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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.

← All of v2.1.219 Home All releases ←olderv2.1.218 v2.1.220newer→
Claude Code v2.1.219

New "Migrating to {{OPUS_NAME}}" migration guide with two breaking changes

You'll notice
Useful3 Signal0
Skills

A migration guide now spells out the two thinking-related changes that break existing API request code.

What

shared/model-migration.md gains a section for the new Opus covering the two behaviours that break existing request code.

Details
  • Breaking change 1: thinking is on by default when thinking is omitted, a silent cost and truncation change measured against max_tokens.
  • Breaking change 2: thinking: {type: "disabled"} is accepted only at effort high or lower; pairing it with xhigh or max returns 400, validated per request.
  • Documents disabled-thinking failure modes: tool calls emitted as plain text, and <thinking> tag leakage.
  • Also notes a 512-token cache minimum, a separate rate-limit bucket, and adds a [BLOCKS]/[TUNE] checklist.
  • Older Opus rows in the migration table now all target the new ID.
Evidence

### Breaking change 2: disabling thinking is capped at

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

See this entry in the whole of v2.1.219 →