Claude Opus 5.5 vs GPT-6 Astra: Which flagship should you use?

Facts checked Oct 22, 2026. Every number links to its source.

Summary

Derived from sourced facts, not from builder opinions

Opus 5.5 costs less than half as much per token ($4 / $20 per million against $10 / $50), but it used more tokens: in Vals AI's Terminal-Bench run it cost $13.20 per test against $9.58 for GPT-6 Astra.
Run through the API in the same harness (Mini-SWE-agent) by Vals AI, Opus 5.5 scored 65.15% on Terminal-Bench 4.0 and GPT-6 Astra 59.60%.
Epoch's capability index has them tied: 168.83 for Opus 5.5 and 168.81 for GPT-6 Astra.
Before paying for Astra, test GPT-6.1 Sol: in Codex, it tied Astra on the official Terminal-Bench 4.0 board at a fifth of the API price.

Choose Claude Opus 5.5 if

You run long coding agents: Opus 5.5 scored higher on Terminal-Bench 4.0 in the same harness.
Your workload is mostly input, such as long documents, where the lower per-token price dominates.
You deploy through Amazon Bedrock, Google Cloud or Microsoft Foundry.
You send prompts over 272K input tokens, which cost 2x input and 1.5x output on GPT-6 Astra.

Choose GPT-6 Astra if

Your team already works in ChatGPT or Codex.
You want a lower measured cost per test: $9.58 against $13.20 in Vals AI's Terminal-Bench run.
Your own tests show Astra ahead on your task, and the price difference does not matter.

Claude Opus 5.5

GPT-6 Astra

Follow both to get an email when either one changes its price, limits or behavior.

Best by task

Agentic coding (Terminal-Bench 4.0, Vals AI, Mini-SWE-agent harness)

Claude Opus 5.5

65.15% against 59.60%, both models run through the API in the same harness.

Cost per test on Terminal-Bench 4.0 (Vals AI, API prices)

GPT-6 Astra

$9.58 per test against $13.20: Opus 5.5 used more tokens.

Chat preference (Arena text)

Claude Opus 5.5

Rank 2 (1516) for Opus 5.5 (High) against rank 73 (1442) for GPT-6 Astra (Max).

Web development (Arena WebDev)

Claude Opus 5.5

Rank 1 (1807) against rank 2 (1775), with confidence ranges that do not overlap.

Overall capability (Epoch Capabilities Index)

Tied

168.83 against 168.81: tied within the confidence intervals.

Cost

One request in an agent loop that rereads a repository, so most input is cached. A full task makes many such requests. Illustrative, not measured.

Input tokens per requestCached input tokensOutput tokensRequests per month

Claude Opus 5.5

$0.124 per request

$124.00 a month for 1,000 requests

GPT-6 Astra

$0.340 per request

$340.00 a month for 1,000 requests

Claude Opus 5.5 costs 64% less for this workload at list prices.

Estimate per request: (input × input rate + cached input × cached rate + output × output rate) ÷ 1,000,000, at list prices on the cited pricing pages. For GPT-6 Astra, a request with more than 272,000 input tokens is charged 2 times the input rates and 1.5 times the output rate. It is not a measured task cost: models use different numbers of tokens, including reasoning tokens, for the same task.

Side by side

	Claude Opus 5.5	GPT-6 Astra
Maker	Anthropic	OpenAI
Model ID	claude-opus-5-5 (Bedrock: anthropic.claude-opus-5-5)	gpt-6-astra (single snapshot, no dated versions)
Released	2026-09-22	2026-09-03 (API changelog) · ChatGPT rollout began the same day
API price per 1M tokens	$4 input / $20 output · 5-minute cache write $5, 1-hour cache write $8, cache read $0.20 · Batch 50% off	$10 input / $50 output · cached input $1 · cache writes $12.50 · prompts over 272K input tokens cost 2x input and 1.5x output · Batch and Flex 50% off · Fast mode 2x
Fast mode price	$8 input / $40 output (up to 2.5x speed, Claude Code and Claude Platform)	Not published
Context window	1M tokens	1,050,000 tokens (max input 922,000)
Max output	128K tokens (300K on Batch API with beta header)	128,000 tokens
Knowledge cutoff	June 2026 (reliable knowledge and training data)	2026-04-30
Reasoning settings	medium · adaptive thinking always on	low, medium, high, xhigh, max (no none)
Where to use it	Claude API, Amazon Bedrock, Google Cloud (Google managed and Anthropic managed), Microsoft Foundry, Claude Platform on AWS · also Claude Code	ChatGPT Plus, Pro, Business and Enterprise · OpenAI API · Amazon Web Services
Retires no sooner than	2027-09-22	Not published
Quality tracking (MarginLab)	Claude Code with Opus 5.5/high on a SWE-Bench-Pro subset: 77% latest day (50 tasks), 76% 7-day, 81% 30-day · baseline still being collected, detection paused (last updated 2026-09-29)	Not published
Quality tracking (Livenerf)	Window 1 (days 11-20) score 0.592 vs baseline 0.584, delta +0.007 plus or minus 0.018: no change	Not published
Codex usage on Plus, per 5 hours	Not published	About 5-45 local messages per 5 hours on Plus and Standard Business
Terminal-Bench 4.0 (official board)	64.85% (plus or minus 3.11)rank 1, Claude Code, max effort, 330 trials, $4,718.65 total	58.18% (plus or minus 2.79)rank 3, Codex, max effort, 330 trials, $3,267.18 total
Terminal-Bench 4.0 (Vals AI, same harness for all)	65.15%rank 1, $13.20 per test	59.60%rank 3, $9.58 per test
Vals Index v2.2	70.18%rank 2, includes tasks a fallback model answered after a refusal (64.16% if those count as failures)	65.02%rank 5
Arena text	Rank 2 (1516)Opus 5.5 (High), 10,322 votes	Rank 73 (1442)GPT-6 Astra (Max), 14,076 votes
Arena WebDev	Rank 1 (1807)Opus 5.5 (Max), 3,119 votes	Rank 2 (1775)GPT-6 Astra (Max), 6,511 votes
Epoch Capabilities Index	168.83(95% CI 165.25 to 173.95), highest in the file	168.81(CI 165.45 to 174.51)

Scores appear side by side only when both were published for the same test. Settings differ between sources, so small gaps can reverse under another harness.

Updates

Both Claude Opus 5.5 and GPT-6 Astra

BenchmarkOct 21, 2026

Vals Index: Sonnet 5.5 edges Opus 5.5 and Gemini 4 Argon

Vals AI's GDP-weighted index of finance, coding, legal and tax tasks ranks Claude Sonnet 5.5 first at 70.85%, Opus 5.5 at 70.18%, Gemini 4 Argon 69.24%, GPT-6 Astra 65.02% and GPT-6.1 Sol 63.94% as of October 21, 2026.

BenchmarkOct 20, 2026

Arena text leaderboard: Gemini 4 Argon first, Opus 5.5 second, GPT-6 Astra 73rd

Arena's October 20, 2026 text leaderboard ranks Gemini 4 Argon (High) first at 1532 (4,900 votes), Claude Opus 5.5 (High) second at 1516, Claude Opus 5 (Max) fourth at 1509, Claude Sonnet 5.5 (xHigh) 28th at 1473, GPT-6.1 Sol (Max) 66th at 1443 and GPT-6 Astra (Max) 73rd at 1441.

BenchmarkOct 19, 2026

Arena WebDev leaderboard: Opus 5.5 first, GPT-6 Astra second

Arena's October 19, 2026 WebDev leaderboard ranks Claude Opus 5.5 (Max) first at 1807 (2,925 votes), GPT-6 Astra (Max) second at 1774, Claude Sonnet 5.5 (xHigh) third at 1768, GPT-6.1 Sol (Max) fourth at 1753, Claude Opus 5 (Max) eighth at 1689 and Gemini 4 Argon (High) tenth at 1678.

Claude Opus 5.5

3 recorded incidents in the last 90 days.

Limits changeOct 29, 2026

Perplexity Agent API xhigh effort raises max steps from 100 to 300

The xhigh level stays on Claude Opus 5.5 with GPT-6.1 Sol as fallback. Context compaction starts at 150,000 tokens, and it no longer sends a system prompt.

Before

100 max steps

After

300 max steps

Who is affected:: Agent API users on xhigh effort or frozen configurations.
What to do:: Copy the updated values if you use a frozen configuration.

Now availableOct 22, 2026

Full Claude Platform available on Google Cloud (Anthropic managed)

Claude Models on Google Cloud (Anthropic managed) is generally available with US and Global endpoints. It gives Google Cloud customers the full Claude Platform feature set with Google Cloud IAM, billing and commitment drawdown. Fable 5.1, Opus 5.5, Sonnet 5.5 and Haiku 5.5 are available, and fast mode for Opus 5.5 costs $8 / $40 per million input and output tokens there.

Who is affected:: Teams that buy Claude through Google Cloud and want features that the Google-managed offering lacks.
What to do:: No action needed. Google Cloud customers who need same-day Claude API features can switch to the Anthropic-managed offering.

Quality report · MeasuredOct 20, 2026

Livenerf finds no change in Opus 5.5 accuracy in its first 10-day window

Window 1 (days 11 to 20, from October 4) scored 0.592 against a 0.584 launch-week baseline, a difference of +0.007 plus or minus 0.018 over 780 samples, which the pre-registered rule calls no change. Median output rose from 588 to 606 tokens. Day 19 ran on API credits because the Max weekly meter was exhausted.

GPT-6 Astra

2 recorded incidents in the last 90 days.

New releaseOct 29, 2026

Perplexity Agent API lists GPT-6 Astra at $10 per million input tokens

The model docs now list openai/gpt-6-astra at $10.00 per million input tokens and $50.00 per million output tokens up to 272k input tokens, with flex and priority tiers.

After

$10.00 input, $50.00 output per million tokens

BenchmarkSep 29, 2026

Epoch: GPT-6.1 Sol matches GPT-6 Astra on FrontierMath tiers 1-3

Epoch's data records gpt-6.1-sol at max effort scoring 0.937 on FrontierMath tiers 1-3 (the same as GPT-6 Astra at max), 1.0 on FrontierMath tier 4 v2 and 0.954 on GPQA Diamond, from runs started September 29, 2026.

Now availableSep 29, 2026

Ultrafast mode available for GPT-6 Astra in the API

Ultrafast mode for GPT-6 Astra became available to API customers in the Responses API, with global processing and US data residency only.

Switching checklist

To test Opus 5.5 from existing OpenAI code, keep the OpenAI SDK, set the base URL to https://api.anthropic.com/v1/ and use a Claude API key from the Claude Console.
Change the model to claude-opus-5-5.
Remove temperature values below 1, which return a 400 error on Claude 4.7 and later models.
Replace tool_choice required or a named function with auto. Opus 5.5 does not support forced tool use and returns an error.
Expect response_format and reasoning_effort to be ignored, and prompt caching to be off, in this compatibility layer.
Move to the Anthropic SDK for production. Anthropic says the compatibility layer is for testing and comparing, and it is not available on Claude Platform on AWS.
Price your real workload with the calculator above. Astra charges more for prompts above 272K input tokens.
Run your own tasks in each maker's agent (Claude Code, Codex), the setting used on the official board.

FAQ

Is Claude Opus 5.5 better than GPT-6 Astra for coding?

On Terminal-Bench 4.0 run by Vals AI with the same harness for both models, Opus 5.5 scored 65.15% and GPT-6 Astra 59.60%.

Which is cheaper, Opus 5.5 or GPT-6 Astra?

Per token, Opus 5.5: one million input plus one million output tokens cost $24 against $60. Per test it can be the other way: in Vals AI's Terminal-Bench run, Opus 5.5 cost $13.20 per test and GPT-6 Astra $9.58.

Which has the larger context window?

About the same: Opus 5.5 takes 1M tokens. GPT-6 Astra takes 1,050,000 with at most 922,000 input.

Which ranks higher on Arena?

Opus 5.5: rank 2 on the text board against rank 73, and rank 1 on WebDev against rank 2.

Can I call Claude Opus 5.5 with the OpenAI SDK?

Yes, for testing. Point the SDK at https://api.anthropic.com/v1/ with a Claude API key. Anthropic says the layer is not meant for most production use.

Which has the newer knowledge cutoff?

Opus 5.5: June 2026. GPT-6 Astra: April 30, 2026.

Can I turn off reasoning on GPT-6 Astra?

No. Its settings run from low to max, with no none option.

Sources and method

Prices, specs and limits come from each maker's own pages. Benchmark scores come from independent benchmark publishers and the makers' announcements, with the setting each was measured in. Changes come from changelogs, deprecation pages and status pages. The summary is written from these facts only.

Claude Opus 5.5 vs GPT-6 Astra: Which flagship should you use?

Summary

Best by task

Cost

Side by side

Updates

Both Claude Opus 5.5 and GPT-6 Astra

Claude Opus 5.5

GPT-6 Astra

Switching checklist

FAQ

Is Claude Opus 5.5 better than GPT-6 Astra for coding?

Which is cheaper, Opus 5.5 or GPT-6 Astra?

Which has the larger context window?

Which ranks higher on Arena?

Can I call Claude Opus 5.5 with the OpenAI SDK?

Which has the newer knowledge cutoff?

Can I turn off reasoning on GPT-6 Astra?

Related comparisons

Sources and method