> Claude Opus 5.5 vs GPT-6 Astra: index scores 200 and 197 (GPT-4 = 100), API prices, context windows and results on the same independent tests, side by side.

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# Compare

Side by side on the same independent tests, with price and context. The link in your address bar shares this exact comparison.

[Claude Opus 5.5](https://themodelindex.org/models/claude-opus-5-5/)

[GPT-6 Astra](https://themodelindex.org/models/gpt-6-astra/)

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Next to Claude Opus 5.5

Best open model, measured

Kimi K3

The version it replaced

Claude Opus 5

Where it started

GPT-4

Presets

Best open vs best closed (measured)

Today vs GPT-4 (2023)

Top three labs

US vs China, best of each

[Claude Opus 5.5](https://themodelindex.org/models/claude-opus-5-5/)

200

188–211

Weights

Closed

Released

Sep 2026

Price in / out

$4.0 / $20

Context

1M

Core tests taken

4 of 8

[GPT-6 Astra](https://themodelindex.org/models/gpt-6-astra/)

197

186–209

Weights

Closed

Released

Sep 2026

Price in / out

$10 / $50

Context

1.05M

Core tests taken

5 of 8

Claude Opus 5.5 and GPT-6 Astra are too close to call (2 points apart, within the uncertainty), ahead on 1 of 4 shared core tests, and costs 2.5× less.

## Test by test · independent results · Epoch AI

Claude Opus 5.5

GPT-6 Astra

_Chart: Test results for the selected models_

Only tests that at least two of these models have taken. Bold rows are the core tests shown on every profile; all of these results inform the index score. Task length (METR) is listed below the chart because it is a duration, not a percentage.

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Source: https://themodelindex.org/compare/claude-opus-5-5-vs-gpt-6-astra/ · The Model Index · data as of 2026-10-01