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.
Updated Sep 2026 · Grounded in real Forward Deployed Engineer interview loops and written to a senior-engineer editorial bar.
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.
Scored on method. The candidate who reproduces each symptom in isolation before touching the code, and who can say why the fix works rather than that it does, passes. The one who rewrites the component from scratch usually ships the same three bugs in a different order. Knowing that TextDecoder needs stream: true is the detail that separates people who have built one of these from people who have read about it.
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