GPT-6 vs Claude Opus 5.5: benchmarks, prices and which to use
GPT-6 Astra, Sol and Luna vs Claude Opus 5.5: specs, token prices, the benchmark results both labs published, and which model fits coding, agents and cost.
Read as MarkdownThere is no single winner. On the benchmark table Anthropic published, Claude Opus 5.5 beats GPT-6 Astra on agentic coding and knowledge work, while GPT-6 Astra leads on business workflows and scientific research. On price, Claude Opus 5.5 sits between GPT-6 Sol and GPT-6 Astra, and GPT-6 Luna is far cheaper than any of them. No lab has published a direct comparison of GPT-6 Sol or GPT-6 Luna with Claude Opus 5.5, so for those pairs your own tests decide.
All four models run on SeedRouter under one key, so you can test them side by side.
How do GPT-6 and Claude Opus 5.5 compare on specs?
| GPT-6 Astra | GPT-6 Sol | GPT-6 Luna | Claude Opus 5.5 | |
|---|---|---|---|---|
| Model ID | gpt-6-astra | gpt-6-sol | gpt-6-luna | claude-opus-5-5 |
| Released | September 3, 2026 | September 22, 2026 | September 22, 2026 | September 22, 2026 |
| Context window | 1,050,000 tokens | 1,050,000 tokens | 1,050,000 tokens | 1M tokens |
| Max output | 128,000 tokens | 128,000 tokens | 128,000 tokens | 128K tokens |
| Effort levels | low to max | none to max | none to max | low to max |
| Default effort | Not documented | medium | medium | medium |
| Reasoning off | No | Yes, at none | Yes, at none | No, thinking is always on |
| Input | Text, images | Text, images | Text, images | Text, images, PDFs |
GPT-6 figures come from OpenAI's model pages for Astra, Sol and Luna; Claude Opus 5.5 figures from Anthropic's model page.
How do their prices compare?
List prices per token, as multiples of GPT-6 Sol's rate on the same line:
| Model | Input | Cached input | Cache writes | Output |
|---|---|---|---|---|
| GPT-6 Astra | 5× | 5× | 5× | 5× |
| Claude Opus 5.5 | 2× | 1× | 2× | 2× |
| GPT-6 Sol | 1× | 1× | 1× | 1× |
| GPT-6 Luna | 0.05× | 0.05× | 0.05× | 0.05× |
The GPT-6 prices are from OpenAI's pricing page and the Claude Opus 5.5 prices from Anthropic's launch post. Claude Opus 5.5 costs 40% of GPT-6 Astra per token and twice GPT-6 Sol. GPT-6 prompts over 272K input tokens are billed at a higher long-context rate. For the rates your account pays, see the GPT-6 API pricing guide and the Claude API pricing guide.
How does Claude Opus 5.5 score against GPT-6 Astra?
Anthropic's Claude Opus 5.5 announcement includes GPT-6 Astra in its table. Claude results are at max effort unless noted; the GPT-6 Astra figures are, in Anthropic's words, "as reported by OpenAI".
| Benchmark (Anthropic's table) | Claude Opus 5.5 | GPT-6 Astra |
|---|---|---|
| Terminal-Bench 4.0 (agentic coding) | 66.4% (xhigh) | 57.9% (high) |
| FrontierCode v1.1 Main (agentic coding) | 54.4% | 53.3% |
| GDPval-AA v2.1 (knowledge work, Elo) | 1846 | 1542 |
| Humanity's Last Exam, with tools | 67.7% | 57.2% |
| AutomationBench (business workflows) | 40.0% | 41.4% |
| Terminal-Bench-Science 0.1 (research) | 58.7% | 64.6% |
Anthropic also compares cost per task: at its default medium effort, Claude Opus 5.5 "beats GPT-6 Astra at max effort for about a fifth of the cost per task" on GDPval-AA, and on FrontierCode it beats Astra "at roughly 20% of the cost per task".
What about GPT-6 Sol and GPT-6 Luna?
OpenAI's GPT-6 Sol and Luna announcement compares them with Claude Opus 5 and the Claude Fable models, not with Claude Opus 5.5, which launched the same day. OpenAI reports, for example:
- On AutomationBench, GPT-6 Sol at xhigh effort scored 33.2% against 26.9% for Claude Opus 5 at max effort, at 9% of Opus 5's cost per task.
- On DeepSWE v1.1, GPT-6 Luna at max effort scored 66.6%, "comparable to Claude Opus 5 and Fable 5 at medium effort", at 93% lower cost per task than Opus 5.
Anthropic reports that Claude Opus 5.5 improves on Claude Opus 5 across its table, so these results do not tell you how GPT-6 Sol or GPT-6 Luna compare with Opus 5.5. Test them on your own tasks.
How do the APIs differ?
- Formats. On SeedRouter, both families answer OpenAI Responses, OpenAI Chat Completions and Anthropic Messages requests with the same key.
- Reasoning. GPT-6 Sol and Luna can skip reasoning at
noneeffort, the only setting that acceptstemperatureandtop_p. Claude Opus 5.5 always thinks; Anthropic's API rejects other sampling values and a disabled thinking setting. - Forced tool calls. Claude Opus 5.5 rejects a forced
tool_choice. On GPT-6, use the Responses API for reasoning with tools. - Documents. Claude Opus 5.5 reads PDFs directly; GPT-6 takes text and images.
Which should I use?
| If you need | Use | Why |
|---|---|---|
| Agentic coding at a mid price | Claude Opus 5.5 | Leads GPT-6 Astra on Anthropic's coding rows at 40% of its token price |
| Business workflows or scientific research | GPT-6 Astra | Leads Claude Opus 5.5 on those rows of Anthropic's table |
| Strong coding and agents at the lowest mid-tier price | GPT-6 Sol | Half of Claude Opus 5.5's token price |
| High-volume classification and extraction | GPT-6 Luna | A fortieth of Claude Opus 5.5's input price |
| PDFs in the prompt | Claude Opus 5.5 | Reads them directly |
Frequently asked questions
Is Claude Opus 5.5 better than GPT-6 Astra?
On four of the six rows Anthropic shared, yes, including both coding benchmarks and knowledge work. GPT-6 Astra leads on business workflows and scientific research. Both sets of scores come from the labs themselves.
Is GPT-6 Sol better than Claude Opus 5.5?
Nobody has published that comparison. OpenAI compared GPT-6 Sol with Claude Opus 5, which Claude Opus 5.5 replaces. GPT-6 Sol costs half as much per token, so it is worth testing on your own tasks.
Can I call GPT-6 and Claude Opus 5.5 with the same code?
Yes. On SeedRouter both take the same OpenAI or Anthropic request formats with one key; change model and remove any field one model rejects, such as temperature on Claude Opus 5.5.
Compare them on your own prompt
Send the same prompt to GPT-6 Astra, GPT-6 Sol and Claude Opus 5.5 in the browser and compare the answers, the token counts and the cost of each.
