Coding Models Can Find the Bad Line, They Just Won't Delete It Source: https://arxiv.org/abs/2607.28887 Paper was published on July 30, 2026 This episode was AI-generated on August 3, 2026. The script was written by an AI language model and the host voices were synthesized by Eleven Labs. The producer is not affiliated with Anthropic or Eleven Labs. Frontier coding models pass SWE-bench by leaving the broken code exactly where it is and building a new path around it — and no test in the suite can tell. When researchers wrote checks that fail if the developer's deleted code is still sitting there, success rates dropped from about 63 percent to about 42, with every model losing between 17 and 24 points. This episode unpacks where inside the model that failure actually lives, why it's a boundary problem rather than a search or intent problem, and why fixing it just trades one failure mode for another. Key Takeaways: - Why this isn't a localization failure: models edit the right file over 92 percent of the time, hit the right enclosing scope about 70 percent, and remove the exact line under 52 percent - The named taxonomy of additive patches — Guard-and-Go (29 percent of passing patches) and Retained Path as Live Fallback (40 percent of typed cases) — and the difference between harmless dead code and a live second route - How a purely source-level absence check, validated to fail on the buggy commit and pass on the real fix, dropped frontier success from about 63 to about 42 percent - The three-rung diagnostic ladder: explicit instructions move the score by roughly nothing, region hints barely help, exact line spans move some models more than thirty points — so it's control, not capability - Why suppressing under-deletion surfaces over-deletion instead: incomplete deletions fall from 114 to 20 while invalid edits after complete removal climb from 14 to 32 - The steelman objection that survives: the absence checks measure conformance to the human developer's solution, not correctness, and nobody counted how many newly failing patches a reviewer would actually reject 00:00 - Two patches, same tests, very different code: The cold open contrasts a human's one-line replacement with a model's version that keeps the line in an else branch, and sets up the 63-to-42 percent collapse and the METR merge-rate gap. 01:22 - Right room, right wall, wall still standing: The obvious explanation — the model never found the code — gets killed by a three-level nesting analysis of file, scope, and exact line. 03:18 - Guard-and-Go, and the pothole with a detour sign: The paper's taxonomy of additive repairs, including the crucial split between unreachable dead code and Retained Path as Live Fallback, plus Exception Capture Bypass. 05:51 - How do you test that code is gone?: Instead of changing the model, the researchers change the grader — writing source-level absence checks, validating them on 34 tasks, and watching every frontier model drop. 07:34 - What if deleting is the entire job?: The CanItDelete benchmark strips away addition and cross-file search entirely — 200 tasks, deterministic occurrence-aware scoring — and the dominant failure mode turns out to be incomplete deletion. 09:34 - Three rungs, and the sting that follows: Explicit instructions do nothing, region hints do almost nothing, exact line spans move everything — establishing a boundary problem, and then showing that fixing under-deletion invites over-deletion. 12:32 - Under one percent of the tokens: A 7B model trained twice under an identical recipe, differing only by about thirteen thousand deletion examples, roughly doubles deletion success and transfers five points to SWE-bench Verified. 14:07 - The objection that survives the whole paper: The hosts push back on what the headline number really measures — conformance to the developer's fix rather than correctness — question the unmeasured maintainability harm and the pilot's missing data-volume control, then ask whether the fix belongs in the graders or the training mixture. Recommended Reading: - SWE-bench: Can Language Models Resolve Real-World GitHub Issues?: The benchmark whose pass/fail grader this episode shows is structurally blind to leftover code — worth reading to see exactly how resolution rate is defined and why absence can't be asserted. (https://arxiv.org/abs/2310.06770) - People systematically overlook subtractive changes: The Nature study behind the episode's closing claim that additive bias isn't a machine quirk — humans reliably add rather than remove when solving problems, and models learned from our text. (https://doi.org/10.1038/s41586-021-03380-y) - Agentless: Demystifying LLM-based Software Engineering Agents: The clearest articulation of the localize-then-repair view of SWE-bench, which makes a useful contrast with this episode's file/scope/line ladder showing the failure is at the line, not the search. (https://arxiv.org/abs/2407.01489) - Measuring the Impact of Early-2025 AI on Experienced Open-Source Developer Productivity: METR's randomized trial on real maintainers, the empirical counterweight to leaderboard scores that the episode's benchmark-versus-merge-rate gap depends on. (https://arxiv.org/abs/2507.09089)