Claude Opus 5.5 vs GPT-6 Astra
Opus 5.5 leads 7 of 9 head-to-head benchmarks and costs 60% less per token — Astra's edge is frontier math and scientific reasoning.
Anthropic's flagship vs OpenAI's most capable model, released nineteen days apart in September 2026. Both offer million-token context windows and configurable reasoning effort, but differ sharply on price, benchmark profile and ecosystem. This Claude Opus 5.5 vs GPT-6 Astra comparison breaks down specs, benchmarks and practical trade-offs for coding, agents and research.
Last updated 2026-09-30 · Vendor data sourced from Anthropic and OpenAI
At a Glance: Opus 5.5 or GPT-6 Astra?
Choose Claude Opus 5.5 when…
You need strong agentic coding, computer use or general knowledge work at a lower price point — Opus leads most benchmarks and costs 60% less per token.
Choose GPT-6 Astra when…
Your workload demands frontier-grade math, scientific reasoning or end-to-end automation where Astra's benchmark leads justify its 2.5x price premium.
Opus 5.5 vs GPT-6 Astra: Key Specifications
API identifiers, pricing, context limits and reasoning behavior — sourced from each vendor's official documentation. Both are reasoning models with configurable effort levels and million-token context windows.
| Spec | Claude Opus 5.5 | GPT-6 Astra |
|---|---|---|
| API model ID | claude-opus-5-5 | gpt-6-astra |
| Released | Sep 22, 2026 | Sep 3, 2026 |
| Tagline | For long-running agentic coding and knowledge work | OpenAI's most capable model for complex reasoning and end-to-end work |
| Latency | Moderate | Slower |
| Input price (per MTok) | $4 | $10 |
| Output price (per MTok) | $20 | $50 |
| Context window | 1M tokens | 1.05M tokens |
| Max output | 128K tokens (300K on Batch API) | 128K tokens |
| Thinking | Adaptive, always on, cannot be disabled | Reasoning model, effort configurable |
| Default effort | medium | medium |
| Knowledge cutoff | Jun 2026 | Apr 2026 |
| Input modalities | Text + images | Text + images |
| Retirement | Not sooner than Sep 22, 2027 | At least 6 months notice per OpenAI policy |
Pricing: Opus 5.5 Costs 60% Less per Token
At list price, Opus 5.5 is $4 / $20 vs Astra at $10 / $50 per million tokens (MTok) for input / output — 60% cheaper on both. A typical 100K-input, 20K-output request costs $0.80 on Opus and $2.00 on Astra.
| Pricing Tier | Claude Opus 5.5 | GPT-6 Astra |
|---|---|---|
| Standard API (per MTok) | $4 / $20 | $10 / $50 |
| Batch API (50% off) | $2 / $10 | $5 / $25 |
| Cache write (5 min) | $5 | $12.50 |
| Cache write (1 hour) | $8 | $12.50 |
| Cache read | $0.20 (0.05x base) | $1 (0.1x base) |
Cache reads differ significantly: Opus at $0.20 per MTok (5% of base) vs Astra at $1.00 per MTok (10% of base). For prompt-heavy workloads with high cache-hit rates, Opus's 5x cheaper cache reads compound the savings. Note: OpenAI uses a single cache-write tier ($12.50 per MTok regardless of TTL), while Anthropic offers two tiers — $5 for 5-minute and $8 for 1-hour cache TTL.
Both models offer fast modes, but pricing differs dramatically. Opus fast mode costs $8 / $40 per MTok, up to 2.5x speed — still cheaper than Astra's standard $10 / $50. Astra fast mode runs $20 / $100 per MTok.
Long-context surcharge (Astra only): prompts exceeding 272K input tokens trigger Astra's long-context pricing at $20 / $75 per MTok. Opus has no long-context surcharge at any prompt length.
Benchmark Results: Opus 5.5 Leads 7 of 9
Head-to-head scores where both vendors published results on the same benchmark. Effort levels and evaluation setups may differ between vendors — treat as directional, not perfectly apples-to-apples.
| Benchmark | Claude Opus 5.5 | GPT-6 Astra | Lead | Note |
|---|---|---|---|---|
| Terminal-Bench 4.0 | 66.4% | 57.9% | Opus +8.5 pp | Agentic coding |
| FrontierCode 1.1 Main | 54.4% | 53.3% | Opus +1.1 pp | — |
| Humanity's Last Exam | 67.7% | 57.2% | Opus +10.5 pp | With tools |
| OSWorld 2.1 | 81.8% | 72.6% | Opus +9.2 pp | Computer use, partial scoring |
| DeepSWE 1.1 | 74.2% | 74.1% | Tied | 113-task agentic coding |
| BenchCAD | 96.2% | 95.9% | Opus +0.3 pp | With Python tool |
| HealthBench Professional | 65.6% | 63.4% | Opus +2.2 pp | Length-adjusted |
| AutomationBench | 40.0% | 41.4% | Astra +1.4 pp | End-to-end automation |
| Terminal-Bench-Science 0.1 | 58.7% | 64.6% | Astra +5.9 pp | Scientific agentic coding |
Takeaway: Opus 5.5 dominates agentic coding (Terminal-Bench 4.0, +8.5 pp), computer use (OSWorld 2.1, +9.2 pp) and broad knowledge (Humanity's Last Exam, +10.5 pp). Astra leads on scientific agentic coding and end-to-end automation — narrower categories where its premium may be justified.
Independent Evaluations & Third-Party Data
Cross-vendor comparisons from independent aggregators. Numbers may differ from vendor tables due to different evaluation setups, effort settings and scoring methodologies.
BenchLM category averages (Sep 29, 2026):
| Category | Claude Opus 5.5 | GPT-6 Astra |
|---|---|---|
| Overall | 86.94 | 88.22 |
| Agentic | 87.9 | 70.7 |
| Coding | 83.6 | 74.2 |
| Reasoning | 82.4 | 89.5 |
| Knowledge | 88.5 | 85.4 |
Astra's overall BenchLM score (88.22) is slightly higher than Opus's (86.94), but their 90% confidence intervals overlap. Opus leads decisively on agentic (+17.2) and coding (+9.4); Astra leads on reasoning (+7.1). View the full BenchLM comparison
Further reading and hands-on comparisons:
Opus 5.5 vs GPT-6 Astra: Which to Choose by Use Case
Not every task needs the most expensive model. Use this table to match your workload to the model that gives you the best balance of quality, speed and cost.
| Scenario | Recommended | Why |
|---|---|---|
| Agentic coding | Opus 5.5 | Terminal-Bench 4.0 lead (+8.5 pp) and 60% lower cost per long-running coding session |
| Code review | Opus 5.5 | Higher coding benchmark scores across the board; lower per-token cost for multi-file reviews |
| Computer use / GUI agents | Opus 5.5 | OSWorld 2.1 lead (+9.2 pp); no long-context surcharge for extended GUI sessions |
| Frontier math and reasoning | GPT-6 Astra | Saturates FrontierMath Tier 4 (97.6%) and ARC-AGI-3 (99.9%); higher BenchLM reasoning score |
| Scientific research agents | GPT-6 Astra | Terminal-Bench-Science 0.1 lead (+5.9 pp); purpose-built for research-grade workloads |
| End-to-end automation | GPT-6 Astra | Slight AutomationBench lead (+1.4 pp), though both score around 40% |
| Document / slide generation | Opus 5.5 | Adequate quality for content work at 60% less cost; faster latency |
| High-volume batch processing | Opus 5.5 | $2 / $10 batch pricing vs $5 / $25 — half the cost at comparable quality |
| Long-context workloads (>272K) | Opus 5.5 | No long-context surcharge; Astra reprices at $20/$75 past 272K tokens |
Switching Between Claude Opus 5.5 and GPT-6 Astra
Different APIs, different ecosystems. Claude and GPT use different message formats, tool-calling conventions and streaming protocols. Migrating a workload between them requires prompt and schema adaptation — there is no drop-in swap.
Context and reasoning are not portable. Thinking blocks, tool results and conversation history from one vendor cannot be passed to the other. Treat a vendor switch as starting a new conversation.
Reasoning effort maps similarly. Both models support low, medium, high, xhigh and max effort levels. Both default to medium. Higher effort increases quality and cost on both, but the token-per-task ratio differs — always benchmark on your own workload before comparing cost.
Knowledge cutoffs differ. Opus 5.5 has a June 2026 cutoff; Astra has April 2026. For tasks requiring the latest information, Opus has a two-month advantage.
Platform availability differs. Opus 5.5 is available on Claude API, AWS Bedrock, Google Cloud, Microsoft Foundry and Claude Platform on AWS. Astra is on OpenAI API, Azure OpenAI and ChatGPT plans. Multi-cloud routing through OpenRouter can abstract the vendor difference for some use cases.
The Bottom Line
Claude Opus 5.5 is the stronger all-round choice for most developers: it leads 7 of 9 head-to-head benchmarks — including the biggest agentic coding, computer use and knowledge tests — while costing 60% less per token, with no long-context surcharge and a more recent knowledge cutoff. Reserve GPT-6 Astra for workloads where frontier math, scientific reasoning or end-to-end automation is the primary requirement and Astra's benchmark leads on those tasks outweigh the price premium. For everything else, Opus delivers equal or better quality at a fraction of the cost.
Opus 5.5 vs GPT-6 Astra: FAQ
What are the main differences between Claude Opus 5.5 and GPT-6 Astra?
Both are frontier reasoning models with million-token-class context windows (1M vs 1.05M) and 128K max output. The key differences are vendor (Anthropic vs OpenAI), price ($4/$20 vs $10/$50 per million tokens), knowledge cutoff (June 2026 vs April 2026), and benchmark profile: Opus leads 7 of 9 head-to-head benchmarks including agentic coding and computer use, while Astra leads AutomationBench and Terminal-Bench-Science.
Is Claude Opus 5.5 better than GPT-6 Astra?
Opus 5.5 leads 7 of 9 comparable benchmarks and costs 60% less per token. Its strongest leads are on agentic coding (Terminal-Bench 4.0, +8.5 pp), computer use (OSWorld 2.1, +9.2 pp) and Humanity's Last Exam (+10.5 pp). Astra leads AutomationBench (+1.4 pp) and Terminal-Bench-Science (+5.9 pp), and has the higher BenchLM overall score (88.22 vs 86.94, though intervals overlap). For most coding and knowledge workloads Opus is the better value; for frontier math and scientific reasoning Astra may have an edge.
How much cheaper is Claude Opus 5.5 than GPT-6 Astra?
60% cheaper per token: $4 / $20 (Opus) vs $10 / $50 (Astra) per million input / output tokens. A typical 100K-input, 20K-output request costs $0.80 on Opus and $2.00 on Astra. Opus is also cheaper on batch ($2/$10 vs $5/$25), cache reads ($0.20 vs $1.00) and cache writes ($5/$8 vs $12.50). Even Opus fast mode ($8/$40) costs less than Astra's standard pricing.
Which model is better for coding?
Opus 5.5 leads the two most popular agentic coding benchmarks: Terminal-Bench 4.0 (66.4% vs 57.9%) and FrontierCode 1.1 (54.4% vs 53.3%). BenchLM's coding average also favors Opus (83.6 vs 74.2). On DeepSWE 1.1 the models are essentially tied (74.2% vs 74.1%). Astra leads Terminal-Bench-Science 0.1 (64.6% vs 58.7%), a benchmark focused on scientific agentic coding. For general-purpose coding, Opus offers stronger benchmark scores at a lower price.
Which model is better for math and science?
Astra has the edge on research-grade math and science. It saturates FrontierMath Tier 4 at 97.6% and ARC-AGI-3 at 99.9%, and leads Terminal-Bench-Science 0.1 (64.6% vs 58.7%). BenchLM's reasoning average is higher for Astra (89.5 vs 82.4). If frontier math and abstract reasoning are core to your workload, Astra may justify its premium.
Do Claude Opus 5.5 and GPT-6 Astra have the same context window?
Nearly identical. Opus 5.5 has a 1M-token (1,000,000) context window while Astra has a 1.05M-token (1,050,000) context window. Both support up to 128K output tokens. Astra triggers long-context pricing ($20/$75 per MTok) past 272K input tokens; Opus has no long-context surcharge.
Can I switch between Claude Opus 5.5 and GPT-6 Astra?
Not within a single API conversation. They are from different vendors with different APIs, message formats and tool-calling conventions. Migrating a workload requires adapting prompts and tool schemas. Context and thinking blocks are not portable across vendors.
Does GPT-6 Astra have a fast mode like Opus 5.5?
Yes, both offer a fast mode. Opus 5.5 fast mode costs $8 / $40 per MTok with up to 2.5x speed (Anthropic, research preview). Astra fast mode costs $20 / $100 per MTok. Even Astra's standard pricing ($10/$50) is higher than Opus fast mode.
Can I use Claude Opus 5.5 or GPT-6 Astra for free?
Neither model is available on a free plan. Opus 5.5 requires a claude.ai Pro, Max, Team or Enterprise subscription (the free plan defaults to Sonnet 5.5). GPT-6 Astra requires ChatGPT Plus, Pro, Team or Enterprise — the free ChatGPT tier does not include it. Both models are available via their respective APIs on a pay-per-token basis with no subscription required.
When were Opus 5.5 and GPT-6 Astra released?
GPT-6 Astra launched on September 3, 2026; Claude Opus 5.5 followed on September 22, 2026. Astra's knowledge cutoff is April 2026; Opus's is June 2026. Anthropic guarantees Opus availability until at least September 2027; OpenAI's deprecation policy gives at least 6 months notice.