Claude Preserved Thinking: How the API Protects Thinking Blocks

1 day ago

2 min read

⚡ TL;DR From Fable 5.1, Opus 5.5, Sonnet 5.5 and Haiku 5.5, new API accounts can't alter context around thinking blocks. It fights illicit distillation; Claude apps handle it automatically.

Developers editing earlier turns may hit a new error. Claude preserved thinking explains why.

Developer update. 🧠

🧠 What Claude Preserved Thinking Changes

For Claude Fable 5.1, Opus 5.5, Sonnet 5.5 and Haiku 5.5, API accounts created on or after August 31, 2026 can't change prior context around a thinking block. Altering it returns an error unless you opt into a mode that drops those blocks.

🛡️ Why

Editing earlier turns is a common technique in illicit distillation, which tries to copy a model's reasoning to train other models.

👥 Who's Affected

Existing accounts aren't covered yet; rollout expands with future models. Claude Code, Cowork and Claude.ai need no changes.

💡 Pro tip Pass thinking blocks back unchanged in multi-turn requests to avoid errors.

🔗 Account Linking

A thinking block can only be read by the account that created it or a linked account. From an unlinked account, the API drops it silently, making responses slower and more token-heavy.

Claude preserved thinking protects model integrity. Need your API integration updated? Lord Of The Wix can do it. ✨

❓ Claude preserved thinking FAQ

What is preserved thinking?

A rule blocking edits to context around thinking blocks.

Which accounts are affected?

API accounts created on or after August 31, 2026.

Why was it introduced?

To protect against illicit distillation.

Do Claude apps need changes?

No.

What happens with unlinked accounts?

Thinking blocks are silently dropped.

🙏 Credit: This article is based on public information from the Claude Help Center. Claude is a product of Anthropic. Source: support.claude.com.

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