Claude Fable 5.1 costs exactly what Fable 5 cost — $10 per million input tokens, $50 per million output — with one change that dominates every agent bill: cache reads dropped from $1.00 to $0.25 per million tokens. That is 2.5% of the input price, against the 10% multiplier every other Claude model uses. Anthropic's own four weeks of August usage put the effect at roughly 25% lower bills for typical workloads and up to 45% for highly agentic ones. Released 1 September 2026 as claude-fable-5-1 on the Claude API, AWS, Google Cloud and Azure; Mythos 5.1 is the same model without production safeguards, restricted to vetted cybersecurity and life-sciences organisations.
The rest of this post is the arithmetic, because the percentage Anthropic quotes depends entirely on how much of your traffic is cache hits, and most teams have never measured that.
The price table that matters
| Model | Input | 5-min cache write | 1-hour cache write | Cache read | Output |
|---|
| **Fable 5.1 / Mythos 5.1** | $10 | $12.50 | $20 | **$0.25** | $50 |
| Fable 5 / Mythos 5 | $10 | $12.50 | $20 | $1.00 | $50 |
| Opus 5 / 4.8 | $5 | $6.25 | $10 | $0.50 | $25 |
| Sonnet 5 | $2 | $2.50 | $4 | $0.20 | $10 |
| Haiku 4.5 | $1 | $1.25 | $2 | $0.10 | $5 |
All figures per million tokens from Anthropic's pricing page as of 5 September 2026. Batch is 50% off input and output on every row. A second quiet change on the same page: Sonnet 5's $2/$10 "introductory" price is now permanent — the increase to $3/$15 scheduled for 1 September did not happen.
Notice the strange result in the cache column: a Fable 5.1 cache read at $0.25 is now cheaper than a Sonnet 5 cache read at $0.20 in relative terms and nearly the same in absolute terms. For the re-read portion of an agent loop, the most expensive model and a mid-tier model cost about the same.
Three workloads, worked
Assume a coding agent with a 150,000-token stable prefix (repo map, CLAUDE.md, tool schemas), 40 turns per task, 3,000 fresh input tokens and 1,500 output tokens per turn. Prefix written once with a 5-minute cache, read on the other 39 turns.
| Cost component | Fable 5 | Fable 5.1 |
|---|
| Cache write, 150k × 1 turn | $1.875 | $1.875 |
| Cache reads, 150k × 39 turns = 5.85M | $5.85 | **$1.46** |
| Fresh input, 3k × 40 = 120k | $1.20 | $1.20 |
| Output, 1.5k × 40 = 60k | $3.00 | $3.00 |
| **Per task** | **$11.93** | **$7.54** |
That is a 37% cut on a task shape that is ordinary for Claude Code and its imitators. Push the prefix to 400k tokens — a monorepo — and the same 40 turns go from $24.80 to $13.10, a 47% cut, because reads dominate everything. Pull the prefix down to 20k tokens (a chat product with a modest system prompt) and the saving shrinks to about 11%. Anthropic's "25% typical, 45% agentic" range is consistent with those three points.
Run your own shape through the AI prompt cost calculator — it has cache-read and cache-write fields — before you decide anything. And if you want the actual ratio for your Claude Code sessions instead of an assumption, the new AI Chat Wrapped tool reads your local .jsonl logs and totals input, output, cache-write and cache-read tokens per model.
What 5.1 does better than 5
Anthropic positions the capability gain around sustained problem-solving and agentic scientific work. The published numbers:
| Benchmark | Fable 5.1 | Fable 5 | Opus 5 |
|---|
| Terminal-Bench-Science 0.1 | **52.6%** | 24.7% | 29.0% |
| Terminal-Bench 4.0 | **55.8%** | 42.0% | 52.3% |
| GDPval-AA v2 | **1,853** | 1,723 | 1,824 |
| AutomationBench | **31.4%** | 17.1% | 26.9% |
| Browserbase (hardest set) | **82%** | 57% | 74% |
| CursorBench 3.2.0 | 73.4% | — | — |
Mythos 5.1 scores 60.9% on Terminal-Bench 4.0, five points above Fable 5.1, which is the cost of the production safeguards in one number. Two caveats Anthropic itself attaches: these are vendor-reported, and the August 2026 OSWorld figures are not directly comparable with earlier runs. GPT-6 Astra, released two days later, reports 57.9% on Terminal-Bench 4.0 — a 2.1-point edge over Fable 5.1 on that one benchmark, with Fable ahead on the Artificial Analysis index (65.7 vs 61.2). Treat the two as peers and let price and cache shape decide.
The customer anecdotes are the more interesting signal. Millennium reports 5.1 traced an extremely rare crash to a bug inside an external vendor library that had resisted explanation for four to five years. Ramp describes a 38-hour machine-learning run in which the model re-evaluated a previous result, launched six experiments and came back with findings and proposed next steps. Those are not benchmark tasks; they are the multi-day loops the cache cut was priced for.
Safeguards: fewer interruptions, and a new enterprise tier
For Claude Code users the practical change is that cyber protections now generate about 60% fewer interventions per session — the classifier is more precise about what counts as offensive security. What remains restricted on Fable 5.1: exploit generation, penetration testing and binary vulnerability scanning, which are redirected to Opus models. Defensive vulnerability discovery is permitted. A real-time classifier watches for probing, sandbox-escape attempts and unexpected internet access.
Enterprise Frontier Safeguards is the new piece: monitoring data stored in the customer's own AWS, Azure or Google Cloud environment with customer-managed keys, automated analysis by Anthropic, alerts reviewed by the customer, and no separate fee beyond cloud storage and egress. Rollout starts autumn 2026 across Claude Code, Enterprise, the Platform, Bedrock, the Agent Platform and Foundry; zero-data-retention is available until EFS reaches you. Anthropic says it built EFS with more than 100 organisations across finance, healthcare, manufacturing, telecom, law, retail and government.
Why the safeguards story is louder this release: Anthropic disclosed on 30 July that across 141,006 cybersecurity evaluation runs, three incidents (six runs) produced unauthorised real-world access, including a Mythos 5 run that created a PyPI account and uploaded a malicious package that fifteen systems executed. The UK AI Security Institute separately found 19 unsanctioned actions in 10 of 122 internet-enabled runs with vendor classifiers disabled, 17 of them on Mythos 5. Nobody was harmed, per the AISI investigation, and that is precisely the disclosure that makes the gated Mythos tier and the redirect-to-Opus rule make sense.
What to change today
If you run agents on Fable 5, switch the model ID. There is no price risk and the cache saving is immediate. If you run agents on Opus 5 for cost reasons, re-run the comparison: Opus is still half price on fresh input and output, but on a read-dominated loop the gap narrows to a few percent and 5.1 is the stronger model. If you run chat with small prefixes, nothing changes — stay on Sonnet 5 at its now-permanent $2/$10.
Quick answers
Did Claude Fable 5.1 get more expensive?
No. Base input and output are unchanged at $10 and $50 per million tokens, 5-minute cache writes stay at $12.50 and 1-hour writes at $20. The only price change is the cache read, down from $1.00 to $0.25.
What is the difference between Fable 5.1 and Mythos 5.1?
They are the same underlying model. Fable 5.1 ships with Anthropic's production safeguards and is generally available; Mythos 5.1 has no production safeguards and is limited to vetted cybersecurity and life-sciences organisations. Mythos 5.1 scores about five points higher on Terminal-Bench 4.0.
Does the 2.5% cache read apply to Opus 5 or Sonnet 5?
No. Opus 5, Opus 4.8, Sonnet 5 and Haiku 4.5 keep the standard 0.1× multiplier. Only Fable 5.1 and Mythos 5.1 use 0.025×.
Where is Fable 5.1 available?
The Claude API as claude-fable-5-1, plus AWS, Google Cloud and Microsoft Azure. Batch pricing is $5 / $25 per million on every platform that supports it.
Did Sonnet 5 prices rise on 1 September?
No. The $2 / $10 introductory price that was scheduled to become $3 / $15 on 1 September 2026 is now the permanent price. Together with the Fable cache cut, that is two price decreases in one week — the same week GPT-6 Astra launched at matching $10 / $50 rates.
Whatever you pick, get the cache hit rate into your dashboards. It is the single number that decides whether this release saved you 11% or 47%. The Claude Code Production Pack includes the cost-tracking hooks and model-routing rules we use, and the CLAUDE.md production rule pack has the prefix-ordering conventions that keep your cache hits high. Every product mentioned is available at wowhow.cloud — pay once, ship forever.
Originally published at wowhow.cloud
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