Introduction
Claude Opus 5.5 is Anthropic's newest AI model, released September 22, 2026, positioned as a faster, cheaper replacement for Claude Opus 5. About 90 minutes after launch, OpenAI answered with two new models, GPT-6 Sol and GPT-6 Luna. This guide covers what changed in Opus 5.5, how it compares to Claude Fable 5.1 and OpenAI's new lineup, and which model fits which task.
What Is Claude Opus 5.5 and How Much Does It Cost?
Opus 5.5 costs 4 dollars per million input tokens and 20 dollars per million output tokens, down from 5 and 25 for Opus 5, a roughly 40% drop in typical running cost, with output generating over 30% faster. It is available now on Claude.ai, Claude Code, the API, AWS Bedrock, Google Cloud Vertex AI, and Microsoft Azure. Sonnet 5.5 and Haiku 5.5 follow in the coming weeks.
What Changed in Claude Opus 5.5?
Claude Opus 5.5's biggest change is how it communicates, a direct response to criticism of Opus 5's writing style. Independent tracking found Opus 5's responses averaging 510 words against 158 for Opus 4.5, sentences 58% longer, and phrases like "load-bearing" appearing twice as often, burying the actual answer inside paragraphs of hedging.
Thariq Shihipar, who leads Claude Code, confirmed Opus 5.5 is the direct result of that feedback, using less jargon and putting the most important information first. Independent tester Simon Willison said the model appears to directly address the biggest complaints about Opus 5's style, a change users notice before checking any benchmark.
How Good Is Claude Opus 5.5 at Real Work?
Claude Opus 5.5 performs strongly on long, complex tasks rather than single quick answers. One tester completed a 680,000-line code migration in under a day, work that would normally take an engineering team weeks. Another audited and fixed a 200,000-line codebase in under three hours versus more than 20 hours for Opus 5, using 2.5 times fewer tokens.
On formal benchmarks, Opus 5.5 leads Fable 5.1 and GPT-6 Astra on GDPval-AA, a test of real professional work across 44 occupations, and leads on Terminal-Bench 4.0 and CursorBench 4.0 for coding. It trails Astra on Terminal-Bench-Science, a scientific research benchmark. Anthropic's own caveat: at this capability level, benchmark gaps are a less reliable guide to real-world difference, and the gap to Fable 5.1 is narrower than scores suggest.
How Does Claude Opus 5.5 Compare to GPT-6 Sol and Luna?
The three target different budgets and task complexity, not the same use case. Sol costs 2 dollars input and 10 dollars output per million tokens, half of Opus 5.5's rate. Luna costs 10 cents input and 50 cents output, a fraction of the others. Astra remains the most expensive and most restricted, gated for its cybersecurity capability.
For demanding coding and knowledge work, reviewers still shortlist Opus 5.5 first. For everyday coding on a tighter budget, Sol is a genuine contender at a fraction of the cost per task. For high-volume, low-complexity work like classification, Luna is difficult to beat on price. In live testing by The Neuron, Sol reached a working result faster than Opus 5.5 on one build task, a real trade-off between reasoning effort, quality, and time. Track cost per successful task, including retries, rather than the launch-day score.
Why Is Claude Opus 5.5 Considered Safer Than Previous Models?
Claude Opus 5.5 is Anthropic's first model since CEO Dario Amodei publicly called for pacing frontier AI development. Anthropic reports its best-ever score on its internal alignment audit, with containment-boundary attempts down roughly 85% versus Opus 5, all logged as low severity. It was evaluated by METR and Frontier Design, and ships with safeguards previously reserved for Anthropic's most capable systems, since its biology and cybersecurity capability is rated comparable to Claude Mythos 5.1. METR's own verdict: an incremental improvement over Fable 5.1, unlikely alone to fully automate AI research and development.
Which AI Model Should You Actually Use?
For demanding coding, long migrations, or work that needs to be right the first time, Opus 5.5 is the strongest current option and now costs meaningfully less than two months ago. For everyday coding on a budget, test GPT-6 Sol before assuming you need the pricier model. For high-volume, simple work, GPT-6 Luna will likely handle it for a fraction of the price. If your task needs frontier-level scientific reasoning, GPT-6 Astra still holds an edge. None of these models is a universal winner, the right one depends on the task and what a wrong answer actually costs you.
Frequently Asked Questions
Is Claude Opus 5.5 better than GPT-6 Astra?
Not universally. Opus 5.5 leads on coding and general knowledge work, while Astra leads on autonomous scientific research and remains OpenAI's most capable model overall, at a higher price with tighter access restrictions.
Is Claude Opus 5.5 cheaper than Opus 5?
Yes. Opus 5.5 costs 4 dollars per million input tokens and 20 dollars per million output tokens, versus 5 and 25 for Opus 5, roughly a 40% reduction in typical running cost.
What is the difference between GPT-6 Sol and GPT-6 Luna?
Sol is priced at 2 dollars input and 10 dollars output per million tokens and suits everyday coding and agent tasks. Luna costs 10 cents input and 50 cents output and suits high-volume, low-complexity work like classification and routing.
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