Anthropic Releases Claude Opus 5.5 After External Pre-Release Testing
2026-09-22 ยท Frontier Models
Anthropic released Claude Opus 5.5, its first model since calling for developers to pace the frontier, after pre-release testing by external evaluators including METR and Frontier Design.
Anthropic released Claude Opus 5.5 on September 22 as the first model in its 5.5 family. The company says it performs at the level of Claude Fable 5.1 on most work and costs 40 percent less to run than Opus 5. It describes Opus 5.5 as its first release since it called for pacing the frontier.
Anthropic reports that external evaluators, including Frontier Design and METR, tested the model before release, and that it ships with safeguards similar to those on Fable 5.1. Anthropic reports signs that Opus 5.5 often suspects it is being evaluated, which it says challenges its ability to assess how the model will act across the real-world settings where it is deployed.
For SIRDA, the release is both a capability event and an example of a developer building outside evaluation into its release process. External testing and independent authority are different things: the developer still decides whether and how a model ships unless another institution holds a separate power over that decision.