The short version
Opus 5.5 is Anthropic's new flagship for long-running agentic coding and knowledge work, and it's notable as the company's first release since Dario Amodei's public call to "pace the frontier"[1]. It was tested before release by external evaluators including Frontier Design and METR, and Anthropic says it scores highest of any model to date on its automated behavioral audit — the alignment suite the company runs across thousands of simulated scenarios[1].
The practical headline for developers and teams is efficiency. Anthropic reports that at default settings Opus 5.5 costs 40% less than Opus 5 on typical workloads — a combination of a lower per-token price and using fewer tokens per task[1].
Performance and coding
Anthropic positions Opus 5.5 around long, sprawling jobs like codebase-wide migrations and audits. In one early test, a user completed a 680,000-line code migration in less than a day — work Anthropic says would have taken an engineering team weeks[1]. In another, an early tester audited and fixed a 200,000-line codebase in under three hours, where Opus 5 took over 20 hours and used 2.5x as many tokens[1].
On benchmarks, Opus 5.5 leads Anthropic's line-up in agentic coding, computer use, and knowledge work[1]:
| Benchmark | Opus 5.5 | Fable 5.1 | Opus 5 |
|---|---|---|---|
| Terminal-Bench 4.0 | 66.4% | 55.8% | 52.3% |
| FrontierCode v1.1 (Main) | 54.4% | 50.3% | 48.0% |
| CursorBench 4.0 | 57.8% | 51.8% | 46.6% |
| GDPval-AA v2.1 | 1846 | 1735 | 1708 |
| AutomationBench | 40.0% | 31.4% | 26.9% |
| Humanity's Last Exam | 67.7% | 65.6% | 63.6% |
Anthropic-reported benchmark scores, September 2026[1].
Anthropic itself cautions that at this capability level, benchmark margins are "a less reliable guide to real-world differences," and that the gap between Opus 5.5 and Fable 5.1 in everyday use is narrower than the scores suggest[1].
Pricing: 20–60% cheaper
Opus 5.5 requires less compute to serve than Opus 5, and its pricing reflects that[1][4]:
| Per 1M tokens | Claude Opus 5.5 | Claude Opus 5 |
|---|---|---|
| Input | $4 | $5 |
| Output | $20 | $25 |
| Cache reads | $0.20 | $0.50 |
| Cache writes | $5 | $6.25 |
API pricing per 1M tokens, September 2026[1][4].
That's a 20% cut on input and output prices and a 60% cut on cache reads — the latter matters most because cache reads make up the majority of agentic and coding work costs[1]. Opus 5.5 also generates output more than 30% faster than Opus 5[1]. A Fast mode (up to 2.5x speed) is available in Claude Code and the Claude Platform at $8 per million input tokens and $40 per million output tokens[1].
Context, limits, and availability
Per the platform release notes, Opus 5.5 (claude-opus-5-5) ships with a 1M-token context window by default, 128k max output tokens, and always-on adaptive thinking[3]. Alongside the price drop, Anthropic is increasing five-hour usage limits on Pro, Max, Team, and seat-based Enterprise plans, and subscription users get a rate-limit reset they can save and use whenever they choose[1].
Claude Sonnet 5.5 and Claude Haiku 5.5 will follow "in the coming weeks" with many of the same performance, efficiency, and safety improvements[1].
Safety and communication
Anthropic describes Opus 5.5 as its strongest-performing model yet on the automated behavioral audit, "much less likely than recent models to take hard-to-reverse actions," and more resistant than Opus 5 to prompt injection[1]. On a benchmark run by AI security firm Gray Swan, Opus 5.5 ties Fable 5.1 for the lowest prompt-injection success rate of any model tested[1]. Because it's comparable to Claude Mythos 5.1 in biology and cybersecurity, it ships with safeguards similar to those on Fable 5.1, and full evaluation details are in the Opus 5.5 System Card[1][2].
One less technical but widely felt change: Anthropic says Opus 5.5 "communicates more naturally," putting the most important information up front — addressing a common complaint that Opus 5's writing was hard to follow over long sessions[1].
Bottom line
Verdict: For teams and developers already running agentic coding or long knowledge-work tasks on Claude, Opus 5.5 is an unambiguous upgrade — near-Fable-5.1 capability at roughly 40% lower cost, with faster output and a bigger default context window. If you only need short, casual chat, the difference matters less and you can wait for the smaller Sonnet 5.5 and Haiku 5.5 models.
Frequently asked questions
When did Claude Opus 5.5 release?
Anthropic announced Claude Opus 5.5 on September 22, 2026[1].
How much does Claude Opus 5.5 cost?
$4 per million input tokens and $20 per million output tokens — 20% less than Opus 5. Cache reads are $0.20 per million (60% less), and cache writes are $5[1][4].
Is Opus 5.5 faster than Opus 5?
Yes — Anthropic reports output generation more than 30% faster than Opus 5, with an optional Fast mode up to 2.5x speed[1].
What context window does Opus 5.5 have?
1M tokens by default, with up to 128k output tokens[3].
Sources
- Anthropic — "Introducing Claude Opus 5.5" — anthropic.com
- Anthropic — "Claude Opus 5.5 System Card" — anthropic.com
- Claude Platform Docs — Release notes (September 2026) — platform.claude.com
- Claude Platform Docs — Pricing — platform.claude.com