wroclove.rb 2026 lightning talk following the speaker's earlier event-sourcing talk. Starts with a buggy demo of a small app the speaker built for drawing command/event/read-model diagrams in a mermaid-like text-based syntax, with DSL for test scenarios. Pivots to the real subject: the app's backend is a Rack-mounted REST API with an OpenAPI spec, built on a new Ruby gem called step. Walks through step's design (service metadata + endpoints with query/payload schemas, railway-oriented step pipelines, sub-pipelines, schemas-as-steps that can validate before/after authentication, content negotiation, authentication schemes from the OpenAPI spec). Shows generating an OpenAPI JSON description and rendering it with Swagger. Closes with the punchline: a single additional line decorates the same REST service as an MCP endpoint, turning every REST endpoint into an MCP tool. Demo: Claude reads the domain model conversationally and can even implement it based on the MCP description.