Claude Fable 5.1 vs GPT-6 Astra
Claude Fable 5.1 and GPT-6 Astra compared on price, context window, cache pricing, long-context surcharges, endpoints and published benchmarks, with both frontier models available through one AIsa key.
Model comparisonUpdated
The short answer
Claude Fable 5.1 and GPT-6 Astra are the two September 2026 frontier releases and they list at the same headline price: $10 per million input tokens and $50 per million output. The differences are in the fine print. Fable 5.1 charges a quarter as much for cache reads and keeps flat pricing across its full 1M context, while Astra doubles input and cache rates and raises output by half once a prompt passes 272K tokens. On published benchmarks Astra leads terminal, computer-use and frontier-math suites, and Fable 5.1 leads tool-augmented Humanity's Last Exam and the Artificial Analysis index. Pick Fable 5.1 for long-context, cache-heavy agent loops where cost compounds; pick Astra for computer-use and terminal-heavy automation or OpenAI-shaped stacks. Both are on AIsa under one key and one budget.
Choose Claude Fable 5.1 when
- Long-running agent loops that resend a large prompt: cache reads cost $0.25 versus $1 per million tokens.
- Prompts routinely above 272K tokens, where Astra's long-context surcharge applies and Fable 5.1's price does not change.
- Deep research and document, spreadsheet and slide work; Fable 5.1 leads the tool-augmented Humanity's Last Exam.
- Anthropic-style Messages API integrations with adaptive thinking and per-message effort control.
Choose GPT-6 Astra when
- Computer-use and terminal-heavy automation: Astra posts the higher OSWorld 2.0 and Terminal-Bench 4.0 scores.
- Frontier math and hard scientific benchmarks such as FrontierMath Tier 4.
- OpenAI-shaped codebases that need Chat Completions, the Responses API or built-in tools like web search and code interpreter.
- Five reasoning effort levels from low to max when you want to trade cost for depth per request.
Claude Fable 5.1 vs GPT-6 Astra at a glance
| Dimension | Claude Fable 5.1 | GPT-6 Astra |
|---|---|---|
| Provider | Anthropic | OpenAI |
| Release date | 2026-09-01 | 2026-09-03 |
| Context window | 1,000,000 tokens | 1,050,000 tokens |
| Input price | $10 / M tokens | $10 / M tokens |
| Output price | $50 / M tokens | $50 / M tokens |
| Cache read price | $0.25 / M tokens | $1 / M tokens |
| Cache write price | $12.5 / M tokens | $12.5 / M tokens |
| Endpoints on AIsa | /v1/messages | /v1/messages, /v1/chat/completions, /v1/responses |
| Capabilities | coding, text, vision | coding, text, vision |
| Billing | Pay as you go | Pay as you go |
| Max output | 128K tokens | 128K tokens |
| Knowledge cutoff | June 2026 | April 2026 |
| Long-context pricing | Flat across the full 1M window | Above 272K input tokens: 2x input and cache rates, 1.5x output |
| Reasoning control | Adaptive thinking, always on; effort defaults to high, adjustable per message (beta) | Effort levels low, medium, high, xhigh, max |
| Terminal-Bench 4.0 (vendor-published) | 55.8% | 57.7% |
| OSWorld 2.0 (vendor-published) | 77.9% partial / 41.7% strict | 72.6% |
| Humanity's Last Exam, with tools (vendor-published) | 65.0% | 57.2% |
| FrontierMath Tier 4 (vendor-published) | 87.8% | 97.6% |
| Restricted sibling | Claude Mythos 5.1, same model without dual-use safeguards, invitation only | None announced |
Pricing and endpoints come from the AIsa model catalog as of Sep 9, 2026. Benchmark rows are the vendors' own published figures. Open a model page for live rates.
Pricing on AIsa: same list price, different fine print
Both models cost $10 per million input tokens and $50 per million output tokens on AIsa, the same as the vendors' list prices. If your workload is short prompts with short answers, the bills are identical.
Two things change that picture for agents. First, cache reads: Fable 5.1 charges $0.25 per million cached tokens, Astra $1. An agent that resends a 200K-token system prompt and tool schema on every turn pays four times more for that prefix on Astra. Second, Astra's long-context tier: once a prompt exceeds 272K input tokens, input and cache rates double and output goes to $75 per million. Fable 5.1 keeps one price across its entire 1M window.
Cache-write prices are the same at $12.50 per million. Prices in the table come from the AIsa catalog and were checked against both vendors' pricing pages on 2026-09-09.
Context and output limits
Both accept roughly a million tokens of input and produce up to 128K tokens of output. Astra's window is nominally 1,050,000 tokens, Fable 5.1's 1,000,000. For whole-repository or archive-scale work the practical difference is not the window size but what you pay to fill it, which is where the long-context surcharge above matters.
What the published benchmarks say
The numbers below are the vendors' own launch figures, so treat them as upper bounds on your workload rather than a ranking. Astra is ahead on agentic terminal and computer-use suites: Terminal-Bench 4.0 at 57.7% versus 55.8%, OSWorld 2.0 at 72.6% under OpenAI's protocol, and a large lead on FrontierMath Tier 4 at 97.6% versus 87.8%. OpenAI also reports Astra finishing OSWorld tasks in roughly half the time of GPT-5.6 Sol.
Fable 5.1 is ahead on tool-augmented Humanity's Last Exam at 65.0% versus 57.2%, and on the Artificial Analysis Intelligence Index at 65.7 versus 61.2. Anthropic positions it for long-horizon agentic coding, multistep research and document work, and reports typical cost reductions of 25% and up to 45% on agentic workloads from the cheaper cache reads.
On general coding the field is tight. DeepSWE v1.1 has Astra at 74.1% and Fable 5.1 at 67.4%, with Claude Opus 5 and Gemini 3.8 Flash within a point of Astra, so the choice for everyday coding should be driven by cost and integration rather than these deltas.
Endpoints and integration on AIsa
GPT-6 Astra is served on AIsa through three endpoint shapes: OpenAI Chat Completions, the Responses API and an Anthropic-style Messages route. That makes it the drop-in for OpenAI-shaped code and for teams that rely on OpenAI's built-in tools such as web search, file search, code interpreter and computer use.
Claude Fable 5.1 is served through the Anthropic Messages endpoint. It uses adaptive thinking that is always on, with effort defaulting to high and adjustable per message. If you are migrating from Fable 5, note that forced tool use now returns an error and thinking blocks are tied to the model that produced them.
Availability and safeguards
Both models are marked trial-unavailable on AIsa, so a funded wallet is required. Anthropic ships Fable 5.1 with additional dual-use safeguards and offers the unfiltered twin, Claude Mythos 5.1, only to Project Glasswing participants. OpenAI launched Astra first to enterprises in its gated Daybreak program, with API access following within days.
Because both are in the same AIsa catalog, an agent can route planning to one and execution to the other under one key, one wallet and the same budget limits and approval thresholds.
Common questions
- Which is cheaper, Claude Fable 5.1 or GPT-6 Astra?
- The list price is the same: $10 per million input tokens and $50 per million output tokens on AIsa. Fable 5.1 is cheaper in practice for cache-heavy agent loops, because cache reads cost $0.25 versus $1 per million, and for prompts above 272K tokens, where Astra charges double for input and 1.5 times for output.
- Which has the larger context window?
- They are effectively equal. GPT-6 Astra lists 1,050,000 tokens and Claude Fable 5.1 lists 1,000,000. Both allow up to 128K output tokens.
- Which is better for coding?
- Vendor benchmarks put GPT-6 Astra slightly ahead on Terminal-Bench 4.0 and DeepSWE v1.1, and Claude Fable 5.1 ahead on the Artificial Analysis index and tool-augmented Humanity's Last Exam. For everyday coding the gap is small; cost per agent turn and your existing SDK matter more.
- What is Claude Mythos 5.1?
- The same model as Claude Fable 5.1 without the additional dual-use safety measures, available by invitation through Anthropic's Project Glasswing. It shares Fable 5.1's specifications and pricing. AIsa serves Fable 5.1.
- Can I call both through the same API key?
- Yes. Both are listed in the AIsa model catalog, billed to the same prepaid wallet and subject to the same budget limits and approval thresholds.
Run both models from one key.
Every model in the catalog bills to the same wallet under the same budget limits. Switching is a string change.