CARD · Card record
Claude Fable 5.1 tops the AA index; cache reads cut to 2.5% of input
Anthropic shipped Claude Fable 5.1, successor to Fable 5 for long-running agentic coding, knowledge work, and research, alongside Mythos 5.1 for Project Glasswing participants. Defaults: 1M-token context, 128k max output, always-on adaptive thinking at $10/$50 per MTok with prompt-cache reads cut to $0.25/MTok (0.025x base input versus 0.1x elsewhere). Live on Claude API, Bedrock, AWS, Google Cloud, Microsoft Foundry, and OpenRouter at matching prices. Artificial Analysis now ranks it #1 of 192 (index 66 vs 62 for Fable 5); cost per index task is $3.69 vs $3.14, about 18% higher.
01
THE STORY · The original
Read the explanation and its limits.
- Originalofficial statementhttps://platform.claude.com/docs/en/release-notes/overview
Why it is worth understanding
The Anthropic Claude line advances on three verifiable axes at once — capability (AA #1), cost structure (cache reads at 0.025x of input price), and availability (five platforms plus OpenRouter at identical prices) — an immediate calibration point for agent cost math.
Conditions and limitations
Evidence is Anthropic's official release notes (vendor's own account) plus the Artificial Analysis leaderboard (third-party measurement, fetched and checked by BitShovel on Sep 2, 2026), cross-reported the same day by aihot and AlphaSignal. The ~18% per-task delta is computed from AA's $3.69 vs $3.14, slightly under the 20% rounding in aggregator headlines. An index rank is not a substitute for independent evaluation, and leaderboard attention is not proof of fit.
Important additions and corrections
Dates below mark changes to our coverage.
- Correction ·
Corrected the sourcing: release coverage and user experience do not independently verify historical rankings or pricing.
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02
THE THREAD · Full timeline
2 events, every row traceable.
A card accumulates records as its story develops: new corroborators, market contrasts, material changes at the original — each dated and linked to its archive day.
- Added to BitShovelEntered the BitShovel radar: 50 entries visible on AIHOT featured at collection.
- RecordCorroboration added: Simon Willison's weblog carried the same story this window (title match).
03
SOURCES · Sources and evidence
Check each link's purpose and limits.
Sources and evidence · 5 links
Each link has a purpose and scope. Link counts do not establish independent confirmation; direct support applies only to the statements specified below.
- Official Claude release notes, surfaced via AIHOT and AlphaSignalScope not documented
A per-link statement of support has not been attached.
- AIHOT featured (discovery)Discovery or relay
Carries and links to the September 1 Claude release notes.
Limits: Establishes coverage of the official release, not independent verification of benchmarks, pricing, or practical results.
Evidence for this assessment
https://aihot.virxact.com/items/cmtj04igz04xproel00pt2ojuRetained evidence and full review record (JSON) - AlphaSignalDiscovery or relay
The retained same-topic news entry points to AA’s Claude Fable 5.1 measurement page.
Limits: This relays a measurement source. The headline’s 20% is not a substitute for AA’s methodology and contemporaneous data.
- Artificial AnalysisScope not documented
A per-link statement of support has not been attached.
- Simon Willison's weblogBackground
Provides the author’s hands-on examples of animation and SVG generation with this version.
Limits: One author’s examples do not establish AA leadership, cache pricing, or general productivity gains.
Evidence for this assessment
https://simonwillison.net/2026/Sep/1/claude-fable-5-1/Retained evidence and full review record (JSON)
04
EDITORIAL REVIEW
Review of this card's wording and evidence.
Editorial review · Corrected
· Beijing time (UTC+8)
Corrects the source attribution from cross-checking to a relay path and distinguishes release coverage from usage context. AA’s live page cannot retrospectively establish the earlier numeric observation.
This dates our review of the card's wording and evidence, not a project release or product update.