Experiences developing structural biology tools with AI coding assistance

Large language models (LLMs) offer significant potential to accelerate scientific software development, yet their application to structural biology algorithms requires rigorous evaluation. This challenge is examined through the development of xcif, a new native parser for the crystallographic information file (CIF) format, designed to replace a stable but slow parser (ucif) in the Computational Crystallography Toolbox (cctbx). Using an expert-led, artificial intelligence (AI)-assisted workflow, implementation time was reduced from an estimated several months to approximately one week of active human effort. However, this efficiency relied entirely on domain knowledge to specify constraints, intercept plausible but flawed machine-generated code and design adversarial tests. The machine assistance did not replace programming expertise but rather shifted the human engineering burden from writing code to independent validation. Applying the new xcif parser across a corpus of approximately 550 000 archive files confirmed that it produces output identical to both the legacy implementation and GEMMI, a modern high-performance parser. xcif parses roughly an order of magnitude faster than the legacy cctbx parser and 1.3-1.5 times faster than GEMMI, while reducing peak memory use for larger files. This case study demonstrates that AI assistance provides a highly practical, cost-effective pathway for the renewal of mature structural biology infrastructure, provided that the development is strictly governed by expert oversight and objective verification.