Claude Fable 5.1 vs GPT-6 Astra: API Cost Compared
The verdict, in numbers: Claude Fable 5.1 and GPT-6 Astra publish identical list rates ($10.00 / $50.00), but identical list rates do not mean identical bills: on our 200-step agent workload the two land 1.8x apart ($7.49 vs $13.46), driven entirely by the cache-read row.
This page compares list rates and workload cost only. It does not tell you which model is better — capability belongs to your own evaluation. What it does give you is the half of the decision that can be verified: what each provider will actually charge, read from their own documentation on September 18, 2026.
Rate cards side by side
| Item | Claude Fable 5.1 | GPT-6 Astra |
|---|---|---|
| Provider | Anthropic | OpenAI |
| Input / 1M | $10.00 | $10.00 |
| Output / 1M | $50.00 | $50.00 |
| Cache read / 1M | $0.25 | $1.00 |
| Context window | 1M | 1.05M |
Sources: Anthropic pricing ↗ and OpenAI pricing ↗, both checked September 18, 2026.
The same 200-step agent task, priced twice
One coding-agent task: 200 steps, a stable 40,000-token prefix written to cache on step 1 and re-read on the remaining 199, 500 output tokens per step. Cache-write billing follows each provider’s own rules (cache writes bill at 1.25x the input rate; cache writes bill at 1.25x the uncached input rate).
| Component | Claude Fable 5.1 | GPT-6 Astra |
|---|---|---|
| Cache write (once) | $0.50 | $0.50 |
| Cache reads (199 × 40K) | $1.99 | $7.96 |
| Output (200 × 500) | $5.00 | $5.00 |
| Total per task | $7.49 | $13.46 |
A heavy month on each
50M fresh input, 150M cached input, 10M output tokens in a month:
| Component | Claude Fable 5.1 | GPT-6 Astra |
|---|---|---|
| Fresh input (50M) | $500.00 | $500.00 |
| Cached input (150M) | $37.50 | $150.00 |
| Output (10M) | $500.00 | $500.00 |
| Monthly total | $1,037.50 | $1,150.00 |
What this comparison does and does not say
Claude Fable 5.1: Top-end Claude model for long-horizon autonomous agents; 1M context, 128K max output, adaptive thinking always on.
GPT-6 Astra: Current OpenAI flagship for computer use, coding, and multi-step professional work; 128K max output.
Cost is the verifiable half of the decision; quality is the half only your workload can answer. Run both candidates on a representative evaluation set and measure cost per accepted task — the cost planning guide walks through that method, and the calculator lets you re-price this exact comparison with your own token mix.
Frequently asked questions
Is Claude Fable 5.1 cheaper than GPT-6 Astra?
On list rates, Claude Fable 5.1 costs $10.00/$50.00 per 1M input/output tokens versus $10.00/$50.00 for GPT-6 Astra. On a 200-step agent workload (40K cached prefix, 500 output tokens per step), Claude Fable 5.1 totals $7.49 versus $13.46 for GPT-6 Astra. Prices verified September 18, 2026.
Claude Fable 5.1 vs GPT-6 Astra: which has the longer context window?
Claude Fable 5.1 offers 1M tokens; GPT-6 Astra offers 1.05M tokens, per provider documentation checked September 18, 2026.
Where do these Claude Fable 5.1 and GPT-6 Astra prices come from?
Both rate cards were read from the providers' own pricing pages on September 18, 2026: https://platform.claude.com/docs/en/about-claude/pricing and https://developers.openai.com/api/docs/models/gpt-6-astra.