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Chinese AI startup Z.ai, known internationally for its growing lineup of powerful, largely open source GLM series of language models, today released GLM-5.3 with substantial gains in long-horizon coding and a more consequential — and potentially sensitive — jump in cybersecurity capabilities. Already, GLM-5.3's cyber capabilities have found a "potentially serious vulnerability in Cursor," the AI coding startup recently acquired by SpaceX, according to z.ai developer advocate Lou, posting on X. VentureBeat also tagged Cursor for confirmation on X and is awaiting response. GLM-5.3 is available initially only through the company's GLM Coding Plan and ZCode coding environment, while API access and open weights are coming later, "once safety evaluation and hardening are complete," according to the company. Z.ai says it plans to release weights approximately two weeks after launch. For enterprise developers, the notable part of the release is not simply another round of benchmark improvements. Z.ai says GLM-5.3 uses the same base model as GLM-5.2, with the improvements coming entirely from scaling post-training across more environments, more diverse tasks and additional reinforcement-learning compute. That makes GLM-5.3 something of a test of how far a frontier-scale base model can be pushed without another expensive pretraining cycle. “Scaling post-training is all we did for GLM-5.3,” Z.ai wrote in its technical announcement. The results suggest considerable headroom. But they have also produced an unusual problem for an open-model developer: according to Z.ai, cybersecurity capabilities improved faster than anticipated as training scaled, particularly as tasks progressed from vulnerability identification toward constructing complete exploitation chains. Reuters reported Friday that Z.ai is also introducing controls around some of the model's more advanced capabilities, including a “trusted access” approach for sensitive functionality. A large jump in coding without another base model GLM-5.3 builds on the 743-billion-parameter-scale base model behind GLM-5.2 rather than replacing it. Z.ai instead expanded the post-training system it had already assembled around long-horizon reinforcement learning. Those environments increasingly resemble complete engineering jobs rather than isolated programming exercises. Z.ai describes scenarios in which an agent receives access to codebases, documentation, compute clusters, storage systems and experimental results, then has to diagnose problems, modify systems, run experiments and demonstrate a measurable improvement while preserving correctness. Some tasks are designed to approximate several days of work for an experienced engineer. The approach produced sizable generation-over-generation improvements on Z.ai's reported evaluations. GLM-5.3 jumps from 4.6 to 28.3 on Terminal-Bench 3.0, from 46.2 to 66.9 on DeepSWE v1.1, and from 26.2 to 48.2 on AutomationBench. On Agents' Last Exam CLI, it improves from 23.8 to…
