Breaking

Anthropic Publishes Official Prompting Guide for Claude Fable 5

July 2, 2026 at 05:15 EDT

Foundation Models
AI Agents
Software Dev & Coding

Alongside the general availability of its frontier model Claude Fable 5, Anthropic is offering a dedicated official prompting guide, "Prompting Claude Fable 5," as part of its platform documentation. Its explicit warning that the long-standing habit of stacking detailed instructions can actually hurt performance on the new model is drawing developer attention.

June 9, 2026 · Anthropic

Claude Fable 5: the first "Mythos-class" model, tuned for multi-day autonomy

Positioned a tier above Opus 4.8, Fable 5 is built for long-running autonomous tasks — with a dedicated prompting guide that says you now just give it a goal, not a script.

50M

line Ruby codebase migrated in 1 day — a job estimated at 2+ months by hand

3×

higher rate of reaching the final act in Slay the Spire after adding memory

<5%

of sessions fall back to Opus 4.8 on some safety-sensitive queries

Output pricing — about 2× Opus

Per 1M output tokens

~$25

Opus 4.8

$50

Fable 5

Input $10 / 1M · 90% cache discount · 1M-token context · 30-day retention · US-only inference 1.1×

Effort settings

Four levels — higher effort strengthens self-verification but raises latency and cost. Low effort can still beat a legacy model's max.

low

medium

high ★

xhigh ★

★ default range

What developers praise

Long-horizon autonomy over hours to days
Reliable delegation to sub-agents
Reuses past lessons via memory & notes
"Completes in one shot just from a goal"

Cautions

Roughly 2× the price of Opus
Safety false positives fall back to Opus
Needs timeout tuning & async harnesses
Set explicit bounds to curb over-refactoring

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