MPS: Skill for Claude Code

MPS BaseLanguage (Java) Authoring

Loading companion skills

Companion names in this skill are lazy dependencies: load only those relevant to the current task. If this skill came from an MCP server, use the host's skill loader to resolve the companion's unique discovered entry URI on the same host-assigned originating server. If the host has no server-backed skill loader, stop and report that limitation; do not silently fall back to a filesystem copy. If this skill came from a filesystem catalog, load the named sibling from that same catalog at <skills-root>/<skill-name>/SKILL.md, even if remote skill loaders are also available. Do not invent a tool name or server endpoint.

jetbrains.mps.baseLanguage is MPS's Java-equivalent language. You edit it as an AST, not as text. Two authoring paths exist: the Java parser (mps_mcp_parse_java_and_insert) for plain Java, and JSON AST blueprints (mps_mcp_insert_root_node_from_json / mps_mcp_update_root_node_from_json / mps_mcp_update_node) for everything else — especially code that uses BaseLanguage extensions (jetbrains.mps.lang.smodel, jetbrains.mps.baseLanguage.closures, jetbrains.mps.baseLanguage.collections) or needs stable persistent member references.

Critical Directives

• Node equality: always use :eq: and :ne: (concepts NPEEqualsExpression / NPENotEqualsExpression). Never == or .equals() between SNode references.
• Constructor return type: ConstructorDeclaration.returnType MUST be a VoidType node. Leaving it empty fails validation.
• ClassCreator wiring: ClassCreator.baseMethodDeclaration points at the constructor declaration, not the class. InstanceMethodCallOperation.baseMethodDeclaration points at the method declaration.
• new … is a GenericNewExpression plus a creator: the GenericNewExpression is the Expression; its creator child is ClassCreator, ArrayCreator, ArrayCreatorWithInitializer, AnonymousClassCreator, a collections creator, or smodel SNodeCreator (new node<C>()). Creators are AbstractCreators, not Expressions, so none of them can stand alone in ReturnStatement.expression, an argument, or an initializer.
• Inherited methods: use the declaring class ref for baseMethodDeclaration, not the subclass that calls the method. E.g. addActionListener is declared on AbstractButton, so use the AbstractButton class ref.
• Expressions in statement lists: any Expression must be wrapped in ExpressionStatement to be valid inside a StatementList.
• Variable declarations: in method bodies wrap LocalVariableDeclaration in LocalVariableDeclarationStatement. In ForStatement.variable use LocalVariableDeclaration directly, no wrapper.
• Prefer primitives: use string (StringType) over String (ClassifierType) where possible; same for int, boolean, etc.
• Compatibility: BaseLanguage core is Java 7 (including generics). When building AST directly (JSON blueprints), there is no lambda/record syntax — use the jetbrains.mps.baseLanguage.closures extension for closures (blueprints in the mps-collections-and-closures skill). The Java parser (mps_mcp_parse_java_and_insert) additionally accepts the Java 8+ syntax MPS recognizes — most notably lambdas, which it maps to closures ClosureLiterals (auto-imported when postProcess.importUsedLanguages is on); a lambda only type-checks against a matching functional-type target. Constructs the parser does not recognize (e.g. records) still fail to parse. See references/parse-java-tips.md.
• Surgical edits: when a single child changes prefer mps_mcp_update_node over rewriting the whole root — full-root rewrites churn persistent IDs and break incoming refs.
• Ask the running typesystem, not its rules: use mps_mcp_query_types OPERATION_TYPE for expressions such as int + long or String + int, IS_SUBTYPE for assignability, and COERCE to ClassifierType before reading classifier.targetReference for classifier members. See mps-mcp-workflow/references/analysis-tools/type-queries.md.