encoding

No JSON libraries allowed, and the test data contains commas, newlines, and your escape character itself. Length-prefix encoding solves it in fifteen lines, and knowing why it beats escaping is the actual interview.

This is the shape of the TypeScript debugging round at the agent companies: a forty-line stream handler that mostly works. Each bug has a one-line demonstration, and none of them is in the model. One is a stale closure, one is a decoder splitting a multibyte character, and one is a render per token.