A practitioner's log of specific Opus 5 regressions — silently dropping injected CLAUDE.md/rule files and editing files without reading them first — that explains why agent setups tuned for earlier models suddenly misbehave and what to force explicitly.
After trying out Opus 5 for a few hours today, I honestly think Anthropic's benchmark scores are a complete fraud.
Sure, the model might have improved in a few areas, but it has regressed unbelievably on so many weird, basic things.
The single worst issue: the system prompt never explicitly tells it to ignore auto-injected files like CLAUDE.md or *.rule.md.
Yet, as soon as the model sees them, it basically decides, "Eh, these aren't important," and ignores them for the rest of the session.
Unless you explicitly force it or hit some weird edge case that triggers it to read the files, you constantly have to re-educate it on what it's supposed to do from scratch.
Anthropic claims the model works fine without complex system prompts.
You can tell from intercepted requests that the system prompt really is shorter, but because the model lacks context, it ends up completely ignoring traditional tools like the read tool, text edit tool, monitor, etc. The chain reaction is that it just blindly writes weird scripts to do batch edits based on pure guesswork, without ever checking if its changes are correct.
Text edit used to require a read call before editing, which prevented hallucinations and guaranteed safety during concurrent modifications.
If I remember right, Claude Code dropped that requirement in later versions.
But now it has gotten way worse: it modifies files without reading or verifying a single thing.
In environments where you can't run the code right away, this is a total disaster.
Who can write inside the lines with their eyes closed?
For some reason, this model is extremely allergic to reading files.
Is it trying to save input tokens?
It regularly skips reading files, running ls, or running grep, and just hallucinates what the working directory looks like.
It even pulled this move on me: I have a dedicated trading VM that is totally unrelated to my current workspace, and I have a user-level global trading list skill, which basically lazily injects available trad
