Claude Opus 5.5 Leads Epoch Capabilities Index Over GPT-6 Astra
Anthropic's flagship model took the top spot on Epoch AI's 50-benchmark composite index, edging OpenAI's GPT-6 Astra within heavily overlapping confidence intervals.
In short
- Claude Opus 5.5 scored 167.35 on the Epoch Capabilities Index, leading OpenAI's GPT-6 Astra (166.51) by 0.84 points across more than 50 aggregated evaluations.
- The 95 percent confidence intervals for both models overlap almost entirely (164.0–172.0 for Opus 5.5 versus 163.14–171.05 for Astra), indicating a statistical tie at the frontier.
- Domain-specific scores diverge: GPT-6 Astra recorded an index-record 170 on the Math ECI, while Anthropic models lead software engineering evaluations.
- Anthropic lists Opus 5.5 at $4.00 per million input tokens and $20.00 per million output tokens, compared to $10.00 and $50.00 for GPT-6 Astra.
Anthropic's flagship Claude Opus 5.5 model claimed the top spot on the Epoch Capabilities Index in October 2026, recording a score of 167.35 across more than 50 aggregated evaluations. The result placed the model 0.84 points ahead of OpenAI's GPT-6 Astra, which posted a score of 166.51, according to tracking data reported by OrcaRouter and explainx.ai. While the point estimate gives Anthropic the ranking lead, the published 95 percent confidence intervals span 164.0 to 172.0 for Opus 5.5 and 163.14 to 171.05 for Astra, meaning the two models remain statistically tied at the frontier.
The benchmark results arrive during an active release window in autumn 2026, with three of the top four positions on the index held by Anthropic models. Claude Sonnet 5.5 follows in third place with 165.2 points, while Claude Fable 5.1 registered 164.82 points, according to Epoch AI dataset updates. Together, the top four entries on the composite scale are separated by just 2.53 points.
Understanding the Epoch Capabilities Index
Built by independent research group Epoch AI, the Epoch Capabilities Index (ECI) stitches together individual test results across more than 50 evaluations to build a single composite scale. The index methodology was developed in collaboration with researchers from Google DeepMind's AGI Safety & Alignment team and funded by DeepMind, though Epoch AI maintains full operational independence over the benchmark product.
Unlike traditional benchmarks that measure percentage accuracy against a fixed suite, the ECI is an anchored, continuous scale designed to track relative capabilities as individual benchmarks saturate. Raw scores are anchored with Claude 3.5 Sonnet set at 130 and OpenAI's GPT-5 set at 150. Individual evaluations contribute to a model's score based on difficulty and discriminability parameters fitted across models tested on multiple benchmarks, as explained on Epoch AI's documentation hub.
Because the ECI incorporates a logistic model rather than a simple average, rankings fluctuate when new evaluations are introduced. As reported by Contra Collective, GPT-6 Astra initially recorded a score of 169 at launch, but dropped to 166 in mid-September 2026 as additional software engineering evaluations were added into the composite calculation.
Sub-Domain Discrepancies and Benchmark Breakdown
While Opus 5.5 holds the nominal overall lead, detailed benchmark results show clear divergence across specific operational domains. GPT-6 Astra maintains an advantage in quantitative tasks, recording an all-time record score of 170 on the Math ECI, according to analysis by Contra Collective. On FrontierMath Tier 4, Astra scored 98 percent compared to Opus 5.5's 95 percent, as cataloged by explainx.ai.
Conversely, Anthropic models lead on software engineering metrics. On the Software Engineering ECI (SWE ECI), Claude Fable 5.1 registered a leading score of 167, outpacing Astra's 164. Across individual benchmark results published on Epoch's tracker, Opus 5.5 recorded 91 percent on FrontierMath Tiers 1–3, 91 percent on GPQA Diamond, 77 percent on MirrorCode, and 72 percent on SimpleQA Verified.
On developer tooling evaluations published by Anthropic and reported by Valletta Software, Opus 5.5 registered 66.4 percent on Terminal-Bench 4.0 and 81.8 percent on OSWorld 2.0. However, Anthropic's own documentation notes that Astra outperformed Opus 5.5 on two of nine reported internal comparisons: AutomationBench (41.4 percent for Astra versus 40.0 percent for Opus 5.5) and Terminal-Bench-Science 0.1 (64.6 percent for Astra versus 58.7 percent for Opus 5.5).
Closed Frontier Versus Open Weights
The tight compression among leading proprietary models contrasts sharply with the performance of publicly downloadable weights. According to OrcaRouter, the highest-ranking open-weights model on the ECI is Moonshot's Kimi K3 at 157.61 points (released under a non-commercial license on July 16, 2026), placing it 9.74 points behind Opus 5.5.
Unrestricted open models follow further down the table. DeepSeek V4 Pro sits at 155.38, DeepSeek V4.1 Flash at 155.0, GLM-5.3-Flash at 151.94, and GLM-5.2 at 151.78. Analysis by tech-insider.org notes that while open models from Chinese labs and European builders—such as Mistral Large 4 Preview, released on October 6, 2026—continue to post competitive scores on task-specific metrics, closed frontier models from Anthropic and OpenAI maintain a noticeable margin on general multi-benchmark composites.
Google's publicly listed placement on the index sits further back: Gemini 3.7 Flash recorded 157.3 points to rank 16th, roughly 10 points below the top score, though Google had not posted a higher-tier flagship entry to the public ECI snapshot by mid-October 2026.
Architecture Controls and Pricing Structure
Anthropic officially released Claude Opus 5.5 on September 22, 2026, introducing significant changes to its pricing schedule and model runtime behavior, as reported by Valletta Software. API pricing is set at $4.00 per million input tokens and $20.00 per million output tokens, with prompt cache reads priced at $0.20 per million tokens (0.05x input cost). The model supports a 1-million-token context window and up to 128,000 output tokens.
By comparison, OpenAI lists GPT-6 Astra at $10.00 per million input tokens and $50.00 per million output tokens, with cache reads at $1.00 per million. Astra also increases its pricing above 272,000 tokens to $20.00 for inputs and $75.00 for outputs, according to rate card data compiled by OrcaRouter.
Operationally, Opus 5.5 introduces mandatory extended thinking. API calls attempting to disable thinking return a 400 error, leaving users to manage compute via effort levels ranging from medium up to xhigh and max. Notably, Anthropic's reported benchmark figures on Terminal-Bench and FrontierCode were run at xhigh or max effort, whereas the model defaults to medium in production deployments. Valletta Software notes that while default settings reduce typical task expenses by up to 40 percent compared to Opus 5, running workloads at maximum effort generates more thinking tokens and increases task bills.
Implications for Developers and Enterprise Teams
For engineering leaders selecting models, the 0.84-point gap between Opus 5.5 and Astra serves as an indicator of general parity rather than a decisive procurement mandate. Because confidence intervals overlap substantially, minor adjustments to benchmark test sets or new preview model versions can reorder the top tier at any time.
Procurement decisions increasingly depend on domain requirements and operational costs rather than overall leaderboard rank. Teams building quantitative computing agents find a measurable advantage in Astra's domain-leading Math ECI score of 170. Meanwhile, software development workflows benefit from Anthropic's strength in coding evaluations and its lower token pricing. With developer infrastructure supporting multiple providers, enterprise buyers are using multi-model routing architectures to test both systems against real production workloads rather than relying exclusively on public composite scores, as outlined by OrcaRouter.
Questions people ask
What is the Epoch Capabilities Index?
The Epoch Capabilities Index (ECI) is an aggregated benchmark developed by independent research organization Epoch AI. It synthesizes results from over 50 individual benchmark evaluations onto a single relative capability scale anchored to Claude 3.5 Sonnet at 130 and GPT-5 at 150.
Is Claude Opus 5.5 decisively more capable than GPT-6 Astra?
No. While Opus 5.5 holds a 0.84-point lead in point estimates (167.35 versus 166.51), their published 95 percent confidence intervals (164.0–172.0 for Opus 5.5 and 163.14–171.05 for Astra) overlap heavily, indicating a statistical tie at the frontier.
How does API pricing compare between Opus 5.5 and GPT-6 Astra?
Claude Opus 5.5 lists at $4.00 per million input tokens and $20.00 per million output tokens, with cache reads at $0.20 per million across its full 1M context. GPT-6 Astra lists at $10.00 per million input tokens and $50.00 per million output tokens (with higher rates above 272,000 tokens), making Opus 5.5 2.5 times cheaper on base token rates.
Which model performs better on math and software engineering?
GPT-6 Astra leads math benchmarks, holding an index record Math ECI of 170 and scoring 98 percent on FrontierMath Tier 4. In software engineering, Anthropic models lead, with Claude Fable 5.1 reaching 167 on SWE ECI compared to Astra's 164.
Sources
- Epoch Capabilities IndexEpoch AI
- Claude Opus 5.5 Tops the Epoch Capabilities Index by 0.84 PointsOrcaRouter, Oct 1, 2026
- Claude Opus 5.5 Tops the Epoch Capabilities Index With 167 — What the Score Meansexplainx.ai, Oct 3, 2026
- Claude Opus 5.5 vs GPT 6 Astra: What the Epoch Capabilities Index Actually Shows (October 2026)Contra Collective, Oct 2, 2026
- Claude Opus 5.5: the 20% price cut that is really 40%Valletta Software, Sep 22, 2026
- Claude Opus 5.5 Tops Epoch AI Leaderboard [2026]tech-insider.org, Oct 12, 2026
How this story was made: the newsroom picked it up from Google Search, gathered the full text of the sources above, and drafted it with AI assistance. Every factual claim was then checked against those sources before publishing (37 claims checked). Illustrations marked as AI-generated are not photographs. Spotted an error? Tell us.
Published October 13, 2026 at 01:13 UTC
