JetBrains MPS is a development environment for creating domain-specific languages, which are programming languages designed for a particular field or task. It provides editors with features such as completion, semantic checks, and type checking, and can generate code in languages including Java and XML.
Getting it into your agent
One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.
npx agentmods add skills/jetbrains/mps/mps-baselanguagenpx skills add JetBrains/MPS --skill mps-baselanguagegit clone --depth 1 https://github.com/JetBrains/MPSWrote this? Show the measurements
A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.
[](https://agentmods.dev/skills/jetbrains/mps/mps-baselanguage)<a href="https://agentmods.dev/skills/jetbrains/mps/mps-baselanguage"><img src="https://agentmods.dev/badge/skills/jetbrains/mps/mps-baselanguage.svg" alt="Measured on agentmods" height="20"></a>What it costs to keep this loaded
Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.
| Model | Per session | Once invoked |
|---|---|---|
| Fable 5 | $0.00094 | $0.01818 |
| Opus 5 | $0.00047 | $0.00909 |
| Sonnet 5 | $0.00019 | $0.00364 |
| Haiku 4.5 | $0.00009 | $0.00182 |
Grade A, and why
mps-baselanguage scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 5d ago.
A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.
Nothing flagged
None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.
How it starts
The opening of the file, as written. The whole thing — 62 lines — stays where its author put it; the contents beside it link to each section on GitHub.
MPS BaseLanguage (Java) Authoring
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:(conceptsNPEEqualsExpression/NPENotEqualsExpression). Never==or.equals()between SNode references. - Constructor return type:
ConstructorDeclaration.returnTypeMUST be aVoidTypenode. Leaving it empty fails validation. ClassCreatorwiring:ClassCreator.baseMethodDeclarationpoints at the constructor declaration, not the class.InstanceMethodCallOperation.baseMethodDeclarationpoints at the method declaration.- Inherited methods: use the declaring class ref for
baseMethodDeclaration, not the subclass that calls the method. E.g.addActionListeneris declared onAbstractButton, so use theAbstractButtonclass ref. - Expressions in statement lists: any
Expressionmust be wrapped inExpressionStatementto be valid inside aStatementList. - Variable declarations: in method bodies wrap
LocalVariableDeclarationinLocalVariableDeclarationStatement. InForStatement.variableuseLocalVariableDeclarationdirectly, no wrapper. - Prefer primitives: use
string(StringType) overString(ClassifierType) where possible; same forint,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.closuresextension for closures. The Java parser (mps_mcp_parse_java_and_insert) additionally accepts the Java 8+ syntax MPS recognizes — most notably lambdas, which it maps toclosuresClosureLiterals (auto-imported whenpostProcess.importUsedLanguagesis 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. Seereferences/parse-java-tips.md. - Surgical edits: when a single child changes prefer
mps_mcp_update_nodeover rewriting the whole root — full-root rewrites churn persistent IDs and break incoming refs.
What ships with it
7 files beside SKILL.md in the same directory: the scripts, references and assets a skill reads on demand. Not counted in the per-session cost; read them before you install if any of them is executable.
What this file has done since we first saw it
Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.
- 5d ago First seen · 62 lines · 94 tokens per session scan A 31274d492f50
mps-baselanguage is a skill published in the GitHub repository JetBrains/MPS (1,658 stars, last pushed yesterday), licensed Apache-2.0. It adds 94 tokens to every session and 1,818 once invoked, about $0.0005 per session on Opus 5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-30.
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