Anthropic has rebranded Workbench as Playground! It has been in development for quite a while and is now more conventional.

> "Workbench is now Playground - Explore the Claude API by using it. Test models and Messages API features without writing code."

> "Start from working examples for tools, citations, and caching
See each request and response, with tokens, cost, and latency
Copy the SDK code when something works"