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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 v2.1.219

Bundled Claude API skill docs rewritten around Opus 5

You'll notice
Useful3 Signal0
Skills

The bundled Claude API skill docs now target the new Opus, covering thinking defaults, fallbacks and fast mode.

/claude-api
What

The vendored claude-api skill documents were retargeted from a hardcoded model ID to placeholders resolving to the new Opus, with new guidance on thinking defaults, refusal fallbacks and fast mode.

Details
  • Python, Go and Ruby guides, the error-code reference, the managed-agents reference and the top-level skill all replace claude-opus-4-8 with {{OPUS_NAME}}/{{OPUS_ID}}, and add a {{PREV_OPUS_NAME}}/{{PREV_OPUS_ID}} row.
  • On the new Opus, thinking is on by default, so omitting thinking runs adaptive; {type: "disabled"} is accepted only at effort high or lower and returns 400 at xhigh or max.
  • A new pitfall section covers the two failure modes of disabling thinking.
  • Refusal fallbacks gain a scalar form fallbacks: "default" under beta header server-side-fallback-2026-07-01; the array form keeps -2026-06-01 and mixing the two returns 400.
  • Fast mode is documented as new-Opus/4.8 only and Claude API only, with Opus 4.7 fast mode removed rather than deprecated, plus a note that Priority Tier does not cover the new Opus.
  • Mid-conversation system messages are documented for several models rather than one.
Evidence

server-side-fallback-2026-07-01

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 →