GPT-6 Astra and Claude Fable 5.1 both list at $10 input and $50 output per 1M tokens. Same sticker does not mean the same bill. Cache reads and long-context rules decide most real workloads. Rates in TokenCalculator checked 17 Sep 2026.
Compare on one prompt
Open the tools compare page or cost calculator, paste one golden prompt, and hold output size fixed. Do not compare list prices alone.
Sticker rates: a tie
| Meter | GPT-6 Astra | Claude Fable 5.1 |
|---|---|---|
| Input / 1M | $10.00/1M | $10.00/1M |
| Output / 1M | $50.00/1M | $50.00/1M |
| Cache read / 1M | $1.00/1M | $0.250/1M |
| Cache write / 1M (5m) | $12.50/1M | $12.50/1M |
| Batch input / output | $5.00/1M / $25.00/1M | $5.00/1M / $25.00/1M |
| Long-context rule | >272K reprices whole request | Standard rates across 1M window |
| Context window | about 1.05M | 1M |
| Tokenizer in TokenCalculator | Exact (o200k) | Approx (confirm with count_tokens) |
Where the invoice diverges
1. Cache reads
Astra cache reads are $1.00 per 1M (10% of input). Fable 5.1 cache reads are $0.25 per 1M (2.5% of input). On agent loops with a large stable system plus tools prefix, Fable usually wins the token meter. On short uncached prompts, costs match.
2. Long-context cliff
Astra doubles input and cache rates and multiplies output by 1.5x once input exceeds 272K, for the entire request. Fable 5.1’s published card keeps standard rates across its 1M window. Million-token windows are not priced the same way.
3. Cost per finished task
Independent harnesses sometimes show Astra finishing benchmark suites with fewer reasoning tokens even when list rates match. Rate cards rank price. Workloads rank how much thinking each model burns. Re-run your own golden tasks before you generalize from a leaderboard.
Workload cheatsheet
- Short, fresh prompts: cost tie at $10 / $50. Pick on quality and tooling, not sticker.
- Cache-heavy agents: lean Fable 5.1 on the meter until your evals say otherwise.
- Prompts past ~272K input: model Astra’s long-context band carefully; Fable keeps flat list rates.
- Batchable offline jobs: both offer ~50% batch. Apply the half-rate card only when latency allows.
Worked numbers (planning only)
Example A, uncached: 5K input + 1K output costs $0.10 on either model. That is (5000 / 1,000,000) x $10 plus (1000 / 1,000,000) x $50.
Example B, cache heavy: 100K cached + 5K fresh + 2K output. Astra about $0.10 cache + $0.05 fresh + $0.10 output = $0.25. Fable about $0.025 cache + $0.05 fresh + $0.10 output = $0.175.
Common mistakes
- Declaring a winner from sticker rates alone
- Ignoring reasoning token spend when comparing "same price" models
- Budgeting Claude with OpenAI Exact token counts
- Assuming Fast mode or US-only multipliers match across vendors
Frequently asked questions
Is GPT-6 Astra more expensive than Claude Fable 5.1?
Not on sticker input and output. Fable is cheaper on cache reads. Astra can cost more above 272K input. Measured task cost can still favor either side depending on reasoning token use.
Which is cheaper for agents?
If your prefix is stable and cache hits are high, Fable 5.1 usually wins the meter. If your prompts are short and uncached, expect a tie before quality differences.
Do both models have 1M context?
Roughly yes. Astra publishes about 1.05M with a pricing cliff past 272K input. Fable publishes 1M at standard rates across the window.
Can I compare Exact token counts across both?
OpenAI can be Exact in TokenCalculator. Claude stays Approx in-browser. Equalize with provider count APIs when the budget is contractual.
Where is Claude Opus 5 in this comparison?
Opus 5 lists at $5.00/1M / $25.00/1M, half the Fable sticker. Prefer Opus first unless you need Fable-class quality.
Where is GPT-5.6 Sol?
Sol lists at $4.00/1M / $20.00/1M. It is the usual OpenAI step below Astra.
Are these rates live?
No. Curated catalog rates last verified 17 Sep 2026. Confirm on OpenAI and Anthropic pricing pages.
What should I open first?
Paste one production prompt into the cost calculator twice: once on Astra, once on Fable 5.1. Then open the compare pair URL for a side-by-side catalog view.
Next steps
- GPT-6 Astra pricing
- Claude Fable 5.1 pricing
- Astra model page
- Fable 5.1 model page
- How we verify rates
Related Sep 2026 pricing guides
Frontier rate cards, cost-per-task math, and long-context cliffs. Cross-link these when you compare Sol, Luna, Opus 5.5, Grok 4.7, or Astra.