> **TL;DR:** Anthropic's Fable 5.1 (released 2026-09-01) restores the vulnerability-discovery capability that triggered the June 2026 export-control pull of Fable 5, drawing the line the security community asked for (discovery allowed, exploit development blocked) without mentioning June. The headline 25 to 45 percent price cut comes entirely from cheaper cache reads and applies only where usage is billed by token, so subscribers see none of it. Measured from the author's own Claude Code transcripts, 5.1 takes the same number of turns per prompt as Fable 5 but emits about 50 percent more output tokens, which works out to about 21 percent cheaper per prompt at API list price and about 16 percent more expensive on a subscription-shaped meter. The weekly Fable ceiling is unchanged and Claude Code limits drop 17 percent on September 14. OpenAI's GPT-6 Astra, released two days after Fable 5.1, draws the same discovery-allowed, exploit-blocked line, with the unrestricted model gated behind its Daybreak defender program.

In June, three words got Anthropic's best model pulled off the internet. A researcher typed "fix this code" into Fable 5, it found the security holes, patched them, and wrote the tests, and a US export-control directive followed that Friday. [I wrote about it at the time](https://georgediab.com/posts/2026/fix-this-code), and I argued what nearly every security professional argued: you cannot hand a vulnerability-finder to defenders only, so the ban took the tool from the people who follow the law and left it with everyone else.

On Tuesday Anthropic shipped Fable 5.1. [The announcement](https://www.anthropic.com/claude-fable-and-mythos-5-1) says, in its own words, "[Fable 5.1 can now be used to discover software vulnerabilities—though not to develop exploits for them](https://www.anthropic.com/claude-fable-and-mythos-5-1#:~:text=Fable%205.1%20can%20now%20be%20used%20to%20discover%20software%20vulnerabilities)." Claude Code users "[can expect an average of around 60% fewer interventions per session from our cyber safeguards](https://www.anthropic.com/claude-fable-and-mythos-5-1#:~:text=can%20expect%20an%20average%20of%20around%2060%25%20fewer%20interventions%20per%20session)." Penetration testing, exploit generation, and binary vulnerability scanning still get [rerouted to an Opus model](https://code.claude.com/docs/en/model-config). Neither the announcement nor the 212-page system card mentions June.


## Finding vulnerabilities and fixing them was always the defensive job

Katie Moussouris, the one outside expert who read the report behind the June ban, [called finding and fixing vulnerabilities](https://www.lutasecurity.com/post/the-fable-5-export-controls-harm-us-cyber-defense) "the most valuable thing an AI model can do for defensive security." Anthropic's new line draws exactly that boundary: discovery yes, exploit no. Ordinary code review was never the issue, before June or after. The step the ban and the new rule both turn on is narrower. It is spotting the flaw an attacker could use.

Two days later OpenAI drew the same line from the other side. [GPT-6 Astra](https://openai.com/index/gpt-6-astra/) is a model that OpenAI says "[meets the Critical threshold in cybersecurity under our Preparedness Framework](https://openai.com/index/gpt-6-astra/#:~:text=meets%20the%20Critical%20threshold%20in%20cybersecurity%20under%20our%20Preparedness%20Framework)." It scored a [perfect 100% on ExploitBench](https://openai.com/index/gpt-6-astra/#:~:text=achieved%20a%20perfect%20score%20of%20100%25), the test for turning a known vulnerability into a working exploit, and during that testing "[discovered and used two previously unknown zero-day vulnerabilities](https://openai.com/index/gpt-6-astra/#:~:text=discovered%20and%20used%20two%20previously%20unknown%20zero-day%20vulnerabilities)." The version rolling out to Plus and Pro can do "[secure code review and patching](https://openai.com/index/gpt-6-astra/#:~:text=secure%20code%20review%20and%20patching)" and "[will refuse to comply with more advanced cybersecurity tasks such as creating proof-of-concept exploits for vulnerabilities](https://openai.com/index/gpt-6-astra/#:~:text=will%20refuse%20to%20comply%20with%20more%20advanced%20cybersecurity%20tasks%20such%20as%20creating%20proof-of-concept%20exploits%20for%20vulnerabilities)." The version without the refusals goes to vetted defenders through a program called Daybreak, which is Mythos with a budget. OpenAI put [$1 billion in subsidized access](https://openai.com/index/daybreak-for-frontline-defenders/) behind it the same day. Both labs landed on the same boundary in the same week, each with a loosened model for people who can prove who they are. In June the first half of that boundary was enough to get Fable 5 pulled.

I finished Ray Kurzweil's [*The Singularity Is Nearer*](https://www.google.com/books/edition/_/oAriEAAAQBAJ) this week. The chapter on peril walks through the dangerous technologies he expects this century, engineered biology, nanotechnology, and AI, and lands in the same place for each. You cannot uninvent them, so the defense has to be built from the same capability, and it has to be in place first. His example is self-replicating nanobots, but the logic is the June ban's logic with the sign flipped: "Yet even if responsible people design safe nanobots, bad actors could still design dangerous ones. Therefore, we will need a nanotechnology 'immune system' already in place before these scenarios can even become a possibility." A model that finds vulnerabilities is that immune system for software. The June order switched it off and left everything else running.

## At the default setting it feels like Fable 5, and that's by design

I ran 5.1 in Claude Code for two days before writing this. Solid. Nothing drastic in either direction. That matches [Anthropic's own footnote](https://www.anthropic.com/claude-fable-and-mythos-5-1#:~:text=achieves%20results%20similar%20to%20or%20better%20than%20Fable%205): at low or medium effort, 5.1 "achieves results similar to or better than Fable 5's at a much lower cost," and Claude Code defaults to high. The gains live at the top of the dial. [Simon Willison ran his pelican test](https://simonwillison.net/2026/Sep/1/claude-fable-5-1/) at all five effort levels. Low and medium showed no reasoning at all. Xhigh took seven minutes and fifty-one seconds and cost $1.83. Max produced "the best pelican I've seen from any of Anthropic's models" for $3.30, against ten cents at low. If you want the model the benchmark table describes, you pay for it in minutes and in tokens per turn.

## Cheaper is an effort setting, and it isn't for subscribers at all

The headline saving is real and narrow. Cache reads dropped from $1 to $0.25 per million tokens, and Anthropic says that makes 5.1 "[an estimated 25% less than Fable 5 for typical workloads, wherever usage is billed by token](https://www.anthropic.com/claude-fable-and-mythos-5-1#:~:text=wherever%20usage%20is%20billed%20by%20token)," up to about 45% for agentic work. The qualifier is doing the work in that sentence. Subscriptions are not billed by token. [A commenter on r/ClaudeAI](https://www.reddit.com/r/ClaudeAI/comments/1w4juj2/introducing_claude_fable_51_and_claude_mythos_51/) spotted it within hours: "They snuck it in as 'wherever usage is billed by token.'" On Max and Pro, the 75% cut does not exist.

Meanwhile the model got heavier per turn, though not in the part I read. [Artificial Analysis measured](https://artificialanalysis.ai/articles/claude-fable-5-1) 5.1 at max effort using about 1.7 times the output tokens of Fable 5, which makes it 20% more expensive per task on the API even after the cache cut. My own logs agree, at the default setting. Across my last twelve days on Fable 5, a Claude Code turn averaged 957 output tokens. Across my first two days on 5.1 it averaged 1,419, a 48% jump, and each turn wrote about 47% more new context into the cache. None of that is in the replies. The text I see per turn is the same length on both models, within a few percent. The extra is reasoning I never see, up about a fifth, and bigger tool calls, up about two fifths. Cache reads per turn, the one thing that got cheaper, stayed flat. It is not finishing faster to make up for it, either. Counting assistant turns between one prompt of mine and the next, Fable 5 and 5.1 both averaged about fifteen, with the same number of tool calls. So the longer turns are pure addition. The same prompt now produces about half again as many output tokens. I don't pay by the token, so I can't tell you what that does to an API bill. I can do the arithmetic. Run my tokens through Anthropic's list prices and a prompt of mine goes from about $6.40 on Fable 5 to about $5.10 on 5.1, 21% less, because the cheaper reads cover the longer turns. Price the reads at the old rate, which is the shape a subscription meter sees, and the same prompt comes out around $7.40, 16% more. On a Max plan the reads were never the expensive part. The longer turns are.

The arithmetic, per prompt, across every Claude Code session I ran in each window:

| Per prompt | Fable 5 (Aug 20–31, 990 prompts) | Fable 5.1 (Sep 2–3, 142 prompts) |
| --- | --- | --- |
| Assistant turns | 15.6 | 15.2 |
| Tool calls | 7.1 | 6.7 |
| Output tokens | 14.9k | 22.0k |
| Cache writes | 115k | 159k |
| Cache reads | 3.4M | 3.1M |
| Cost at list price | $6.42 | $5.07 |
| Same tokens, reads at the old $1 | $6.42 | $7.42 |

Means over every prompt in the window, subagent turns excluded, cache writes priced at the one-hour rate. Two days of 5.1 is a thin sample, and the 5.1 window had more short question-and-answer prompts than the Fable 5 one, which pulls its averages down, not up.

Then there is the meter itself. Anthropic reset everyone's limits when 5.1 shipped on Tuesday evening. By Thursday afternoon, a day and three quarters into a seven-day window, my Fable allocation read 56% used. This morning Claude Code put up the warning:

![Claude Code's notice: Approaching limit. Resets Tuesday at 8:00 pm. A View usage button below.](/posts/fable-5-1-is-allowed-to-find-vulnerabilities-now/approaching-limit.jpg)

The reset is Tuesday night, so the weekend and Monday run on Opus. Some of that is me. This was a week with one session that ran for two days, subagents fanning out, and a desktop-automation tool stuffing screenshots into context, and Claude Code's own usage screen says so in as many words. But the same logs show the model's share. Compared with an equal stretch of Fable 5 in late August, this week's turns produced almost twice the output tokens and more than twice the cache writes for about a third more cache reads. The expensive parts doubled. The part that got cheaper barely moved, and on a subscription it did not get cheaper anyway.

I don't want to oversell this. Anthropic doesn't publish how Fable draws down a weekly limit, only that it does so "[faster than other Claude models](https://support.claude.com/en/articles/15424964-claude-fable-models-on-your-plan)," and I have no Fable 5 reading of the same meter to set beside this one. But if your Fable allocation feels like it is going faster since Tuesday, the shape of the tokens says you are not imagining it, and the shape of your sessions is probably helping.

## The limit itself didn't move

On Max you can still spend up to 50% of your weekly limit on Fable, and 5.1 inherited that unchanged. [The support page](https://support.claude.com/en/articles/15424964-claude-fable-models-on-your-plan) is explicit that this is a ceiling inside the limit, not an addition: "you can never use more than your weekly limit." Pro runs Fable on credits. Anthropic reset everyone's limits on launch day, which was a decent thing to do. Then on September 14 the temporary 50% boost to Claude Code weekly limits becomes a permanent 25%, which Anthropic itself describes as "[a 17% reduction in weekly limits on Claude Code](https://x.com/ClaudeDevs/status/2093742322525810912)" compared to today. So the model that spends more per turn arrives two weeks before the limit shrinks. In July I built a little skill that runs Fable's habits on Opus so I could save the real thing for problems that need it. It's getting more use this month than it did in August.

OpenAI priced Astra at the same $10 and $50 per million tokens and says it is "[included within the existing subscription allowances](https://openai.com/index/gpt-6-astra/#:~:text=included%20within%20the%20existing%20subscription%20allowances)," with credits for sale on top. OpenAI doesn't say whether there is a ceiling inside the allowance the way there is on Max. It hasn't shown up in my account yet, so I can't say how fast it burns through a week's limit.

## Talks better, still talks too much

[The launch thread on Hacker News](https://news.ycombinator.com/item?id=49525378) passed a thousand points, and most of it was about Opus 5's prose, not about the new model. The top comment on [the Reddit thread](https://www.reddit.com/r/ClaudeAI/comments/1w4juuz/introducing_claude_fable_51_and_claude_mythos_51/) was "please fix Opus 5 and stop its rambling irrelevant responses." An Anthropic engineer replied on HN that 5.1 "is a big improvement in writing style," and [Every's reviewers](https://every.to/vibe-check/fable-5-1-vibe-check) agreed that it "actually speaks like a regular person." The same review reports that asked for 1,000 words it wrote 1,288, and that five of its 27 quotes weren't in the source. I've noticed the voice too. Fable 5 never invented words, but it wrote like a spec sheet: dense technical phrasing, sentences that ran the width of the screen, and a robotic evenness that made every reply cost the same effort to read whether it mattered or not. 5.1 writes the sentence a person would write, breaks the paragraph where a person would, and does what my style instructions say instead of half-doing them. Two days in, its replies are much easier to read than anything Fable 5 gave me, and that is not a small thing when you read a few hundred of them a day. My logs say they are the same length. They just read shorter. Better voice, same appetite for length. It will still make things up when it wants to please you.

And Mythos 5.1, the same model with the safeguards loosened for vetted defenders, is available to "[a set of US organizations](https://www.anthropic.com/claude-fable-and-mythos-5-1#:~:text=only%20available%20to%20a%20set%20of%20US%20organizations)." [Reddit's read](https://www.reddit.com/r/Anthropic/comments/1w4juwx/introducing_claude_fable_51_and_claude_mythos_51/) was blunter than mine: "Why even talk about mythos if none of us are allowed to use it?"

## Still a supply line

Three words got Fable 5 pulled in June. This week the same three words work again, and the word June appears nowhere in the release. I'm glad to have it back. The rest of the release is smaller than the announcement makes it sound. The line Anthropic drew is the one the security community asked for, and OpenAI drew the same one two days later. Both hold only as well as the classifier behind them. The price cut is real for anyone billed by the token and invisible on a subscription. The model takes the same number of turns to finish a job and spends more on each one, almost none of it in what I read, so my week's allocation goes faster, and the week itself gets shorter on the 14th. We'll see what the 14th does to it.