Claude Opus 5.5 vs GPT-6 Astra
OpenAI says GPT-6 Astra continues to be its best model across the board, and Anthropic now tells developers to start with Claude Opus 5.5 for most workloads, so this is the flagship question for late September 2026, with a price gap that does most of the arguing. Opus 5.5 lists $4 per million input tokens and $20 output. Astra lists $10 and $50, two and a half times as much on both sides, and bills any request above 272,000 input tokens at 2x input and cache rates and 1.5x output, which puts its long-context tier at $20, $2 cached, and $75. Opus 5.5 has no long-context surcharge across its 1 million token window, and its cache reads cost $0.20 per million against $1 for Astra cached input. Astra's window is nominally larger at 1,050,000 tokens. On independent measurement Opus 5.5 is ahead: first on the Vals Index at 69.69 percent against 66.61 for Astra, and 58 against 53 on the Artificial Analysis Intelligence Index. Cognition's FrontierCode v1.1 board has them close, 54.6 against 53.3. The vendor evidence splits. Anthropic's launch table shows Opus 5.5 at 66.4 on Terminal-Bench 4.0 against 57.9 for Astra, but the same table has Astra ahead on FrontierSWE v2, 65.5 against 62.3, so even Anthropic's own chart is not a sweep. Some rows cannot be compared at all: Anthropic reported no GPQA Diamond or BrowseComp figure for Opus 5.5, where OpenAI reports 96.0 and 91.5 for Astra, and OpenAI's long-context retrieval result for Astra, 96.3 percent on MRCR v2 8-needle between 512K and 1M tokens, has no Anthropic counterpart.
Head-to-Head Specs
| Spec | Claude Opus 5.5 | GPT-6 Astra |
|---|---|---|
| Provider | Anthropic | OpenAI |
| Input Price | $4.00/1M | $10.00/1M |
| Output Price | $20.00/1M | $50.00/1M |
| Context Window | 1M | 1.1M |
| Released | 2026-09 | 2026-09 |
| Capabilities | text, vision, tool-use, code, reasoning | text, vision, tool-use, code, reasoning |
Benchmark Scores
| Benchmark | Claude Opus 5.5 | GPT-6 Astra | Winner |
|---|---|---|---|
| FrontierCode v1.1 | 54.6 | 53.3 | Claude |
| Terminal-Bench 4.0 | 61.6 | 57.1 | Claude |
| Humanity's Last Exam (tools) | 67.7 | 57.2 | Claude |
See the full benchmark leaderboard for all models.
Category Breakdown
$4/$20 against $10/$50, 40 percent of the Astra rate on both input and output
Opus 5.5 cache reads are $0.20 per 1M against $1 for Astra, a 5x gap on the line that dominates agent loops
Opus 5.5 bills its full 1M window at one rate; Astra moves the whole request to $20/$75 above 272K input tokens
1,050,000 tokens against 1,000,000, though everything past 272K on Astra bills at the higher tier
Vals Index 69.69 against 66.61 and Artificial Analysis 58 against 53, first place on both boards
Cognition measures 54.6 against 53.3, a narrow 1.3 point lead
66.4 against 57.9 on Anthropic's launch table, vendor-run for both columns
Astra leads 65.5 against 62.3 on the same Anthropic table
OpenAI reports GPQA Diamond 96.0 and BrowseComp 91.5 for Astra; Anthropic reported neither for Opus 5.5, so only one side has evidence
OpenAI reports MRCR v2 8-needle at 96.3 percent from 512K to 1M for Astra; Anthropic published no comparable row
Choose Claude Opus 5.5 when:
- ▸Most frontier workloads where cost per token matters, at 40 percent of the Astra rate
- ▸Agentic loops that replay a large cached prompt on every step
- ▸Long-context work past 272K tokens that needs one flat rate
- ▸Long-horizon terminal and coding agents, per the Terminal-Bench 4.0 gap
Choose GPT-6 Astra when:
- ▸Software engineering where FrontierSWE v2 is the closest proxy for your work
- ▸Graduate science reasoning and agentic search, where Astra has published scores
- ▸Long context retrieval where recall past 512K tokens is the failure mode
- ▸Teams standardized on OpenAI tooling, Codex, and ChatGPT Work
Frequently Asked Questions
Which is better, Claude Opus 5.5 or GPT-6 Astra?
It depends on your use case. Claude Opus 5.5 from Anthropic excels at most frontier workloads where cost per token matters, at 40 percent of the astra rate, while GPT-6 Astra from OpenAI is better for software engineering where frontierswe v2 is the closest proxy for your work. See the full comparison above for detailed benchmarks and pricing.
How much does Claude Opus 5.5 cost compared to GPT-6 Astra?
Claude Opus 5.5 costs $4.00 input and $20.00 output per 1M tokens. GPT-6 Astra costs $10.00 input and $50.00 output per 1M tokens.
What is the context window difference between Claude Opus 5.5 and GPT-6 Astra?
Claude Opus 5.5 supports 1M tokens, while GPT-6 Astra supports 1.1M tokens.