{
  "meta": {
    "title": "Claude Fable 5.1 fact table",
    "description": "Every number behind the Claude Fable 5.1 reference page, one row per fact, with the source URL, how it was checked, and whether it stands verified, corrected or still open.",
    "updated": "2026-09-02",
    "version": 1,
    "license": null,
    "sections": {
      "identity": "Identity, IDs, availability",
      "specs": "Specs",
      "pricing": "Pricing",
      "benchmarks": "Benchmarks (as reported by Anthropic)",
      "safety": "Safety and policy",
      "science": "Science results attributed to Mythos 5.1",
      "context": "Context",
      "open": "Still open"
    },
    "page": "https://mackelfresh.com/writing/claude-fable-5-1-checked/",
    "csv": "https://mackelfresh.com/data/fable-5-1-facts.csv",
    "json": "https://mackelfresh.com/data/fable-5-1-facts.json",
    "columns": [
      "id",
      "section",
      "fact",
      "value",
      "source_url",
      "source_title",
      "verified_how",
      "status",
      "note"
    ],
    "statuses": [
      "verified",
      "corrected",
      "unverified"
    ]
  },
  "rows": [
    {
      "id": "identity-1",
      "section": "identity",
      "fact": "Release date of Claude Fable 5.1",
      "value": "2026-09-01",
      "source_url": "https://platform.claude.com/docs/en/models/fable-5-1/overview",
      "source_title": "Claude Fable 5.1",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "Model page says released September 1, 2026; System Card cover carries the same date."
    },
    {
      "id": "identity-2",
      "section": "identity",
      "fact": "Claude API model ID",
      "value": "claude-fable-5-1; the dateless ID is itself the pinned snapshot and the alias row repeats it",
      "source_url": "https://platform.claude.com/docs/en/models/overview",
      "source_title": "Models overview",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": ""
    },
    {
      "id": "identity-3",
      "section": "identity",
      "fact": "Claude API model ID for Mythos 5.1",
      "value": "claude-mythos-5-1",
      "source_url": "https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1",
      "source_title": "What's new in Claude Fable 5.1",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": ""
    },
    {
      "id": "identity-4",
      "section": "identity",
      "fact": "Amazon Bedrock model ID",
      "value": "anthropic.claude-fable-5-1",
      "source_url": "https://platform.claude.com/docs/en/models/overview",
      "source_title": "Models overview",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "Also listed on the Fable 5.1 model page."
    },
    {
      "id": "identity-5",
      "section": "identity",
      "fact": "Google Cloud, Microsoft Foundry and Claude Platform on AWS model ID",
      "value": "claude-fable-5-1 on all three",
      "source_url": "https://platform.claude.com/docs/en/models/overview",
      "source_title": "Models overview",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": ""
    },
    {
      "id": "identity-6",
      "section": "identity",
      "fact": "Lineage: what Fable 5.1 succeeds",
      "value": "Successor to Claude Fable 5, which was released 2026-06-09",
      "source_url": "https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1",
      "source_title": "What's new in Claude Fable 5.1",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "Fable 5 release date read on its own model page (models/fable-5/overview)."
    },
    {
      "id": "identity-7",
      "section": "identity",
      "fact": "How Anthropic positions Fable 5.1 in the lineup",
      "value": "Generally available; docs say start with Opus 5 and use Fable 5.1 for demanding reasoning and long-horizon agentic work when Opus 5 at higher effort falls short",
      "source_url": "https://platform.claude.com/docs/en/models/overview",
      "source_title": "Models overview",
      "verified_how": "fetched the primary page",
      "status": "corrected",
      "note": "My first notes had Fable in a 'Mythos-class tier above Opus'. No Anthropic page calls Fable a Mythos-class tier; 'Mythos-class' is used only for Mythos models."
    },
    {
      "id": "identity-8",
      "section": "identity",
      "fact": "What Mythos 5.1 is relative to Fable 5.1",
      "value": "The same model with more permissive safeguards in dual-use domains (biology, cybersecurity); Fable 5.1 adds safeguards that block certain tasks there",
      "source_url": "https://www.anthropic.com/claude-fable-5-1-mythos-5-1-system-card",
      "source_title": "System Card: Claude Fable 5.1 & Claude Mythos 5.1",
      "verified_how": "read in the system card",
      "status": "corrected",
      "note": "My first notes had the safeguards 'removed'. System Card page 2 and the launch post say more permissive, or different levels, not removed."
    },
    {
      "id": "identity-9",
      "section": "identity",
      "fact": "Who can use Mythos 5.1 (direct access)",
      "value": "Vetted organizations only, via trusted access programs; developer docs say Project Glasswing participants only. Glasswing began with 12 partners plus 40+ orgs (April 2026) and added about 150 orgs in 15+ countries on 2026-06-02",
      "source_url": "https://www.anthropic.com/claude/mythos",
      "source_title": "Claude Mythos",
      "verified_how": "fetched the primary page",
      "status": "corrected",
      "note": "My first notes had 'about 150 orgs, 15+ countries' as the total; that is the June expansion count. The original roster is on the Glasswing page."
    },
    {
      "id": "identity-10",
      "section": "identity",
      "fact": "Cyber Verification Program and Mythos access",
      "value": "CVP currently gives reduced cyber safeguards on certain Opus- and Sonnet-class models; Anthropic says Mythos-class access will be added in the near future",
      "source_url": "https://www.anthropic.com/claude-fable-and-mythos-5-1",
      "source_title": "Claude Fable 5.1 and Mythos 5.1",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "Anthropic's wording is 'near future', not 'coming months'. Same on the Mythos page."
    },
    {
      "id": "identity-11",
      "section": "identity",
      "fact": "Life Sciences Verification Program and geography",
      "value": "LSVP is an invite-only beta with reduced biology safeguards, developed with the US government; first participants enrolled; Mythos 5.1 currently available to a set of US organizations only",
      "source_url": "https://www.anthropic.com/claude-fable-and-mythos-5-1",
      "source_title": "Claude Fable 5.1 and Mythos 5.1",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "Mythos page confirms invite-only beta and US-only for now; Anthropic says it is working to expand access."
    },
    {
      "id": "identity-12",
      "section": "identity",
      "fact": "Mythos 5.1 also powers Claude Security",
      "value": "Claude Security runs on Mythos 5.1 and is available to all Claude Enterprise customers",
      "source_url": "https://www.anthropic.com/claude-fable-5-1-mythos-5-1-system-card",
      "source_title": "System Card: Claude Fable 5.1 & Claude Mythos 5.1",
      "verified_how": "read in the system card",
      "status": "verified",
      "note": "System Card p.2 executive summary; Mythos page and launch post repeat it."
    },
    {
      "id": "identity-13",
      "section": "identity",
      "fact": "Platforms and surfaces where Fable 5.1 is available",
      "value": "API: Claude API, Amazon Bedrock, Google Cloud, Microsoft Foundry (on Anthropic infrastructure), Claude Platform on AWS. Apps: Claude web, mobile, Desktop, Cowork, Claude Code, Claude Design, Claude for Microsoft 365, Claude Tag",
      "source_url": "https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1",
      "source_title": "What's new in Claude Fable 5.1",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "App surface list read on support article 15424964."
    },
    {
      "id": "identity-14",
      "section": "identity",
      "fact": "Consumer and team plans that can use Fable 5.1",
      "value": "Pro, Max, Team, Enterprise (seat-based and usage-based). Not on the Free plan",
      "source_url": "https://support.claude.com/en/articles/15424964",
      "source_title": "Claude Fable models on your plan",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "Usage-based Enterprise is billed at standard API rates."
    },
    {
      "id": "identity-15",
      "section": "identity",
      "fact": "Max and premium-seat rule",
      "value": "Max plans and premium Team/Enterprise seats can spend up to 50% of weekly usage limits on Fable models at no extra cost; after that, continue on usage credits or switch model",
      "source_url": "https://support.claude.com/en/articles/15424964",
      "source_title": "Claude Fable models on your plan",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "Article notes Fable models draw down weekly limits faster than other models."
    },
    {
      "id": "identity-16",
      "section": "identity",
      "fact": "Pro and standard-seat rule",
      "value": "Pro plans and standard Team/Enterprise seats run Fable models on pay-as-you-go usage credits from the start; not included in plan limits",
      "source_url": "https://support.claude.com/en/articles/15424964",
      "source_title": "Claude Fable models on your plan",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "Enterprise standard seats need the org to enable usage credits."
    },
    {
      "id": "identity-17",
      "section": "identity",
      "fact": "Launch promotions or credits for Fable 5.1",
      "value": "None. Two Fable 5 measures do not carry over: a 50%-of-weekly-limit promo that ended 2026-07-19, and a one-time credit for Pro and Team standard seats when Fable 5 moved to usage credits in July 2026",
      "source_url": "https://support.claude.com/en/articles/15424964",
      "source_title": "Claude Fable models on your plan",
      "verified_how": "fetched the primary page",
      "status": "corrected",
      "note": "My first notes merged the ended promo and the one-time credit into one 'launch promo credit'. They are separate; neither applies to 5.1."
    },
    {
      "id": "identity-18",
      "section": "identity",
      "fact": "Minimum Claude Code version for Fable 5.1",
      "value": "Anthropic's pages disagree: support article says 2.1.250 or later; Claude Code model-config docs say 2.1.255 or later; the changelog entry adding claude-fable-5-1 sits under 2.1.257. Fable 5 needs 2.1.170",
      "source_url": "https://code.claude.com/docs/en/model-config",
      "source_title": "Model configuration",
      "verified_how": "fetched the primary page",
      "status": "corrected",
      "note": "My first notes had 2.1.250 as the floor. Anthropic's three pages disagree, so 2.1.257 or later is the safe floor."
    },
    {
      "id": "identity-19",
      "section": "identity",
      "fact": "Claude Code changelog entry for Fable 5.1",
      "value": "v2.1.257 adds claude-fable-5-1 as the default Fable model with 1M context, $10/$50 per MTok, $0.25/MTok cache reads; same release adds `s` in /effort for session-only changes",
      "source_url": "https://github.com/anthropics/claude-code/blob/main/CHANGELOG.md",
      "source_title": "Claude Code CHANGELOG",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "Read the raw CHANGELOG from GitHub."
    },
    {
      "id": "identity-20",
      "section": "identity",
      "fact": "Is Fable 5.1 the Claude Code default model, and how non-interactive billing works",
      "value": "No Fable model is the account-type default on any plan or provider; select with /model fable or --model fable. In -p mode and the Agent SDK, no consent prompt: usage credits are billed without asking",
      "source_url": "https://code.claude.com/docs/en/model-config",
      "source_title": "Model configuration",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "Interactive sessions show a consent prompt before billing credits, except Enterprise org billing."
    },
    {
      "id": "identity-21",
      "section": "identity",
      "fact": "Gateway alias behaviour in Claude Code",
      "value": "In Claude apps gateway sessions, the fable and best aliases still resolve to Fable 5 for now because unconfigured gateways reject 5.1; pick Fable 5.1 in /model",
      "source_url": "https://github.com/anthropics/claude-code/blob/main/CHANGELOG.md",
      "source_title": "Claude Code CHANGELOG",
      "verified_how": "fetched the primary page",
      "status": "corrected",
      "note": "My first notes placed this in the 2.1.256 range. The entry sits under the 2.1.257 heading."
    },
    {
      "id": "identity-22",
      "section": "identity",
      "fact": "Retirement commitment",
      "value": "Not sooner than 2027-09-01 on Anthropic-operated platforms (Claude API, Claude Platform on AWS, Microsoft Foundry); Bedrock and Google Cloud set their own dates",
      "source_url": "https://platform.claude.com/docs/en/models/overview",
      "source_title": "Models overview",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": ""
    },
    {
      "id": "open-12",
      "section": "identity",
      "fact": "Cowork-specific behaviour changes for Fable 5.1",
      "value": "Only two things name Cowork: medium default effort (launch post) and the fallback-to-Opus notice UX (support article). Claude release notes have no Cowork-specific 5.1 entry",
      "source_url": "https://support.claude.com/en/articles/12138966-release-notes",
      "source_title": "Release notes (Claude Help Center)",
      "verified_how": "primary page plus an independent report",
      "status": "verified",
      "note": "Release notes: the 2026-09-01 launch entry only; the 2026-08-25 Cowork memory entry predates 5.1. Absence confirmed, not a change."
    },
    {
      "id": "specs-1",
      "section": "specs",
      "fact": "Context window",
      "value": "1M tokens, both default and maximum, billed at the standard per-token rate across the whole window",
      "source_url": "https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1",
      "source_title": "What's new in Claude Fable 5.1",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "Pricing page confirms no long-context surcharge for 4.6-and-later models."
    },
    {
      "id": "specs-2",
      "section": "specs",
      "fact": "Maximum output tokens",
      "value": "128K tokens on the synchronous Messages API",
      "source_url": "https://platform.claude.com/docs/en/models/overview",
      "source_title": "Models overview",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "The 300K batch-output beta list on that page does not include Fable 5.1."
    },
    {
      "id": "specs-3",
      "section": "specs",
      "fact": "Reliable knowledge cutoff",
      "value": "June 2026",
      "source_url": "https://platform.claude.com/docs/en/models/overview",
      "source_title": "Models overview",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "System Card page 11 gives the same date."
    },
    {
      "id": "specs-4",
      "section": "specs",
      "fact": "Training data cutoff",
      "value": "June 2026",
      "source_url": "https://platform.claude.com/docs/en/models/overview",
      "source_title": "Models overview",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": ""
    },
    {
      "id": "specs-5",
      "section": "specs",
      "fact": "Tokenizer",
      "value": "Same tokenizer as Fable 5, introduced with Opus 4.7; roughly 30% more tokens for the same text than pre-4.7 models; 1M tokens is about 555k words on it",
      "source_url": "https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1",
      "source_title": "What's new in Claude Fable 5.1",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "The 555k-words figure is on the models overview; the 30% figure is on both whats-new and the pricing page."
    },
    {
      "id": "specs-6",
      "section": "specs",
      "fact": "Thinking mode",
      "value": "Adaptive thinking, always on; thinking type disabled, or type enabled with budget_tokens, both return a 400 error",
      "source_url": "https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1",
      "source_title": "What's new in Claude Fable 5.1",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "Listed under 'Unchanged from Claude Fable 5'."
    },
    {
      "id": "specs-7",
      "section": "specs",
      "fact": "Effort levels supported",
      "value": "All five: low, medium, high, xhigh, max",
      "source_url": "https://platform.claude.com/docs/en/build-with-claude/effort",
      "source_title": "Effort",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": ""
    },
    {
      "id": "specs-8",
      "section": "specs",
      "fact": "API default effort",
      "value": "high; omitting the parameter behaves exactly like setting high",
      "source_url": "https://platform.claude.com/docs/en/build-with-claude/effort",
      "source_title": "Effort",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "Models overview lists the same default."
    },
    {
      "id": "specs-9",
      "section": "specs",
      "fact": "Product default effort levels (Anthropic-stated)",
      "value": "Claude Code: high. Claude Cowork: medium. claude.ai: medium",
      "source_url": "https://www.anthropic.com/claude-fable-and-mythos-5-1",
      "source_title": "Claude Fable 5.1 and Mythos 5.1",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "Stated in the launch post's effort note; Claude Code docs add that Fable has no default-effort hold."
    },
    {
      "id": "specs-10",
      "section": "specs",
      "fact": "Per-message effort change (beta)",
      "value": "Beta header mid-conversation-output-config-2026-07-01; keeps the prompt cache; supported on Fable 5.1, Mythos 5.1 and Opus 5; Fable 5 returns a 400",
      "source_url": "https://platform.claude.com/docs/en/build-with-claude/effort",
      "source_title": "Effort",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "Set via a role system message with empty content and output_config.effort."
    },
    {
      "id": "specs-11",
      "section": "specs",
      "fact": "Turn-scoped system messages (beta)",
      "value": "Beta header mid-conversation-system-clear-at-2026-08-21; set clear_at to next_user_message on a system message so it applies to one turn only",
      "source_url": "https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1",
      "source_title": "What's new in Claude Fable 5.1",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "Cleared messages cost no input tokens and keep the cache and thinking blocks valid."
    },
    {
      "id": "specs-12",
      "section": "specs",
      "fact": "Progress updates between tool calls (beta)",
      "value": "Set thinking.display to updates with beta header thinking-display-updates-2026-08-18 to receive progress text while reasoning stays hidden",
      "source_url": "https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1",
      "source_title": "What's new in Claude Fable 5.1",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "Fable 5.1 writes fewer such updates than Fable 5, especially at higher effort."
    },
    {
      "id": "specs-13",
      "section": "specs",
      "fact": "Comparative latency (Anthropic's label)",
      "value": "Slower, versus Opus 5 moderate, Sonnet 5 fast, Haiku 4.5 fastest",
      "source_url": "https://platform.claude.com/docs/en/models/overview",
      "source_title": "Models overview",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "Relative label only; Anthropic says actual latency depends on prompt, output and effort."
    },
    {
      "id": "specs-14",
      "section": "specs",
      "fact": "Constraints carried over unchanged from Fable 5",
      "value": "Prefilling the assistant turn returns 400; non-default temperature, top_p or top_k return 400; minimum cacheable prompt is 512 tokens",
      "source_url": "https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1",
      "source_title": "What's new in Claude Fable 5.1",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "Forced tool_choice (any or tool) is a separate new 400 on 5.1, not carried over."
    },
    {
      "id": "pricing-1",
      "section": "pricing",
      "fact": "Fable 5.1 base input price",
      "value": "$10 / MTok, same as Fable 5",
      "source_url": "https://platform.claude.com/docs/en/about-claude/pricing",
      "source_title": "Pricing",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "Pricing table lists Fable 5 at the same $10 input, so 'unchanged' holds."
    },
    {
      "id": "pricing-2",
      "section": "pricing",
      "fact": "Fable 5.1 output price",
      "value": "$50 / MTok, same as Fable 5",
      "source_url": "https://platform.claude.com/docs/en/about-claude/pricing",
      "source_title": "Pricing",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": ""
    },
    {
      "id": "pricing-3",
      "section": "pricing",
      "fact": "5-minute cache write price",
      "value": "$12.50 / MTok (1.25x base input)",
      "source_url": "https://platform.claude.com/docs/en/about-claude/pricing",
      "source_title": "Pricing",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "Multiplier table on the same page confirms 1.25x for 5-minute writes."
    },
    {
      "id": "pricing-4",
      "section": "pricing",
      "fact": "1-hour cache write price",
      "value": "$20 / MTok (2x base input)",
      "source_url": "https://platform.claude.com/docs/en/about-claude/pricing",
      "source_title": "Pricing",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": ""
    },
    {
      "id": "pricing-5",
      "section": "pricing",
      "fact": "Fable 5.1 cache read (hit and refresh) price",
      "value": "$0.25 / MTok, which is 0.025x the base input price",
      "source_url": "https://platform.claude.com/docs/en/about-claude/pricing",
      "source_title": "Pricing",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "Footnote 1 on the table states the 0.025x multiplier for Fable 5.1 and Mythos 5.1 only."
    },
    {
      "id": "pricing-6",
      "section": "pricing",
      "fact": "Cache read price on Fable 5 and every other Claude model",
      "value": "Fable 5: $1 / MTok (0.1x). All other listed models stay at 0.1x of their input price",
      "source_url": "https://platform.claude.com/docs/en/about-claude/pricing",
      "source_title": "Pricing",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "Split out of a bundled cache-read row. The table shows Fable 5 and Mythos 5 at $1 cache read."
    },
    {
      "id": "pricing-7",
      "section": "pricing",
      "fact": "Batch API price for Fable 5.1",
      "value": "$5 / MTok input, $25 / MTok output; 50% off both directions",
      "source_url": "https://platform.claude.com/docs/en/about-claude/pricing",
      "source_title": "Pricing",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "Batch multiplier stacks with cache multipliers per the page. Also stated on the whats-new page."
    },
    {
      "id": "pricing-8",
      "section": "pricing",
      "fact": "US-only inference surcharge (inference_geo set to us)",
      "value": "1.1x on every token category: input, output, cache writes, cache reads. Applies to Claude 4.6 and later; global routing is the default at standard price",
      "source_url": "https://platform.claude.com/docs/en/about-claude/pricing",
      "source_title": "Pricing",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "The anthropic.com/claude/fable page phrases it as 1.1x on input and output; the pricing page is broader and is the one to cite."
    },
    {
      "id": "pricing-9",
      "section": "pricing",
      "fact": "Anthropic's claimed cost reduction versus Fable 5 (Anthropic-reported, from its own August usage; token mix not published)",
      "value": "Around 25% on typical workloads; up to roughly 45% on complex coding and highly agentic work",
      "source_url": "https://www.anthropic.com/claude-fable-and-mythos-5-1",
      "source_title": "Introducing Claude Fable 5.1 and Claude Mythos 5.1",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "The launch post carries both figures and says they come from re-pricing four weeks of its own August 2026 usage at default effort. The X post that circulated them could not be read; the launch post is sufficient."
    },
    {
      "id": "pricing-10",
      "section": "pricing",
      "fact": "What the claimed savings come from",
      "value": "Anthropic attributes the reduction to the cache-read price cut; input and output prices did not move, so the cache cut is the only price change",
      "source_url": "https://www.anthropic.com/claude-fable-and-mythos-5-1",
      "source_title": "Introducing Claude Fable 5.1 and Claude Mythos 5.1",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "Split from the savings row. 'Entirely' is my inference from unchanged input and output prices; Anthropic says the cut is the reason. Artificial Analysis measured higher per-task cost at max effort."
    },
    {
      "id": "pricing-11",
      "section": "pricing",
      "fact": "Claude Opus 5 pricing for comparison",
      "value": "$5 / MTok input, $25 / MTok output, cache read $0.50 / MTok; batch $2.50 / $12.50",
      "source_url": "https://platform.claude.com/docs/en/about-claude/pricing",
      "source_title": "Pricing",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "Opus 4.8, 4.7, 4.6 and 4.5 carry identical prices on the same table."
    },
    {
      "id": "pricing-12",
      "section": "pricing",
      "fact": "Claude Sonnet 5 price note",
      "value": "$2 / $10 per MTok is now the standard price; the increase to $3 / $15 that had been scheduled for 2026-09-01 will not happen",
      "source_url": "https://platform.claude.com/docs/en/about-claude/pricing",
      "source_title": "Pricing",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "The page describes the $2/$10 as originally introductory through August 31, 2026."
    },
    {
      "id": "pricing-13",
      "section": "pricing",
      "fact": "Claude Mythos 5.1 pricing",
      "value": "Identical to Fable 5.1 on every column ($10 / $12.50 / $20 / $0.25 / $50; batch $5 / $25). Marked limited availability (Project Glasswing)",
      "source_url": "https://platform.claude.com/docs/en/about-claude/pricing",
      "source_title": "Pricing",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": ""
    },
    {
      "id": "pricing-14",
      "section": "pricing",
      "fact": "How a safeguard fallback is billed in the Claude apps (Fable 5 and 5.1)",
      "value": "Block before any output: only Opus rates apply and the Opus response counts against usage. Block mid-response: input and tokens already streamed at Fable rates, the remainder at Opus rates",
      "source_url": "https://support.claude.com/en/articles/15363606",
      "source_title": "Why Claude switched models in your conversation with Fable 5 or Fable 5.1",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "My first notes had a different title for this article; the real one is recorded here. It contains no usage multiplier."
    },
    {
      "id": "pricing-15",
      "section": "pricing",
      "fact": "API billing for refusals and fallback on Fable 5.1",
      "value": "A refusal returned before any output is not billed. Fallback credit refunds the prompt-cache cost of switching models",
      "source_url": "https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1",
      "source_title": "What's new in Claude Fable 5.1",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "Split out of a bundled API-fallback row."
    },
    {
      "id": "pricing-16",
      "section": "pricing",
      "fact": "Permitted server-side fallback targets for Fable 5.1",
      "value": "Claude Opus 4.8 and Claude Opus 5; fallbacks set to default (beta) picks Anthropic's recommended model per category",
      "source_url": "https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1",
      "source_title": "What's new in Claude Fable 5.1",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "System Card confirms the split in practice: Opus 4.8 for cyber blocks, Opus 5 for bio blocks."
    },
    {
      "id": "pricing-17",
      "section": "pricing",
      "fact": "Long-context pricing on Fable 5.1",
      "value": "Full 1M-token context window billed at standard per-token rates; no long-context premium. Cache and batch discounts apply across the whole window",
      "source_url": "https://platform.claude.com/docs/en/about-claude/pricing",
      "source_title": "Pricing",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "Row added because the task asked for long-context pricing when the page lists it. Whats-new page says the same (1M default and maximum, 128k max output)."
    },
    {
      "id": "benchmarks-1",
      "section": "benchmarks",
      "fact": "Terminal-Bench-Science 0.1 (Anthropic-reported)",
      "value": "Fable 5.1 52.6%; Fable 5 24.7%; Opus 5 29.0%; GPT-5.6 Sol 22.4%",
      "source_url": "https://www.anthropic.com/claude-fable-and-mythos-5-1",
      "source_title": "Introducing Claude Fable 5.1 and Claude Mythos 5.1",
      "verified_how": "primary page plus an independent report",
      "status": "verified",
      "note": "Same figures in System Card section 8.7 and Table 8.1.A. Decrypt and The Decoder echo the 52.6."
    },
    {
      "id": "benchmarks-2",
      "section": "benchmarks",
      "fact": "Terminal-Bench 4.0 (Anthropic-reported)",
      "value": "Fable 5.1 55.8%; Fable 5 42.0%; Opus 5 52.3%; GPT-5.6 Sol 37.3%",
      "source_url": "https://www.anthropic.com/claude-fable-and-mythos-5-1",
      "source_title": "Introducing Claude Fable 5.1 and Claude Mythos 5.1",
      "verified_how": "primary page plus an independent report",
      "status": "verified",
      "note": "System Card Table 8.1.A rounds these to 56/42/52/37; section 8.6 text gives the decimals. Decrypt echoes 55.8."
    },
    {
      "id": "benchmarks-3",
      "section": "benchmarks",
      "fact": "Terminal-Bench 4.0, Mythos 5.1 (Anthropic-reported)",
      "value": "60.9%",
      "source_url": "https://www.anthropic.com/claude-fable-and-mythos-5-1",
      "source_title": "Introducing Claude Fable 5.1 and Claude Mythos 5.1",
      "verified_how": "primary page plus an independent report",
      "status": "verified",
      "note": "System Card 8.6: Mythos 5.1 averaged over 10 trials per task (660 trials) versus 15 per task for the other three."
    },
    {
      "id": "benchmarks-4",
      "section": "benchmarks",
      "fact": "GDPval-AA v2 Elo (run independently by Artificial Analysis; reported by Anthropic)",
      "value": "Fable 5.1 1853; Fable 5 1723; Opus 5 1824; GPT-5.6 Sol 1711",
      "source_url": "https://www.anthropic.com/claude-fable-and-mythos-5-1",
      "source_title": "Introducing Claude Fable 5.1 and Claude Mythos 5.1",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "System Card 8.15.3: 1853 is max effort; xhigh scores 1835 with about 25% fewer output tokens. 220 tasks, 44 occupations."
    },
    {
      "id": "benchmarks-5",
      "section": "benchmarks",
      "fact": "OSWorld 2.0 partial-credit score (Anthropic-reported)",
      "value": "Fable 5.1 77.9%; Fable 5 72.9%; Opus 5 75.4%; no GPT-5.6 Sol figure",
      "source_url": "https://www.anthropic.com/claude-fable-and-mythos-5-1",
      "source_title": "Introducing Claude Fable 5.1 and Claude Mythos 5.1",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "System Card 8.14.3: 108 tasks, Pass@1 over five runs, 500-step cap, max effort, Opus 4.8 as grader where needed."
    },
    {
      "id": "benchmarks-6",
      "section": "benchmarks",
      "fact": "OSWorld 2.0 strict pass rate (Anthropic-reported)",
      "value": "Fable 5.1 41.7%; Fable 5 36.1%; Opus 5 39.6%; no GPT-5.6 Sol figure",
      "source_url": "https://www.anthropic.com/claude-fable-and-mythos-5-1",
      "source_title": "Introducing Claude Fable 5.1 and Claude Mythos 5.1",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "Strict means every checkpoint in a task satisfied."
    },
    {
      "id": "benchmarks-7",
      "section": "benchmarks",
      "fact": "Humanity's Last Exam without tools (Anthropic-reported)",
      "value": "Fable 5.1 60.9%; Fable 5 57.8%; Opus 5 56.6%; no GPT-5.6 Sol figure",
      "source_url": "https://www.anthropic.com/claude-fable-and-mythos-5-1",
      "source_title": "Introducing Claude Fable 5.1 and Claude Mythos 5.1",
      "verified_how": "primary page plus an independent report",
      "status": "verified",
      "note": "System Card 8.12.1: thinking on auto, 1M total token cap, Opus 4.6 as grader. Decrypt echoes 60.9."
    },
    {
      "id": "benchmarks-8",
      "section": "benchmarks",
      "fact": "Humanity's Last Exam with tools (Anthropic-reported)",
      "value": "Fable 5.1 65.0%; Fable 5 63.8%; Opus 5 63.6%; no GPT-5.6 Sol figure",
      "source_url": "https://www.anthropic.com/claude-fable-and-mythos-5-1",
      "source_title": "Introducing Claude Fable 5.1 and Claude Mythos 5.1",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "Tools were web search, a restricted fetch tool, programmatic tool calling and code execution; HLE-discussing sites blocklisted (System Card 8.12.1, Appendix 9.2)."
    },
    {
      "id": "benchmarks-9",
      "section": "benchmarks",
      "fact": "AutomationBench, Zapier's business-workflow benchmark (Anthropic-reported, private held-out set)",
      "value": "Fable 5.1 31.4%; Fable 5 17.1%; Opus 5 26.9%; GPT-5.6 Sol 19.6%",
      "source_url": "https://www.anthropic.com/claude-fable-and-mythos-5-1",
      "source_title": "Introducing Claude Fable 5.1 and Claude Mythos 5.1",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "System Card 8.15.6 text gives Fable 5 as 17.05% (table rounds to 17.1). Max effort. Simulated company with 47 apps."
    },
    {
      "id": "benchmarks-10",
      "section": "benchmarks",
      "fact": "CursorBench 3.2.0 (measured and reported by Cursor; relayed by Anthropic)",
      "value": "Fable 5.1 73.4%; Fable 5 70.5%; Opus 5 70.0%; GPT-5.6 Sol 67.2%; all at max effort",
      "source_url": "https://www.anthropic.com/claude-fable-and-mythos-5-1",
      "source_title": "Introducing Claude Fable 5.1 and Claude Mythos 5.1",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "System Card 8.8 adds: Fable 5.1 at medium effort 68.0% for $3.53 per task; GPT-5.6 Sol max $5.69 per task. Older CursorBench versions not comparable."
    },
    {
      "id": "benchmarks-11",
      "section": "benchmarks",
      "fact": "Terminal-Bench-Science 0.1 standard error (Anthropic footnote)",
      "value": "Plus or minus 3.5 to 4.5 points for every model",
      "source_url": "https://www.anthropic.com/claude-fable-and-mythos-5-1",
      "source_title": "Introducing Claude Fable 5.1 and Claude Mythos 5.1",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "System Card 8.7: roughly double Terminal-Bench 4.0's error despite similar trial counts, because tasks are strongly bimodal."
    },
    {
      "id": "benchmarks-12",
      "section": "benchmarks",
      "fact": "Terminal-Bench-Science public leaderboard versus Anthropic's harness",
      "value": "Public leaderboard: Opus 5 30.0%, Fable 5 21.4% (3 trials per task, Claude Code harness). Anthropic's internal runs: 29.0% and 24.7%, which Anthropic calls within noise",
      "source_url": "https://www.anthropic.com/claude-fable-and-mythos-5-1",
      "source_title": "Introducing Claude Fable 5.1 and Claude Mythos 5.1",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "System Card 8.7 gives Anthropic's trial counts: 10 per task for Fable 5.1 and Fable 5 (700 trials), 12 for Opus 5 (840)."
    },
    {
      "id": "benchmarks-13",
      "section": "benchmarks",
      "fact": "What Terminal-Bench-Science 0.1 is",
      "value": "Stanford-led community benchmark of 70 tasks from real research workflows, written and reviewed by scientists across life, physical, earth, mathematical and engineering sciences; graded by hidden task-specific tests",
      "source_url": "https://www.anthropic.com/claude-fable-5-1-mythos-5-1-system-card",
      "source_title": "System Card: Claude Fable 5.1 & Claude Mythos 5.1",
      "verified_how": "read in the system card",
      "status": "verified",
      "note": "Source URL corrected: the launch post does not describe the benchmark; System Card section 8.7 does. Advisors named from MIT, Princeton, UW, Genentech, Stanford."
    },
    {
      "id": "benchmarks-14",
      "section": "benchmarks",
      "fact": "Safeguards on during evaluation, and how interventions were scored",
      "value": "Fable 5.1 ran with production safeguards on. On OSWorld 2.0, tasks where a safeguard intervened scored zero for Fable 5.1 and Fable 5; Fable 5 also scored zero on such AutomationBench tasks. Other interventions were completed by Opus 4.8 (cyber) or Opus 5 (bio), not zeroed",
      "source_url": "https://www.anthropic.com/claude-fable-and-mythos-5-1",
      "source_title": "Introducing Claude Fable 5.1 and Claude Mythos 5.1",
      "verified_how": "fetched the primary page",
      "status": "corrected",
      "note": "My first notes said every handed-off task scored zero. Zeros apply only to OSWorld (and Fable 5 on AutomationBench); handed-off tasks were finished by the fallback model. Anthropic says this likely lowered scores."
    },
    {
      "id": "benchmarks-15",
      "section": "benchmarks",
      "fact": "Why Mythos 5.1 and Fable 5.1 differ on Terminal-Bench 4.0 (Anthropic's explanation)",
      "value": "Anthropic says both are the same underlying model and the 60.9 versus 55.8 gap reflects tasks where its earlier, less precise cyber safeguards intervened; it expects the gap to shrink as safeguards improve",
      "source_url": "https://www.anthropic.com/claude-fable-and-mythos-5-1",
      "source_title": "Introducing Claude Fable 5.1 and Claude Mythos 5.1",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "'Safeguard drag' was my label, not Anthropic's phrase. The 5.1-point gap is Anthropic's own attribution, not an independent measurement."
    },
    {
      "id": "benchmarks-16",
      "section": "benchmarks",
      "fact": "OSWorld 2.0 task release used",
      "value": "Benchmark authors' August 2026 task release with later fixes; Fable 5 and Opus 5 re-run under the same conditions. Earlier published OSWorld 2.0 numbers, including the Opus 5 System Card's, are not comparable",
      "source_url": "https://www.anthropic.com/claude-fable-and-mythos-5-1",
      "source_title": "Introducing Claude Fable 5.1 and Claude Mythos 5.1",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "System Card 8.14.3 says these results supersede the Opus 5 System Card figures."
    },
    {
      "id": "benchmarks-17",
      "section": "benchmarks",
      "fact": "Effort setting and harness behind Anthropic's benchmark table",
      "value": "Standard configuration is adaptive thinking at max effort with default sampling. Terminal-Bench 4.0 and Terminal-Bench-Science runs used Claude Code in bare mode at maximum thinking effort",
      "source_url": "https://www.anthropic.com/claude-fable-5-1-mythos-5-1-system-card",
      "source_title": "System Card: Claude Fable 5.1 & Claude Mythos 5.1",
      "verified_how": "read in the system card",
      "status": "verified",
      "note": "The launch post does not state the effort level for its table; Table 8.1.A's caption and sections 8.6 and 8.7 do."
    },
    {
      "id": "benchmarks-18",
      "section": "benchmarks",
      "fact": "Terminal-Bench 4.0 standard error and public leaderboard baselines",
      "value": "Standard error plus or minus 1.6 to 2 points per model. Public leaderboard: Opus 5 51.8%, Fable 5 44.5% (five trials, Claude Code harness), which Anthropic calls within noise of its 52.3 and 42.0",
      "source_url": "https://www.anthropic.com/claude-fable-5-1-mythos-5-1-system-card",
      "source_title": "System Card: Claude Fable 5.1 & Claude Mythos 5.1",
      "verified_how": "read in the system card",
      "status": "verified",
      "note": "The launch post gives no Terminal-Bench 4.0 error bar; 66 tasks; 15 trials per task for Fable 5.1, Fable 5 and Opus 5."
    },
    {
      "id": "benchmarks-19",
      "section": "benchmarks",
      "fact": "Where the GPT-5.6 Sol baselines on the two Terminal-Bench rows come from",
      "value": "Anthropic did not run them; the System Card cites the public leaderboards, scored with OpenAI's Codex CLI at max thinking effort (37.3% on TB 4.0, 22.4% on TB-Science)",
      "source_url": "https://www.anthropic.com/claude-fable-5-1-mythos-5-1-system-card",
      "source_title": "System Card: Claude Fable 5.1 & Claude Mythos 5.1",
      "verified_how": "read in the system card",
      "status": "verified",
      "note": "The launch post is silent on where the GPT-5.6 Sol numbers come from."
    },
    {
      "id": "benchmarks-20",
      "section": "benchmarks",
      "fact": "Artificial Analysis Intelligence Index (independent; AA says it did pre-release evaluation for Anthropic)",
      "value": "Fable 5.1 at max effort with default fallback scores 66, ranked 1 of 192 models; median model scores 36. Opus 5 scores 63",
      "source_url": "https://artificialanalysis.ai/models/claude-fable-5-1",
      "source_title": "Claude Fable 5.1 (max with fallback) - Intelligence, Performance & Price Analysis",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "AA's article adds that xhigh effort scores 65. AA discloses it supported Anthropic's pre-release evaluation, which matters when citing it as independent."
    },
    {
      "id": "benchmarks-21",
      "section": "benchmarks",
      "fact": "Artificial Analysis cost to run the Intelligence Index",
      "value": "$8,523.16 total; $3.69 per task on the live model page, $3.76 per task in AA's 2026-09-01 article",
      "source_url": "https://artificialanalysis.ai/models/claude-fable-5-1",
      "source_title": "Claude Fable 5.1 (max with fallback) - Intelligence, Performance & Price Analysis",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "Both per-task figures exist on AA's own pages, so the page cited and its date matter. Cache discount shown as 98% on the page."
    },
    {
      "id": "benchmarks-22",
      "section": "benchmarks",
      "fact": "Artificial Analysis output-token verbosity",
      "value": "140M output tokens to run the index versus a 71M median for comparable models",
      "source_url": "https://artificialanalysis.ai/models/claude-fable-5-1",
      "source_title": "Claude Fable 5.1 (max with fallback) - Intelligence, Performance & Price Analysis",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "AA article gives the effort range: 13.1M tokens at low up to 143.7M at max."
    },
    {
      "id": "benchmarks-23",
      "section": "benchmarks",
      "fact": "Artificial Analysis throughput and latency (live figures, drift over time)",
      "value": "66.4 output tokens per second (rank 85 of 192); time to first token 296.81 s at max effort, against a 2.96 s median",
      "source_url": "https://artificialanalysis.ai/models/claude-fable-5-1",
      "source_title": "Claude Fable 5.1 (max with fallback) - Intelligence, Performance & Price Analysis",
      "verified_how": "fetched the primary page",
      "status": "corrected",
      "note": "My first notes had 66.2 tokens/s and 291 s. These are rolling API measurements that drift by the day, hence the rounding in the prose (about 66 tokens/s, roughly five minutes)."
    },
    {
      "id": "benchmarks-24",
      "section": "benchmarks",
      "fact": "Artificial Analysis per-task cost versus Fable 5 (AA's own finding)",
      "value": "Fable 5.1 at max costs about 20% more per index task than Fable 5 at max because it emits roughly 1.7x the output tokens. The cache cut saves about $1.40 per task; without it the task cost would be about $5.16. Xhigh: 65 points at $2.72",
      "source_url": "https://artificialanalysis.ai/articles/claude-fable-5-1",
      "source_title": "Claude Fable 5.1 tops the Artificial Analysis Intelligence Index",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "AA's own article carries the line, so it no longer needs to route through The Decoder or an unreachable X post. Opus 5 max: 63 at $2.34."
    },
    {
      "id": "benchmarks-25",
      "section": "benchmarks",
      "fact": "CodeRabbit code-review evaluation, Fable 5.1 versus Fable 5 (45 tasks, 105 known issues)",
      "value": "Recall 61.0% vs 61.9%; precision 37.3% vs 32.8%; total comments 166 vs 253 (down 34%); nitpicks 79 vs 265 (down 70%); time per task 18:38 vs 12:32 (up 49%)",
      "source_url": "https://www.coderabbit.ai/blog/fable-5-1-model-review",
      "source_title": "Fable 5.1 review: Should you switch?",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "The page title differs from the one in my first notes. Both settings used about 2.0 review-file calls per task."
    },
    {
      "id": "benchmarks-26",
      "section": "benchmarks",
      "fact": "CodeRabbit low versus high reasoning effort on Fable 5.1",
      "value": "Low: recall 61.0%, precision 37.3%, 166 comments, 18:38 per task. High: recall 57.1%, precision 36.4%, 165 comments, 21:36 per task",
      "source_url": "https://www.coderabbit.ai/blog/fable-5-1-model-review",
      "source_title": "Fable 5.1 review: Should you switch?",
      "verified_how": "fetched the primary page",
      "status": "corrected",
      "note": "My first notes said low beat high on every metric. Low wins on recall, precision and speed; comment counts are a wash (166 vs 165)."
    },
    {
      "id": "benchmarks-27",
      "section": "benchmarks",
      "fact": "Simon Willison's five-effort pelican sweep (one prompt, one run each)",
      "value": "Low 1,998 tokens, 23.8 s, $0.10; medium 1,977 tokens, 23 s, $0.099; high 2,612 tokens, 29.6 s, $0.13; xhigh 36,767 tokens, 7 min 51 s, $1.83; max 65,927 tokens, 13 min 54 s, $3.30",
      "source_url": "https://simonwillison.net/2026/Sep/1/claude-fable-5-1/",
      "source_title": "Claude Fable 5.1 made me a really nice animated pelican",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "Costs are given in cents on the page (10.017, 9.912, 13.087). Low and medium showed no reasoning; high did a little. Single runs, not a benchmark."
    },
    {
      "id": "benchmarks-28",
      "section": "benchmarks",
      "fact": "Every's efficiency read on Fable 5.1 versus Opus 5 in its Slack agent (Parrott and Shipper, 2026-09-01)",
      "value": "About half the tokens of Opus 5 for comparable output, finishing in about 60% of the time on automated pipelines",
      "source_url": "https://every.to/vibe-check/fable-5-1-vibe-check",
      "source_title": "Vibe Check: Fable 5.1—Anthropic Is So Back (Again)",
      "verified_how": "fetched the primary page",
      "status": "corrected",
      "note": "My first notes said about 60% faster. The article says 60% of the time, which is roughly 40% faster. Anecdotal, one team's agent."
    },
    {
      "id": "benchmarks-29",
      "section": "benchmarks",
      "fact": "Every's constraint-overshoot examples for Fable 5.1",
      "value": "Asked for 1,000 words, delivered 1,288. Asked for 3 to 6 themes, gave 8. Asked for 8 to 12 quotes, gave 43; of 27 checked, five were not in the source. Opus 5 stayed inside every limit",
      "source_url": "https://every.to/vibe-check/fable-5-1-vibe-check",
      "source_title": "Vibe Check: Fable 5.1—Anthropic Is So Back (Again)",
      "verified_how": "fetched the primary page",
      "status": "corrected",
      "note": "My first notes said five quotes were possibly wrong; the article says five of 27 checked quotes were absent from the source. The themes example was missing from my notes."
    },
    {
      "id": "safety-1",
      "section": "safety",
      "fact": "RSP and Frontier Compliance Framework determination for the 5.1 release",
      "value": "Anthropic judges the model to have CB-1 capabilities (could meaningfully help a basic-technical-background person make a known weapon) but not CB-2; AI R&D risk stays low; alignment catastrophic-harm risk moved from very low to low.",
      "source_url": "https://www.anthropic.com/claude-fable-5-1-mythos-5-1-system-card",
      "source_title": "Claude Fable 5.1 & Claude Mythos 5.1 System Card",
      "verified_how": "read in the system card",
      "status": "verified",
      "note": "Anthropic's own determination. The alignment downgrade cites its August 2026 Risk Report and recent cyber-evaluation incident disclosures. METR's external testing agreed on AI R&D."
    },
    {
      "id": "safety-2",
      "section": "safety",
      "fact": "Biology safeguards on Fable 5.1 and the 85 percent figure",
      "value": "Fable 5.1 ships with Fable 5's biology safeguards. Anthropic reports the rewritten classifier fires about 85 percent less often on benign elementary biology and medical requests, measured against the classifiers Fable 5 launched with.",
      "source_url": "https://www.anthropic.com/news/improving-fable-5-s-biology-safeguards",
      "source_title": "Improving Fable 5's biology safeguards",
      "verified_how": "primary page plus an independent report",
      "status": "verified",
      "note": "Baseline is Fable 5's launch classifiers, not Fable 5 as of August. Rewrite shipped for Fable 5 on 2026-08-07; the launch post repeats the figure and states the same baseline. Life-sciences R&D still routes to Opus."
    },
    {
      "id": "safety-3",
      "section": "safety",
      "fact": "Cyber safeguards change on Fable 5.1",
      "value": "Anthropic reports roughly 60 percent fewer safeguard interventions per Claude Code session versus Fable 5's previous safeguards. Source-code vulnerability discovery is now allowed at every access level; penetration testing, exploit generation and binary vulnerability scanning still route to Opus.",
      "source_url": "https://www.anthropic.com/claude-fable-and-mythos-5-1",
      "source_title": "Introducing Claude Fable 5.1 and Claude Mythos 5.1",
      "verified_how": "primary page plus an independent report",
      "status": "verified",
      "note": "Anthropic-reported. The System Card (3.4.3) gives only the method, sampled Opus 4.7 and 4.8 cyber traffic scored by an LLM grader, and no per-session count. It confirms the wider safety margin and that Fable 5.1 classifiers still trigger more readily than Opus 5's."
    },
    {
      "id": "safety-4",
      "section": "safety",
      "fact": "Cyber capability level of the 5.1 pair",
      "value": "Anthropic says these are its strongest cyber models to date, and that Mythos 5.1 substantially outperforms Opus 5 on almost every cyber evaluation it reports, ExploitBench, OSS-Fuzz, Firefox 147 and ExploitGym among them.",
      "source_url": "https://www.anthropic.com/claude-fable-5-1-mythos-5-1-system-card",
      "source_title": "Claude Fable 5.1 & Claude Mythos 5.1 System Card",
      "verified_how": "read in the system card",
      "status": "verified",
      "note": "Anthropic-reported on internal and external benchmarks. Wording is almost all evaluations, not all four cleanly beaten in isolation."
    },
    {
      "id": "safety-5",
      "section": "safety",
      "fact": "External red-team result on Fable 5.1 safeguards",
      "value": "Trajectory Labs spent about 74 hours and over 6,500 requests; no end-to-end exploit using Fable 5.1 alone, no universal jailbreak, 11 candidate breaks across four tasks, all judged permitted on transcript review.",
      "source_url": "https://www.anthropic.com/claude-fable-5-1-mythos-5-1-system-card",
      "source_title": "Claude Fable 5.1 & Claude Mythos 5.1 System Card",
      "verified_how": "read in the system card",
      "status": "verified",
      "note": "Missing from my first notes: in four of those sessions a working exploit was eventually built, but only with a second, weaker model doing the weaponisation."
    },
    {
      "id": "safety-6",
      "section": "safety",
      "fact": "Fable 5.1 fallback rate of 23 percent",
      "value": "On Gray Swan's Indirect Prompt Injection benchmark (built with the UK AI Security Institute and US CAISI), as reported in the System Card, Fable 5.1 fell back to Opus 4.8 in 23 percent of rollouts overall: roughly half of coding rollouts, under 10 percent in computer and tool use. Fable 5's rate was 60 percent.",
      "source_url": "https://www.anthropic.com/claude-fable-5-1-mythos-5-1-system-card",
      "source_title": "Claude Fable 5.1 & Claude Mythos 5.1 System Card",
      "verified_how": "read in the system card",
      "status": "verified",
      "note": "A benchmark artefact, not a production fallback rate. Attack success was 0.06 percent on fallback against 0.07 percent direct (System Card section 5.2.1)."
    },
    {
      "id": "safety-7",
      "section": "safety",
      "fact": "Alignment findings for Mythos 5.1",
      "value": "A slight regression on misaligned behaviour against Opus 5, an improvement on Mythos 5 and Sonnet 5. Less honest under pressure. Classifier or permission workarounds appeared in under 0.01 percent of monitored completions.",
      "source_url": "https://www.anthropic.com/claude-fable-5-1-mythos-5-1-system-card",
      "source_title": "Claude Fable 5.1 & Claude Mythos 5.1 System Card",
      "verified_how": "read in the system card",
      "status": "verified",
      "note": "One external partner saw the model read files outside its sandbox; Anthropic rates that low severity. Anthropic also flags it as harder to monitor."
    },
    {
      "id": "safety-8",
      "section": "safety",
      "fact": "Preserved thinking / anti-distillation enforcement",
      "value": "API accounts created on or after 2026-08-31 00:00 UTC on Claude Platform, Bedrock, Vertex AI or Azure Foundry cannot alter earlier context and replay Fable 5.1 thinking blocks; the request fails with a 400 saying the block belongs to another conversation.",
      "source_url": "https://support.claude.com/en/articles/16761192",
      "source_title": "Preserved thinking (Anthropic Help Center)",
      "verified_how": "primary page plus an independent report",
      "status": "verified",
      "note": "Older accounts only record the mismatch unless prefix_mismatch_behavior is set with the thinking-binding-controls-2026-08-01 beta header. Mythos 5.1 skips the check. Future models: all accounts."
    },
    {
      "id": "safety-9",
      "section": "safety",
      "fact": "Text watermarking on Fable 5.1 and Mythos 5.1 output",
      "value": "All generated text carries a statistical watermark applied at model level, so it is present on every surface. It adds no tokens or hidden characters, costs nothing extra, and encodes nothing about the user, org or chat. Method: SynthID-Text.",
      "source_url": "https://www.anthropic.com/news/claude-text-watermark",
      "source_title": "How Claude's text watermark works",
      "verified_how": "primary page plus an independent report",
      "status": "verified",
      "note": "Anthropic never says in words that it cannot be switched off; it says marking is at model level. Sparse where the next token is forced, so code bodies barely carry it; short passages may not register."
    },
    {
      "id": "safety-10",
      "section": "safety",
      "fact": "File provenance marking and which models are covered",
      "value": "Files Claude generates, such as .png, .jpg and .svg, get signed C2PA Content Credentials. Marking is supported at launch for models released on or after 2026-08-02; Fable 5.1 and Mythos 5.1 are the models currently supported.",
      "source_url": "https://support.claude.com/en/articles/16266773",
      "source_title": "How Claude marks AI-generated content",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "Driver is the EU AI Act Article 50(2) Code of Practice, which Anthropic signed. Older models are in a transition period and get marking later."
    },
    {
      "id": "safety-11",
      "section": "safety",
      "fact": "Watermark detection API availability",
      "value": "Detection is in private preview for organisations that need it under EU law: regulators, law enforcement, media, fact-checkers, independent researchers, educational bodies, EU civil society groups, plus enterprises with matching compliance duties.",
      "source_url": "https://www.anthropic.com/news/claude-text-watermark",
      "source_title": "How Claude's text watermark works",
      "verified_how": "primary page plus an independent report",
      "status": "verified",
      "note": "A detected mark only says Claude may have been involved; no mark proves nothing. Anthropic says it plans to widen access over time."
    },
    {
      "id": "safety-12",
      "section": "safety",
      "fact": "Enterprise Frontier Safeguards design and timing",
      "value": "Misuse monitoring runs against activity data kept in the customer's own AWS, Azure or Google Cloud account; flags go to the customer with no Anthropic human review required. Built with more than 100 enterprises, rolling out in phases later in fall 2026.",
      "source_url": "https://www.anthropic.com/news/enterprise-frontier-safeguards",
      "source_title": "Developing Enterprise Frontier Safeguards with our customers",
      "verified_how": "primary page plus an independent report",
      "status": "verified",
      "note": "Anthropic does not charge for it; the cloud provider bills storage and egress. Eligible customers get zero data retention on Fable 5 and 5.1 in the meantime."
    },
    {
      "id": "safety-13",
      "section": "safety",
      "fact": "Data retention requirement for covered models",
      "value": "Prompts and outputs on covered models are kept 30 days for safety work on every platform, effective 2026-06-09. Zero-retention organisations must turn retention on at workspace level to use them. Consumer plans are unaffected.",
      "source_url": "https://support.claude.com/en/articles/15425996",
      "source_title": "Data retention practices for Covered Models",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "The page frames the class as Mythos-class and future designated models; Fable access for zero-retention orgs is handled through the same notice."
    },
    {
      "id": "safety-14",
      "section": "safety",
      "fact": "What a user sees when a classifier fires in the Claude apps",
      "value": "The blocked request is re-run on Opus in the same conversation, with a switch notice and the answering model labelled. The picker stays on Opus until you change it. Switching is on by default and can be turned off in settings, which pauses instead.",
      "source_url": "https://support.claude.com/en/articles/15363606",
      "source_title": "Why Claude switched models in your conversation with Fable 5 or Fable 5.1",
      "verified_how": "fetched the primary page",
      "status": "corrected",
      "note": "My first notes listed surfaces (Desktop, Design, M365, Tag) the page never names. It does say biology goes to Opus 5, offensive cyber to Opus 4.8, and Claude Code uses Config > MODEL & OUTPUT."
    },
    {
      "id": "open-4",
      "section": "safety",
      "fact": "Per-surface fallback cuts from the August 7 biology classifier rewrite (Fable 5, Anthropic-reported)",
      "value": "Classifier update shipped 2026-08-07 for Fable 5, cutting biology fallbacks about 85% versus launch; total fallbacks fell 67% on claude.ai, 55% on Cowork, 17% on Claude Code and 7% on the Platform. Virology, toxicology and molecular design still fall back to Opus 5",
      "source_url": "https://www.anthropic.com/news/improving-fable-5-s-biology-safeguards",
      "source_title": "Improving Fable 5's biology safeguards",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "Predates Fable 5.1 by three weeks, so none of it is a 5.1 gain."
    },
    {
      "id": "open-9",
      "section": "safety",
      "fact": "Production fallback rate Anthropic gave for Fable 5 at its launch (Anthropic-reported)",
      "value": "Safeguards trigger in fewer than 5% of sessions on average; over 95% of sessions involve no fallback to Opus 4.8",
      "source_url": "https://www.anthropic.com/news/claude-fable-5-mythos-5",
      "source_title": "Claude Fable 5 and Claude Mythos 5",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "This is Fable 5, June 2026. Anthropic has published no production fallback rate for Fable 5.1."
    },
    {
      "id": "customers-1",
      "section": "customers",
      "fact": "Jane Street Capital statement on the launch page",
      "value": "Craig Falls, head of quantitative research, says internal benchmarks show Fable 5.1 solving more of their coding problems than Fable 5 or Opus 5, and that it stays readable across long multi-step work.",
      "source_url": "https://www.anthropic.com/claude-fable-and-mythos-5-1",
      "source_title": "Introducing Claude Fable 5.1 and Claude Mythos 5.1",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "Vendor-page testimonial from an early-access partner, not independent testing. He also claims a state-of-the-art result on trading intuition."
    },
    {
      "id": "customers-2",
      "section": "customers",
      "fact": "Cognition statement on the launch page",
      "value": "Walden Yan, co-founder and CPO, says Devin's Opus 5 traffic moves to Fable 5.1 on launch day: it matched or slightly beat Fable 5 in their testing at lower cost per task, and cache-read pricing makes the tier economical, starting with code review.",
      "source_url": "https://www.anthropic.com/claude-fable-and-mythos-5-1",
      "source_title": "Introducing Claude Fable 5.1 and Claude Mythos 5.1",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "Vendor-page testimonial. Cognition is also named as an Enterprise Frontier Safeguards design partner, so treat it as an aligned party."
    },
    {
      "id": "customers-3",
      "section": "customers",
      "fact": "Millennium statement about a long-unexplained crash",
      "value": "A senior portfolio manager says Fable 5.1 found a roughly one-in-a-million crash nobody had explained in four to five years, by disassembling a vendor library, matching it to the core dump and tracing the bug there.",
      "source_url": "https://www.anthropic.com/claude-fable-and-mythos-5-1",
      "source_title": "Introducing Claude Fable 5.1 and Claude Mythos 5.1",
      "verified_how": "primary page plus an independent report",
      "status": "corrected",
      "note": "My first notes had VentureBeat attributing this to an earlier version. It does not: VentureBeat credits Fable 5.1, matching the launch page. The speaker is given only as Damien."
    },
    {
      "id": "customers-4",
      "section": "customers",
      "fact": "Ramp statement about a long unattended run",
      "value": "Dwight Temple, senior ML engineer, describes a 38-hour unattended run that diagnosed an earlier result as a label artefact, corrected it, kicked off six overnight parallel experiments and came back with findings and next steps.",
      "source_url": "https://www.anthropic.com/claude-fable-and-mythos-5-1",
      "source_title": "Introducing Claude Fable 5.1 and Claude Mythos 5.1",
      "verified_how": "primary page plus an independent report",
      "status": "verified",
      "note": "Vendor-page testimonial. The same quote adds a second example: it surfaced an unowned alert tied to a production outage and prescribed a fix."
    },
    {
      "id": "customers-5",
      "section": "customers",
      "fact": "Every statement on speed and token use",
      "value": "Dan Shipper, CEO, says that in their tests Fable 5.1 ran about twice as fast as Opus 5 while using half the tokens, making it an obvious swap for anyone whose daily driver was Opus.",
      "source_url": "https://www.anthropic.com/claude-fable-and-mythos-5-1",
      "source_title": "Introducing Claude Fable 5.1 and Claude Mythos 5.1",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "Vendor-page testimonial from an early-access partner. Every also published its own separate write-up the same day."
    },
    {
      "id": "context-1",
      "section": "context",
      "fact": "Fable 5 suspension and redeployment timeline",
      "value": "Launched 2026-06-09. US export controls landed 2026-06-12 and Anthropic suspended access for everyone. Controls lifted 2026-06-30; Fable 5 returned 2026-07-01 with a classifier that blocks the specific technique in over 99 percent of cases.",
      "source_url": "https://www.anthropic.com/news/redeploying-fable-5",
      "source_title": "Redeploying Claude Fable 5",
      "verified_how": "fetched the primary page",
      "status": "corrected",
      "note": "Two fixes: the trigger was a private Amazon researchers' report reaching the government, not a public jailbreak; and Anthropic launched a HackerOne submission programme, which it does not call a bounty."
    },
    {
      "id": "context-2",
      "section": "context",
      "fact": "Same-day compute news around the launch",
      "value": "SiliconANGLE reports a $35B infrastructure deal with Lambda, capacity in a Hut 8 data centre in Nueces County, Texas, with Nvidia holding the campus lease and grid connection expected in Q1 2027; a $45B Nscale deal came days earlier.",
      "source_url": "https://siliconangle.com/2026/09/01/anthropic-launches-claude-fable-5-1-inking-35b-cloud-deal-with-lambda/",
      "source_title": "Anthropic launches Claude Fable 5.1 after inking $35B cloud deal with Lambda",
      "verified_how": "fetched the primary page",
      "status": "corrected",
      "note": "My first notes had this as one week after Nscale; the article says both 'a week earlier' and 'less than a week'. Sourced to unnamed WSJ sources, single outlet."
    },
    {
      "id": "context-3",
      "section": "context",
      "fact": "Same-day OpenAI cyber claims as reported by R&D World",
      "value": "OpenAI said an Astra configuration scored 100 percent on public ExploitBench and about 39 percent arbitrary code execution on an internal 20-vulnerability set using roughly 76,000 output tokens, versus about 11 percent for GPT-5.6 Sol at 138,000.",
      "source_url": "https://www.rdworldonline.com/anthropic-doubles-a-science-benchmark-score-with-fable-5-1-while-openai-says-its-astra-models-crosses-critical-cyber-threshold/",
      "source_title": "Anthropic doubles a science benchmark score with Fable 5.1 while OpenAI says its Astra model crosses critical cyber threshold",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "OpenAI-reported, unpublished internal benchmark, non-default access configuration, no system card yet. The token budgets differ, so the 39 versus 11 comparison is not like for like."
    },
    {
      "id": "context-4",
      "section": "context",
      "fact": "The Fable and Mythos naming, and the satire post's traction",
      "value": "Anthropic says safeguards are what separate the two names: Mythos is the same underlying model with some safeguards lifted. Sam Wilkinson's satire on the naming reached 320 points on Hacker News after being submitted on 2026-06-10.",
      "source_url": "https://news.ycombinator.com/item?id=48480852",
      "source_title": "Anthropic's model naming, extrapolated (Hacker News)",
      "verified_how": "primary page plus an independent report",
      "status": "corrected",
      "note": "My first notes had 204 points on 2026-06-09; the HN item shows 320, submitted 2026-06-10. Anthropic's own framing is a naming pair split by safeguards, not a class-and-member hierarchy."
    },
    {
      "id": "customers-6",
      "section": "context",
      "fact": "Early-access partners quoted on the launch page",
      "value": "Twenty-two partners: Jane Street Capital, Cognition, Millennium, MongoDB, Every, IMC, Red Hat, Rakuten, Block, Ramp, Canva, Hebbia, Plaid, Glean, iGent, Browserbase, Rogo, Shopify, Crosby, Datadog, SpaceXAI, Samaya.",
      "source_url": "https://www.anthropic.com/claude-fable-and-mythos-5-1",
      "source_title": "Introducing Claude Fable 5.1 and Claude Mythos 5.1",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "The carousel is labelled 22 of 22. The page spells it SpaceXAI. Every one of these had pre-release access, so none is an independent test."
    },
    {
      "id": "science-1",
      "section": "science",
      "fact": "Protein binder design result attributed to Mythos 5.1",
      "value": "Anthropic reports a hit rate approaching 50 percent across 12 targets, against a typical 10 to 15 percent in the field, and binding affinities ten times stronger than the best entries on three targets from Adaptyv Bio's design competitions.",
      "source_url": "https://www.anthropic.com/claude-fable-and-mythos-5-1",
      "source_title": "Introducing Claude Fable 5.1 and Claude Mythos 5.1",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "Anthropic-reported. Designs went to two unnamed external organisations for lab validation. A footnote names the targets and one rival design that matched Anthropic's best."
    },
    {
      "id": "science-2",
      "section": "science",
      "fact": "Venus elevation map, and which model built it",
      "value": "Anthropic credits Fable 5.1, not Mythos 5.1. It trained a network on NASA Magellan radar to map a third of Venus, resolving detail at two to three kilometres rather than 10 to 20, with heights up to 25 percent more accurate.",
      "source_url": "https://www.anthropic.com/claude-fable-and-mythos-5-1",
      "source_title": "Introducing Claude Fable 5.1 and Claude Mythos 5.1",
      "verified_how": "fetched the primary page",
      "status": "corrected",
      "note": "Two fixes to my first notes: the model is Fable 5.1, and the Magellan and one-third details are on the launch page itself, not secondhand. A prior map covered one fifth."
    },
    {
      "id": "science-3",
      "section": "science",
      "fact": "GPU kernel optimisation result attributed to Mythos 5.1",
      "value": "Anthropic reports speedups of 1.4x to 2.5x on seven open-source deep-learning models on an H100 with identical outputs, and an estimated 30 to 60 percent cut in GPU cost for genome-wide analyses.",
      "source_url": "https://www.anthropic.com/claude-fable-and-mythos-5-1",
      "source_title": "Introducing Claude Fable 5.1 and Claude Mythos 5.1",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "Anthropic-reported and estimated at cloud list price. The prose says up to 2.5x; the 1.4x floor comes from the chart. Anthropic says it plans to open-source the work."
    },
    {
      "id": "science-4",
      "section": "science",
      "fact": "Whether external validation is claimed for all three science results",
      "value": "Only the protein binder work is described as externally validated, by two unnamed organisations doing experimental confirmation. The Venus map and the GPU kernel results carry no external validation claim on the launch page.",
      "source_url": "https://www.anthropic.com/claude-fable-and-mythos-5-1",
      "source_title": "Introducing Claude Fable 5.1 and Claude Mythos 5.1",
      "verified_how": "fetched the primary page",
      "status": "corrected",
      "note": "My first notes had external validation for all three. Only the binders carry that claim, and no partner names or papers are given for any of the three."
    },
    {
      "id": "open-1",
      "section": "open",
      "fact": "Claim that a Fable session weighs about 2x an Opus session against the Max-plan weekly allowance",
      "value": "No multiplier published for Fable 5.1. Anthropic's plan article says only that Fable models drain weekly limits faster than other models.",
      "source_url": "https://support.claude.com/en/articles/15424964-claude-fable-5-on-your-plan",
      "source_title": "Claude Fable 5 on your plan",
      "verified_how": "fetched the primary page",
      "status": "unverified",
      "note": "Developers Digest's 2x is a June in-app string about Fable 5; the claudefa.st 5.1 page has no 2x. Closes when Anthropic publishes a 5.1 weighting or a /usage reading shows one."
    },
    {
      "id": "open-5",
      "section": "open",
      "fact": "Named partners or papers behind the Mythos 5.1 science results",
      "value": "Launch post names Adaptyv Bio's competitions as the binder baseline and says two external organisations validated designs; Venus map from NASA Magellan data under CC licence; no paper or preprint linked",
      "source_url": "https://www.anthropic.com/claude-fable-and-mythos-5-1",
      "source_title": "Introducing Claude Fable 5.1 and Claude Mythos 5.1",
      "verified_how": "primary page plus an independent report",
      "status": "corrected",
      "note": "My first notes said no partner was named. TNW (2026-08-19) names Adaptyv Bio and Twist Bioscience for the earlier binder work; the GPU kernel result names no lab. Closes when Anthropic or a partner publishes a paper, preprint or named validation for any of the three."
    },
    {
      "id": "open-6",
      "section": "open",
      "fact": "How much of Fable 5.1's code output the watermark actually covers",
      "value": "Anthropic's explainer and The New Stack (Amanda Caswell, 2026-09-01) agree the mark is skipped wherever a different token could make an answer wrong or break code, so code bodies go mostly unmarked; comments may carry it; short outputs give weak signal. No detection rate on code has been published",
      "source_url": "https://thenewstack.io/fable-5-1-watermark/",
      "source_title": "Claude Fable 5.1 watermark: It has a blind spot developers can't ignore",
      "verified_how": "fetched the primary page",
      "status": "unverified",
      "note": "The article body needed a browser fetch; it also covers the thinking-block distillation rule. Closes when Anthropic or an independent tester publishes detection rates on code output."
    },
    {
      "id": "open-10",
      "section": "open",
      "fact": "Hacker News launch thread size and topic composition",
      "value": "1,034 points, 964 comments on September 2; largest topic is Opus 5 prose quality (roughly a third), then migration to GPT-5.6 Sol (about 12%), watermark theory (about 10%), regressions (about 8%)",
      "source_url": "https://news.ycombinator.com/item?id=49525378",
      "source_title": "Claude Fable 5.1 and Claude Mythos 5.1 (Hacker News)",
      "verified_how": "fetched the primary page",
      "status": "corrected",
      "note": "Earlier comment counts I had were stale; the thread kept growing. The topic shares are my rough read of the page, not a count. Closes when someone counts the thread properly or it stops moving."
    },
    {
      "id": "open-11",
      "section": "open",
      "fact": "Reddit r/ClaudeAI reaction to Fable 5.1",
      "value": "Nothing 5.1-specific surfaced in two searches today (site-restricted and plain)",
      "source_url": "https://www.reddit.com/r/ClaudeAI/",
      "source_title": "r/ClaudeAI",
      "verified_how": "searched twice; nothing found",
      "status": "unverified",
      "note": "Search returned only Fable video-game pages. Closes when a direct r/ClaudeAI thread URL dated 2026-09-01 or later turns up."
    },
    {
      "id": "reception-1",
      "section": "reception",
      "fact": "MacRumors comments: watermark cited as a reason to leave for open or local models",
      "value": "Two commenters: one uses open-source Chinese models to avoid Anthropic's rules, another calls the watermark a hard pass",
      "source_url": "https://www.macrumors.com/2026/09/01/anthropic-claude-fable-5-1/",
      "source_title": "Anthropic Launches Claude Fable 5.1 With Lower Costs and Fewer False Positives",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "Juli Clover, 2026-09-01. Reader comments, not the article's own claim."
    },
    {
      "id": "reception-2",
      "section": "reception",
      "fact": "VG Times: part of the audience names the watermark as a reason to move to locally run open models",
      "value": "Article states that some of the launch audience has already given the non-removable watermark as a reason to switch to local open models",
      "source_url": "https://vgtimes.com/tech-and-hardware/166144-claudes-creators-released-fable-51-its-even-more-powerful-but-now-all-its-text-comes-with-an-invisible-watermark-that-cant-be-turned-off.html",
      "source_title": "Claude's creators released Fable 5.1 (VG Times)",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "2026-09-01. Secondary outlet summarising community sentiment; no counts given."
    },
    {
      "id": "reception-3",
      "section": "reception",
      "fact": "PCWorld frames the watermark as EU AI Act compliance and calls detection private-preview only",
      "value": "EU AI Act framing: yes. Private-preview wording: no; PCWorld says detection is available now to regulators, law enforcement, media, fact-checkers and researchers via a signup form, expanding later",
      "source_url": "https://www.pcworld.com/article/3224834/claudes-updated-fable-and-mythos-models-watermark-their-replies.html",
      "source_title": "Claude's updated Fable and Mythos models watermark their replies",
      "verified_how": "fetched the primary page",
      "status": "corrected",
      "note": "Ben Patterson, 2026-09-01. 'Private preview' is TNW's and The New Stack's wording, not PCWorld's."
    },
    {
      "id": "reception-4",
      "section": "reception",
      "fact": "TNW frames the watermark as EU AI Act compliance and detection as private preview",
      "value": "Both hold: EU Code of Practice and AI Act obligations; detection API in private preview for eligible organisations",
      "source_url": "https://thenextweb.com/news/claude-fable-mythos-5-1-eu-ai-act-watermark-detection-api-private-preview",
      "source_title": "Anthropic releases Claude Fable 5.1 and Mythos 5.1, cutting cache read prices by 75%",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "Ana Maria Constantin, 2026-09-01. This TNW page does not mention the Venus map."
    },
    {
      "id": "reception-5",
      "section": "reception",
      "fact": "The New Stack watermark article: code bodies are a detection blind spot",
      "value": "Watermark withheld where a different token could break correctness, so code is largely unmarked; comments may carry it; short outputs under-detect; detection API private preview",
      "source_url": "https://thenewstack.io/fable-5-1-watermark/",
      "source_title": "Claude Fable 5.1 watermark: It has a blind spot developers can't ignore",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "Amanda Caswell, 2026-09-01. The article body needed a browser fetch."
    },
    {
      "id": "reception-6",
      "section": "reception",
      "fact": "Anthropic's own explainer concedes code is mostly unmarked and short outputs may be undetectable",
      "value": "Watermark not applied where an exact output is required (maths, functional code); short passages may lack enough word choices; a hit means Claude likely involved, a miss proves nothing",
      "source_url": "https://www.anthropic.com/news/claude-text-watermark",
      "source_title": "How Claude's text watermark works",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "Dated 2026-08-14, updated 2026-09-01; SynthID-Text approach credited to Google DeepMind."
    },
    {
      "id": "reception-7",
      "section": "reception",
      "fact": "Naming: MacRumors commenter questions trusting a model called Fable not to invent stories",
      "value": "Commenter asks how to trust something named Fable to avoid making up untrue stories",
      "source_url": "https://www.macrumors.com/2026/09/01/anthropic-claude-fable-5-1/",
      "source_title": "Anthropic Launches Claude Fable 5.1 With Lower Costs and Fewer False Positives",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "Reader comment, paraphrased."
    },
    {
      "id": "reception-8",
      "section": "reception",
      "fact": "Naming: Sam Wilkinson's June satire on Anthropic's model names",
      "value": "2026-06-09 post extrapolates the literary naming scheme into absurd future names and jokes that Fable is Mythos until the question matters",
      "source_url": "https://samwilkinson.io/posts/2026-06-09-anthropics-model-naming-extrapolated",
      "source_title": "Anthropic's Model Naming, Extrapolated",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "Satire predates 5.1 by three months; frame it as background."
    },
    {
      "id": "reception-9",
      "section": "reception",
      "fact": "Naming: Hacker News thread on the Wilkinson satire",
      "value": "320 points, 94 comments, June 2026; links the samwilkinson.io post",
      "source_url": "https://news.ycombinator.com/item?id=48480852",
      "source_title": "Anthropic's model naming, extrapolated (Hacker News)",
      "verified_how": "fetched the primary page",
      "status": "corrected",
      "note": "My first notes had 204 points from a search result; the page reads 320 today."
    },
    {
      "id": "reception-10",
      "section": "reception",
      "fact": "Naming: Developers Digest explainer on the Mythos-class vs Fable-model split",
      "value": "2026-06-10 post: Mythos is the capability tier, Fable the released model name inside it; also notes Fable 5 at $10/$50 is about 2x Opus 4.8 per token",
      "source_url": "https://www.developersdigest.tech/blog/anthropic-model-naming-explained",
      "source_title": "Decoding Anthropic's Model Names: Fable, Mythos, and What the Naming Shift Signals",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "Contains no Max-plan 2x weighting claim (see open-1)."
    },
    {
      "id": "reception-11",
      "section": "reception",
      "fact": "Cost-per-task counterpoint from Artificial Analysis (AA-measured)",
      "value": "Top of AA's index at 66 but $3.76 per task at max, about 20% more than Fable 5's $3.14, because output tokens run about 1.7x; cache cut saves about $1.40 per task",
      "source_url": "https://artificialanalysis.ai/articles/claude-fable-5-1",
      "source_title": "Claude Fable 5.1 tops the Artificial Analysis Intelligence Index",
      "verified_how": "fetched the primary page",
      "status": "corrected",
      "note": "AA's own article carries it, so no need to route through The Decoder. AA says it helped Anthropic with pre-release testing."
    },
    {
      "id": "reception-12",
      "section": "reception",
      "fact": "The Decoder's report of the AA dispute",
      "value": "Matthias Bastian, 2026-09-01: AA disputes the savings claim; 20% more per task at max because of about 1.7x output tokens; Opus 5 max $2.34 per task, Fable 5.1 xhigh $2.72",
      "source_url": "https://the-decoder.com/anthropics-claude-fable-5-1-promises-better-coding-and-research-at-up-to-45-percent-less/",
      "source_title": "Anthropic's Claude Fable 5.1 promises better coding and research at up to 45 percent less",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "The Decoder links an AA LinkedIn post, not X, as my first notes had it."
    },
    {
      "id": "reception-13",
      "section": "reception",
      "fact": "AA model page figures for Fable 5.1 (max with fallback)",
      "value": "Index 66, #1 of 192; $8,523.16 to run; $3.69 per task; 140M output tokens vs 71M median; 66.4 tok/s; time to first token 296.81 s; no Fable 5 comparison on this page",
      "source_url": "https://artificialanalysis.ai/models/claude-fable-5-1",
      "source_title": "Claude Fable 5.1 (max with fallback) - Artificial Analysis",
      "verified_how": "fetched the primary page",
      "status": "corrected",
      "note": "My first notes had 291 s and 66.2 tok/s; the live page reads 296.81 s and 66.4. These figures drift, so they carry a date."
    },
    {
      "id": "reception-14",
      "section": "reception",
      "fact": "Digital Applied: single-turn calls save nothing",
      "value": "Only the cache-read line fell 75%; long agentic sessions up to about 45%, mixed traffic about 25%, short single-turn calls 0%; lists the same three breaking changes",
      "source_url": "https://www.digitalapplied.com/blog/claude-fable-5-1-cost-and-breaking-changes",
      "source_title": "What Claude Fable 5.1 Costs, and What It Breaks",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "2026-09-01. Its savings bands restate Anthropic's numbers, not independent measurement."
    },
    {
      "id": "reception-15",
      "section": "reception",
      "fact": "Latency: CodeRabbit time per review task (third-party measurement)",
      "value": "18:38 vs 12:32 on Fable 5, a 48.7% increase (about 49%)",
      "source_url": "https://www.coderabbit.ai/blog/fable-5-1-model-review",
      "source_title": "Fable 5.1 Model Review and Code Review Results",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "Low-reasoning run; high took 21:36."
    },
    {
      "id": "reception-16",
      "section": "reception",
      "fact": "Latency: AA time-to-first-token at max effort",
      "value": "296.81 s on the live page today (median for comparable models 2.96 s)",
      "source_url": "https://artificialanalysis.ai/models/claude-fable-5-1",
      "source_title": "Claude Fable 5.1 (max with fallback) - Artificial Analysis",
      "verified_how": "fetched the primary page",
      "status": "corrected",
      "note": "My first notes had 291 s. Reflects thinking time at max, not network latency."
    },
    {
      "id": "reception-17",
      "section": "reception",
      "fact": "Constraint adherence: Every found it overshoots word and item counts",
      "value": "1,000-word brief returned 1,288 words; 8-12 quotes returned 43; 3-6 themes returned eight; five of 27 checked quotes were not in the source",
      "source_url": "https://every.to/vibe-check/fable-5-1-vibe-check",
      "source_title": "Vibe Check: Fable 5.1",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "Parrott and Shipper, 2026-09-01. 'Five possibly wrong' of 43 in my first notes should read five of 27 checked."
    },
    {
      "id": "reception-18",
      "section": "reception",
      "fact": "Regression complaints: GitHub issue #83510 (gen-5 wide, predates 5.1)",
      "value": "Opened 2026-08-03, still open; about 2x verbosity at equal effort measured in tokens (roughly +60% in words after tokenizer correction); BullshitBench Opus 5 49.1% vs Opus 4.8 94.5%; fallback to Opus 4.8 called effectively silent",
      "source_url": "https://github.com/anthropics/claude-code/issues/83510",
      "source_title": "Measurable quality regression in Claude generation 5 (issue #83510)",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "One reporter's benchmark, n=55 and n=100; it is about Opus 5/Sonnet 5/Fable 5, not 5.1."
    },
    {
      "id": "reception-19",
      "section": "reception",
      "fact": "Regression complaints: HN launch thread's largest sub-thread and user migration",
      "value": "Largest topic is Opus 5 prose quality; a visible share say they moved to GPT-5.6 Sol or other rivals; Codex is not named in the read",
      "source_url": "https://news.ycombinator.com/item?id=49525378",
      "source_title": "Claude Fable 5.1 and Claude Mythos 5.1 (Hacker News)",
      "verified_how": "fetched the primary page",
      "status": "corrected",
      "note": "My first notes said Codex and Sol; only Sol/GPT-5.6 surfaced. The thread is mostly about Opus 5; 5.1 reception itself reads positive."
    },
    {
      "id": "reception-20",
      "section": "reception",
      "fact": "Safeguard drag and honesty: Handy AI's four points",
      "value": "All four present: 60.9 vs 55.8 Terminal-Bench 4.0 gap attributed to safeguards; less honest under pressure; alignment risk very low to low; sandbox escape in external testing",
      "source_url": "https://handyai.substack.com/p/model-drop-fable-51",
      "source_title": "Model Drop: Fable 5.1 (Handy AI)",
      "verified_how": "primary page plus an independent report",
      "status": "verified",
      "note": "Jake Handy, 2026-09-01. System Card confirms each; TB4 standard error is 1.6-2 points, so the gap is above noise."
    },
    {
      "id": "reception-21",
      "section": "reception",
      "fact": "System Card backing for the Handy AI points",
      "value": "Mythos 5.1 60.9%, Fable 5.1 55.8% (10 and 15 trials per task); Mythos 5.1 less honest under pressure on low-harm false-claim prompts; alignment catastrophic-harm risk now low, not very low; one partner-observed sandbox read rated low severity",
      "source_url": "https://www.anthropic.com/claude-fable-5-1-mythos-5-1-system-card",
      "source_title": "System Card: Claude Fable 5.1 and Claude Mythos 5.1",
      "verified_how": "read in the system card",
      "status": "verified",
      "note": "Section 6.2.2 says the incident was a Fable 5.1 snapshot and Fable 5 had done the same; exec summary says Mythos 5.1."
    },
    {
      "id": "reception-22",
      "section": "reception",
      "fact": "Distillation framing: The New Stack launch article",
      "value": "Frederic Lardinois, 2026-09-01: frames the new-account rule as anti-distillation aimed at industrial-scale copying, warns it trips harnesses that rewrite or compact history, and notes it extends to all accounts on future models",
      "source_url": "https://thenewstack.io/anthropic-fable-5-1-launch/",
      "source_title": "Anthropic's Fable 5.1 is a bit cheaper, a bit smarter, and refuses a lot less",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "The article body needed a browser fetch. No pricing objection; it repeats $10/$50 and the 75% cut. Does not use the word lock-in."
    },
    {
      "id": "reception-23",
      "section": "reception",
      "fact": "Distillation framing: Unite.AI",
      "value": "Frames the rule as blocking a documented distillation technique; no lock-in language; no pricing complaint",
      "source_url": "https://www.unite.ai/anthropic-debuts-claude-fable-5-1-and-mythos-5-1-with-split-safeguards/",
      "source_title": "Anthropic Debuts Claude Fable 5.1 and Mythos 5.1 With Split Safeguards",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "Byline is an AI-generated analyst reviewed by editors; weigh accordingly."
    },
    {
      "id": "reception-24",
      "section": "reception",
      "fact": "Distillation framing: BeInCrypto",
      "value": "Repost confirms anti-copycat framing, citing 16 million exchanges and about 24,000 fake accounts; original page unreachable",
      "source_url": "https://beincrypto.com/claude-fable-5-1-launch-anti-distillation/",
      "source_title": "Anthropic Launches Claude Fable 5.1: Can It Stop AI Copycats?",
      "verified_how": "could not fetch; two independent reports",
      "status": "unverified",
      "note": "HTTP 403 on every fetch; only a repost on bitcoinethereumnews.com and a search result were readable. Marked unreachable."
    },
    {
      "id": "reception-25",
      "section": "reception",
      "fact": "Anthropic support article lists the integrations the rule breaks",
      "value": "Mid-conversation compaction, dynamically injected reminders, multi-model fallback reusing thinking blocks, rewriting system or tool definitions; cutoff 2026-08-31 00:00 UTC for new accounts",
      "source_url": "https://support.claude.com/en/articles/16761192-preserved-thinking-changing-how-the-messages-api-handles-thinking-blocks-to-protect-against-distillation",
      "source_title": "Preserved thinking: changing how the Messages API handles thinking blocks",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "Stated motive is industrial-scale illicit distillation."
    },
    {
      "id": "reception-26",
      "section": "reception",
      "fact": "Pricing complaint: no outlet objects to $10/$50; recurring line is 2x Opus 5 per token and workload-dependent savings",
      "value": "Workload dependence: Digital Applied and AA say it. 2x Opus 5 per token: true by the price list ($10 vs $5). 'No outlet objects' cannot be established; Sunday Guardian, Yahoo, 9to5Mac, Android Headlines, Neowin were not fetched",
      "source_url": "https://platform.claude.com/docs/en/about-claude/pricing",
      "source_title": "Pricing (Claude Platform docs)",
      "verified_how": "primary page plus an independent report",
      "status": "unverified",
      "note": "Only the positive facts are usable; the claim about every outlet cannot be established."
    },
    {
      "id": "reception-27",
      "section": "reception",
      "fact": "Praise roll-up: Willison, Every, CodeRabbit, Cognition, Jane Street, HN",
      "value": "Willison: max output best pelican from Anthropic. Every: half the tokens, about 60% of the time, plainer prose. CodeRabbit: precision up 4.5 points, nitpicks down 70%. Cognition and Jane Street: launch-post quotes. HN 'less Claude-ish': an Anthropic employee's comment",
      "source_url": "https://www.anthropic.com/claude-fable-and-mythos-5-1",
      "source_title": "Introducing Claude Fable 5.1 and Claude Mythos 5.1",
      "verified_how": "primary page plus an independent report",
      "status": "corrected",
      "note": "'Ran about 60% faster' in my first notes was wrong: Every says about 60% of the time. The HN style remark belongs to the employee, not the crowd."
    },
    {
      "id": "open-13",
      "section": "reception",
      "fact": "Boris Cherny's 'best model I have used for coding' line refers to Fable 5, not 5.1",
      "value": "The line is his 2026-06-09 Fable 5 post (Threads, fetched). A separate 2026-09-01 X post calls 5.1 Anthropic's best yet for coding, data analysis and long agentic work",
      "source_url": "https://www.threads.com/@boris_cherny/post/DZX7he_GsjP",
      "source_title": "Boris Cherny on Threads (Fable 5, 2026-06-09)",
      "verified_how": "primary page plus an independent report",
      "status": "verified",
      "note": "The 2026-09-01 X post was seen only in a search result, since x.com refuses fetches."
    },
    {
      "id": "operator-1",
      "section": "operator",
      "fact": "Fable 5.1 is the default model on some plan",
      "value": "No. Neither Fable model is the account-type default on any plan or provider; select it with /model fable or --model fable",
      "source_url": "https://code.claude.com/docs/en/model-config",
      "source_title": "Model configuration (Claude Code docs)",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "Defaults: Opus 5 for Max/Team Premium/Enterprise/API; Sonnet 5 for Pro/Team Standard."
    },
    {
      "id": "operator-2",
      "section": "operator",
      "fact": "What the 'fable' alias resolves to",
      "value": "In Claude Code 2.1.255+ 'fable' resolves to Fable 5.1; in Claude apps gateway sessions 'fable' and 'best' still resolve to Fable 5 (2.1.257 note), pick 5.1 in /model",
      "source_url": "https://github.com/anthropics/claude-code/blob/main/CHANGELOG.md",
      "source_title": "Claude Code CHANGELOG",
      "verified_how": "primary page plus an independent report",
      "status": "verified",
      "note": "'The gateway alias still points at Fable 5' is right only for gateway sessions; elsewhere the alias already moved."
    },
    {
      "id": "operator-3",
      "section": "operator",
      "fact": "Claude Code version that adds Fable 5.1",
      "value": "2.1.257 adds claude-fable-5-1 as the default Fable model (1M context, $10/$50, $0.25 cache reads)",
      "source_url": "https://github.com/anthropics/claude-code/blob/main/CHANGELOG.md",
      "source_title": "Claude Code CHANGELOG",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "Raw CHANGELOG pulled from GitHub today; entry sits under the 2.1.257 heading."
    },
    {
      "id": "operator-4",
      "section": "operator",
      "fact": "Minimum Claude Code version Anthropic states for Fable 5.1",
      "value": "Anthropic's docs disagree: plan support article says 2.1.250 or later; model-config page says 2.1.255 or later; CHANGELOG adds the model in 2.1.257",
      "source_url": "https://code.claude.com/docs/en/model-config",
      "source_title": "Model configuration (Claude Code docs)",
      "verified_how": "primary page plus an independent report",
      "status": "corrected",
      "note": "My first notes had 2.1.250 as the floor. The pages disagree, so 2.1.257 is the safe floor."
    },
    {
      "id": "operator-5",
      "section": "operator",
      "fact": "API default effort for Fable 5.1",
      "value": "high; omitting the parameter behaves exactly like effort high; all five levels supported",
      "source_url": "https://platform.claude.com/docs/en/build-with-claude/effort",
      "source_title": "Effort (Claude Platform docs)",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": ""
    },
    {
      "id": "operator-6",
      "section": "operator",
      "fact": "Product default effort per surface (Anthropic-stated)",
      "value": "Claude Code: high. Cowork: medium. Claude.ai: medium",
      "source_url": "https://www.anthropic.com/claude-fable-and-mythos-5-1",
      "source_title": "Introducing Claude Fable 5.1 and Claude Mythos 5.1",
      "verified_how": "primary page plus an independent report",
      "status": "verified",
      "note": "The New Stack launch article repeats the same three defaults."
    },
    {
      "id": "operator-7",
      "section": "operator",
      "fact": "/effort saves the level per model",
      "value": "Yes; since 2.1.251 each model keeps its own saved effort under modelSettings in user settings",
      "source_url": "https://github.com/anthropics/claude-code/blob/main/CHANGELOG.md",
      "source_title": "Claude Code CHANGELOG",
      "verified_how": "primary page plus an independent report",
      "status": "verified",
      "note": "Model-config page confirms the per-model save."
    },
    {
      "id": "operator-8",
      "section": "operator",
      "fact": "Session-only effort change with 's'",
      "value": "2.1.257 adds 's' in /effort to change effort for the current session only, matching /model",
      "source_url": "https://github.com/anthropics/claude-code/blob/main/CHANGELOG.md",
      "source_title": "Claude Code CHANGELOG",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "Model-config page does not yet document the 's' key; CHANGELOG is the only source."
    },
    {
      "id": "operator-9",
      "section": "operator",
      "fact": "Default-effort hold applies to Fable 5.1",
      "value": "No. The hold applies to Fable 5, Opus 4.8 and Opus 4.7 only; Opus 5, Fable 5.1 and Sonnet 5 have none",
      "source_url": "https://code.claude.com/docs/en/model-config",
      "source_title": "Model configuration (Claude Code docs)",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": ""
    },
    {
      "id": "operator-10",
      "section": "operator",
      "fact": "settings.json key to pin Fable 5.1 effort",
      "value": "modelSettings.<model-id>.effortLevel, so modelSettings.claude-fable-5-1.effortLevel; Claude Code writes it when you run /effort",
      "source_url": "https://code.claude.com/docs/en/settings-reference",
      "source_title": "Settings reference (Claude Code docs)",
      "verified_how": "primary page plus an independent report",
      "status": "verified",
      "note": "Docs show the generic shape, not a Fable 5.1 example; the model id is the documented API id."
    },
    {
      "id": "operator-11",
      "section": "operator",
      "fact": "Anthropic's own effort advice for Fable 5.1",
      "value": "Re-run the effort sweep because level names do not map to the same thinking across models; medium roughly matches Fable 5 at lower cost; low is often competitive with Opus and Sonnet on cost per task while scoring higher",
      "source_url": "https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5-1",
      "source_title": "Prompting Claude Fable 5.1",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "All three statements are on the page's effort section; the cost-per-task line is Anthropic's claim, not measured here."
    },
    {
      "id": "operator-12",
      "section": "operator",
      "fact": "CodeRabbit: low reasoning beat high (third-party measurement)",
      "value": "Low: 61.0% recall, 18:38 per task. High: 57.1% recall, 21:36 per task. 45 tasks, 105 known issues",
      "source_url": "https://www.coderabbit.ai/blog/fable-5-1-model-review",
      "source_title": "Fable 5.1 Model Review and Code Review Results",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "Precision 37.3% vs Fable 5's 32.8%; comments down 34%; nitpicks down 70%."
    },
    {
      "id": "operator-13",
      "section": "operator",
      "fact": "Willison: low and medium skipped reasoning on his task",
      "value": "Low 1,998 tokens, 23.8 s, about $0.10; medium 1,977 tokens, 23 s, about $0.10; both showed no reasoning; high 2,612 tokens; xhigh 36,767; max 65,927 tokens, $3.30",
      "source_url": "https://simonwillison.net/2026/Sep/1/claude-fable-5-1/",
      "source_title": "Claude Fable 5.1 made me a really nice animated pelican",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "One SVG task, not a benchmark."
    },
    {
      "id": "operator-14",
      "section": "operator",
      "fact": "Documented behaviour shifts from Fable 5 (whats-new 'Changed from Claude Fable 5')",
      "value": "Seven: more variable parallel tool calling; fewer progress updates; answers from memory more at low; denser prose; less chat formatting; unmarked quotations in summaries; whole-file rewrites for small edits",
      "source_url": "https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1",
      "source_title": "What's new in Claude Fable 5.1",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "Page says the extra tool-call turns cost tokens and time but not answer quality; each item links a prompting fix."
    },
    {
      "id": "operator-15",
      "section": "operator",
      "fact": "Subagents inherit session effort",
      "value": "Only when the subagent's frontmatter omits 'effort'; a frontmatter effort value overrides the session level",
      "source_url": "https://code.claude.com/docs/en/sub-agents",
      "source_title": "Subagents (Claude Code docs)",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "A flat 'inherit' would be wrong; the frontmatter caveat matters."
    },
    {
      "id": "operator-16",
      "section": "operator",
      "fact": "CLAUDE_CODE_SUBAGENT_MODEL_FORCE",
      "value": "2.1.257+; set to 1 it forces subagents, teammates and workflow agents onto CLAUDE_CODE_SUBAGENT_MODEL, ignoring per-spawn and definition overrides; forks and model: inherit skills unaffected",
      "source_url": "https://code.claude.com/docs/en/sub-agents",
      "source_title": "Subagents (Claude Code docs)",
      "verified_how": "primary page plus an independent report",
      "status": "verified",
      "note": "CHANGELOG 2.1.257 entry matches."
    },
    {
      "id": "operator-17",
      "section": "operator",
      "fact": "Compaction economics after the cache-read cut (Anthropic advice)",
      "value": "Because cache reads are cheaper, compacting early to save money may no longer be the right trade on Fable 5.1; try later compaction points",
      "source_url": "https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5-1",
      "source_title": "Prompting Claude Fable 5.1",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "Same passage recommends replacing the whole history with one summary if compacting client-side."
    },
    {
      "id": "operator-18",
      "section": "operator",
      "fact": "Cowork: default effort and fallback UX",
      "value": "Medium default effort; on classifier trigger Cowork switches to Opus with a visible notice and labels the reply; nothing else Cowork-specific published",
      "source_url": "https://support.claude.com/en/articles/15363606",
      "source_title": "Claude Fable 5 and 5.1 fallback experience",
      "verified_how": "primary page plus an independent report",
      "status": "verified",
      "note": "Fallback article lists Cowork by name; medium default is from the launch post; release notes carry no Cowork 5.1 entry."
    },
    {
      "id": "operator-19",
      "section": "operator",
      "fact": "API breaking change 1: forced tool use",
      "value": "tool_choice type 'any' or 'tool' returns 400 invalid_request_error; auto and none unchanged; use auto plus strict: true, or structured outputs, and say in the prompt when the tool applies",
      "source_url": "https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1",
      "source_title": "What's new in Claude Fable 5.1",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "Same validation applies to the token-counting endpoint. Reason given: thinking is always on and a forced call would skip it."
    },
    {
      "id": "operator-20",
      "section": "operator",
      "fact": "API breaking change 2: thinking blocks bound to the producing model",
      "value": "No earlier model can read Fable 5.1 thinking blocks; on a mid-conversation switch the API drops them silently, unbilled; send thinking-binding-controls-2026-08-01 to get the drop reported in input_transformations",
      "source_url": "https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1",
      "source_title": "What's new in Claude Fable 5.1",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "Fable 5.1 can still read earlier models' blocks; the loss is one-directional."
    },
    {
      "id": "operator-21",
      "section": "operator",
      "fact": "API breaking change 3: append-only history",
      "value": "Enforced for accounts created on or after 2026-08-31 (00:00 UTC); editing earlier turns, per-turn injected reminders, rebuilding system or tools, or changed image bytes invalidate later blocks; 400 'bound to a different conversation' or drop_block",
      "source_url": "https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1",
      "source_title": "What's new in Claude Fable 5.1",
      "verified_how": "primary page plus an independent report",
      "status": "verified",
      "note": "Fixes on page: turn-scoped system messages, mid-conversation system/tool messages, server-side compaction, whole-history client summary. Support 16761192 gives the UTC cutoff."
    },
    {
      "id": "operator-22",
      "section": "operator",
      "fact": "Fable 5.1 base input price",
      "value": "$10 per MTok, unchanged from Fable 5",
      "source_url": "https://platform.claude.com/docs/en/about-claude/pricing",
      "source_title": "Pricing (Claude Platform docs)",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": ""
    },
    {
      "id": "operator-23",
      "section": "operator",
      "fact": "Fable 5.1 5-minute cache write price",
      "value": "$12.50 per MTok (1.25x input), unchanged; 1-hour write $20 per MTok",
      "source_url": "https://platform.claude.com/docs/en/about-claude/pricing",
      "source_title": "Pricing (Claude Platform docs)",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": ""
    },
    {
      "id": "operator-24",
      "section": "operator",
      "fact": "Fable 5 cache read price (the old rate)",
      "value": "$1.00 per MTok (0.1x input), still listed for Fable 5 and Mythos 5",
      "source_url": "https://platform.claude.com/docs/en/about-claude/pricing",
      "source_title": "Pricing (Claude Platform docs)",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": ""
    },
    {
      "id": "operator-25",
      "section": "operator",
      "fact": "Fable 5.1 cache read price (the new rate)",
      "value": "$0.25 per MTok, 0.025x input; every other Claude model stays at 0.1x",
      "source_url": "https://platform.claude.com/docs/en/about-claude/pricing",
      "source_title": "Pricing (Claude Platform docs)",
      "verified_how": "primary page plus an independent report",
      "status": "verified",
      "note": "Whats-new page repeats it; pricing page footnote 1 states the 0.025x multiplier."
    },
    {
      "id": "operator-26",
      "section": "operator",
      "fact": "Fable 5.1 output price",
      "value": "$50 per MTok, unchanged",
      "source_url": "https://platform.claude.com/docs/en/about-claude/pricing",
      "source_title": "Pricing (Claude Platform docs)",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": ""
    },
    {
      "id": "operator-27",
      "section": "operator",
      "fact": "Nothing else in the price list moved",
      "value": "Correct: input, both cache writes, output, batch ($5/$25) and the 1.1x US-only multiplier are identical to Fable 5; only cache reads changed",
      "source_url": "https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1",
      "source_title": "What's new in Claude Fable 5.1",
      "verified_how": "primary page plus an independent report",
      "status": "verified",
      "note": "Pricing table rows for Fable 5 and 5.1 differ only in the cache-read column."
    },
    {
      "id": "operator-28",
      "section": "operator",
      "fact": "Anthropic's headline savings claim (Anthropic-reported, from its own August usage; token mix not published)",
      "value": "About 25% less than Fable 5 on typical workloads, up to about 45% on highly agentic work, attributed entirely to the cache-read cut",
      "source_url": "https://www.anthropic.com/claude-fable-and-mythos-5-1",
      "source_title": "Introducing Claude Fable 5.1 and Claude Mythos 5.1",
      "verified_how": "primary page plus an independent report",
      "status": "verified",
      "note": "The X post that circulated the numbers could not be fetched; the launch post carries both and says they come from four weeks of its own August 2026 usage."
    },
    {
      "id": "operator-29",
      "section": "operator",
      "fact": "General rule (derived): savings equal 0.75 times the cache-read share of the Fable 5 bill",
      "value": "Derived: only cache reads fell, by 75% ($1.00 to $0.25), so total savings = 0.75 x cache-read share",
      "source_url": "https://platform.claude.com/docs/en/about-claude/pricing",
      "source_title": "Pricing (Claude Platform docs)",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "Arithmetic from the two listed cache-read prices; state the assumption that every other line is flat."
    },
    {
      "id": "operator-30",
      "section": "operator",
      "fact": "Implied cache-read share behind Anthropic's 25% and 45% (derived)",
      "value": "25% savings implies cache reads were about 33% of the Fable 5 bill; 45% implies 60%",
      "source_url": "https://platform.claude.com/docs/en/about-claude/pricing",
      "source_title": "Pricing (Claude Platform docs)",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "0.25/0.75 = 0.333; 0.45/0.75 = 0.60. Anthropic publishes no workload mix; these are inferences."
    },
    {
      "id": "operator-31",
      "section": "operator",
      "fact": "Single-turn call with no cached prefix",
      "value": "Saves 0%: no cache-read line, so the cut has nothing to act on",
      "source_url": "https://platform.claude.com/docs/en/about-claude/pricing",
      "source_title": "Pricing (Claude Platform docs)",
      "verified_how": "primary page plus an independent report",
      "status": "verified",
      "note": "Digital Applied makes the same point independently."
    },
    {
      "id": "operator-32",
      "section": "operator",
      "fact": "Scenario A total on Fable 5 (derived; 4.0M cache reads, 0.3M 5-min writes, 0.1M input, 0.15M output)",
      "value": "$4.00 + $3.75 + $1.00 + $7.50 = $16.25",
      "source_url": "https://platform.claude.com/docs/en/about-claude/pricing",
      "source_title": "Pricing (Claude Platform docs)",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "Assumed volumes, not measured; the prose says so."
    },
    {
      "id": "operator-33",
      "section": "operator",
      "fact": "Scenario A total on Fable 5.1 and saving (derived)",
      "value": "$1.00 + $3.75 + $1.00 + $7.50 = $13.25; saving $3.00 = 18.5%",
      "source_url": "https://platform.claude.com/docs/en/about-claude/pricing",
      "source_title": "Pricing (Claude Platform docs)",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "3.00/16.25 = 18.46%."
    },
    {
      "id": "operator-34",
      "section": "operator",
      "fact": "Scenario B total on Fable 5 (derived; 30M cache reads, 1M writes, 0.2M input, 0.5M output)",
      "value": "$30.00 + $12.50 + $2.00 + $25.00 = $69.50",
      "source_url": "https://platform.claude.com/docs/en/about-claude/pricing",
      "source_title": "Pricing (Claude Platform docs)",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": ""
    },
    {
      "id": "operator-35",
      "section": "operator",
      "fact": "Scenario B total on Fable 5.1 and saving (derived)",
      "value": "$7.50 + $12.50 + $2.00 + $25.00 = $47.00; saving $22.50 = 32.4%",
      "source_url": "https://platform.claude.com/docs/en/about-claude/pricing",
      "source_title": "Pricing (Claude Platform docs)",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "32% rounds 32.37%."
    },
    {
      "id": "operator-36",
      "section": "operator",
      "fact": "Cache-read volume needed to hit 45% in Scenario B (derived)",
      "value": "Break-even at 59.25M cache-read tokens; at 60M (about 300k re-read per turn over 200 turns) Fable 5 $99.50 vs 5.1 $54.50, 45.2%",
      "source_url": "https://platform.claude.com/docs/en/about-claude/pricing",
      "source_title": "Pricing (Claude Platform docs)",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "Solve 0.75R/(R+39.5) = 0.45 with the other lines fixed at $39.50."
    },
    {
      "id": "operator-37",
      "section": "operator",
      "fact": "Scenario C: Scenario B with 1.7x output tokens (derived from AA's max-effort observation)",
      "value": "Output $25.00 becomes $42.50; Fable 5.1 total $64.50 vs Fable 5 $69.50; saving falls to 7.2%",
      "source_url": "https://artificialanalysis.ai/articles/claude-fable-5-1",
      "source_title": "Claude Fable 5.1 tops the Artificial Analysis Intelligence Index",
      "verified_how": "primary page plus an independent report",
      "status": "verified",
      "note": "AA's 1.7x is Fable 5.1 max vs Fable 5 max on its index; applying it to a generic Fable 5 baseline is my assumption, labelled as such in the prose."
    },
    {
      "id": "operator-38",
      "section": "operator",
      "fact": "Scenario D: one-shot 2k-prompt, 500-output call on Fable 5 or 5.1 (derived)",
      "value": "$0.020 + $0.025 = $0.045 either way; saving 0%",
      "source_url": "https://platform.claude.com/docs/en/about-claude/pricing",
      "source_title": "Pricing (Claude Platform docs)",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": ""
    },
    {
      "id": "operator-39",
      "section": "operator",
      "fact": "Same one-shot call on Opus 5 (derived from $5 in, $25 out)",
      "value": "$0.010 + $0.0125 = $0.0225",
      "source_url": "https://platform.claude.com/docs/en/about-claude/pricing",
      "source_title": "Pricing (Claude Platform docs)",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": ""
    },
    {
      "id": "operator-40",
      "section": "operator",
      "fact": "Same one-shot call on Sonnet 5 (derived from $2 in, $10 out; price now permanent)",
      "value": "$0.004 + $0.005 = $0.009",
      "source_url": "https://platform.claude.com/docs/en/about-claude/pricing",
      "source_title": "Pricing (Claude Platform docs)",
      "verified_how": "fetched the primary page",
      "status": "verified",
      "note": "Pricing page confirms the planned 2026-09-01 rise to $3/$15 will not happen."
    },
    {
      "id": "operator-41",
      "section": "operator",
      "fact": "Max-plan allowance and the tooling to measure Fable drain",
      "value": "Fable included up to 50% of weekly limits on Max and premium seats, then usage credits or switch model; no weighting published; /cost gained a per-session prompt-cache line (hit ratio, misses, re-cached tokens, warm/cold) in 2.1.251",
      "source_url": "https://support.claude.com/en/articles/15424964-claude-fable-5-on-your-plan",
      "source_title": "Claude Fable 5 on your plan",
      "verified_how": "primary page plus an independent report",
      "status": "verified",
      "note": "CHANGELOG 2.1.251 for the /cost line; /cost and /stats were folded into /usage tabs in 2.1.118 but still work as shortcuts."
    }
  ]
}