Opus 5.5 vs GPT-6 Astra: what is verified and what is not

Before writing this comparison, we googled it, the way anyone would. The first page of results: nine posts with nearly identical titles, «Opus 5.5 vs GPT-6 Astra: Benchmarks, Pricing», from sites like kingy.ai, myclaw.ai, alphacorp.ai, orcarouter.ai. Two of them disagree on Astra's context window badly enough that one calls it the larger of the two and the other makes it effectively smaller than Opus's. That is not a minor discrepancy in a comparison built entirely on numbers.

So what follows is only what traces to anthropic.com, to OpenAI's own system card, or to a published price sheet. Where no such source exists, the post says so outright.

Three numbers from Anthropic - and one of them is barely a win

In its own Opus 5.5 announcement, Anthropic gives three direct comparisons to GPT-6 Astra on its own benchmarks: Terminal-Bench 4.0 - 66.4% versus 57.9%, FrontierCode - 54.4% versus 53.3%, GDPval-AA v2.1 - 1846 versus 1542 Elo. These are the company's own numbers about its own product, on benchmarks it chose to show - worth keeping in mind while reading them.

The gap on Terminal-Bench and GDPval is real. On FrontierCode it is 1.1 percentage points - practically a tie. Any article promising a clean sweep across every metric is already simplifying what Anthropic itself says.

The sentence worth reading closely

«At default effort (medium), Opus 5.5 beats GPT-6 Astra at max effort for about a fifth of the cost per task» - Anthropic

The detail easy to miss: this is not the same effort level on both sides - it is Opus's cheaper mode against Astra's most expensive one. Honest about how the model actually gets used by default, but not an answer to «which is smarter at full power», which is the question every spam article's headline pretends to answer.

Price and context - independently verifiable

Unlike benchmarks, price does not need to be taken on faith - it sits in both vendors' published sheets. Opus 5.5: $4 / $20 per million tokens. GPT-6 Astra: $10 / $50. Both models carry a 1 million token context window - confirmed on both sides and never in dispute, unlike what the two comparison sites we checked claimed.

Why the rest of the comparisons online are not worth reading as fact

One aggregator we checked separately shows Astra scoring things like ExploitBench 100.0% and ARC-AGI-3 99.9%, with no source link and no stated methodology. Round, near-perfect numbers on a brand-new model in its first week are a typical sign of text generated to answer an «X vs Y» search query, not of an actual benchmark run.

That is not a reason to skip such articles entirely - it is a reason to check whether a number's source is named at all before letting it into a decision about which model touches production code.

Bottom line

On the three benchmarks Anthropic itself published, Opus 5.5 leads - by a wide margin on some, within a rounding error on others. Price and context window check out independently: Opus 5.5 costs two and a half times less for nearly the same context window. An independent, methodologically transparent head-to-head between the two does not exist yet - and any article claiming otherwise to a tenth of a percent is more likely inventing that precision than measuring it.