Claude Fable 5.1: Anthropic's Fable Flagship, Fully Specified

Claude Fable 5.1 is the generally available Fable-class flagship, live since September 1, 2026, and it sits in the middle of three names readers routinely mix up. The no-suffix Claude Fable 5 is the June 2026 original that Fable 5.1 supersedes at the same per-token price. Claude Fable 5.2 is a third name that has never shipped and has no vendor page anywhere — including the vendor's own documentation, pricing table and newsroom, none of which mention it. And Claude Mythos 5.1 is not a separate model either: it is the same weights as Claude Fable 5.1 sold through a restricted access program. This page is the reference for the one model that a reader can actually call today: its envelope, its full rate card, what the vendor documents about effort and thinking, every published benchmark figure and the setting it was run at, the independent read, and what it costs on OrcaRouter. It also covers the version question underneath the name — the successor number readers search for that has no model behind it, Claude Fable 5.5 — and sets the Fable rate card beside what Claude Opus 5.5 and Claude Sonnet 5.5 cost for the same work.

The Fable family: three names, two models, one suffix trap

The suffix confusion is not cosmetic — the three strings differ by a single digit and lead to different envelopes, different prices and, in one case, to nothing at all. Sorting them out first is what makes the rest of this page readable.

• Claude Fable 5 — released June 9, 2026 as Anthro​pic's first Fable-class model. It is still served and still priced identically to its successor: $10 per million input tokens and $50 per million output, with cache reads at $1.00 per million. It is the model the .1 release replaces, and it is not the model this page is about.

• Claude Fable 5.1 — this model. Released September 1, 2026, same $10 / $50 headline as Claude Fable 5, with cache reads cut to $0.25 and a shorter list of behavioral changes than the version bump suggests. Anthro​pic's own documentation describes it as the version to reach for "for demanding reasoning and long-horizon agentic work."

• Claude Fable 5.2 — does not exist as a purchasable or documented model. Anthro​pic's newsroom lists no such announcement; its model overview, pricing page and Fable 5.1 model page do not carry the name; Artificial Analysis has no entry for it; and OrcaRouter's catalogue returns "model not found." Write-ups you may have seen are pre-release reporting, which is a different article from this one and is linked below rather than re-litigated here.

• Claude Mythos 5.1 — the same underlying model as Claude Fable 5.1, with safeguards tuned for cybersecurity and life-sciences work instead of general release. It is available only through Anthro​pic's trusted-access programs (Project Glasswing and the Cyber Verification Program), currently only to a set of US organizations, and it is priced identically to Claude Fable 5.1. It is not in OrcaRouter's catalogue, and this page is not about it.

Each of those three Fable strings belongs to a model — two shipping today, one that never shipped. A fourth circulates in search demand and in social posts, and it belongs to none. Claude Fable 5.5 is the subject of a claim made on October 2, 2026, in two posts — the second quoting the first — that a successor Fable model would outrun GPT-6 Astra, a flagship its vendor had already finished and then declined to ship, a cancellation reported in the press on September 28, 2026. Two posts is the whole sourcing chain, and neither carries a screenshot, a registry entry, a vendor statement, a staff comment, a date, a price or a benchmark. Where the claim came from and the withheld model it names are covered in a separate article on this blog, Claude Fable 5.5 Beat GPT-6.1 Astra Before It Exists, rather than restated here.

What can be checked is the present tense, and it was checked on October 7, 2026. No catalogue that can be read carries an identifier for Claude Fable 5.5. The vendor's models overview lists Claude Fable 5.1, Claude Opus 5.5, Claude Sonnet 5.5 and Claude Haiku 4.5 and stops; the vendor's sitemap ends at the fable-5-1 path; Artificial Analysis has no page for it; and our own catalogue returns "model not found" for anthropic/claude-fable-5.5. A context window, a rate card and a benchmark for Claude Fable 5.5 therefore cannot be quoted, here or anywhere else, and this page quotes none. An unannounced model attracts the most confident claims precisely because nothing exists to check them against, and the absence of a page is not evidence against a future one — labs do not document what they have not announced. It does mean the version question has a factual answer today, and that answer is Claude Fable 5.1: generally available since September 1, 2026, and specified below.

What shipped on September 1, 2026, and what the model actually is

Anthro​pic shipped Claude Fable 5.1 and Claude Mythos 5.1 in a single announcement on September 1, 2026, describing them as "the world's most advanced models for coding and knowledge work." That is a vendor claim, quoted as such. The verifiable part of the release is narrower and more useful: one model, two safeguard profiles, and a price change concentrated entirely in cached input.

The envelope, per Anthro​pic's own model page and API documentation: a 1,000,000-token context window at standard per-token pricing — no long-context premium, so a 900,000-token request bills at the same rate as a 9,000-token one — and up to 128,000 output tokens. Input is text and images; output is text. Tool use, vision, structured outputs and multilingual work are all supported. The reliable-knowledge and training-data cutoffs are both June 2026.

The API surface: claude-fable-5-1 on the first-party Claude API and on Claude Platform on AWS, Goo​gle Cloud and Microsoft Foundry; anthropic.claude-fable-5-1 on Amazon Bedrock. Anthro​pic commits to supporting it "not sooner than September 1, 2027," which is the horizon to plan a migration against rather than a deprecation date.

One deployment constraint is worth knowing before you standardize on it: Claude Fable 5.1 runs with production safeguards that can refuse a request outright, and Anthro​pic's fallback guidance matters in practice — a refusal returns a normal HTTP 200 whose stop reason is refusal, not an error. The announcement also sets out a data-retention path: Anthro​pic says its new Enterprise Frontier Safeguards system, which keeps data in customer-controlled cloud infrastructure, will be phased in for enterprise customers from later this fall, and that until then eligible customers can run Fable 5.1 with zero data retention.

The full rate card, and where the money actually moved

Every figure in this section is from Anthro​pic's own pricing documentation, read on September 28, 2026. Base token pricing is unchanged from Claude Fable 5: $10.00 per million input tokens and $50.00 per million output tokens. The change is in caching.

• Input — $10.00 per million tokens, identical to Claude Fable 5.

• Output — $50.00 per million tokens, identical to Claude Fable 5.

• Cache read (a hit) — $0.25 per million tokens, down from $1.00 on Claude Fable 5. Anthro​pic states this as a 75% reduction, and it is the entire mechanism behind the release's cost story: a cache hit now costs 2.5% of the standard input price instead of 10%.

• Cache write — $12.50 per million tokens for a five-minute window, $20.00 per million for a one-hour window. These are unchanged from Claude Fable 5, so the write side of the trade is not part of the discount.

• Batch API — a 50% discount on both directions, so $5.00 per million input and $25.00 per million output for asynchronous work.

• Data residency — requesting US-only inference carries Anthro​pic's standard 1.1x multiplier on every token category.

Anthro​pic models the result in its announcement as roughly 25% lower cost on typical workloads and up to around 45% on highly agentic ones, measured at default effort over four weeks of August 2026 usage. Those two percentages are vendor estimates from Anthro​pic's own usage data, not independently audited figures, and they hold only for workloads that re-read context. A single-turn request with no cache reuse costs exactly what it did on Claude Fable 5.

Placed against its own family, the headline is the part a buyer has to square. Claude Fable 5.1 is the most expensive of Anthro​pic's current flagships on input: $10.00 per million input tokens and $50.00 per million output, against $4.00 per million input and $20.00 per million output for Claude Opus 5.5, released September 22, 2026, and $2.00 per million input and $10.00 per million output for Claude Sonnet 5.5, released September 28, 2026. Those are the vendor's list prices, and they are also what you pay on OrcaRouter — the provider list price is passed through with 0% markup rather than marked up, so a vendor price change lands here the same day it lands on the vendor's card.

Artificial Analysis's measurements put the same three models in the opposite order to the price list. On Intelligence Index v4.3.2 — the revision the independent-read section below quotes, re-read on October 7, 2026 — Claude Opus 5.5 scores 57.62, Claude Sonnet 5.5 scores 56.00 and Claude Fable 5.1 scores 53.35, which is the 58, 56 and 53 that index displays once rounded. All three sit on one revision of one index measured by one harness, so they are comparable to each other and to nothing else. On it, the most expensive of the three measures lowest: Claude Opus 5.5 sits 4.27 points above Claude Fable 5.1 while costing less than half as much in both directions, and Claude Sonnet 5.5 sits 2.65 points above it at a fifth of the input price and a fifth of the output price. Those figures are Artificial Analysis's own measurements, taken against the vendors' list prices rather than anything Anthro​pic reported, and neither is a verdict on your workload — an index summarises ten evaluations. Anthro​pic's own documentation still recommends starting with Claude Opus 5.5 for most workloads and reserving Claude Fable 5.1 for demanding reasoning and long-horizon agentic work. What the Fable premium buys is the envelope set out above — 1,000,000 tokens of context at standard pricing, up to 128,000 tokens of output — and the long-horizon behaviour the benchmark section is about. What it does not buy, on this index, is a higher score than the cheaper flagships in its own family.

Effort, thinking, and the setting each benchmark was run at

This is the part of the spec sheet most coverage skips, and it is the part that decides whether two published scores are comparable at all.

Claude Fable 5.1 uses adaptive thinking, and thinking is always on — Anthro​pic's model page lists it as "Adaptive (always on)," and there is no mode that turns it off. The older fixed-budget form of extended thinking is not merely discouraged here: the parameter is rejected. On Claude 4.7-and-later models, a request that sets thinking: {type: "enabled", budget_tokens: N} returns a 400 error, and Anthro​pic's documentation names Claude Fable 5.1 as one of the models affected. The replacement is a depth dial rather than a token budget.

That dial is output_config.effort, and Claude Fable 5.1 supports all five levels: low, medium, high, xhigh and max. The default is high, which is also what you get by omitting the parameter entirely. Anthro​pic's own guidance is to start at high, step up to xhigh or max for the most capability-sensitive agentic and coding work, and step down to medium or low for routine or latency-sensitive work once your evals show quality holds. Anthro​pic also states that at Low or Medium effort Fable 5.1 reaches results similar to or better than Claude Fable 5 at much lower cost — and that the model defaults to High effort in Claude Code and to Medium in Claude Cowork and on Claude.ai, so the same model name can behave differently depending on which surface you call it from.

Now the benchmark-setting question, answered precisely. Anthro​pic publishes each of these benchmarks as an accuracy-versus-cost curve plotted at all five effort levels, and its headline comparison table does not attach a single effort level to a single number. The one figure in Anthro​pic's material that is pinned to an effort level is CursorBench 3.2.0: the partner quote in the same announcement reports 73.4% "at max effort," which matches the 73.4% in the table. Everything else should be read as the curve, not the point — which is also why a bare score comparison against a different lab's model, evaluated at a different effort and a different harness, tells you very little.

The benchmark sheet, and who ran it

Every number in this section is Anthro​pic-reported and has not been reproduced independently. Two caveats from Anthro​pic's own footnotes belong with them: the Terminal-Bench-Science 0.1 standard error is ±3.5–4.5 points per model, and the OSWorld 2.0 scores are on the benchmark authors' August 2026 task release, so they are not comparable to previously published OSWorld 2.0 results.

• Terminal-Bench-Science 0.1 — Claude Fable 5.1: 52.6% vs Claude Fable 5: 24.7%, Claude Opus 5: 29.0%, GPT-5.6 Sol: 22.4%. The "more than double" framing holds, and Anthro​pic notes its harness reproduces the public leaderboard's Opus 5 result (30.0%) and Claude Fable 5 result (21.4%) within noise.

• Terminal-Bench 4.0 — Claude Fable 5.1: 55.8% vs Claude Fable 5: 42.0%, Claude Opus 5: 52.3%, GPT-5.6 Sol: 37.3%. Claude Mythos 5.1 reaches 60.9%, a gap Anthro​pic attributes to tasks where the less precise cyber safeguards on the gated model intervened.

• Humanity's Last Exam — Claude Fable 5.1: 60.9% without tools and 65.0% with them, vs Claude Fable 5 at 57.8% / 63.8% and Claude Opus 5 at 56.6% / 63.6%.

• GDPval-AA v2 — Claude Fable 5.1: 1,853 vs Claude Fable 5: 1,723, Claude Opus 5: 1,824, GPT-5.6 Sol: 1,711.

• AutomationBench — Claude Fable 5.1: 31.4% vs Claude Fable 5: 17.1%, Claude Opus 5: 26.9%, GPT-5.6 Sol: 19.6%.

• CursorBench 3.2.0 — Claude Fable 5.1: 73.4% at max effort, vs Claude Fable 5: 70.5%, Claude Opus 5: 70.0%, GPT-5.6 Sol: 67.2%.

• OSWorld 2.0 — Claude Fable 5.1: 77.9% partial and 41.7% strict, vs Claude Fable 5 at 72.9% / 36.1% and Claude Opus 5 at 75.4% / 39.6%.

• Partner-run, first-party-reported — Browserbase measured 82% of its hardest browser-agent tasks completed in about ten minutes each, against 74% for Claude Opus 5 and 57% for Claude Fable 5; Crosby's RedlineBench contract-redlining score moved from 47.9 to 57.0 over Fable 5; Samaya's FrontierFinance rubric moved from 49.2% to 55.9%.

Read the sheet with one structural fact in mind, because Anthro​pic states it plainly: Claude Fable 5.1 and Claude Fable 5 were both evaluated with production safeguards enabled, and on tasks where those safeguards intervened both models scored a zero — on OSWorld 2.0 for each, and on AutomationBench for Claude Fable 5. Anthro​pic says cyber tasks that triggered an intervention were completed by Claude Opus 4.8 and biology tasks by Claude Opus 5, and that this "likely reduces the performance" of the Fable models on those benchmarks. A refusal, in other words, is scored as a failure, which is a fair reading of an agent that stops — and also a reason the numbers move when the safeguards change.

The independent read: what a third party measured, and at what effort

Independent figures here come from Artificial Analysis, whose Intelligence Index is the most widely cited third-party aggregate and who publish the exact configuration each result was produced under. As of September 28, 2026, its page for Claude Fable 5.1 carries the Artificial Analysis Intelligence Index v4.3.2 at 53 under the configuration "Adaptive Reasoning, Max Effort, Default Fallback," at a weighted cost of $7.63 per Intelligence Index task and 69.7 output tokens per second.

The effort ladder is the interesting part, and it is the thing a single headline number hides. Artificial Analysis publishes a separate page for each effort level: 47 at low ($2.37 per task), 49 at medium ($2.98), 51 at high ($3.91), 53 at xhigh ($5.98) and 53 at max ($7.63). Two things follow. First, the effort dial is real and it is priced: dropping from max to high gives up two index points and costs 49% less per task, and dropping to low gives up six points for 69% less. Second, xhigh and max land on the same headline score at different costs, which is the ordinary shape of these curves and a reason to measure your own workload rather than buy the top setting by default. The same index revision puts Claude Fable 5 at 50 ($8.75 per task), Claude Opus 5 at 51 ($5.86) and Claude Opus 5.5 at 58 ($5.98).

Two honesty notes on those comparisons. They are all one vendor's harness on one index revision, so they are comparable to each other and to nothing else. And they do not reproduce Anthro​pic's ordering: on Anthro​pic's vendor-reported sheet Claude Fable 5.1 clears its whole comparison field, while on Artificial Analysis's index a cheaper Anthro​pic model released three weeks later scores five points higher. Both can be true, because the index and the vendor sheet weight different work — and because a headline index is a summary of ten evaluations, not a verdict on your task.

Calling it: the version question under one key

The practical reason this page exists is that "claude fable 5.1" is a query people type on its own, usually to answer one of three things: which of the Fable names is current, what it costs, and how to try it without standing up a second vendor contract. The first two are answered above from Anthro​pic's own pages. The third is where a router earns its place.

Claude Fable 5.1 is on OrcaRouter as anthropic/claude-fable-5.1 at Anthro​pic's list price with 0% markup — $10.00 per million input tokens, $50.00 per million output, cache reads at $0.25 and cache writes at $12.50 — alongside Claude Fable 5 in the same catalogue. Both are reachable from one key, one base URL and one billing line, through either the OpenAI-compatible /v1/chat/completions endpoint or Anthro​pic's own /v1/messages shape, so an existing Claude SDK client can point at the new model by changing two values. That is the cheapest possible version test: run the same prompt against Claude Fable 5 and Claude Fable 5.1 at high and xhigh effort, compare cost per completed task rather than cost per request, and let your own evals decide whether the .1 is worth moving for. Because the markup is zero, a vendor price change — as with the cache-read cut on September 1 — is live on our side the same day rather than waiting on a re-rate.

If you are choosing between the Fable line and something cheaper, note that Anthro​pic itself points most workloads at Claude Opus 5.5 in its own documentation, reserving Fable 5.1 for demanding reasoning and long-horizon agentic work. And if what you actually need is the gated biology or cyber capability, that is Claude Mythos 5.1 through Anthro​pic's access programs — not something this page, or our catalogue, can route you to.

Why this is a reference page and not a launch story

Claude Fable 5.1 shipped on September 1, 2026 — four weeks before this page was written — so the launch is not the news, and this piece is deliberately not a second announcement. The argument for the page is different and, we think, more durable: it is the canonical reference for a model that is live in our catalogue today, the page a reader reaches after searching the model's own name, and its warrant is standing search demand that we can see first-hand rather than the freshness of a launch. The September 1 release date is used here to date the model, not as an event to report. Our launch-day coverage and the pre-launch leak reporting remain separate pieces, and this page links to them rather than restating either.