Regression: Opus 4.5 "Shadow Downgrade" Significant reasoning quality drop & aggressive context compaction

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Type of Behavior Issue

Other unexpected behavior

What You Asked Claude to Do

Starting approximately January 12, 2026, I have observed a sharp degradation in the quality of claude code responses when using the Claude 4.5 Opus model. The model appears to be suffering from severe "laziness," refusal to follow negative constraints, and rapid context loss compared to performance levels from early January.

What Claude Actually Did

Starting approximately January 12, 2026, I have observed a sharp degradation in the quality of claude code responses when using the Claude 4.5 Opus model. The model appears to be suffering from severe "laziness," refusal to follow negative constraints, and rapid context loss compared to performance levels from early January.

Expected Behavior

Starting approximately January 12, 2026, I have observed a sharp degradation in the quality of claude code responses when using the Claude 4.5 Opus model. The model appears to be suffering from severe "laziness," refusal to follow negative constraints, and rapid context loss compared to performance levels from early January.

Files Affected

Starting approximately January 12, 2026, I have observed a sharp degradation in the quality of claude code responses when using the Claude 4.5 Opus model. The model appears to be suffering from severe "laziness," refusal to follow negative constraints, and rapid context loss compared to performance levels from early January.

Permission Mode

Accept Edits was ON (auto-accepting changes)

Can You Reproduce This?

Yes, every time with the same prompt

Steps to Reproduce

Starting approximately January 12, 2026, I have observed a sharp degradation in the quality of claude code responses when using the Claude 4.5 Opus model. The model appears to be suffering from severe "laziness," refusal to follow negative constraints, and rapid context loss compared to performance levels from early January.

Claude Model

Opus

Relevant Conversation

Impact

Medium - Extra work to undo changes

Claude Code Version

2.1.19

Platform

Anthropic API

Additional Context

_No response_