Claude Haiku 5.5 is built for high-volume AI work where speed and cost matter. This guide explains its pricing, adjustable effort setting, benchmark gains, API usage, and role alongside larger models in agent systems.
Claude Opus 5.5 is Anthropic’s first Claude 5.5 model, built for long-running coding agents, research, and business workflows. It combines higher benchmark performance with lower token use, faster output, clearer communication, and tighter safeguards.
Claude Fable 5.1 and Claude Mythos 5.1 share the same underlying model but serve different audiences through different safeguards. This guide breaks down the pricing, benchmark caveats, scientific research examples, privacy design, and access rules behind the September 2026 release.