How does it work? is just a documentation specification like spec kit?

Nope, it's a structured spec agents construct using a CLI or MCP (you can also interact with the spec using a web UI). It's CUE, and validated against a schema. Instead of taking your conversation and generating a markdown document that agents might (but often don't) respect, the agent populates the spec in the service from your conversation, then when you're done you can use the CLI to automatically generate a bunch of code.