[MODEL] Opus 4.6 Performance Degraded since end of March

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

The claude code (Opus 4.6 - Max Effort) showing degraded in performance especially in reading the code files and instructions. It was working perfectly before by reading all the files and info (Claude.md) before proceed to edit, and it's now directly edit the code and cause the system to break.

What Claude Actually Did

1. Skip the Claude.md, Lessons.md and Progress.md and directly edit which violating all the rules set previously.
2. And the code reviewer cannot find out the BUGS.

Expected Behavior

Claude should have:

1. Read all the files
2. Read the instructions
3. Read the code and past issues
4. Start to edit
5. Identify the part to edit
6. Edit it

Files Affected

Permission Mode

Accept Edits was ON (auto-accepting changes)

Can You Reproduce This?

Yes, every time with the same prompt

Steps to Reproduce

1. Just start any previous Claude Code project and ask it to edit the function (ex. Edit the Function Call to the API with the new field added in)

Claude Model

Opus

Relevant Conversation

Impact

Critical - Data loss or corrupted project

Claude Code Version

2.1.107 (Claude Code)

Platform

Anthropic API

Additional Context

This only happen since the end of March.