# My AI agent diagnosed its own bug. The diagnosis was plausible, specific, and wrong. > Source: > Published: 2026-08-25 17:09:19+00:00 Article pages were shifting during load. Lighthouse reported **CLS 0.303**, where anything above 0.1 counts as poor. My ops agent wrote the cause into its log: Article pages CLS 0.303, caused by `.prose img` missing`width` /`height` . Fixing requires fetching intrinsic dimensions for 125 images — separate task. **That diagnosis looks entirely correct.** Images without declared dimensions give the browser nothing to reserve space with, so content jumps when they load. It's the canonical cause of layout shift; every article about CLS leads with it. When I sat down to fix it the next day, I did one thing first: **I counted how many images were actually missing dimensions.** The answer was **4**. The other **155 already had them.** Not in the physics — image dimensions genuinely do affect CLS. It went wrong by **applying a familiar cause without checking the scene.** The real problem: those 155 images had dimensions, and the dimensions were **wrong**. These posts were written in 2017. The HTML looks like this: ``` ``` The author meant *"display this at 500×500."* They did **not** mean *"this image is 500×500."* The image is actually **1361×738**. So here's the sequence: `width="500" height="500"` , reserves a square based on that 1:1 ratio**Wrong dimensions are worse than no dimensions.** Without them, the browser knows it doesn't know. With wrong ones, it confidently reserves the wrong space. I fetched intrinsic dimensions for all 119 unique image URLs and compared: **145 of 159 were wrong. 91%.** If the log had said "cause unknown," I'd have investigated. Instead it gave me something **plausible, specific, and immediately actionable.** Following it, I'd have added dimensions to those 4 images, watched CLS barely move, and started suspecting something else entirely — **continuing to search from an already-misled starting point.** **A wrong diagnosis costs more than no diagnosis, because it consumes the attention you would otherwise have spent doubting.** The shape of the mistake is worth naming: it **treated the common cause as this case's cause.** Missing dimensions genuinely is the most likely explanation for CLS, so it was a good guess. The failure wasn't the guess. It was **not spending one command to test the guess before committing to it.** That command takes under ten seconds: ``` # how many images have no width/height? grep -o ']*>' *.md | grep -vc 'width=' ``` Worth stating plainly, because it reads that way if I don't. The same system wrote that log, and in the same pass it got a lot right: it recorded the exact CLS figure, identified the affected selector, filed the work as its own task, and left a note in the status file saying *"new articles must always carry real width/height so this doesn't accumulate again."* **That preventive note was correct. The diagnosis was wrong. Both came from the same thinking.** People do this constantly. A familiar symptom triggers the most common cause, and subsequent observations get bent toward it. The difference is that **a system writes its guess down as a confident single line, files it, and tomorrow you read that line as an established fact.** **1. Measure before fixing.** Not the severity — I already had that (CLS 0.303). Measure **whether the cause I assume actually holds here.** It's usually one command. Ten seconds would have redirected this entire task. **2. Separate "measured" from "inferred" in the log.** `CLS 0.303` and `selector .prose img` were measured. `because they lack width/height` was inferred. They sat on the same line and read as equally reliable the next morning. Inferences now have to be labelled as inferences. **3. When corrected, amend the original diagnosis explicitly.** The log now says *"the original diagnosis was wrong"* rather than quietly swapping in the right answer. **Next time I need to know more than the correct answer — I need to know how I got it wrong.** There's a tail to this. After replacing every declared size with the real one, one page went from 0.303 to **0.212**. Not zero. Another round of digging: that 0.212 was **manufactured by the local environment.** The images are on imgur, imgur's hotlink protection rejects a localhost referer, the images fail, the browser renders alt text at a different size, and the layout shifts again. **Measured in production: 0.** So the same number got explained once by a wrong diagnosis and produced once by a wrong environment. Both times I nearly acted on it.