GPT-5.5 Pro versus GPT-6 Astra

The match-up at a glance

This comparison pits two frontier models, the ones you pick when quality comes first against each other: GPT-5.5 Pro and GPT-6 Astra, both built by OpenAI. The first dates from April 23, 2026, the second from September 4, 2026.

All figures below come from public sources checked on October 22, 2026: models.dev and OpenRouter for prices and capabilities, Epoch AI for scores.

Which one is more capable?

On our capability index, GPT-5.5 Pro scores 96.9 out of 100 and GPT-6 Astra 94.7. GPT-5.5 Pro leads by 2.2 points.

On the 3 benchmarks reported for both, GPT-5.5 Pro wins 0 times and GPT-6 Astra 2:

• GPQA Diamond (PhD-level science questions): GPT-5.5 Pro 93.9%, GPT-6 Astra 95.8%
• OTIS Mock AIME (competition mathematics problems): GPT-5.5 Pro 100%, GPT-6 Astra 100%
• FrontierMath (very hard research-level mathematics): GPT-5.5 Pro 87.7%, GPT-6 Astra 93.7%

Budget

GPT-5.5 Pro costs $30 per million input tokens and $180 per million output tokens; GPT-6 Astra costs $10 and $50.

For a typical agent task (read 20,000 tokens, write 2,000), expect $0.96 with GPT-5.5 Pro versus $0.30 with GPT-6 Astra. GPT-6 Astra is 3.2 times cheaper, a gap that adds up quickly over 1,000 tasks: $660 difference.

Run the numbers with your own volumes in the agent cost calculator.

Context and capabilities

Both models have the same context window: 1,050,000 tokens, roughly 1,575 pages.

Reasoning mode: GPT-5.5 Pro yes, GPT-6 Astra yes. Tool calling: GPT-5.5 Pro yes, GPT-6 Astra yes. Image input: GPT-5.5 Pro yes, GPT-6 Astra yes.

Neither publishes its weights: both are used through an API.

Knowledge cutoff: December 1, 2025 for GPT-5.5 Pro, April 30, 2026 for GPT-6 Astra.

In short

Our reading of the data:

• Choose GPT-5.5 Pro for a higher capability index.
• Choose GPT-6 Astra for a lower cost and a more recent release.

This verdict relies on public figures; it does not replace a trial on your own tasks.