Opus 5.5 vs GPT-6 (Astra & Sol): Benchmarks, Price, Which to Use
Updated 2026-10-08
Short answer: Claude Opus 5.5 beats OpenAI's flagship GPT-6 Astra on terminal coding and expert-knowledge tests and costs 2.5x less; Astra is stronger on formal math, some science workflows and office automation. Opus 5.5 (Anthropic, September 22, 2026) costs $4 per million input tokens and $20 per million output. GPT-6 Astra (OpenAI, early September 2026) costs $10 and $50. If price is the deciding factor, OpenAI's cheaper GPT-6 Sol tier ($2/$10) is the model to compare instead.
"GPT-6" is a family, not one model: Astra is the flagship, Sol the cost-effective high-end tier (now GPT-6.1 Sol), and Luna the fast, low-cost tier. Most "Opus 5.5 vs GPT-6" searches mean Astra, so that's the main comparison here.
Claude Opus 5.5 vs GPT-6 Astra at a glance
| Claude Opus 5.5 | GPT-6 Astra | |
|---|---|---|
| Maker | Anthropic | OpenAI |
| Released | September 22, 2026 | Early September 2026 |
| API price (input / output per 1M) | $4 / $20 | $10 / $50 |
| Cached input read per 1M | $0.20 | $1.25 |
| Fast mode | $8 / $40 | $20 / $100 |
| Context window | ~1M tokens | ~1M tokens |
Prices as published by both vendors and compiled by Vellum (September 23, 2026). Check the official pricing pages before budgeting.
Benchmarks: who wins where
Vendor-reported results collected by Vellum:
| Benchmark | Opus 5.5 | GPT-6 Astra |
|---|---|---|
| Terminal-Bench 4.0 (terminal coding) | 66.4% | 57.7% |
| FrontierCode v1.1 (full-repo coding) | 54.4% | 53.3% |
| Humanity's Last Exam, with tools | 67.7% | 57.2% |
| AutomationBench (office apps) | 40.0% | 41.4% |
| Terminal-Bench Science 0.1 | 58.7% | 64.6% |
| FrontierMath Tier 4 | – | 97.6% |
OSWorld 2.0 (desktop computer use) was reported at 81.8% for Opus 5.5 on Anthropic's evaluation and 72.6% for Astra on OpenAI's, but the harnesses differ, so don't read that gap literally.
The pattern: Opus 5.5 is ahead on messy, real-world coding and broad expert questions; Astra is ahead on closed, formally checkable problems like competition math and specialised science pipelines.
Price: 2.5x per token
A coding task that reads 200,000 tokens and writes 20,000:
- Claude Opus 5.5: 0.2 × $4 + 0.02 × $20 = $1.20
- GPT-6 Astra: 0.2 × $10 + 0.02 × $50 = $3.00
- GPT-6.1 Sol (OpenAI's cheaper tier): $0.60
Token counts differ by model too: early enterprise testers reported Opus 5.5 writing noticeably shorter outputs on the same coding tasks, which widens its cost advantage in agent loops. Cached context ($0.20 vs $1.25 per million) matters for long-running agents.
Which should you use?
Choose Claude Opus 5.5 if:
- You build coding agents that work in a terminal (Claude Code, Codex-style CLIs).
- You need broad expert reasoning across science, law and policy.
- You want flagship quality at a lower price than Astra.
Choose GPT-6 Astra if:
- Your work is formal math, proofs or specialised scientific computing.
- You automate spreadsheets, slides and CRM tasks end to end (slight AutomationBench edge).
- You're already deep in OpenAI's stack and need its top model.
Consider GPT-6.1 Sol if cost per task decides: it's half Opus 5.5's price, ties it on the DeepSWE coding benchmark and loses on most others – details in GPT-6.1 vs Opus 5.5. Within OpenAI's line-up, see GPT-6.1 Sol vs GPT-6 Astra.
How to decide for your own work
- Pick 20–50 real tasks from your backlog.
- Run each model with the same prompt, tools and effort level.
- Record pass rate, tokens and time.
- Compare cost per solved task.
- Route: cheap model first, flagship for failures – this works across vendors too.
FAQ
Is Claude Opus 5.5 better than GPT-6?
Against GPT-6 Astra, Opus 5.5 leads on Terminal-Bench 4.0 (66.4% vs 57.7%), FrontierCode and Humanity's Last Exam, while Astra leads on formal math, Terminal-Bench Science and AutomationBench. Opus 5.5 is also 2.5x cheaper per token.
Opus 5.5 vs Astra: which is cheaper?
Opus 5.5: $4/$20 per million input/output tokens. GPT-6 Astra: $10/$50. Opus is 2.5x cheaper.
Which GPT-6 model should I compare with Opus 5.5?
On quality, GPT-6 Astra (both are flagships). On price, GPT-6.1 Sol, which costs half as much as Opus 5.5.
When were Opus 5.5 and GPT-6 released?
GPT-6 Astra in early September 2026; Claude Opus 5.5 on September 22, 2026; GPT-6 Sol on September 22 and GPT-6.1 Sol on September 29, 2026.
Which is better for coding?
For terminal-based agent coding, Opus 5.5 scored higher on the benchmarks above. For high-volume coding where cost matters, test GPT-6.1 Sol as well. More on the Claude model: Claude Opus 5.5 explained.