Anthropic releases Claude Fable 5.1 and Mythos 5.1, cuts cache reads 75%
Anthropic launched Claude Fable 5.1 (GA) and Mythos 5.1 (restricted) on September 1, with list price unchanged at $10/$50 per 1M tokens and cache reads cut from $1.00 to $0.25.
Anthropic released Claude Fable 5.1 and Claude Mythos 5.1 on September 1. Both models share a single engine but use different safeguard profiles. Fable 5.1 is the general-purpose frontier model with API ID claude-fable-5-1, a 1M-token context window, 128K maximum output, adaptive thinking always on, and a knowledge cutoff extended to June 2026.
Mythos 5.1 is available only through the Cyber Verification Program and the new Life Sciences Verification Program. It can discover software vulnerabilities without developing exploits and handles biological queries that Fable's safeguards block.
Pricing holds at $10 input and $50 output per 1M tokens. The biggest change is cache reads, cut from $1.00 to $0.25 per 1M tokens, a 75% reduction. Typical workloads run roughly 25% cheaper, with cache-heavy agent runs reaching 45% savings.
Fable 5.1 leads Claude Opus 5 on every benchmark Anthropic published, with Terminal-Bench-Science at 52.6%, more than double Fable 5's 24.7%. Anthropic says low and medium effort settings now match Fable 5 results.