> The Digital Twin Universe is our answer: behavioral clones of the third-party services our software depends on. We built twins of Okta, Jira, Slack, Google Docs, Google Drive, and Google Sheets, replicating their APIs, edge cases, and observable behaviors.

Came to the same conclusion. I have an integration heavy codebase and it could hardly test anything if tests weren't allowed to call external services. So there are fake implementations of every API it touches: Anthropic, Gemini, Sprites, Brave, Slack, AgentMail, Notion, on and on and on. 22 fakes and climbing. Why not? They're essentially free to generate, it's just tokens.

I didn't go as far as recreating the UI of these services, though, as the article seems to be implying based on those screenshots. Just the APIs.