[MODEL] Mostly Opus 4.7: constant spelling deviations, mixing American and British spelling

Preflight Checklist

• [x] I have searched existing issues for similar behavior reports
• [x] This report does NOT contain sensitive information (API keys, passwords, etc.)

Type of Behavior Issue

Other unexpected behavior

What You Asked Claude to Do

I asked: "I want you to write a product specification (see "C:\[...redacted...]") in prose. You may read code and plans, but don't run anything".

What Claude Actually Did

It happened not only with this prompt, it started, I feel, with Opus 4.7. So Claude carries out the command, but spelling in both code and Markdown is absolutely messy: it can be, for example, initialize() and, several lines below, initialise(), customization vs. customisation, localization vs. localisation, etc.
It never happened before, and not only does it make the text mixed, but sometimes it causes compile errors (again, take the example of initialize() vs. initialise()).

Expected Behavior

Please bring back consistency, I personally prefer American spelling, but it must be consistent.

Files Affected

It's not relevant, lots of code and Markdown files.

Permission Mode

Accept Edits was ON (auto-accepting changes)

Can You Reproduce This?

Yes, every time with the same prompt

Steps to Reproduce

I don't guarantee 100% reproducibility, but:

1. Ask the default model to write quite a lengthy piece of code or Markdown, based on a repo you already have (like implement a feature).
2. Observe what it outputs.

Claude Model

Opus

Relevant Conversation

Impact

Medium - Extra work to undo changes

Claude Code Version

2.1.119 (Claude Code)

Platform

Anthropic API

Additional Context

As I've already told you, it is quite a sudden regression, started out of the blue, probably with introduction of Opus 4.7.
My host operating system is Windows, if it's important, no locale was changed. I tested in Windows per se and in WSL.