# One Fable 5.1 price moved. The rest of the launch numbers, checked

Robert MacKelfresh · 2026-09-02 · canonical: https://mackelfresh.com/writing/claude-fable-5-1-checked/

Every Claude Fable 5.1 launch number, re-read against Anthropic's pricing page, its docs and the System Card, with a source beside each row and the unsettled claims listed. CSV and JSON included.

## In short

Claude Fable 5.1 is the same model as Claude Mythos 5.1, shipped with tighter safeguards around biology and cybersecurity. One price moved: cache reads fell from $1 to $0.25 per million tokens; input stays at $10 and output at $50. I checked each Anthropic figure against its own pages and listed what I could not settle as open.

Anthropic released Claude Fable 5.1 and Claude Mythos 5.1 on [September 1, 2026](https://www.anthropic.com/claude-fable-and-mythos-5-1). The coverage that followed mostly re-ran the launch post's numbers. I wanted each number beside the document it came from, with a verdict on whether it held, so I built that from the pricing page, the model docs, the support articles and the System Card PDF.

Where coverage and source disagreed, the source won and the row is marked corrected; where I couldn't settle a claim, the row sits under Still open with what would close it. I haven't benchmarked the model. Every performance figure here is Anthropic's or a named third party's, and each row says which.

This is a reference page, not a review. Rows move as evidence arrives, and the changelog records every move.

## What actually moved

Start with what the two names hide. Anthropic's [System Card](https://www.anthropic.com/claude-fable-5-1-mythos-5-1-system-card) describes Fable 5.1 and Mythos 5.1 as the same model: Fable 5.1 carries safeguards that block certain biology and cybersecurity tasks, and Mythos 5.1 runs with more permissive ones. Not removed, more permissive. [Mythos 5.1](https://www.anthropic.com/claude/mythos) goes to vetted organisations through trusted-access programmes; everyone else gets Fable.

The price list changed in exactly one place. Cache reads dropped from $1.00 per million tokens on Fable 5 to $0.25 on Fable 5.1, per the [pricing page](https://platform.claude.com/docs/en/about-claude/pricing). Input is still $10, output $50, a five-minute cache write $12.50, batch $5 in and $25 out. At $10 and $50 it's still twice Opus 5's $5 and $25 per million tokens.

Fable 5.1 takes [all five levels](https://platform.claude.com/docs/en/build-with-claude/effort); leave the parameter out and you get high. Claude Code defaults to high; Cowork and claude.ai default to medium. Anthropic's [prompting guide](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5-1) says to re-run your effort sweep rather than carry settings over, and claims medium roughly matches Fable 5 at lower cost. That's Anthropic's claim.

Nothing switches to Fable 5.1 on its own. It isn't the default on any plan or provider; you choose it with `/model fable`. Interactive sessions ask before billing usage credits; `-p` runs and the Agent SDK don't. Max plans and premium seats include Fable up to half the weekly limit, Pro plans and standard seats run it on pay-as-you-go credits, and the Free plan doesn't get it. Anthropic's pages also disagree on the Claude Code version you need: the [plan article](https://support.claude.com/en/articles/15424964) says 2.1.250, the [model-config page](https://code.claude.com/docs/en/model-config) says 2.1.255, and the [changelog](https://github.com/anthropics/claude-code/blob/main/CHANGELOG.md) adds the model under 2.1.257. Use 2.1.257 or later.

## The cache-read arithmetic

Only one line moved, so the saving depends on how much of your Fable 5 bill was cache reads. The read price fell 75 percent and every other line stayed flat, so the saving equals 0.75 times the cache-read share of the old bill. Anthropic's launch post puts the reduction at around 25 percent on typical workloads and up to roughly 45 percent on complex coding and highly agentic work, and says both figures come from re-pricing four weeks of its own August usage at default effort: typical meaning Enterprise, Claude Code and API traffic, agentic meaning context-heavy tool work where cache reads dominate. Run those backwards and cache reads were about a third of the typical bill and 60 percent of the agentic one. Anthropic publishes no token mix, so the shares are my inference.

The four scenarios below use assumed token volumes chosen to bracket common patterns, not readings from my own sessions. Prices: $10 input, $12.50 five-minute cache write, $1.00 (Fable 5) or $0.25 (Fable 5.1) cache read, $50 output, all per million tokens.

| Scenario | Assumed volumes | Fable 5 | Fable 5.1 | Saving |
|---|---|---|---|---|
| A. A working Claude Code session | 4.0M cache reads, 0.3M cache writes, 0.1M uncached input, 0.15M output | $16.25 | $13.25 | 18.5% |
| B. A long unattended agent run | 30M cache reads, 1M cache writes, 0.2M uncached input, 0.5M output | $69.50 | $47.00 | 32.4% |
| B, pushed to Anthropic's 45 percent | B with 60M cache reads | $99.50 | $54.50 | 45.2% |
| C. B at max effort with 1.7x output | B with 0.85M output on Fable 5.1 | $69.50 | $64.50 | 7.2% |
| D. A one-shot call with no cache | 2,000 input, 500 output | $0.045 | $0.045 | 0% |

Scenario A is 40 turns re-reading a 100k-token cached prefix each, which is the 4.0M; the whole $3.00 saving is the read line falling from $4.00 to $1.00. Scenario B is 200 turns re-reading about 150k each. Anthropic's 45 percent needs roughly 60M cache-read tokens, about 300k re-read on each of 200 turns (break-even 59.25M). Scenario C applies [Artificial Analysis's finding](https://artificialanalysis.ai/articles/claude-fable-5-1) that Fable 5.1 at max effort emitted about 1.7 times the output tokens of Fable 5 at max on its index, which is how AA arrived at 20 percent more per task rather than 45 percent less. Applying that ratio to Scenario B is my assumption; AA measured it on its own benchmark, and discloses pre-release evaluation work for Anthropic. Scenario D, the short single call most automations actually make, costs $0.045 on either Fable, $0.0225 on Opus 5 and $0.009 on Sonnet 5.

On a Max plan none of this shows up in dollars; what changes is how fast Fable eats the half of your weekly limit it's allowed. Anthropic has published no weighting for 5.1, only that Fable models drain limits faster, so the 2x figure that circulates sits in the open table. Claude Code 2.1.251 added a prompt-cache line to `/cost`, and the prompting guide says early compaction to save money may no longer be the right trade.

## Three numbers that need a footnote before you repeat them

**The 85 percent biology figure isn't a Fable 5.1 improvement.** The launch post says biology safeguards fire about 85 percent less often on benign elementary biology and medical requests. Read fast, that's a 5.1 gain. It isn't. Baseline: the classifiers Fable 5 launched with in June. Anthropic [published the rewrite for Fable 5 on August 7](https://www.anthropic.com/news/improving-fable-5-s-biology-safeguards), and Fable 5.1 inherits it. Cyber is the new part: Anthropic reports roughly 60 percent fewer safeguard interventions per Claude Code session, its own number, with no independent measurement I could find.

**Half of coding rollouts falling back to Opus is a benchmark number, not a production one.** On the Gray Swan prompt-injection benchmark the System Card reports, Fable 5.1 fell back to Opus 4.8 in 23 percent of rollouts: about half of coding rollouts, under 10 percent in computer and tool use. Fable 5 fell back 60 percent of the time on the same benchmark. Those are adversarial test runs. That number travels well, so here's the caveat in advance. Anthropic's only published production figure is Fable 5's at launch, [under 5 percent of sessions](https://www.anthropic.com/news/claude-fable-5-mythos-5); there is none for 5.1.

**The benchmark table has error bars.** On Terminal-Bench-Science 0.1, Fable 5.1 posts 52.6 percent against Fable 5's 24.7. Anthropic's footnote puts the standard error at plus or minus 3.5 to 4.5 points for every model. A 28-point gap survives that; the ordering underneath doesn't, since Opus 5's 29.0 and Fable 5's 24.7 are 4.3 points apart. On Terminal-Bench 4.0 the error is 1.6 to 2 points and Fable 5.1's 55.8 leads Opus 5's 52.3 by 3.5, just above noise. Two of the GPT-5.6 Sol figures, the Terminal-Bench pair, weren't run by Anthropic; the System Card takes them from the public leaderboards, scored with OpenAI's Codex CLI at max effort. Fable 5.1 also ran with production safeguards on; on OSWorld 2.0 any task where a safeguard intervened scored zero. Between Mythos 5.1's 60.9 and Fable 5.1's 55.8, the gap is Anthropic's own attribution to those safeguards, not an independent measurement.

## What breaks in an unattended workflow

Forced tool use is gone. A `tool_choice` of type `any` or `tool` now returns a 400; `auto` and `none` still work, per the [what's new page](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1). That's the standard way to guarantee an extraction call. Anthropic's fix is `auto` with `strict: true`, or structured outputs, plus a line in the prompt saying when the tool applies.

Thinking blocks only travel forward. No earlier model can read a Fable 5.1 thinking block, so a router that drops to Opus mid-conversation loses that reasoning; the API drops the blocks silently and doesn't bill them. Send the `thinking-binding-controls-2026-08-01` beta header and the drop is reported in `input_transformations`.

History is append-only for new accounts. Accounts created on or after 2026-08-31 at 00:00 UTC can't edit earlier turns and replay Fable 5.1 thinking blocks, per Anthropic's [preserved-thinking article](https://support.claude.com/en/articles/16761192). That covers per-turn injected reminders, rebuilding the system prompt or tool list mid-session, in-place summarisation and changed image bytes. Anthropic's replacements are turn-scoped system messages, server-side compaction, or a client-side summary that replaces the whole history. Older accounts get the mismatch logged, not enforced, for now.

The what's new page also lists seven behaviour shifts; the two that cost money in a loop are more variable parallel tool calling and whole-file rewrites for small edits. A separate essay will map each of these to the n8n nodes and MCP server patterns they hit, with the failing request and the fix.

## What people are complaining about

The loudest complaint is the watermark. Anthropic says all text from both models carries a statistical watermark applied at model level, SynthID-Text, that encodes nothing about the user. [Its own explainer](https://www.anthropic.com/news/claude-text-watermark) concedes two limits: the mark is withheld wherever a different token could break code or make an answer wrong, so code bodies are mostly unmarked, and short passages may not carry enough signal.

Detection is limited to organisations with duties under EU law; [The Next Web](https://thenextweb.com/news/claude-fable-mythos-5-1-eu-ai-act-watermark-detection-api-private-preview) and [The New Stack](https://thenewstack.io/fable-5-1-watermark/) call that a private preview, while [PCWorld](https://www.pcworld.com/article/3224834/claudes-updated-fable-and-mythos-models-watermark-their-replies.html) says it's open now through a signup form.

On [MacRumors](https://www.macrumors.com/2026/09/01/anthropic-claude-fable-5-1/) one commenter says they already use open-source Chinese models to avoid Anthropic's rules and another calls the watermark a hard pass; [VG Times](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) reports readers naming the watermark as a reason to move to locally run open models. Anthropic itself never says the mark can't be turned off, only that it's applied at model level on every surface. The naming jokes are older: a MacRumors commenter asked how to trust a model called Fable not to make things up, and [Sam Wilkinson's June satire](https://samwilkinson.io/posts/2026-06-09-anthropics-model-naming-extrapolated) is the usual reference.

The regression complaints are mostly about generation 5, not 5.1. [GitHub issue #83510](https://github.com/anthropics/claude-code/issues/83510), opened August 3 and still open, measures about twice the output tokens at equal effort, roughly 60 percent more words once the reporter corrects for the tokenizer, and on the smaller of the reporter's two BullshitBench runs a drop from 94.5 percent on Opus 4.8 to 49.1 on Opus 5. The [Hacker News launch thread](https://news.ycombinator.com/item?id=49525378), 1,034 points and 964 comments on September 2, has Opus 5 prose quality as its largest topic, roughly a third of it by my read, with a visible share saying they've moved to GPT-5.6 Sol; the 5.1 reception inside it reads positive. [CodeRabbit](https://www.coderabbit.ai/blog/fable-5-1-model-review) found precision up from 32.8 to 37.3 percent and nitpicks down 70 percent, but time per review task up 49 percent, and its low-effort run beat high on recall and precision. [Every](https://every.to/vibe-check/fable-5-1-vibe-check) saw about half the tokens of Opus 5, but also a 1,000-word brief that came back at 1,288 words and a request for 8 to 12 quotes that returned 43, five of the 27 checked not present in the source.

## The fact table

Each row carries the fact, the value I settled on, a link to its source, and a Checked column saying how it was checked: fetched, read in the System Card, or confirmed by a second report. Corrected rows say what my first notes or the coverage had wrong. The full set, including the customer statements and the operator and reception rows that have no table here, is downloadable as [CSV](https://mackelfresh.com/data/fable-5-1-facts.csv) and [JSON](https://mackelfresh.com/data/fable-5-1-facts.json).

### Identity, IDs and availability

Model IDs, plan rules and the retirement commitment.

| Fact | Value | Source | Checked |
|---|---|---|---|
| Release date of Claude Fable 5.1 | 2026-09-01 | [Claude Fable 5.1](https://platform.claude.com/docs/en/models/fable-5-1/overview) | fetched the primary page |
| Claude API model ID | claude-fable-5-1; the dateless ID is itself the pinned snapshot and the alias row repeats it | [Models overview](https://platform.claude.com/docs/en/models/overview) | fetched the primary page |
| Claude API model ID for Mythos 5.1 | claude-mythos-5-1 | [What's new in Claude Fable 5.1](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1) | fetched the primary page |
| Amazon Bedrock model ID | anthropic.claude-fable-5-1 | [Models overview](https://platform.claude.com/docs/en/models/overview) | fetched the primary page |
| Google Cloud, Microsoft Foundry and Claude Platform on AWS model ID | claude-fable-5-1 on all three | [Models overview](https://platform.claude.com/docs/en/models/overview) | fetched the primary page |
| Lineage: what Fable 5.1 succeeds | Successor to Claude Fable 5, which was released 2026-06-09 | [What's new in Claude Fable 5.1](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1) | fetched the primary page |
| How Anthropic positions Fable 5.1 in the lineup | 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 (corrected) *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.* | [Models overview](https://platform.claude.com/docs/en/models/overview) | fetched the primary page |
| What Mythos 5.1 is relative to Fable 5.1 | The same model with more permissive safeguards in dual-use domains (biology, cybersecurity); Fable 5.1 adds safeguards that block certain tasks there (corrected) *My first notes had the safeguards 'removed'. System Card page 2 and the launch post say more permissive, or different levels, not removed.* | [System Card: Claude Fable 5.1 & Claude Mythos 5.1](https://www.anthropic.com/claude-fable-5-1-mythos-5-1-system-card) | read in the system card |
| Who can use Mythos 5.1 (direct access) | 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 (corrected) *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.* | [Claude Mythos](https://www.anthropic.com/claude/mythos) | fetched the primary page |
| Cyber Verification Program and Mythos access | 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 | [Claude Fable 5.1 and Mythos 5.1](https://www.anthropic.com/claude-fable-and-mythos-5-1) | fetched the primary page |
| Life Sciences Verification Program and geography | 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 | [Claude Fable 5.1 and Mythos 5.1](https://www.anthropic.com/claude-fable-and-mythos-5-1) | fetched the primary page |
| Mythos 5.1 also powers Claude Security | Claude Security runs on Mythos 5.1 and is available to all Claude Enterprise customers | [System Card: Claude Fable 5.1 & Claude Mythos 5.1](https://www.anthropic.com/claude-fable-5-1-mythos-5-1-system-card) | read in the system card |
| Platforms and surfaces where Fable 5.1 is available | 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 | [What's new in Claude Fable 5.1](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1) | fetched the primary page |
| Consumer and team plans that can use Fable 5.1 | Pro, Max, Team, Enterprise (seat-based and usage-based). Not on the Free plan | [Claude Fable models on your plan](https://support.claude.com/en/articles/15424964) | fetched the primary page |
| Max and premium-seat rule | 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 | [Claude Fable models on your plan](https://support.claude.com/en/articles/15424964) | fetched the primary page |
| Pro and standard-seat rule | 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 | [Claude Fable models on your plan](https://support.claude.com/en/articles/15424964) | fetched the primary page |
| Launch promotions or credits for Fable 5.1 | 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 (corrected) *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.* | [Claude Fable models on your plan](https://support.claude.com/en/articles/15424964) | fetched the primary page |
| Minimum Claude Code version for Fable 5.1 | 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 (corrected) *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.* | [Model configuration](https://code.claude.com/docs/en/model-config) | fetched the primary page |
| Claude Code changelog entry for Fable 5.1 | 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 | [Claude Code CHANGELOG](https://github.com/anthropics/claude-code/blob/main/CHANGELOG.md) | fetched the primary page |
| Is Fable 5.1 the Claude Code default model, and how non-interactive billing works | 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 | [Model configuration](https://code.claude.com/docs/en/model-config) | fetched the primary page |
| Gateway alias behaviour in Claude Code | 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 (corrected) *My first notes placed this in the 2.1.256 range. The entry sits under the 2.1.257 heading.* | [Claude Code CHANGELOG](https://github.com/anthropics/claude-code/blob/main/CHANGELOG.md) | fetched the primary page |
| Retirement commitment | 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 | [Models overview](https://platform.claude.com/docs/en/models/overview) | fetched the primary page |
| Cowork-specific behaviour changes for Fable 5.1 | 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 | [Release notes (Claude Help Center)](https://support.claude.com/en/articles/12138966-release-notes) | primary page plus an independent report |

### Specs

Window, output limit, cutoff, effort levels and the request shapes that return a 400.

| Fact | Value | Source | Checked |
|---|---|---|---|
| Context window | 1M tokens, both default and maximum, billed at the standard per-token rate across the whole window | [What's new in Claude Fable 5.1](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1) | fetched the primary page |
| Maximum output tokens | 128K tokens on the synchronous Messages API | [Models overview](https://platform.claude.com/docs/en/models/overview) | fetched the primary page |
| Reliable knowledge cutoff | June 2026 | [Models overview](https://platform.claude.com/docs/en/models/overview) | fetched the primary page |
| Training data cutoff | June 2026 | [Models overview](https://platform.claude.com/docs/en/models/overview) | fetched the primary page |
| Tokenizer | 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 | [What's new in Claude Fable 5.1](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1) | fetched the primary page |
| Thinking mode | Adaptive thinking, always on; thinking type disabled, or type enabled with budget_tokens, both return a 400 error | [What's new in Claude Fable 5.1](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1) | fetched the primary page |
| Effort levels supported | All five: low, medium, high, xhigh, max | [Effort](https://platform.claude.com/docs/en/build-with-claude/effort) | fetched the primary page |
| API default effort | high; omitting the parameter behaves exactly like setting high | [Effort](https://platform.claude.com/docs/en/build-with-claude/effort) | fetched the primary page |
| Product default effort levels (Anthropic-stated) | Claude Code: high. Claude Cowork: medium. claude.ai: medium | [Claude Fable 5.1 and Mythos 5.1](https://www.anthropic.com/claude-fable-and-mythos-5-1) | fetched the primary page |
| Per-message effort change (beta) | 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 | [Effort](https://platform.claude.com/docs/en/build-with-claude/effort) | fetched the primary page |
| Turn-scoped system messages (beta) | 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 | [What's new in Claude Fable 5.1](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1) | fetched the primary page |
| Progress updates between tool calls (beta) | Set thinking.display to updates with beta header thinking-display-updates-2026-08-18 to receive progress text while reasoning stays hidden | [What's new in Claude Fable 5.1](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1) | fetched the primary page |
| Comparative latency (Anthropic's label) | Slower, versus Opus 5 moderate, Sonnet 5 fast, Haiku 4.5 fastest | [Models overview](https://platform.claude.com/docs/en/models/overview) | fetched the primary page |
| Constraints carried over unchanged from Fable 5 | Prefilling the assistant turn returns 400; non-default temperature, top_p or top_k return 400; minimum cacheable prompt is 512 tokens | [What's new in Claude Fable 5.1](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1) | fetched the primary page |

### Pricing

Every price line, the comparison models, and how a fallback is billed.

| Fact | Value | Source | Checked |
|---|---|---|---|
| Fable 5.1 base input price | $10 / MTok, same as Fable 5 | [Pricing](https://platform.claude.com/docs/en/about-claude/pricing) | fetched the primary page |
| Fable 5.1 output price | $50 / MTok, same as Fable 5 | [Pricing](https://platform.claude.com/docs/en/about-claude/pricing) | fetched the primary page |
| 5-minute cache write price | $12.50 / MTok (1.25x base input) | [Pricing](https://platform.claude.com/docs/en/about-claude/pricing) | fetched the primary page |
| 1-hour cache write price | $20 / MTok (2x base input) | [Pricing](https://platform.claude.com/docs/en/about-claude/pricing) | fetched the primary page |
| Fable 5.1 cache read (hit and refresh) price | $0.25 / MTok, which is 0.025x the base input price | [Pricing](https://platform.claude.com/docs/en/about-claude/pricing) | fetched the primary page |
| Cache read price on Fable 5 and every other Claude model | Fable 5: $1 / MTok (0.1x). All other listed models stay at 0.1x of their input price | [Pricing](https://platform.claude.com/docs/en/about-claude/pricing) | fetched the primary page |
| Batch API price for Fable 5.1 | $5 / MTok input, $25 / MTok output; 50% off both directions | [Pricing](https://platform.claude.com/docs/en/about-claude/pricing) | fetched the primary page |
| US-only inference surcharge (inference_geo set to us) | 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 | [Pricing](https://platform.claude.com/docs/en/about-claude/pricing) | fetched the primary page |
| Anthropic's claimed cost reduction versus Fable 5 (Anthropic-reported, from its own August usage; token mix not published) | Around 25% on typical workloads; up to roughly 45% on complex coding and highly agentic work | [Introducing Claude Fable 5.1 and Claude Mythos 5.1](https://www.anthropic.com/claude-fable-and-mythos-5-1) | fetched the primary page |
| What the claimed savings come from | 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 | [Introducing Claude Fable 5.1 and Claude Mythos 5.1](https://www.anthropic.com/claude-fable-and-mythos-5-1) | fetched the primary page |
| Claude Opus 5 pricing for comparison | $5 / MTok input, $25 / MTok output, cache read $0.50 / MTok; batch $2.50 / $12.50 | [Pricing](https://platform.claude.com/docs/en/about-claude/pricing) | fetched the primary page |
| Claude Sonnet 5 price note | $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 | [Pricing](https://platform.claude.com/docs/en/about-claude/pricing) | fetched the primary page |
| Claude Mythos 5.1 pricing | Identical to Fable 5.1 on every column ($10 / $12.50 / $20 / $0.25 / $50; batch $5 / $25). Marked limited availability (Project Glasswing) | [Pricing](https://platform.claude.com/docs/en/about-claude/pricing) | fetched the primary page |
| How a safeguard fallback is billed in the Claude apps (Fable 5 and 5.1) | 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 | [Why Claude switched models in your conversation with Fable 5 or Fable 5.1](https://support.claude.com/en/articles/15363606) | fetched the primary page |
| API billing for refusals and fallback on Fable 5.1 | A refusal returned before any output is not billed. Fallback credit refunds the prompt-cache cost of switching models | [What's new in Claude Fable 5.1](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1) | fetched the primary page |
| Permitted server-side fallback targets for Fable 5.1 | Claude Opus 4.8 and Claude Opus 5; fallbacks set to default (beta) picks Anthropic's recommended model per category | [What's new in Claude Fable 5.1](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1) | fetched the primary page |
| Long-context pricing on Fable 5.1 | Full 1M-token context window billed at standard per-token rates; no long-context premium. Cache and batch discounts apply across the whole window | [Pricing](https://platform.claude.com/docs/en/about-claude/pricing) | fetched the primary page |

### Benchmarks

Anthropic's launch table with its error bars, then the third-party measurements.

| Fact | Value | Source | Checked |
|---|---|---|---|
| Terminal-Bench-Science 0.1 (Anthropic-reported) | Fable 5.1 52.6%; Fable 5 24.7%; Opus 5 29.0%; GPT-5.6 Sol 22.4% | [Introducing Claude Fable 5.1 and Claude Mythos 5.1](https://www.anthropic.com/claude-fable-and-mythos-5-1) | primary page plus an independent report |
| Terminal-Bench 4.0 (Anthropic-reported) | Fable 5.1 55.8%; Fable 5 42.0%; Opus 5 52.3%; GPT-5.6 Sol 37.3% | [Introducing Claude Fable 5.1 and Claude Mythos 5.1](https://www.anthropic.com/claude-fable-and-mythos-5-1) | primary page plus an independent report |
| Terminal-Bench 4.0, Mythos 5.1 (Anthropic-reported) | 60.9% | [Introducing Claude Fable 5.1 and Claude Mythos 5.1](https://www.anthropic.com/claude-fable-and-mythos-5-1) | primary page plus an independent report |
| GDPval-AA v2 Elo (run independently by Artificial Analysis; reported by Anthropic) | Fable 5.1 1853; Fable 5 1723; Opus 5 1824; GPT-5.6 Sol 1711 | [Introducing Claude Fable 5.1 and Claude Mythos 5.1](https://www.anthropic.com/claude-fable-and-mythos-5-1) | fetched the primary page |
| OSWorld 2.0 partial-credit score (Anthropic-reported) | Fable 5.1 77.9%; Fable 5 72.9%; Opus 5 75.4%; no GPT-5.6 Sol figure | [Introducing Claude Fable 5.1 and Claude Mythos 5.1](https://www.anthropic.com/claude-fable-and-mythos-5-1) | fetched the primary page |
| OSWorld 2.0 strict pass rate (Anthropic-reported) | Fable 5.1 41.7%; Fable 5 36.1%; Opus 5 39.6%; no GPT-5.6 Sol figure | [Introducing Claude Fable 5.1 and Claude Mythos 5.1](https://www.anthropic.com/claude-fable-and-mythos-5-1) | fetched the primary page |
| Humanity's Last Exam without tools (Anthropic-reported) | Fable 5.1 60.9%; Fable 5 57.8%; Opus 5 56.6%; no GPT-5.6 Sol figure | [Introducing Claude Fable 5.1 and Claude Mythos 5.1](https://www.anthropic.com/claude-fable-and-mythos-5-1) | primary page plus an independent report |
| Humanity's Last Exam with tools (Anthropic-reported) | Fable 5.1 65.0%; Fable 5 63.8%; Opus 5 63.6%; no GPT-5.6 Sol figure | [Introducing Claude Fable 5.1 and Claude Mythos 5.1](https://www.anthropic.com/claude-fable-and-mythos-5-1) | fetched the primary page |
| AutomationBench, Zapier's business-workflow benchmark (Anthropic-reported, private held-out set) | Fable 5.1 31.4%; Fable 5 17.1%; Opus 5 26.9%; GPT-5.6 Sol 19.6% | [Introducing Claude Fable 5.1 and Claude Mythos 5.1](https://www.anthropic.com/claude-fable-and-mythos-5-1) | fetched the primary page |
| CursorBench 3.2.0 (measured and reported by Cursor; relayed by Anthropic) | Fable 5.1 73.4%; Fable 5 70.5%; Opus 5 70.0%; GPT-5.6 Sol 67.2%; all at max effort | [Introducing Claude Fable 5.1 and Claude Mythos 5.1](https://www.anthropic.com/claude-fable-and-mythos-5-1) | fetched the primary page |
| Terminal-Bench-Science 0.1 standard error (Anthropic footnote) | Plus or minus 3.5 to 4.5 points for every model | [Introducing Claude Fable 5.1 and Claude Mythos 5.1](https://www.anthropic.com/claude-fable-and-mythos-5-1) | fetched the primary page |
| Terminal-Bench-Science public leaderboard versus Anthropic's harness | 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 | [Introducing Claude Fable 5.1 and Claude Mythos 5.1](https://www.anthropic.com/claude-fable-and-mythos-5-1) | fetched the primary page |
| What Terminal-Bench-Science 0.1 is | 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 | [System Card: Claude Fable 5.1 & Claude Mythos 5.1](https://www.anthropic.com/claude-fable-5-1-mythos-5-1-system-card) | read in the system card |
| Safeguards on during evaluation, and how interventions were scored | 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 (corrected) *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.* | [Introducing Claude Fable 5.1 and Claude Mythos 5.1](https://www.anthropic.com/claude-fable-and-mythos-5-1) | fetched the primary page |
| Why Mythos 5.1 and Fable 5.1 differ on Terminal-Bench 4.0 (Anthropic's explanation) | 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 | [Introducing Claude Fable 5.1 and Claude Mythos 5.1](https://www.anthropic.com/claude-fable-and-mythos-5-1) | fetched the primary page |
| OSWorld 2.0 task release used | 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 | [Introducing Claude Fable 5.1 and Claude Mythos 5.1](https://www.anthropic.com/claude-fable-and-mythos-5-1) | fetched the primary page |
| Effort setting and harness behind Anthropic's benchmark table | 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 | [System Card: Claude Fable 5.1 & Claude Mythos 5.1](https://www.anthropic.com/claude-fable-5-1-mythos-5-1-system-card) | read in the system card |
| Terminal-Bench 4.0 standard error and public leaderboard baselines | 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 | [System Card: Claude Fable 5.1 & Claude Mythos 5.1](https://www.anthropic.com/claude-fable-5-1-mythos-5-1-system-card) | read in the system card |
| Where the GPT-5.6 Sol baselines on the two Terminal-Bench rows come from | 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) | [System Card: Claude Fable 5.1 & Claude Mythos 5.1](https://www.anthropic.com/claude-fable-5-1-mythos-5-1-system-card) | read in the system card |
| Artificial Analysis Intelligence Index (independent; AA says it did pre-release evaluation for Anthropic) | Fable 5.1 at max effort with default fallback scores 66, ranked 1 of 192 models; median model scores 36. Opus 5 scores 63 | [Claude Fable 5.1 (max with fallback) - Intelligence, Performance & Price Analysis](https://artificialanalysis.ai/models/claude-fable-5-1) | fetched the primary page |
| Artificial Analysis cost to run the Intelligence Index | $8,523.16 total; $3.69 per task on the live model page, $3.76 per task in AA's 2026-09-01 article | [Claude Fable 5.1 (max with fallback) - Intelligence, Performance & Price Analysis](https://artificialanalysis.ai/models/claude-fable-5-1) | fetched the primary page |
| Artificial Analysis output-token verbosity | 140M output tokens to run the index versus a 71M median for comparable models | [Claude Fable 5.1 (max with fallback) - Intelligence, Performance & Price Analysis](https://artificialanalysis.ai/models/claude-fable-5-1) | fetched the primary page |
| Artificial Analysis throughput and latency (live figures, drift over time) | 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 (corrected) *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).* | [Claude Fable 5.1 (max with fallback) - Intelligence, Performance & Price Analysis](https://artificialanalysis.ai/models/claude-fable-5-1) | fetched the primary page |
| Artificial Analysis per-task cost versus Fable 5 (AA's own finding) | 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 | [Claude Fable 5.1 tops the Artificial Analysis Intelligence Index](https://artificialanalysis.ai/articles/claude-fable-5-1) | fetched the primary page |
| CodeRabbit code-review evaluation, Fable 5.1 versus Fable 5 (45 tasks, 105 known issues) | 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%) | [Fable 5.1 review: Should you switch?](https://www.coderabbit.ai/blog/fable-5-1-model-review) | fetched the primary page |
| CodeRabbit low versus high reasoning effort on Fable 5.1 | 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 (corrected) *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).* | [Fable 5.1 review: Should you switch?](https://www.coderabbit.ai/blog/fable-5-1-model-review) | fetched the primary page |
| Simon Willison's five-effort pelican sweep (one prompt, one run each) | 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 | [Claude Fable 5.1 made me a really nice animated pelican](https://simonwillison.net/2026/Sep/1/claude-fable-5-1/) | fetched the primary page |
| Every's efficiency read on Fable 5.1 versus Opus 5 in its Slack agent (Parrott and Shipper, 2026-09-01) | About half the tokens of Opus 5 for comparable output, finishing in about 60% of the time on automated pipelines (corrected) *My first notes said about 60% faster. The article says 60% of the time, which is roughly 40% faster. Anecdotal, one team's agent.* | [Vibe Check: Fable 5.1—Anthropic Is So Back (Again)](https://every.to/vibe-check/fable-5-1-vibe-check) | fetched the primary page |
| Every's constraint-overshoot examples for Fable 5.1 | 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 (corrected) *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.* | [Vibe Check: Fable 5.1—Anthropic Is So Back (Again)](https://every.to/vibe-check/fable-5-1-vibe-check) | fetched the primary page |

### Safety and policy

The RSP determination, the safeguard changes, the fallback numbers and the watermark.

| Fact | Value | Source | Checked |
|---|---|---|---|
| RSP and Frontier Compliance Framework determination for the 5.1 release | 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. | [Claude Fable 5.1 & Claude Mythos 5.1 System Card](https://www.anthropic.com/claude-fable-5-1-mythos-5-1-system-card) | read in the system card |
| Biology safeguards on Fable 5.1 and the 85 percent figure | 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. | [Improving Fable 5's biology safeguards](https://www.anthropic.com/news/improving-fable-5-s-biology-safeguards) | primary page plus an independent report |
| Cyber safeguards change on Fable 5.1 | 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. | [Introducing Claude Fable 5.1 and Claude Mythos 5.1](https://www.anthropic.com/claude-fable-and-mythos-5-1) | primary page plus an independent report |
| Cyber capability level of the 5.1 pair | 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. | [Claude Fable 5.1 & Claude Mythos 5.1 System Card](https://www.anthropic.com/claude-fable-5-1-mythos-5-1-system-card) | read in the system card |
| External red-team result on Fable 5.1 safeguards | 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. | [Claude Fable 5.1 & Claude Mythos 5.1 System Card](https://www.anthropic.com/claude-fable-5-1-mythos-5-1-system-card) | read in the system card |
| Fable 5.1 fallback rate of 23 percent | 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. | [Claude Fable 5.1 & Claude Mythos 5.1 System Card](https://www.anthropic.com/claude-fable-5-1-mythos-5-1-system-card) | read in the system card |
| Alignment findings for Mythos 5.1 | 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. | [Claude Fable 5.1 & Claude Mythos 5.1 System Card](https://www.anthropic.com/claude-fable-5-1-mythos-5-1-system-card) | read in the system card |
| Preserved thinking / anti-distillation enforcement | 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. | [Preserved thinking (Anthropic Help Center)](https://support.claude.com/en/articles/16761192) | primary page plus an independent report |
| Text watermarking on Fable 5.1 and Mythos 5.1 output | 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. | [How Claude's text watermark works](https://www.anthropic.com/news/claude-text-watermark) | primary page plus an independent report |
| File provenance marking and which models are covered | 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. | [How Claude marks AI-generated content](https://support.claude.com/en/articles/16266773) | fetched the primary page |
| Watermark detection API availability | 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. | [How Claude's text watermark works](https://www.anthropic.com/news/claude-text-watermark) | primary page plus an independent report |
| Enterprise Frontier Safeguards design and timing | 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. | [Developing Enterprise Frontier Safeguards with our customers](https://www.anthropic.com/news/enterprise-frontier-safeguards) | primary page plus an independent report |
| Data retention requirement for covered models | 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. | [Data retention practices for Covered Models](https://support.claude.com/en/articles/15425996) | fetched the primary page |
| What a user sees when a classifier fires in the Claude apps | 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. (corrected) *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.* | [Why Claude switched models in your conversation with Fable 5 or Fable 5.1](https://support.claude.com/en/articles/15363606) | fetched the primary page |
| Per-surface fallback cuts from the August 7 biology classifier rewrite (Fable 5, Anthropic-reported) | 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 | [Improving Fable 5's biology safeguards](https://www.anthropic.com/news/improving-fable-5-s-biology-safeguards) | fetched the primary page |
| Production fallback rate Anthropic gave for Fable 5 at its launch (Anthropic-reported) | Safeguards trigger in fewer than 5% of sessions on average; over 95% of sessions involve no fallback to Opus 4.8 | [Claude Fable 5 and Claude Mythos 5](https://www.anthropic.com/news/claude-fable-5-mythos-5) | fetched the primary page |

### Science results

Anthropic credits two of the three to Mythos 5.1 and the Venus map to Fable 5.1; only the protein binders carry an external validation claim.

| Fact | Value | Source | Checked |
|---|---|---|---|
| Protein binder design result attributed to Mythos 5.1 | 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. | [Introducing Claude Fable 5.1 and Claude Mythos 5.1](https://www.anthropic.com/claude-fable-and-mythos-5-1) | fetched the primary page |
| Venus elevation map, and which model built it | 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. (corrected) *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.* | [Introducing Claude Fable 5.1 and Claude Mythos 5.1](https://www.anthropic.com/claude-fable-and-mythos-5-1) | fetched the primary page |
| GPU kernel optimisation result attributed to Mythos 5.1 | 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. | [Introducing Claude Fable 5.1 and Claude Mythos 5.1](https://www.anthropic.com/claude-fable-and-mythos-5-1) | fetched the primary page |
| Whether external validation is claimed for all three science results | 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. (corrected) *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.* | [Introducing Claude Fable 5.1 and Claude Mythos 5.1](https://www.anthropic.com/claude-fable-and-mythos-5-1) | fetched the primary page |

### Context

The Fable 5 suspension timeline, the same-day news, the naming, and who had early access.

| Fact | Value | Source | Checked |
|---|---|---|---|
| Fable 5 suspension and redeployment timeline | 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. (corrected) *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.* | [Redeploying Claude Fable 5](https://www.anthropic.com/news/redeploying-fable-5) | fetched the primary page |
| Same-day compute news around the launch | 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. (corrected) *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.* | [Anthropic launches Claude Fable 5.1 after inking $35B cloud deal with Lambda](https://siliconangle.com/2026/09/01/anthropic-launches-claude-fable-5-1-inking-35b-cloud-deal-with-lambda/) | fetched the primary page |
| Same-day OpenAI cyber claims as reported by R&D World | 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. | [Anthropic doubles a science benchmark score with Fable 5.1 while OpenAI says its Astra model crosses critical cyber threshold](https://www.rdworldonline.com/anthropic-doubles-a-science-benchmark-score-with-fable-5-1-while-openai-says-its-astra-models-crosses-critical-cyber-threshold/) | fetched the primary page |
| The Fable and Mythos naming, and the satire post's traction | 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. (corrected) *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.* | [Anthropic's model naming, extrapolated (Hacker News)](https://news.ycombinator.com/item?id=48480852) | primary page plus an independent report |
| Early-access partners quoted on the launch page | 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. | [Introducing Claude Fable 5.1 and Claude Mythos 5.1](https://www.anthropic.com/claude-fable-and-mythos-5-1) | fetched the primary page |

## Still open

Claims I couldn't settle against a primary source, with my current reading and what would close each one.

| Claim | Current reading | Why it is open, and what closes it |
|---|---|---|
| Claim that a Fable session weighs about 2x an Opus session against the Max-plan weekly allowance | No multiplier published for Fable 5.1. Anthropic's plan article says only that Fable models drain weekly limits faster than other models. | 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. |
| Named partners or papers behind the Mythos 5.1 science results | 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 | 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. |
| How much of Fable 5.1's code output the watermark actually covers | 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 | 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. |
| Hacker News launch thread size and topic composition | 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%) | 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. |
| Reddit r/ClaudeAI reaction to Fable 5.1 | Nothing 5.1-specific surfaced in two searches today (site-restricted and plain) | Search returned only Fable video-game pages. Closes when a direct r/ClaudeAI thread URL dated 2026-09-01 or later turns up. |

## How this page changes

Rows move when evidence arrives: a claim that gains a primary source leaves Still open for its topical section, and a row a better source contradicts gets corrected and keeps its note. Each move gets a dated changelog line. If you spot a wrong number or a source I missed, reach me through the profiles on the home page.

## Sources

1. [Introducing Claude Fable 5.1 and Claude Mythos 5.1](https://www.anthropic.com/claude-fable-and-mythos-5-1) — Anthropic, 2026-09-01. Accessed 2026-09-02
2. [System Card: Claude Fable 5.1 & Claude Mythos 5.1](https://www.anthropic.com/claude-fable-5-1-mythos-5-1-system-card) — Anthropic, 2026-09-01. Accessed 2026-09-02
3. [Pricing](https://platform.claude.com/docs/en/about-claude/pricing) — Claude Platform Docs. Accessed 2026-09-02
4. [Models overview](https://platform.claude.com/docs/en/models/overview) — Claude Platform Docs. Accessed 2026-09-02
5. [What's new in Claude Fable 5.1](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1) — Claude Platform Docs, 2026-09-01. Accessed 2026-09-02
6. [Effort](https://platform.claude.com/docs/en/build-with-claude/effort) — Claude Platform Docs. Accessed 2026-09-02
7. [Prompting Claude Fable 5.1](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5-1) — Claude Platform Docs, 2026-09-01. Accessed 2026-09-02
8. [Claude Mythos](https://www.anthropic.com/claude/mythos) — Anthropic. Accessed 2026-09-02
9. [Claude Fable models on your plan](https://support.claude.com/en/articles/15424964) — Anthropic Help Center. Accessed 2026-09-02
10. [Model configuration](https://code.claude.com/docs/en/model-config) — Claude Code Docs. Accessed 2026-09-02
11. [Claude Code CHANGELOG](https://github.com/anthropics/claude-code/blob/main/CHANGELOG.md) — Anthropic (GitHub). Accessed 2026-09-02
12. [Preserved thinking: changing how the Messages API handles thinking blocks to protect against distillation](https://support.claude.com/en/articles/16761192) — Anthropic Help Center. Accessed 2026-09-02
13. [How Claude's text watermark works](https://www.anthropic.com/news/claude-text-watermark) — Anthropic, 2026-08-14. Accessed 2026-09-02
14. [Improving Fable 5's biology safeguards](https://www.anthropic.com/news/improving-fable-5-s-biology-safeguards) — Anthropic, 2026-08-07. Accessed 2026-09-02
15. [Claude Fable 5 and Claude Mythos 5](https://www.anthropic.com/news/claude-fable-5-mythos-5) — Anthropic, 2026-06-09. Accessed 2026-09-02
16. [Claude Fable 5.1 tops the Artificial Analysis Intelligence Index](https://artificialanalysis.ai/articles/claude-fable-5-1) — Artificial Analysis, 2026-09-01. Accessed 2026-09-02
17. [Fable 5.1 review: Should you switch?](https://www.coderabbit.ai/blog/fable-5-1-model-review) — CodeRabbit, 2026-09-01. Accessed 2026-09-02
18. [Vibe Check: Fable 5.1 — Anthropic Is So Back (Again)](https://every.to/vibe-check/fable-5-1-vibe-check) — Every, 2026-09-01. Accessed 2026-09-02
19. [Claude Fable 5.1 made me a really nice animated pelican](https://simonwillison.net/2026/Sep/1/claude-fable-5-1/) — Simon Willison, 2026-09-01. Accessed 2026-09-02
20. [Measurable quality regression in Claude generation 5 (issue #83510)](https://github.com/anthropics/claude-code/issues/83510) — GitHub (anthropics/claude-code), 2026-08-03. Accessed 2026-09-02
21. [Claude Fable 5.1 and Claude Mythos 5.1 (Hacker News thread)](https://news.ycombinator.com/item?id=49525378) — Hacker News, 2026-09-01. Accessed 2026-09-02
22. [Claude Fable 5.1 watermark: It has a blind spot developers can't ignore](https://thenewstack.io/fable-5-1-watermark/) — The New Stack, 2026-09-01. Accessed 2026-09-02
23. [Anthropic releases Claude Fable 5.1 and Mythos 5.1, cutting cache read prices by 75%](https://thenextweb.com/news/claude-fable-mythos-5-1-eu-ai-act-watermark-detection-api-private-preview) — The Next Web, 2026-09-01. Accessed 2026-09-02
24. [Claude's updated Fable and Mythos models watermark their replies](https://www.pcworld.com/article/3224834/claudes-updated-fable-and-mythos-models-watermark-their-replies.html) — PCWorld, 2026-09-01. Accessed 2026-09-02
25. [Anthropic Launches Claude Fable 5.1 With Lower Costs and Fewer False Positives](https://www.macrumors.com/2026/09/01/anthropic-claude-fable-5-1/) — MacRumors, 2026-09-01. Accessed 2026-09-02
26. [Claude's creators released Fable 5.1](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) — VG Times, 2026-09-01. Accessed 2026-09-02
27. [Anthropic's Model Naming, Extrapolated](https://samwilkinson.io/posts/2026-06-09-anthropics-model-naming-extrapolated) — Sam Wilkinson, 2026-06-09. Accessed 2026-09-02

## Questions

**Is Claude Fable 5.1 the same model as Claude Mythos 5.1?**

Yes, according to Anthropic's System Card. Fable 5.1 is the generally available version, with safeguards that block certain biology and cybersecurity tasks. Mythos 5.1 is the same model with more permissive safeguards, offered only to vetted organisations. Anthropic attributes the gap between them on Terminal-Bench 4.0, 60.9 percent against 55.8, to those safeguards.

**Will my bill really drop 45 percent on Claude Fable 5.1?**

Only if about 60 percent of your Fable 5 spend was cache reads, because cache reads are the one price that changed, from $1 to $0.25 per million tokens. A single-turn call with no cached prefix saves nothing. At max effort, Artificial Analysis measured Fable 5.1 costing about 20 percent more per task than Fable 5, because it produced roughly 1.7 times the output tokens.

**Do I need to change anything in my API integration for Claude Fable 5.1?**

Three things. Forced tool use (tool_choice set to any or to a named tool) now returns a 400; use auto with strict set to true, or structured outputs. Thinking blocks can only be read by Fable 5.1 or newer, so a fallback to Opus loses them. Accounts created on or after 2026-08-31 can't edit earlier turns and replay those blocks.

Last checked 2026-09-02 against the sources above.

## Changes

- 2026-09-02: First version: 187 rows. 154 facts verified against primary sources, 28 corrected from the launch coverage or my first notes, 5 unverified. Five claims sit under Still open with what would close each one.