Claude API: temperature and top_p Now Return 400 on Opus 4.7+; Mythos Preview Retires June 30

TL;DR: Two Anthropic API breaking changes to act on now — temperature/top_p/top_k throw a 400 on Opus 4.7+ and claude-mythos-preview retires June 30.

temperature, top_p, and top_k are deprecated on Opus 4.7 and Opus 4.8. Passing any of these at a non-default value returns a 400 error — not a degraded response, a hard failure. If you carry over API call templates from older Claude models, remove these fields. Anthropic's guidance: use prompting to shape response style rather than numeric sampling parameters. Both claude-opus-4-7 and claude-opus-4-8 are affected; older model strings are not.

claude-mythos-preview retires June 30 (tomorrow). Requests to claude-mythos-preview will fail starting Sunday. Migrate to claude-mythos-5 — see the migration guide linked below.

Why it matters: Any wrapper, framework, or proxy that auto-injects temperature will throw 400s when routed to Opus 4.7 or 4.8 — check SDK option objects and middleware defaults before these failures surface in production.

Sources: Model and API parameter deprecations — Claude Platform Docs | Migration guide — Claude Platform Docs