Type System Discipline

Read the host adapter once per session before following this skill. It maps upstream Cursor-specific tool, model, path, and permission instructions to your host and sets authority limits.

Make the compiler reject impossible states, mismatched primitives, and unhandled variants. Prefer defining errors and special cases out of existence over proliferating handlers. For TypeScript syntax, see the typescript-best-practices skill.

• Brand semantic primitives. UserId and OrderId are strings underneath but should not be interchangeable. Give each its own type, such as a branded intersection in TypeScript or a newtype elsewhere. Validate once at creation, trust the type downstream. If two arguments share a primitive type but mean different things, brand them.
• External data is untyped until parsed. RPC payloads, JSON, IPC messages, CLI args, config files, environment variables, database rows, and in the browser: URL and navigation state, storage and IndexedDB, imported files, postMessage, and third-party SDK objects. Have a parse function at every boundary that turns unstructured input into the typed model. The boundary-discipline principle skill says where to put validation.
• Derive types from authoritative schemas. When a protocol buffer, OpenAPI spec, GraphQL schema, database migration, design-token file, or another file owns a shape, derive from it instead of hand-rolling a parallel type. See the encode-lessons-in-structure principle skill.