Mythos

Where the book uses it

• Chapter 3 — The Missing Researcher

  The dated promise of an AI that does its own research, what the laboratories have published, and what the public record can and cannot yet check.

• Chapter 8 — The Arsenal Behind Every Arsenal

  The state as customer: how the Pentagon buys frontier AI, the contract clauses a laboratory fought over, and what the courts said about a company that would not sign.

Sources that use it

• Anthropic, “Claude Fable 5 and Claude Mythos 5,” 9 June 2026 (the launch page prices both models at “$10 per million input tokens and $50 per million output tokens” and stages the top model off the seat: “From today through June 22, Fable 5 is included on Pro, Max, Team, and seat-based Enterprise plans at no extra cost” and “On June 23, we’ll remove Fable 5 from those plans. Using it after that will require usage credits. If capacity allows, we’ll extend the included window”; the 9 June release also sets the retention floor by capability tier — “we’re making a change to the way we handle business customer data for Fable 5, Mythos 5, and future models with similar or higher capability levels. We will require 30-day retention for all traffic on Mythos-class models” — defines Mythos-class in footnote 1 as “a tier of Claude models that sit above our Opus class in capability”, and states that access to Mythos 5 will be widened “In consultation with the US government”; Mythos 5 is “the same model as Claude Fable 5 but with cyber safeguards lifted”; the launch page read from anthropic.com on 19 September 2026, unchanged; the release deploys Mythos 5 “as an upgrade to Claude Mythos Preview”, says users of the Preview “will be able to upgrade to Claude Mythos 5—the same model as Claude Fable 5 but with cyber safeguards lifted”, and its footnote dates the first Mythos-class model, Claude Mythos Preview, to April through Project Glasswing; the system card of the same day calls Mythos 5 and Fable 5 “two configurations of a new large language model” and scores Mythos Preview as a separate model; re-read on the page as archived at 17:32 UTC on release day). In the Sources
• Anthropic, “Expanding Project Glasswing,” 2 June 2026 (“In early April, we announced that roughly 50 initial partners had access to Claude Mythos Preview”; the program extended “to approximately 150 new organizations. Each one will need to meet our security requirements before they gain access,” based “in more than 15 countries,” after collaboration “with our Project Glasswing partners, the security industry, open-source software maintainers, and the US government”). In the Sources
• Anthropic, “Introducing Claude Fable 5.1 and Claude Mythos 5.1,” September 2026 (the page dates itself to the month; the Internet Archive’s first capture is 1 September 2026, 18:02 UTC) (Fable 5.1 generally available, Mythos 5.1 “available only through our trusted access programs”; the two “are the same model, but with different levels of safeguards”; Fable 5.1 “can now be used to discover software vulnerabilities—though not to develop exploits for them”; the biology access program “developed in partnership with the US government”; Mythos “only available to a set of US organizations”). In the Sources
• Anthropic, “Pricing,” Claude Platform Docs, retrieved 4 September 2026, with the “Steering thinking” and “Fast mode” pages (base input, cache writes, cache hits and output priced apart; the Steering thinking page’s own Pricing section, “Thinking incurs charges for: Tokens Claude uses while thinking (billed as output tokens)” and “You are billed for the full thinking process, not the thinking content visible in the response”; the model table’s five columns, headed “Base input tokens”, “5m cache writes”, “1h cache writes”, “Cache hits and refreshes” and “Output tokens”; the two retired Opus rows, Claude Opus 4.1 and Claude Opus 4, marked retired except on Bedrock and Google Cloud, at “$15 / MTok” base input and “$75 / MTok” output; fast mode for Opus 5 at “$10 / MTok” in and “$50 / MTok” out, the section reading in full, “Fast mode, in research preview, provides significantly faster output for Claude Opus 5 and Claude Opus 4.8 at premium pricing,” with one table row, “Claude Opus 5 / Claude Opus 4.8 $10 / MTok $50 / MTok”; “This tokenizer produces approximately 30% more tokens for the same text”; the page continues, “The exact increase depends on the content and workload shape”; cache hits on Claude Fable 5.1 and Claude Mythos 5.1 “are priced at 0.025x the base input price. All other models use the standard 0.1x multiplier” — $0.25 per million against Opus 5’s $0.50, on a base input of $10 against $5; the section “Long context pricing”: “Claude 4.6 and later models and Claude Mythos Preview include the full 1M token context window at standard pricing. (A 900k-token request is billed at the same per-token rate as a 9k-token request.) Prompt caching and batch processing discounts apply at standard rates across the full context window”; the scope of the tokenizer sentence, stated beside it: “Claude 4.7 and later models and Claude Mythos Preview use a newer tokenizer”, and its closing sentence, “Claude Sonnet 4.6 and earlier models use the previous tokenizer”; the model pricing table as of 20 September 2026: Claude Fable 5.1, Claude Fable 5, Claude Mythos 5.1 and Claude Mythos 5 each at “$10 / MTok” base input and “$50 / MTok” output, and Claude Opus 5 at “$5 / MTok” and “$25 / MTok”; read on 4 September 2026 and re-read on 13, 19, 20 and 21 September 2026, the model table, the long-context section, the tokenizer note and the cache-hit multiplier unchanged on the last reading and the Steering thinking page by then served at platform.claude.com/docs/en/build-with-claude/thinking-steering-and-cost, the live page archived as VERIFY 260913/ch02 fold, VERIFY 260919/T1, VERIFY 260920/FOLD 255 and, with the Steering thinking page, VERIFY 260921/R68 B; re-read 23 September 2026: the model table adds Claude Opus 5.5 at “$4 / MTok” base input and “$20 / MTok” output, its footnote reading “Cache hits and refreshes on Claude Opus 5.5 are priced at 0.05x the base input price”; Claude Opus 5 stays at $5 and $25, Claude Fable 5.1 and Claude Mythos 5.1 at $10 and $50; the fast mode table now reads “Claude Opus 5.5 $8 / MTok $40 / MTok” above the Opus 5 row; archived as VERIFY 260923/FOLD 277; re-read 4 October 2026 for the section “Data residency pricing”: “For Claude 4.6 and later models, specifying US-only inference through the inference_geo parameter incurs a 1.1x multiplier on all token pricing categories, including input tokens, output tokens, cache writes, and cache reads. Global routing (the default) uses standard pricing”; archived as VERIFY 261004/FOLD 552; the model table’s rows: Claude Fable 5.1 and Claude Mythos 5.1 at “$10 / MTok” base input and “$50 / MTok” output, Claude Opus 5 and the Opus 4.5 to 4.8 classes at “$5 / MTok” and “$25 / MTok”). In the Sources
• Anthropic, “Project Glasswing: Securing critical software for the AI era,” 7 April 2026 (the launch partners — Amazon Web Services, Anthropic, Apple, Broadcom, Cisco, CrowdStrike, Google, JPMorganChase, the Linux Foundation, Microsoft, NVIDIA and Palo Alto Networks — plus “a group of over 40 additional organizations that build or maintain critical software infrastructure”; “Anthropic is committing up to $100M in usage credits for Mythos Preview across these efforts, as well as $4M in direct donations to open-source security organizations”; after the preview, Mythos Preview priced to participants at $25/$125 per million input/output tokens; “We do not plan to make Claude Mythos Preview generally available”). In the Sources

And 18 more: search the Sources.

Related terms

• Fable
• METR
• Adoption clock
• Agent
• AGI
• AISI
• Alignment
• Amici
• ARC-AGI-3
• Arsenal behind every arsenal
• Astra
• Automated AI researcher