Coding Agents Invent Facts When Denied Them. All 4 of My Probes Returned a False Zero.

A new arXiv paper watched coding agents get denied the facts they needed. They did not stop. They invented. On August 17th, a group of researchers posted a paper to arXiv with an unglamorous title and a genuinely unsettling core finding. The paper is "The Working Set of a Coding Agent: Coherence Debt in Repository-Scale Tasks" (arXiv:2608.16630), by Bardia Mohammadi, Lars Klein, Aman Chadha, Akhil Arora, and Laurent Bindschaedler. Before going further, one honesty note that will hold for this whole piece: I have read the paper's abstract, not its full text, and every quotation below comes from that abstract. It is enough for what this essay is about, because what this essay is about is one sentence. The setup first. The authors model repository-scale coding as reconstructing a web of coupled facts. Every edit an agent makes needs certain facts, and each fact arrives through one of two channels: it is either in the recent context, or it is in the model's memorized knowledge. Facts covered by neither channel are what the authors call coherence debt. Their experiment supplies and withholds each channel deliberately, injecting faults across "seven models and five harnesses" (abstract), and then watches what the agents do when a needed fact simply is not there. The comfortable prediction is that a competent agent, denied a fact, stops and says so. Here is what the authors report instead: "A missing fact produces wrong work rather than absent work" (abstract). The agent asked to act, acts. In the paper's words, "an agent asked to act acts, fabricating the file or guessing the value" (abstract). That much is alarming in a familiar way. Everyone who works with these systems has a story about an invented function or a guessed constant. The abstract has sharper findings than the fabrication itself, though. When the researchers renamed a real library to defeat memorized knowledge, the failure was collective: "all seven fail in the same place, passing and missing the same tests