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Claude Fable 5 and the Mythos class

Anthropic releases the first public Mythos-class model, Fable 5, with cyber and biology safeguards; three days later the US restricts it and access is revoked worldwide for three weeks.

category
model
significance
4 of 5
people
Dario Amodei
organisations
Anthropic, US Department of Commerce

what had to happen · 66 events back to 1943

Every event this one built on, transitively, in order. The organism has the same path lit. Direct influences are marked.

00 · One neuron · 1

  1. 1943A logical calculus of nervous activity

I · Foundations · 11

  1. 1948A mathematical theory of communication
  2. 1949Cells that fire together wire together
  3. 1950Programming a computer for playing chess
  4. 1950Computing machinery and intelligence
  5. 1958The perceptron learns
  6. 1959Samuel's checkers program coins 'machine learning'
  7. 1960ADALINE and the least-mean-squares rule
  8. 1965Moore's law
  9. 1966ELIZA
  10. 1969Perceptrons
  11. 1970Reverse-mode automatic differentiation

W1 · The first winter · 2

  1. 1974Werbos applies backpropagation to neural networks
  2. 1980The Neocognitron

II · Connection · 3

  1. 1982The Hopfield network
  2. 1985The Boltzmann machine
  3. 1986Backpropagation

W2 · The second winter · 6

  1. 1988Temporal-difference learning
  2. 1989Q-learning
  3. 1989LeNet reads handwritten postcodes
  4. 1990Finding structure in time
  5. 1991The vanishing gradient problem
  6. 1992TD-Gammon reaches world-class backgammon

III · Statistics and data · 10

  1. 1997Long short-term memory
  2. 1998MNIST and LeNet-5
  3. 1999The first GPU
  4. 2003A neural probabilistic language model
  5. 2006Deep belief networks and the word 'deep'
  6. 2007CUDA
  7. 2009Deep learning moves to GPUs
  8. 2009ImageNet
  9. 2010Rectified linear units
  10. 2010DeepMind is founded

IV · Deep learning · 11

  1. 2012Google Brain's network discovers cats
  2. 2012Dropout
  3. 2012AlexNet wins ImageNet
  4. 2013Deep Q-networks play Atari
  5. 2014Google buys DeepMind
  6. 2014Superintelligence
  7. 2014Attention
  8. 2014Sequence to sequence learning
  9. 2015Batch normalisation
  10. 2015Residual networks
  11. 2015OpenAI is founded

V · Transformers · 10

  1. 2017The Asilomar AI Principles
  2. 2017Attention is all you need
  3. 2017Deep reinforcement learning from human preferences
  4. 2018GPT: generative pre-training
  5. 2019GPT-2 and the model too dangerous to release
  6. 2020Scaling laws for neural language models
  7. 2020GPT-3
  8. 2020Learning to summarise from human feedback
  9. 2021Anthropic is foundeddirect
  10. 2021GitHub Copilot writes code

VI · Everyone · 10

  1. 2022InstructGPT
  2. 2022ChatGPT
  3. 2023Claude
  4. 2023GPT-4
  5. 2023'Pause Giant AI Experiments'
  6. 2023The US executive order on AI
  7. 2023The Bletchley Declaration
  8. 2024Claude 3 catches GPT-4
  9. 2024The EU AI Act enters into force
  10. 2024Claude learns to use a computer

VII · Agents · 2

  1. 2025Claude 4 and Claude Codedirect
  2. 2025America's AI Action Plandirect

Anthropic had previewed a model it called Mythos to a closed group of security organisations under a programme named Project Glasswing, because the model could find and fix software vulnerabilities at a level the company judged unsafe to release. On 9 June 2026 it released the same model to the public as Claude Fable 5, identical in capability but with classifiers that route requests touching cybersecurity, biology and the training of other models to a less capable model instead. Anthropic called it the most capable system it had ever made available. Mythos 5, without the safeguards, stayed with the Glasswing customers.

On 12 June the US Department of Commerce wrote to the company prohibiting access to either model for any non-American national on national-security grounds. Unable to enforce that at the level of individual users, Anthropic revoked access for every customer. Restoration began for American organisations on 26 June and the restrictions were lifted altogether on 30 June. It was the first time a government had pulled a deployed frontier model, and the first test of what the Action Plan's language about American leadership meant when the leading model was also the most dangerous one.

Fable 5.1 and Mythos 5.1 followed on 1 September 2026. The June episode is on this timeline as the point at which the capability the safety field had warned about, a model that could do serious harm in the wrong hands, stopped being hypothetical and became an export-control matter.

what it led to · 1 events downstream, through 2026

Built on it directly:

  1. 2026Claude Fable 5.1VII

sources · 3

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