Opus 5 feels like a downgrade from Opus 4.8 overall. It, along with Fable, really has a problem following instructions and staying in scope, and their prose keeps growing, both in explaining what it did and in writing multiline code comments (some comments read like a changelog, e.g. `// sky is blue (changed from red on 2026-01-01 per TCK-234 by @Foo)`).

Every time I ask it to do something, it does 80% of the job, goes off on "side quests" beyond the scope, and then leaves something out of the core ask (and when you tell it to finish, it does the same thing again).

The only advantage of Opus 5 over 4.8 is the better cutoff date for working with 3rd-party tools, though both do a very bad job of "this tool is constantly updated, I should look for the latest version first".

I have a rule in claude.md:

``` ## Writing rules

*Describe the code as it is now — no residue, no change-narration.* Artifacts (docs, plans, comments, commit messages) should describe the current code statically, as if it had always been this way. Two facets of one rule: (1) never describe a dismissed alternative or a corrected/replaced choice; (2) even when nothing was rejected, don't narrate continuity or evolution relative to some earlier state. Mention a former state only when the current choice is genuinely hard to understand without it, and then only as an explanation of the current choice.

This governs descriptions of the code and main documentation. It does not apply to work-tracking artifacts in `doc/tasks/`. ```

Which makes comments and docs bearable but I'll be damned how it loves to overload work tracking document with every little detail.