Use when writing or modifying tests inside MPS `@tests` models — `NodesTestCase` (typesystem, constraints, scopes, dataflow, generator output), `EditorTestCase` (intentions, actions, keystrokes, side-transforms, completion), `MigrationTestCase` (migration scripts), `BTestCase` (plain JUnit on hand-written Java/Kotlin runtime), inline annotations such as `has error` / `has type` / `ScopesTest`, label-based `node<label>` cross-references, caret markers, and running tests via MCP / in MPS. Reach for this skill whenever the task involves authoring or fixing tests in a `tests`-stereotype model, or interpreting failures from one.