Projectional Agent Toolkit – JetBrains MPS for Agents

Entry point for working with JetBrains MPS (Meta Programming System) models, languages, generators, and the MPS MCP tools.

Read this together with AGENTS.md whenever the task involves MPS artifacts or MPS MCP tooling.

Critical Directives

⚠️ STOP — If you are reading raw .mps or .mpl XML files, you are way off track. MPS model files are opaque serialized XML: you cannot safely understand or edit them as plain text, and doing so will corrupt the model. Use mps_mcp_* tools instead. If they are not available, ask the user to start MPS and enable the MPS MCP server before continuing.

• Always prefer MPS MCP tools over hand-editing .mps / .mpl XML. Hand-edits silently corrupt model files.
• Preserve node IDs. Prefer mps_mcp_update_root_node_from_json or surgical edits (mps_mcp_update_node, …) over delete-and-reinsert. Deleting destroys persistent IDs and breaks incoming references.
• Copy-then-modify beats reconstruct-from-JSON. To create a node that closely resembles an existing one, duplicate it with mps_mcp_alter_nodes COPY_NODE and adjust the copy with mps_mcp_update_node, instead of printing the original and re-authoring it as a JSON blueprint. The copy is guaranteed structurally valid and needs no blueprint authoring at all.
• Reload after compiled-aspect changes. After modifying typesystem, constraints, behavior, editor, or any other compiled aspect, call mps_mcp_reload_all (or rebuild the language module via mps_mcp_alter_nodes MAKE) before validating with mps_mcp_check_root_node_problems on a consumer model. Without a reload, cached compiled rules continue to fire — checking will report stale errors or miss new ones.
• The Structure-to-Aspect/Scaffold Prerequisite Chain (Ordering & Recovery). When editing a language's structure aspect (e.g., creating concepts/enums or modifying properties, children, or references via mps_mcp_alter_structure), you MUST adhere to the following strict sequence of cross-tool steps before invoking dependent tools (such as mps_mcp_get_concept_details or mps_mcp_scaffold_editor):
  1. Structure Edit: Modify the structure aspect (using mps_mcp_alter_structure).
  2. Clean Make/Rebuild: Run mps_mcp_alter_nodes with MAKE and rebuild=true targeting the language module (a model-scoped make can leave a never-before-deployed language's runtime hollow). Note: a plain mps_mcp_reload_all alone is not sufficient because the generated structure aspect descriptor classes on disk must be regenerated before the ClassLoader reloads them.
  3. Verify Descriptors: Retrieve the compiled concept details using mps_mcp_get_concept_details and verify that the descriptorStatus is not "hollow" and no warnings exist.
  4. Scaffold / Aspect Editing: Once the descriptors are verified as compiled and active, proceed to scaffold the editor (mps_mcp_scaffold_editor) or define other aspects (behavior, constraints, typesystem, generator, etc.). Failing to follow this ordering results in stale or hollow descriptors where dependent tools silently see empty properties, children, or references. Shortcut: mps_mcp_alter_structure CREATE_CONCEPTS with make: true already does steps 2–3 — it clean-rebuilds, verifies every created descriptor against the runtime, and auto-recovers a never-deployed language (one module-scoped rebuild, recoveryStage: "module-rebuild"). Trust its makeStatus; run the manual chain only for structure edits made through tools that do not make (UPDATE_*/RENAME_* operations, mps_mcp_update_node).
• Surgical edits over full-root rewrites. When a single child changes, prefer mps_mcp_update_node over re-running mps_mcp_update_root_node_from_json on the whole root.
• Validate frequently. A successful insertion ("ok": true) does not mean the AST is semantically valid — always follow with mps_mcp_check_root_node_problems.

Tool name note: MPS MCP tools are named with a mps_mcp_ prefix (e.g. mps_mcp_query_nodes, mps_mcp_alter_nodes, mps_mcp_get_concept_details). Your MCP client wraps these with a server-specific prefix (e.g. mcp__mps-mcp-server__<env>___), which varies by environment. Match tools by the stable mps_mcp_* suffix.

Which project the tools act on (subdirectory & multi-project checkouts). The mps_mcp_* tools operate on the MPS project currently open in the running MPS instance. That project often lives in a subdirectory of your repository (e.g. <repo>/tools/BigProject, as in mbeddr or MPS-extensions), and one checkout may even hold several MPS projects. The host resolves the project from your client's workspace roots, then falls back to the single open project. If a tool call is rejected with "no project opened" or "multiple projects opened", call mps_mcp_list_open_projects, then supply the intended project's mpsProjectBaseDirectory — the folder MPS actually opened, i.e. a path at or inside it, not the repository root — via the host's project-path argument. This is the opposite of mps_mcp_initialize_project_for_agents, whose targetDirectory is the repository / workspace root (where .agents/, .claude/, AGENTS.md, and CLAUDE.md belong), which may be an ancestor of the MPS project directory.

Multi-project repository rules. All open MPS projects share the same module repository. Write targets — models, modules, root nodes, and nodes being mutated — must belong to the project selected by projectPath; tools refuse writes to another open project's elements. Read/reference/dependency targets may come from another open MPS project when they are explicitly named or imported: model/module dependencies, model used languages/devkits, node concepts, and reference targets can point across projects, and the foreign elements are treated like read-only library/stub elements. Returned JSON for an element from another open project includes containingProject: { name, mpsProjectBaseDirectory } and editableFromCurrentProject: false; nested references use prefixes such as conceptContainingProject, targetContainingProject, or typeContainingProject. These markers appear only for elements owned by another open project — their absence does not imply an element is editable (a read-only library/stub in the current project carries readOnly: true but no containingProject), so decide editability from readOnly together with these markers.

When in doubt, ask which project. When several MPS projects are open under one repository / VCS root, the mps_mcp_* tools accept any valid project path and act relative to that project. Same-name resolution prefers the selected project's own elements before falling back to the shared repository, but an explicit reference may still resolve to another open project. If the intended project is not obvious from the user's request or the current editor focus (mps_mcp_get_current_editor_root_node), do not guess — show the mps_mcp_list_open_projects entries and ask the user which project the query or change should run against.

Companion Skills

All MPS skills live in a per-harness directory loaded by the agent host (e.g. .agents/skills/<skill-name>/SKILL.md for AGENTS.md-aware hosts, .claude/skills/<skill-name>/SKILL.md for Claude Code). Load whichever ones apply to your current task.

Installing / refreshing the catalog. These mps-* skills and the project's AGENTS.md/CLAUDE.md are installed and refreshed by the mps_mcp_initialize_project_for_agents MCP tool. It installs into a targetDirectory — normally your repository / workspace root (the folder containing .git), which may be an ancestor of the MPS project directory when the project sits in a subdirectory; leave targetDirectory empty to let the tool derive that root from the open project's enclosing VCS folder, and do not pass it projectPath. If no mps-* skills exist yet, the project has not been initialized for agents — tell the user and offer to run it. If the catalog looks stale or incomplete, offer to refresh it (with approval): delete every mps-* skill folder from .agents/skills/ and .claude/skills/ — keeping any project-local *-dsl skills — re-run the tool, then carefully merge its returned agentsFileText into AGENTS.md/CLAUDE.md, preserving project-specific sections. See that tool's description for the exact refresh contract.

Key Concepts

MPS is a projectional editor and a language workbench. Unlike text-based IDEs, MPS works with an Abstract Syntax Tree (AST) directly. JSON is used to represent MPS nodes and their properties in a structured format for the MPS tools.

• Modules: The top-level containers in an MPS project.
  • Solution: contains user code (models).
  • Language: defines a new language - structure (concepts and interface concepts), editor, etc.
  • Generator: defines how to transform one language to another (usually to Java/BaseLanguage). May belong to a language or be independent.
  • DevKit: a bundle of languages and other devkits. DevKits can also export solutions and languages. Importing a DevKit into a module or model automatically makes all its exported languages and solutions available. This is the preferred way to manage common sets of languages and dependencies.
• Models: contained within modules. They hold a collection of Root Nodes.
• Nodes: the basic building blocks of the AST. Nodes are organized hierarchically.
  • Root Nodes: the top-level nodes in a model.
• Concepts: define the "type" of a node (like a class in OOP). They define properties, children, and references. Concepts are defined in the structure aspect of a language. Like in OOP, a concept can extend another concept and implement multiple interface concepts, which leads to subconcept-superconcept relationships and affects assignability of nodes into child or reference roles.
• Aspects: different parts of a language definition (Structure, Editor, Typesystem, Constraints, etc.). Technically, each aspect is a dedicated model inside a language's module.
• Some MPS modules and models can be read-only.
• MPS modules and models define dependencies between each other. DevKits can re-export dependencies on solutions and other devkits. If a module depends on a DevKit, it implicitly depends on all solutions exported by that DevKit.
• MPS models specify 'used languages'. If model A uses language L, nodes in model A can be instances of concepts from language L. Using a DevKit in a model automatically includes all languages exported by that DevKit (and any devkits it extends).

Common Workflow — Initialize a Session

1. Check generated DSL skills under .agents/skills/*-dsl/ before starting unfamiliar DSL work. Use the relevant project-local DSL skill when it exists; if it is missing or stale, load mps-dsl-memory to create or refresh it.
2. Anchor on the user's focus — call mps_mcp_get_current_editor_root_node so you know which root the user is looking at.
3. Identify the task family: defining a language → load mps-language-aspects-overview. Editing user code → load mps-node-editing and (often) mps-baselanguage. Investigating an unfamiliar language → load mps-language-analysis.
4. Follow the matching aspect skill for the specific aspect you're touching.
5. Validate frequently with mps_mcp_check_root_node_problems. Rebuild/reload after compiled-aspect changes.

Essential Skills (Detail)

Open references/finding-things.md for the protocol on finding models, modules, and languages (including shortened-name resolution like j.m.l.core → jetbrains.mps.lang.core).

Open references/node-editing-rules.md for the full rulebook on adding/updating nodes (concept selection, role types, cardinality, assignability, persistent IDs, surgical edits, reload after compiled-aspect changes).

Open references/reference-formats.md for the reference-format protocol: node refs (r:/i:), concept refs (c:), and the critical "never use a concept ref where a node ref is expected" rule.

Open references/bulk-creation.md for the print-shallow-then-add-children staged construction workflow used when subtrees exceed the JSON size limit.

Open references/analysis-tools.md for the inventory of analysis operations (mps_mcp_print_node, mps_mcp_check_root_node_problems with its quickFixes/autoApplyQuickFixes, mps_mcp_list_node_intentions / mps_mcp_apply_intention, mps_mcp_alter_nodes FIX_REFERENCES, etc.).

Open references/editor-workflow.md for the MPS Concept Editor workflow (scaffold → componentize → refine → validate) and the editor procedural guidelines.

Open references/mcp-tools-index.md for the complete inventory of MPS MCP tools grouped by Project/Structure, Modules and Models, Root Nodes and Nodes, Console, and Language Definition.

Boundaries

• Do not edit serialized .mps model files as plain text unless the user explicitly asks for it.
• Do not edit .mpl module descriptors manually if an MCP wiring tool (mps_mcp_module_dependency, mps_mcp_update_module, …) covers the change.
• Do not delete-and-reinsert a node to "change" it when surgical tools exist.