Mythos Benchmark: Claude Fable 5 Workflow ROI — Cromus
The Cromus Mythos Benchmark Framework is an open standard for measuring Claude Mythos (Capybara) workflow ROI. The Capybara/Mythos tier ships as two models at identical pricing: Claude Fable 5 (generally available June 9, 2026) and Claude Mythos 5 (open preview since July 15, 2026), both priced at $10 / $50 per million input/output tokens — roughly 2x Claude Opus 4.8. It applies Cromus's 4-step measurement framework to decide where Capybara-tier capability actually lowers total workflow cost.
The framework follows four steps: Step 0 — Workflow Classification maps your AI touchpoints to determine which tasks justify Mythos-tier capability vs. existing Claude models. Step 1 — Baseline Cost establishes your current fully-loaded workflow cost and models Mythos substitution scenarios. Step 2 — Croms quantifies the preventable waste that Mythos could eliminate (or introduce through overprovisioning). Step 3 — TCWO calculates the total cost of workflow ownership with Mythos integration.
The Monitoring Protocol provides a post-deployment checklist for tracking Mythos performance against predictions, including cost variance tracking, quality regression detection, and adoption metrics.
The benchmark uses Cromus's Croms metric to prevent the common mistake of deploying a frontier model everywhere — ensuring Mythos is only used where its capabilities justify the cost.