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DisclaimerUnofficial, and not affiliated with Anthropic. Nearly all of this is read straight out of what ships: npm bundles, captured prompts, published docs. Anthropic's own notes go in verbatim, marked as theirs. The rest is my reading, and every entry carries the strings behind it. If one looks wrong, vote it down and say why.

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Claude Code v2.1.251 28 Aug 2026

Keeping another vendor's thinking text across a model upgrade

Not switched on

Useful2 Signal4

Models Notable not in their notes

Reasoning text from a non-Claude model can survive a model switch, off unless enabled.

Preserving other-vendor reasoning across upgrades is off unless an experiment or env var turns it on.

CLAUDE_CODE_LUMINOUS_WHISTLE

What

A setting decides whether reasoning text produced by a non-Claude model survives an upgrade to a different model instead of being discarded. It is off unless the tengu_luminous_whistle experiment turns it on, and CLAUDE_CODE_LUMINOUS_WHISTLE overrides either way. Even then it only applies when a separate precondition holds.

Details

Decided once per session and reset on clear, resume, spare claim and remote attach.
Logs api_keep_thinking_on_upgrade when it either keeps or rejects the thinking text.

Evidence

tengu_luminous_whistle

Strings lifted out of the shipped bundle, so the claim above can be checked against them.

See this entry in the whole of v2.1.251 →