Two Opus 5.5 API changes can turn previously valid requests into HTTP 400 errors: thinking is now always adaptive (fixed thinking budgets or disabling thinking are rejected), and forced tool-call requirements are retired entirely.
Anthropic states Opus 5.5 matches Fable 5.1-level performance while running about 30% faster and about 40% cheaper per task than Opus 5, per Vercel's changelog.
Opus 5.5 has a 1M-token context windowThe maximum amount of text a model can consider at once — its working memory for the current conversation or task.Full definition →, returns up to 128K output tokens, and carries a June 2026 knowledge cutoffThe date after which a model saw no training data — everything later has to reach it through search, tools, or the context window.Full definition →.
AI Gateway offers opt-in Regional inferenceRunning a trained model to get answers — the phase where AI is actually used, as opposed to trained.Full definition → (pinning requests to a geographic zone) and Zero Data Retention controls, and passes through provider pricing with no platform fee, including on Bring-Your-Own-Key requests.
Terms in this piece · Glossary
agent harness — The scaffolding around a model that turns it into a working agent — the loop, the tools it can call, and the rules for when to stop.
context window — The maximum amount of text a model can consider at once — its working memory for the current conversation or task.
knowledge cutoff — The date after which a model saw no training data — everything later has to reach it through search, tools, or the context window.
inference — Running a trained model to get answers — the phase where AI is actually used, as opposed to trained.
Why it matters
Existing integrations that disable thinking, set fixed thinking budgets, or force a specific tool call will start returning HTTP 400 errors under Opus 5.5, so agent harnessThe scaffolding around a model that turns it into a working agent — the loop, the tools it can call, and the rules for when to stop.Full definition → code needs updating before upgrading.
