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-language-modularitynpx skills add JetBrains/MPS --skill mps-language-modularitygit 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-language-modularity)<a href="https://agentmods.dev/skills/jetbrains/mps/mps-language-modularity"><img src="https://agentmods.dev/badge/skills/jetbrains/mps/mps-language-modularity.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 | $0.00179 | $0.01885 |
| Opus 5 | $0.00089 | $0.00942 |
| Sonnet 5 | $0.00036 | $0.00377 |
| Haiku 4.5 | $0.00018 | $0.00188 |
Grade A, and why
mps-language-modularity 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 4d 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 — 80 lines — stays where its author put it; the contents beside it link to each section on GitHub.
MPS Language Modularity & Composition
The design tier for multi-language MPS work: before authoring aspects, decide how the languages relate. This skill turns a 2×2 taxonomy into a decision procedure and routes each choice into the aspect skills that implement it.
The MPS-specific lesson: syntax composition is the easy part. MPS is projectional — no parser, no grammar to become ambiguous — so combining notations needs far less work than in parser-based tools (only alias clashes and completion UX still need a glance). The real difficulty is semantic: scopes, type systems, generator contracts, and migration. Choosing a modularity kind is mostly choosing where the dependency lives and how generated code interacts.
Critical directives
- Spend effort on semantics, not syntax. Scopes, type bridges, generator contracts, and migration are where modular languages break.
- Two questions decide the kind (below). Answer them before touching any aspect.
- MPS "extends" ≠ the design category. At the module level MPS calls almost any inter-language use "extension"; that tool-level dependency is independent of whether your design is referencing/extension/reuse/embedding. See
references/design-principles.md. - References vs. containment is the structural fork. Model a foreign element as a reference → referencing. Contain a foreign concept as a child → inline composition (extension vs embedding). Hard to reverse — pick deliberately.
- Reuse must be designed in. Abstract hooks, behavior contracts, and generator placeholders must exist before you need them; after-the-fact reuse (especially interwoven generation) is severely limited.
- The exported surface is an API. Behavior-method names, interface concepts, placeholder IDs, mapping labels, and generated abstract signatures are cross-language contracts — stabilize, version, and test them.
The two questions (the 2×2)
The four-kind taxonomy (referencing / extension / reuse / embedding) was introduced by Markus Voelter in his work on language composition (DSL Engineering). This skill adapts it to MPS practice.
What ships with it
6 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.
- 4d ago First seen · 80 lines · 179 tokens per session scan A d0678570b76d
mps-language-modularity is a skill published in the GitHub repository JetBrains/MPS (1,658 stars, last pushed today), licensed Apache-2.0. It adds 179 tokens to every session and 1,885 once invoked, about $0.0009 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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