StepFun-Prover Preview is a large-scale language model designed for formal theorem proving via tool-integrated reasoning. Using a reinforcement learning pipeline that integrates tool-based interactions, StepFun-Prover achieves robust performance in generating Lean 4 proofs with minimal sampling. This approach allows the model to iteratively improve its proofs based on real-time environmental feedback, emulating human-like problem-solving strategies. On the miniF2F-test benchmark, StepFun-Prover achieves a pass@1 success rate of 70.0%. Beyond improving benchmark performance, we introduce an end-to-end training framework for developing tool-integrated reasoning models, suggesting promising directions for automated theorem proving and mathematical AI assistants.