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Tuesday, September 1, 2026 · UTC
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Anthropic's Claude Fable 5.1 arrives in GitHub Copilot

Anthropic's Claude Fable 5.1 model is now available for GitHub Copilot users.

TruthFoundry News Desk
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Stands on 11 placed sources from 3 publishers.
Anthropic has announced that Claude Fable 5.1 is now available in GitHub Copilot. [1] Anthropic launched the Claude Fable 5.1 AI model on the Harvey platform on September 1, 2026. [2] Claude Code v2.1.257 added a Containment Escape rule to auto mode, which prevents automatic approval of cloud metadata-credential fetches, egress evasion, and cross-tenant reach unless the environment marks them expected. [3] Claude Code v2.1.257 added Claude Fable 5.1 as the default Fable model, featuring 1M context and pricing of $10/$50 per million tokens with $0.25 per million token cache reads. [4] Claude Fable 5.1 is the latest model in Anthropic's Mythos class and is designed for long-horizon, autonomous coding and knowledge-work tasks. [5] Zero data retention access is available for certain eligible enterprise customers for Claude Fable 5.1. [6] Anthropic is rolling out its Enterprise Frontier Safeguards, allowing eligible customers to store data within their own infrastructure or on Harvey's systems under a time-limited exemption. [7] Early testing indicates that Claude Fable 5.1 excels in delivering quality results even at lower reasoning levels, using roughly half the tokens compared to higher settings. [8] Harvey clarified that the Claude Fable 5.1 model processes all data in the United States, with no regional processing options available. [9] Claude Code v2.1.257 fixed a security issue where plugins could read files outside their own directory through symlinked command, agent, skill, hooks, or component paths; such paths are now refused with an error. [10] Claude Code v2.1.257 added the CLAUDE_CODE_SUBAGENT_MODEL_FORCE environment variable to apply the subagent model to every subagent, ignoring per-spawn and agent-definition model overrides. [11]
What this stands on
  1. Anthropic has announced that Claude Fable 5.1 is now available in GitHub Copilot. · The GitHub Blog
  2. Anthropic launched the Claude Fable 5.1 AI model on the Harvey platform on September 1, 2026. · Blockchain.News
  3. Claude Code v2.1.257 added a Containment Escape rule to auto mode, which prevents automatic approval of cloud metadata-credential fetches, egress evasion, and cross-tenant reach unless the environment marks them expected. · GitHub
  4. Claude Code v2.1.257 added Claude Fable 5.1 as the default Fable model, featuring 1M context and pricing of $10/$50 per million tokens with $0.25 per million token cache reads. · GitHub
  5. Claude Fable 5.1 is the latest model in Anthropic's Mythos class and is designed for long-horizon, autonomous coding and knowledge-work tasks. · The GitHub Blog
  6. Zero data retention access is available for certain eligible enterprise customers for Claude Fable 5.1. · The GitHub Blog
  7. Anthropic is rolling out its Enterprise Frontier Safeguards, allowing eligible customers to store data within their own infrastructure or on Harvey's systems under a time-limited exemption. · Blockchain.News
  8. Early testing indicates that Claude Fable 5.1 excels in delivering quality results even at lower reasoning levels, using roughly half the tokens compared to higher settings. · Blockchain.News
  9. Harvey clarified that the Claude Fable 5.1 model processes all data in the United States, with no regional processing options available. · Blockchain.News
  10. Claude Code v2.1.257 fixed a security issue where plugins could read files outside their own directory through symlinked command, agent, skill, hooks, or component paths; such paths are now refused with an error. · GitHub
  11. Claude Code v2.1.257 added the CLAUDE_CODE_SUBAGENT_MODEL_FORCE environment variable to apply the subagent model to every subagent, ignoring per-spawn and agent-definition model overrides. · GitHub
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