Initial impressions of Claude Fable 5

Anthropic released Claude Fable 5 and Claude Mythos 5, two new large language models with a 1 million token context window and 128,000 maximum output tokens, priced at $10 per million input tokens and $50 per million output tokens. Fable 5 includes stricter safety guardrails that trigger frequently, while Mythos 5 offers the same capabilities without those classifiers. Early testing shows Fable 5 outperforms the previous Opus 4.8 model on knowledge tasks, though it is slower and more expensive to run.

I didn't have early access to today's Claude Fable 5 https://www.anthropic.com/news/claude-fable-5-mythos-5 release, but I've spent the past ~5.5 hours putting it through its paces. My initial impressions are that this is something of a beast . It's slow, expensive and has been quite happily churning through everything I've thrown at it so far. As is frequently the case with current frontier models the challenge is finding tasks that it can't do. First, let's review the key characteristics. Anthropic claim that Claude Fable 5 https://www.anthropic.com/news/claude-fable-5-mythos-5 offers the same performance as Claude Mythos 5, except with much more strict guardrails in place to prevent it being used for harmful things. Those guardrails trigger often enough that the Claude API has new mechanisms for letting you know when you hit them, and even has a new option https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback to request it falls back to another model automatically if something gets rejected. Claude Mythos 5 is out today as well, Anthropic say it https://platform.claude.com/docs/en/about-claude/models/introducing-claude-fable-5-and-claude-mythos-5 "Shares Claude Fable 5's capabilities without the safety classifiers". The models have a 1 million token context window, 128,000 maximum output tokens and a knowledge cut-off date of January 2026. They are priced at twice the price of Claude Opus 4.5/4.6/4.7/4.8: $10/million input tokens and $50/million output tokens. There's no increase in price for longer context usage. Other than that the upgrade guide https://platform.claude.com/docs/en/about-claude/models/introducing-claude-fable-5-and-claude-mythos-5 is substantially thinner than the similar guide for Opus 4.8 https://platform.claude.com/docs/en/about-claude/models/whats-new-claude-4-8 . The best way to describe Fable is that it feels big . Not just in terms of speed and cost, but also in how much it knows. Here's an example prompt, which I used to compare Fable's knowledge with Opus 4.8 typo included : List all of Simon Willion's open source projects, most recent first, each with a rough date of when they were first released Opus 4.8's reply started like this https://gist.github.com/simonw/e77abf645156ade8c5c610f524226dcb response-1 : I don't have a reliable, comprehensive, and date-verified list of all of Simon Willison's open source projects, and I want to be honest about that rather than risk giving you inaccurate dates or fabricated entries. Here's what I can tell you with reasonable confidence: Well-known projects by Simon Willison: LLM ~2023 – A command-line tool and Python library for interacting with large language models. Datasette ~2017 – A tool for exploring and publishing data, probably his most prominent recent project. sqlite-utils ~2018-2019 – A Python library and CLI tool for manipulating SQLite databases, often used alongside Datasette. Django 2005 – He is one of the co-creators of the Django web framework, along with Adrian Holovaty, originally developed at the Lawrence Journal-World newspaper.He also maintains a large ecosystem of Datasette pluginsand many smaller utilities often with names like shot-scraper , various -to-sqlite tools for importing data, etc. . That's not bad. Here's the start of the response https://gist.github.com/simonw/e77abf645156ade8c5c610f524226dcb response for the exact same prompt from Fable 5 - both models were running without access to search: I think you mean Simon Willison— the prolific open source developer, co-creator of Django, and creator of Datasette.An important caveat upfront: a truly complete list isn't feasible. Simon Willison is famously prolific — he has published hundredsof repositories Datasette alone has 100+ plugins, many written by him , and my knowledge has a cutoff date, so I'll be missing his most recent work. Below are hisnotableprojects with approximate initial release dates, most recent first: Notable projects approximate dates files-to-prompt April 2024 – concatenates files into a single prompt for LLMsdatasette-extract 2024 – plugin for extracting structured data using LLMsLLM May–June 2023 – CLI tool and Python library for interacting with large language models, plus a large ecosystem of plugins llm-gpt4all, llm-mistral, llm-claude, etc., 2023 onward symbex June 2023 – search Python code for symbolsttokandstrip-tags May 2023 – CLI tools for token counting and HTML cleanup for LLM pipelinesdatasette-lite May 2022 – Datasette running entirely in the browser via WebAssembly/Pyodideshot-scraper March 2022 – automated website screenshots via Playwrights3-credentials November 2021 – CLI for creating scoped AWS S3 credentialsdjango-sql-dashboard 2021 – SQL reporting dashboards for DjangoDogsheep suite 2019 – personal analytics tools: twitter-to-sqlite, github-to-sqlite, healthkit-to-sqlite, dogsheep-beta, etc.sqlite-utils 2018 – CLI and Python library for manipulating SQLite databasesDatasette November 2017 – his flagship project; tool for exploring and publishing datacsvs-to-sqlite 2017 – convert CSV files to SQLiteVarious early tools ~2007–2010 – soupselect, json-head, geocoders, and othersDjango developed 2003–2005, open-sourced July 2005 – co-created with Adrian Holovaty at the Lawrence Journal-World Here's GPT-5.5 for good measure https://gist.github.com/simonw/8d05ce836d44bd8543055c9614f7c478 . It listed even more projects than Fable did In the past I've stated that I don't care about how much models know - I want them to be able to manipulate text and code in useful ways and actively look up the information they need via search tools, not bake it into their weights. But knowledge like this is a reasonably good proxy for model size - you can cram a whole lot more details about the world into a larger number of parameters. Does knowing more stuff mean the model is better at the tasks we pose to it? I can certainly imagine how a coding model with deeper knowledge of modern libraries and patterns could crunch through coding tasks more effectively. Is Fable really bigger than Opus? Anthropic haven't said anything about model size, so all we have are tea-leaves, but the speed, pricing and my own poking at its knowledge make me think that it's a large model. Maybe the largest yet from any vendor. Anthropic made Fable 5 available across all of their surfaces - the Claude.ai https://claude.ai/ chat interface, Claude Code for web, Claude Code CLI and Claude Cowork as well. The model is available "until June 22nd" on the subscription plans I'm on $100/month Max at the moment , after which it will be billed extra. Claude.ai is often under-estimated. Since September 2025 https://simonwillison.net/2025/Sep/9/claude-code-interpreter/ every chat has had access to a full container environment to run code, including the ability to install additional packages and even clone repositories directly from GitHub. Last week I released micropython-wasm https://simonwillison.net/2026/Jun/6/micropython-in-a-sandbox/ , a Python library that uses wasmtime https://wasmtime.dev/ to run a custom build of MicroPython in WebAssembly to act as a sandbox for untrusted Python code. I decided to see if Fable could upgrade that to running full Python instead. I started with this prompt: Clone simonw/micropython-wasm from GitHub and research how this could use a full Python as opposed to MicroPython Fable identified that it could use Brett Cannon's cpython-wasi-build https://github.com/brettcannon/cpython-wasi-build/releases builds for this, but was unable to download them itself due to environment restrictions. So I grabbed the two zip files from that page and uploaded them to Claude: Here's the Brett Cannon builds python-3.zip , build-python-3.zip as attachments And that was that. It churned away for a few minutes and got the entire thing working. Part of the response included: I tried the cleaner single-zip-stdlib approach to shrink the filesystem surface, but CPython's getpath bootstrap fails to find encodings from inside a zip without more prefix finessing — the directory-preopen approach works reliably, so that's what the PoC uses. The zip path is solvable but needs PYTHONHOME /frozen-getpath work. So I said: Try a bit more at the single-zip-stdlib problem Then a little later: I want a wheel that has the whole system in it, the Python wrappers and the WASM files and the stdlibrary, so I can do uv run --with path-to-whl python -c "demo code" ... and it gave me this 13.9MB cpython wasm-0.1.0-py3-none-any.whl https://static.simonwillison.net/static/cors-allow/2026/cpython wasm-0.1.0-py3-none-any.whl file. You can try running Python code in a sandbox using that wheel URL and uv like this: uv run --with https://static.simonwillison.net/static/cors-allow/2026/cpython wasm-0.1.0-py3-none-any.whl \ cpython-wasm -c 'print 45 56 ' Here's the full chat transcript https://claude.ai/share/a73b8b8b-8ebc-4fef-9e5c-7438e5e7ae35 . This was a very strong start. Before I'd realized it was Fable day, my stretch goal for today was to add a new feature to Datasette Agent https://agent.datasette.io/ : I wanted tool calls within that agent software to gain the ability to pause mid-execution and request approval directly from the user. This felt like a suitably meaty task to throw at the new model. Over the course of the day Fable not only solved that problem https://github.com/datasette/datasette-agent/pull/20 , it also identified and then implemented four issues in my underlying LLM library that would help support this kind of advanced pause-resume mechanism in tool calls. It got everything working first using somewhat gnarly hacks, but the moment I told it that changes to LLM itself were in scope it set to work unraveling the hacks and turning them into supported features of LLM instead. My stretch goal turned into LLM 0.32a3 https://llm.datasette.io/en/latest/changelog.html a3-2026-06-09 , almost entirely written by Fable. Here are the release notes: Driven by the needs of Datasette Agent 's human-in-the-loop ask user feature, made the following improvements to how tool calls work: - Tool implementations can declare a parameter named llm tool call in order to be passed the llm.ToolCall object for the current invocation. This allows them to access the current llm tool call.tool call id . See Accessing the tool call from inside a tool . 1480 - Every tool call is now guaranteed a unique tool call id - providers that do not supply one get a synthesized tc -prefixed ULID. 1481 - Tools can raise a llm.PauseChain exception to cleanly pause the tool chain, useful for things like waiting for human approval. The exception propagates to the caller with .tool call and .tool results completed sibling results attached, and no model call is made with a placeholder result. See Pausing a chain from inside a tool . 1482 - Failure semantics for concurrent tool execution: async sibling tool calls always run to completion before a pause or hook exception propagates. 1482 - Chains can now resume from a messages= history ending in unresolved tool calls: the calls are executed through the normal before call / after call machinery before the first model call, skipping any that already have results. The execute tool calls method also accepts a new optional tool calls list= argument for executing an explicit list of ToolCall objects in place of the calls requested by the response. See Resuming a chain with pending tool calls . 1482 - Fixed a bug where the async tool executor silently dropped calls to tools not present in tools= - these now return Error: tool "..." does not exist results, matching the sync executor. 1483 I'm really impressed with the quality of API design, tests, code and documentation that Fable put together for this. I spent several hours on it today, but it feels like several days' worth of work. I recently started using AgentsView https://agentsview.io to help track my local LLM usage across all of the different coding agents. I published a TIL today https://til.simonwillison.net/llms/agentsview-custom-model-price about adding custom Fable pricing to that tool, which I expect will not be necessary in the very near future. After setting the price, I ran this command to start a localhost web server to explore my usage: uvx agentsview serve Here's the treemap showing the breakdown of my Fable usage across various projects today: I used $110.42 worth of tokens today, all as part of my $100/month subscription. I ran "Generate an SVG of a pelican riding a bicycle" against all five thinking effort levels with Fable. Here are the results https://tools.simonwillison.net/markdown-svg-renderer url=https%3A%2F%2Fgist.github.com%2Fsimonw%2F94fde31c34a0400c1d29f57e6a708e6b , including the token cost for each one: It's interesting that high ended up using fewer tokens than medium for this particular run. Here are the Opus 4.8 pelicans https://simonwillison.net/2026/May/28/claude-opus-4-8/ and-some-pelicans for comparison. Tags: ai https://simonwillison.net/tags/ai , generative-ai https://simonwillison.net/tags/generative-ai , llms https://simonwillison.net/tags/llms , anthropic https://simonwillison.net/tags/anthropic , claude https://simonwillison.net/tags/claude , llm-pricing https://simonwillison.net/tags/llm-pricing , pelican-riding-a-bicycle https://simonwillison.net/tags/pelican-riding-a-bicycle , llm-release https://simonwillison.net/tags/llm-release , claude-mythos https://simonwillison.net/tags/claude-mythos