A Copilot-assisted TypeScript runtime loaded .prompty files through gray-matter with the library's default frontmatter engines, so a file that opened with a JavaScript frontmatter block ran its code during parsing. The repair rejects JavaScript frontmatter outright.

The TypeScript Prompty loader reads a .prompty file, splits its frontmatter from the markdown body and builds a typed prompt object. The split used gray-matter with its default engine set, which treats a frontmatter block opened with ---js as JavaScript and evaluates it. The file being parsed is the attacker-controlled artifact, so a prompt file from an untrusted source executed code with the privileges of the Node.js host process at load time, before any template rendering happened. The repair hands gray-matter an explicit engines map whose js and javascript entries throw, so executable frontmatter is refused rather than evaluated, and the accompanying regression test writes a ---js prompt that tries to create a file and then asserts the file never appears.