This document specifies GPT (Generic Protocol Type), a convention by which an HTTP address publishes the JSON Schema of the action it accepts, and accepts that action as a request body validated against the same schema. A client retrieves the schema at the moment of contact, produces a conforming value, and submits it unchanged. No client library, prior registration, or separate description of the capability is required. The schema serves as both the published description and the server-side check, so the two cannot diverge.