OpenAI sells both as flagships, but GPT-6 Astra is the newer model (September 2026) and GPT-5.5 pro is the premium-compute option of the previous generation. Their pricing structures differ more than their headline rates.
GPT-5.5 pro costs 3× GPT-6 Astra on input and 3.6× on output, and OpenAI lists no cached-input price for it. On agent loops that re-send the same context, Astra pays $1.00 per cached 1M tokens while 5.5 pro pays its full $30.00 — roughly a 10× gap on a cache-heavy coding session.
| GPT-5.5 pro | GPT-6 Astra | |
|---|---|---|
| Provider | OpenAI | OpenAI |
| Tier | Flagship | Flagship |
| Input / 1M tokens | $30.00 | $10.00 |
| Output / 1M tokens | $180.00 | $50.00 |
| Cache hit / 1M | — | $1.00 |
| Cache write / 1M | — | $12.50 |
| Context window | 1M | 1M |
| Notes | Higher-compute GPT-5.5 for hard problems (slowest); no cached-input discount; >272K tokens: $60/$270 | New top flagship (Sep 3, 2026); rolling out to Plus/Pro/Business/Enterprise; >272K tokens: $20/$75, cache $2 |
GPT-6 Astra is 3.0× cheaper on input tokens ($10.00 vs $30.00 per 1M). GPT-6 Astra is 3.6× cheaper on output tokens ($50.00 vs $180.00 per 1M).
| Workload | GPT-5.5 pro | GPT-6 Astra | Difference |
|---|---|---|---|
| Support chatbot 10,000 conversations × (2,000 input + 400 output tokens) | $1,320 | $400 | GPT-6 Astra saves $920 |
| Document Q&A 1,000 questions × (30,000 input + 800 output tokens) | $1,044 | $340 | GPT-6 Astra saves $704 |
| Coding agent session 4M input tokens (90% cache hits) + 150,000 output tokens | $147 | $15.10 | GPT-6 Astra saves $132 |
Standard rates per 1M tokens from data on this site. The coding-agent row assumes 90% of input is read from the prompt cache (full input price where a model has no cache rate). Cache-write fees, batch discounts and long-context surcharges listed in the notes are not included. Updated November 2, 2026.
you have a small number of hard, single-shot problems where your own testing shows it outperforms Astra, and volume is low enough that its rates don't matter.
you run anything at volume or in a loop — chat, agents, RAG — where cache pricing and lower rates compound, or you want OpenAI's current flagship.
Prompt caching is what makes agentic workloads affordable: the same system prompt, tools and files are re-sent every turn. OpenAI's pricing page lists no cached-input rate for GPT-5.5 pro, so every re-sent token is billed at $30.00. Astra charges $12.50 to write a 30-minute cache and $1.00 to read it.
Astra's rate rises to 2× input and 1.5× output above 272K tokens. OpenAI also lists a higher long-context rate for GPT-5.5 pro, so very long prompts multiply the gap rather than closing it.
OpenAI's Batch and Flex tiers halve input and output for both models, which helps for evaluations or offline analysis. GPT-5.5 pro is not offered in OpenAI's Fast tier. If you need 5.5 pro for a hard subset of tasks, route only those to it and send the rest to Astra or GPT-6.1 Sol ($2.00/$10.00).
Yes: $30.00 vs $10.00 per 1M input tokens and $180.00 vs $50.00 per 1M output tokens, with no cached-input discount on 5.5 pro.
OpenAI's pricing page lists no cached-input price for GPT-5.5 pro, so repeated context is billed at the full input rate.
Yes. GPT-6 Astra launched on September 3, 2026 as OpenAI's new top flagship.
OpenAI's Batch and Flex tiers are half the standard input and output price for GPT-5.5 pro.
How we choose: prices come from each vendor's official pricing page and are re-checked every week. We take no affiliate fees, so rankings aren't for sale.