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A streaming chat UI in React drops tokens, shows garbled characters, and lags on long replies. Find the three bugs and fix them.

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.

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Next in this trackSimulate an infection spreading across a grid over timePremium Next in this trackTag every part of a generated sentence that came from a source, merging overlaps and listing the sources that contributed.Premium Next in this trackShortest path across a grid to collect every key, where locked doors open only once you hold the matching key.Premium

◆ WORK SAMPLE · 60 MIN · RUNS LOCALLY

Now do it: Repair a customer's streaming assistant

Three symptoms filed as one ticket, with three separate mechanisms behind them, plus a fourth defect nobody reported because it is invisible until the server fails. Fix the handler until the grader is green, then explain the state model.

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FEDITOR'S NOTE

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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