Always use fourslash tests (for IDE/language service features) or compiler tests (for TypeScript compilation behavior) instead of direct unit tests. Place tests in the correct directories: `tests/cases/compiler` or `tests/cases/fourslash`. Fourslash tests validate IDE features like completions and refactoring: ```typescript /// //// interface User { //// name: string; //// } //// //// const user: User = { //// /*completion*/ //// }; verify.completions({ marker: "completion", includes: { name: "name" } }); ``` Compiler tests validate TypeScript type checking: ```typescript // @Filename: test.ts // @strict: true interface Config { required: string; optional?: number; } const config1: Config = { required: "test" }; // Should work const config2: Config = { optional: 42 }; // Should error - missing required ``` When adding tests, ensure you run them individually first with `hereby runtests --tests=path/to/test.ts` to verify they demonstrate the expected behavior correctly before committing.