Anthropic's Frontier Red Team published a research post on September 29, 2026 analysing Z.ai's open-weight GLM-5.3. The post describes two automated evaluations. On ExploitBench, which targets known vulnerabilities in Chrome's V8 engine, Anthropic reports GLM-5.3 building end-to-end exploits in 50 of 410 attempts and Claude Mythos Preview in 56 of 410. Separately, on what Anthropic calls its internal Binary Exploitation benchmark, which tests whether models can find and exploit vulnerabilities in open source projects participating in Google's OSS-Fuzz, full credit is awarded only for a full control-flow hijack. The post states that it evaluated several models on 100 tasks selected at random from that benchmark, and found GLM-5.3 developing full control-flow hijacks in 4% of trials and Claude Mythos Preview in 6%. The post adds that although GLM-5.3 performs below Claude Mythos Preview on this measure, earlier models such as Claude Opus 4.6 and GLM-5.2 do not succeed in any of them. Anthropic states all tested models were run in isolated, sandboxed environments attacking only offline targets it had set up. The post also references a separate NIST CAISI assessment of September 17, 2026, which Anthropic says found GLM-5.3 to be the most cyber-capable open-weight model released to date while lagging the US frontier by about four months, and says its own capability findings broadly match CAISI's.