Creator @datasetteproj, co-creator Django. PSF board. Hangs out with @natbat. He/Him. Mastodon: fedi.simonwillison.net/@simo… Bsky: simonwillison.net

Joined November 2006
3,934 Photos and videos
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The US government, citing national security authorities, has issued an export control directive to suspend all access to Fable 5 and Mythos 5 by any foreign national, whether inside or outside the United States, including foreign national Anthropic employees. The net effect of this order is that we must abruptly disable Fable 5 and Mythos 5 for all our customers to ensure compliance. Access to all other Claude models is not affected. We apologize for this disruption to our customers. We believe this is a misunderstanding and are working to restore access as soon as possible. Read our full statement: anthropic.com/news/fable-myt…
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I ran a script every minute against the API to see how long I'd maintain access to claude-fable-5 - I lost access 14 minutes ago:
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I got fed up of waiting for OpenAI to bring their much improved gpt-realtime-2 voice conversation model to the ChatGPT product, so I upgraded my OpenAI-WebRTC playground tool to use it and to let you paste in a document to have a conversation about, too simonwillison.net/2026/Jun/1…
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After two days with Claude Fable 5 the best way I can describe it is "relentlessly proactive" - here's an example where I dropped in a screenshot of a bug and it span up custom CORS Python servers and used pyobjc-framework-Quartz to capture screenshots simonwillison.net/2026/Jun/1…
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This right here is a pretty elegant solution to the problem of wanting to capture measurements from JavaScript in the system Safari without being able to directly access the DOM
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New Datasette release: 1.0a33, which finally brings documents the ?_extra= JSON API mechanism and brings it to the row and query pages in addition to the table pages (Most of the code in this release was built with the help of Claude Fable 5) datasette.io/blog/2026/api-e…

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Since the JSON extras API is a little hard to explain without an example I also had Fable 5 and GPT-5.5 collaborate on this custom API explorer tool for trying out the new feature tools.simonwillison.net/data…
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It's fun to look back at this Twitter conversation about the then-new ChatGPT Code Interpreter from three years ago - with hindsight this was our first glimpse of a coding agent, before we knew what a coding agent was
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If you're a programmer and you're still thinking that all of this ChatGPT stuff is a waste of your time, I strongly suggest reviewing this example It's over-hyped, sure - but it's not something anyone in our profession should continue to ignore
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Very pleased to hear Anthropic have walked back this policy simonwillison.net/2026/Jun/1…
BREAKING NEWS: Anthropic's latest model will NOT help you if it thinks your ML research/ML engineering is interesting, and/or will secretly degrade its IQ so that the average engineer won't notice. We are already seeing Anthropic's latest model's moderation filters our GPU inference research and programming 😭
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Don't miss the exact text though: "We’re changing Fable 5’s safeguards for frontier LLM development to make them visible" - make them visible means they're undoing the truly egregious (dare I say "unaligned") decision to have the model lie about its refusals, it will still refuse
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More details directly from Anthropic
We’re rolling out changes to make Fable 5’s safeguards for frontier LLM development visible. Starting this week, flagged requests will visibly fall back to Opus 4.8—the same as our safeguards for cyber and bio. You will see this every time it happens. On the API, any flagged requests will return a reason for their refusal (coming to server-side fallback in the next few days). We wanted to deploy Fable 5 to our users quickly and safely. Visible safeguards can be probed, so they have to be robust, which takes time to get right. Invisible safeguards can be targeted more narrowly, allowing us to ship quickly with very few false positives. We went with invisible safeguards for this reason—and that was the wrong tradeoff. You should have visibility into the safeguards we have in place, and why. We’re sorry for not getting the balance right. Making the safeguards visible makes them easier to work around, so keeping them robust to jailbreaks will unfortunately mean more false positives while we improve the classifiers. We're also tuning our bio and cyber classifiers to trigger less often on harmless requests. We know this is frustrating and we’ll do our best to keep this period as short as possible. If you think a request has been mistakenly flagged: run /feedback in Claude Code, click thumbs-down on the fallback in Claude.ai or Cowork, or file the safeguard appeal form for API requests. Your reports help us tune these classifiers and we appreciate your feedback. support.claude.com/en/articl…
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Simon Willison retweeted
I believe what Anthropic is doing, gating the ability to do certain harmless things like LLM research, and with incredibly sensitive filters that even medical questions are often blocked, is *deeply* wrong. They got open research, the Transformer, GPT2, ...
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Simon Willison retweeted
In good faith and with no judgment (mistakes happen), I truly hope that Anthropic will hear the feedback and change course on this. Anthropic is a company that has been raising awareness about AI manipulation which is a very important topic! You don’t want to go down as the first company to enable and open the door for human-designed AI manipulation at scale (giving intentionally bad answers to users without them knowing is the highest form of manipulation in my opinion). One way to avoid that is just at the very least to always keep disclosing the limitations and safeguards. More generally I want to emphasize that there are millions of AI builders out there using your tools for good every single day and the more you can keep helping them, the better for the world! Thank you, it’s not too late to fix this!
BREAKING NEWS: Anthropic's latest model will NOT help you if it thinks your ML research/ML engineering is interesting, and/or will secretly degrade its IQ so that the average engineer won't notice. We are already seeing Anthropic's latest model's moderation filters our GPU inference research and programming 😭
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Wrote up my initial impressions of Claude Fable 5 - it has a big model smell: slow, expensive and capable of crunching through pretty much everything I threw at it simonwillison.net/2026/Jun/9…
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Here are Fable's pelicans for the different thinking effort levels, plus how much each one cost to generate via the Claude API
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A TIL on using agentsview.io to calculate token spending with Claude Fable 5 despite that model not yet being included in the AgentsView pricing database til.simonwillison.net/llms/a…
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Simon Willison retweeted
BREAKING: Anthropic just dropped Claude Fable 5—this is Mythos, made safe for public release. It is the best coding model in the world. We've been testing it internally @every for the last week or so across coding, writing, marketing, editing, and more—here's our vibe check: - It broke our benchmarks. Fable scored a 91/100 on our Senior Engineer benchmark—this is human senior engineer level. The previous high score was Opus 4.8 at 63. GPT-5.5 is a 62. - It's a one-shot wonder. You can set it and forget for hours or overnight on huge coding tasks, and come back to completed work. It cleared entire production bug backlogs, built a playable 3D, and even made a 2-minute animated film—all one-shot. - Taste and attention to detail. In coding and knowledge work tasks, it has much better taste and attention to detail than we've ever seen. It gets subtle things right, adds little features you might not have thought of, and generally understands the assignment in ways that surprised us. - Great use of context. We set it loose analyzing customer feedback surveys and our website data and it came back with a crisp, clean report that identified a. our biggest problem and b. a concrete testable solution—and then we sent it off to build that. - It's best for power users. If you're already used to orchestrating multiple agents in your work, this model can do things that you've never seen before. If you're a knowledge worker or vibe coder with a more basic setup, you're not going to notice a huge difference—in fact, it probably isn't the right model for you. - It's very slow, token-hungry. Using this thing for regular knowledge work is like squashing an ant with a rocket launcher. It also routinely uses 500k to 1M tokens on tasks. That's why it's best for your heaviest jobs—but not as good for tasks like collaborative writing. - It's expensive. It's about twice as expensive as Opus, and it's also incredibly token hungry—so expect it to be something you'll use sparingly unless your company pays for it. Overall, I think of it like a warp drive for coding: It can get you across the galaxy in a few hours, when it used to take months or years. But it's not appropriate for getting around town—you need something faster, cheaper, and more maneuverable. The ceiling is extraordinarily high on this model though. Even our most advanced testers like @kieranklaassen felt like they were only scratching the surface of it. Want our full vibe check with all of our testing and benchmarks? Read it on @every: every.to/vibe-check/anthropi…
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Simon Willison retweeted
I've had access to Fable for a bit. A genuine jump in capability, I could feed it a 15 page design document for a project and it would work for 9 hours and deliver terrific results. But working with it is weird & weirder is coming Lots of examples: open.substack.com/pub/oneuse…
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That's both OpenAI and Anthropic with confidential S-1s filed with the SEC - Anthropic filed theirs on June 1st
We recently submitted a confidential S-1. We expect it to leak so we’re just announcing it. We have not decided on timing yet; it may be a while because there are things we want to do that are likely easier as a private company. But it’s a complicated set of tradeoffs and this gives us the option to go public sooner if that ends up being best. This announcement is being made pursuant to Rule 135 under the Securities Act of 1933, as amended, and does not constitute an offer to sell or the solicitation of an offer to buy any securities. Any offers, solicitations of offers to buy, or any sales of securities will be made in accordance with the registration requirements of the Securities Act.
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