Claude Fable 5 with OrchestKit
How OrchestKit supports Claude Fable 5: correct pricing, a spend-consent gate for agent pins, advisor warnings, and the deliberate opt-in path (ORK_FABLE_OK).
Claude Fable 5 with OrchestKit
Claude Fable 5 (claude-fable-5, released 2026-06-09) is Anthropic's Mythos-class frontier model: state-of-the-art on coding, vision, and knowledge work, with week-long autonomous-run capability and a 1M-context variant (claude-fable-5[1m]). It bills at $10 input / $50 output per MTok — 2x Opus 4.8 output pricing — and in under 5% of sessions a safety classifier transparently falls back to Opus 4.8 (cyber/bio topics; you are informed when it happens).
OrchestKit supports Fable 5 fully, while treating it as a deliberate spend decision rather than a default.
Dated note: Fable 5 is included at no extra cost in Pro/Max/Team subscriptions until June 22, 2026; after that it requires usage credits. Everything below is evergreen.
What OrchestKit does for you
| Surface | Behavior |
|---|---|
| Pricing | claude-fable-5 (+ fable alias and the [1m] variant) is priced correctly in cost estimation — previously unknown models silently fell back to sonnet pricing, underbilling by ~70% |
| Model vocabulary | One source of truth (src/hooks/src/lib/models.vocab.json) feeds agent-frontmatter validation, docs generation, the skill content lint, and pricing — model: fable is a first-class, CI-validated value |
| Spend consent | Spawning an agent pinned to fable triggers a permission prompt with the cost spelled out — approve per-spawn, or pre-consent with ORK_FABLE_OK=1 |
| Cost advice | An agent that merely inherits fable from your session gets a downgrade warning when its task is low/medium complexity (~70%/60% savings vs. sonnet, derived from real pricing) |
| Tier rules | fable joins opus as a premium tier: CI reserves it for high-complexity agents (security/safety/system-design/event-driven/workflow) |
Choosing where Fable 5 runs
There are three distinct decisions, and OrchestKit treats them differently:
- Your session model (
/model) — entirely yours; OrchestKit never interferes.inheritagents follow it. - An agent pinned to fable (
model: fablein a spawn or agent frontmatter) — this commits future runs to frontier pricing, so the spend-consent gate asks first. - Pre-consented automation — set
ORK_FABLE_OK=1in the environment (or CI) when you have deliberately decided a workload is worth frontier pricing.
# One-off: approve the permission prompt when it appears.
# Session/CI-wide pre-consent:
export ORK_FABLE_OK=1The cost-neutral default for agent fleets is model: inherit — quality follows whatever you chose for the session (including fable when you chose it), prompt-cache forking keeps working, and nothing is silently committed to frontier pricing.
Fallback behavior to know about
Fable 5's safety classifiers fall back to Opus 4.8 in a small fraction of sessions, most likely on cyber/bio-adjacent content. For security-audit workloads this means a fable-pinned auditor could silently switch models mid-run — one reason OrchestKit keeps its own security agents on an explicit opus pin rather than fable.
Requirements
- Claude Code 2.1.170+ (OrchestKit's support floor) — Fable 5 access shipped in CC 2.1.170.
- No OrchestKit configuration is needed for any of the above; it ships in the
orkplugin.
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