Anthropic cut over 80% of a harness system prompt for newer models

Cut 80% of the harness system prompt for newer models. Vendor-side setup change reported in the news.

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what to steal

Re-read your system prompt after each model upgrade; newer models need less.

Our takeaway from the source, not a quote.

Drawn from this card's sourced fields. Amber = older model named by the source. Rose = cut or replaced.

anthropic-cut-system-prompt.toml

# Cut 80% of the harness system prompt for newer models
# source: https://onthewire.ai/article/anthropic-cut-over-80-of-claude-code-s-system-prompt-for-its-newest-models-many-

[rig]
who     = "Vendor-side setup change reported in the news"
author  = "Anthropic (reported by onthewire.ai)"
date    = "2026-07-26"

harness = ["Claude Code"]
models  = ["Opus 5", "Fable 5"]

[keep]
[cut]
  - System prompt

⚠ Opus 5, Fable 5: older model(s), named by the source on 2026-07-26. See current models.

thing	status	why (per source)
System prompt	over 80% removed	Reported for its newest models

Source: https://onthewire.ai/article/anthropic-cut-over-80-of-claude-code-s-system-prompt-for-its-newest-models-many- · checked 2026-10-14

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steal → Mine your session logs for what actually ran, then cut the rest.

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