# GPT-6 vs Claude Opus 5.5: benchmarks, prices and which to use By SeedRouter · Published 2026-09-28 · Updated 2026-09-28 There 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](https://developers.openai.com/api/docs/models/gpt-6-astra), [Sol](https://developers.openai.com/api/docs/models/gpt-6-sol) and [Luna](https://developers.openai.com/api/docs/models/gpt-6-luna); Claude Opus 5.5 figures from [Anthropic's model page](https://platform.claude.com/docs/en/models/opus-5-5/overview). ## 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](https://developers.openai.com/api/docs/pricing) and the Claude Opus 5.5 prices from Anthropic's [launch post](https://www.anthropic.com/claude-opus-5-5). 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](https://seedrouter.ai/blog/gpt-6-api-pricing) and the [Claude API pricing guide](https://seedrouter.ai/blog/claude-api-pricing). ## How does Claude Opus 5.5 score against GPT-6 Astra? Anthropic's [Claude Opus 5.5 announcement](https://www.anthropic.com/claude-opus-5-5) 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](https://openai.com/index/introducing-gpt-6-sol-and-luna/) 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 `none` effort, the only setting that accepts `temperature` and `top_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](https://seedrouter.ai/models/gpt-6-astra), [GPT-6 Sol](https://seedrouter.ai/models/gpt-6-sol) and [Claude Opus 5.5](https://seedrouter.ai/models/claude-opus-5-5) in the browser and compare the answers, the token counts and the cost of each.