-performance, scalable workflow for both interaction and extraction of proofs, tactics, and tactic states. We open source our implementation on GitHub.