Claude Opus 5.5 Review: Benchmarks, Pricing & Migration (2026)

Claude Opus 5.5 launched 22 September 2026 at $4/$20 with a 1M context window and adaptive thinking always on. Full specs, benchmarks context, pricing against GPT-6, and the migration checks that matter.

Published 23 September 2026 by Jake Hissitt, Founder of Stob.AI.

Anthropic released Claude Opus 5.5 on 22 September 2026 as model ID claude-opus-5-5 .

It is cheaper than the model it replaces, keeps the million-token context window, and changes enough in its default behaviour that existing integrations need checking before you switch.

The specifications Attribute Claude Opus 5.5 Model ID claude-opus-5-5 Input price $4.00 per 1M tokens Output price $20.00 per 1M tokens Cache reads $0.20 per 1M tokens Cache writes (5 minute) $5.00 per 1M tokens Batch discount 50% Context window 1,000,000 tokens Max synchronous output 128,000 tokens Knowledge cutoff June 2026 Availability Generally available, 22 September 2026 Against Claude Opus 5 at $5 / $25, that is a 20% price cut at the same context size.

Against Claude Fable 5.1 at $10 / $50, it is 60% cheaper and, for most production work, close enough on quality that Fable becomes the escalation model rather than the default.

Adaptive thinking is always on The biggest behavioural change is that extended thinking is no longer something you switch on.

Adaptive thinking runs by default at medium effort, and the model decides how much deliberation each step deserves.

That is good for quality and bad for anyone who budgeted output tokens assuming no thinking.

Thinking tokens bill as output.

Re-measure your per-request cost before assuming the headline price cut flows straight to your invoice.

Migration checklist Disabled thinking.

Integrations that explicitly disabled extended thinking need reviewing against the new default behaviour.