Failure modes of Claude Code in a supervised but code-blind 60h project

100% human-written. Copyedited by Claude. *Epistemic status: experience report of one ~60h project with Claude Code, plus longer for the write-up. * Scope: I supervised Claude's thinking but didn't look at the code. Spec was an exploratory, high-level design refined iteratively. * Method: I introduced runtime and design self-checks as Claude failure modes appeared, and ran manual regression tests against a known result. I noted the behaviors and quotes myself; I linked them to existing research and Anthropic's docs when I found correspondence; the rest I listed as potential research questions. The project arc is consistent with YC alum, AI tooling founder Dex Horthy's public account. * Background/bias: 18+ years as a software engineer in critical infrastructure, lately moving towards R&D in formal methods. I am skeptical of human software engineering practices, so my bar for LLM code is just "about as good as human code". I tried taking the LLM-coding claims at face value. I discussed parts of the post with two AI-adjacent researchers and two senior software engineers, all broadly bullish on LLM usage, yet they recognized several of these failure modes from their own use. Below is a summary focused on what I think is most relevant to LW. Full post at the link. I guided Claude Code (Pro plan; Opus 4.8, Sonnet 5, Opus 5, Opus 5.5; at or above Anthropic's effort recommendations, whose inconsistencies I document) to build a semantic fuzzer to find bugs in Obsidian Sync. I supervised its thinking but never read the code. Handwaving a lot, we got a functional prototype in ~30h, which I estimate would take ~40h by hand. The next ~30h I tried to finish off this prototype for publication, which turned into a bug treadmill: at every step, Claude kept breaking as much as it fixed. I gave up and declared the project finished at the ~60h mark. The fuzzer works: It does find sequences of operations on synced notes that you can repeat manually to cause data loss on your o