Opus 5 system prompt: checking Anthropic’s 80% claim

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Alex Getman @alexgetmancom · 25 Jul 2026, 17:22 UTC · Updated 31 Oct 2026, 20:59 UTC

Tests & comparisons

Anthropic said it cut 80% of the system prompt for Opus 5

I checked the claim:

Opus 4.8: 130K characters

Opus 5: 135K characters

So the prompt actually got longer. Maybe they meant some specific section inside the system prompt. Or maybe not.

Screenshot of an English article titled "The new rules of context engineering for Claude 5 generation models". Text claims over 80% of Claude Code's system prompt was removed for advanced models.

Anthropic Claude

Русская версия Markdown

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