Borrowing it
Nothing to install: this file belongs to ymm-oss/fsl. Take a copy, put it at the same path in your own repository, and replace the rules that are about this project with yours.
curl -O https://raw.githubusercontent.com/ymm-oss/fsl/main/.claude/skills/add-language-feature/SKILL.mdgit clone --depth 1 https://github.com/ymm-oss/fslWrote 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/ymm-oss/fsl/add-language-feature)<a href="https://agentmods.dev/skills/ymm-oss/fsl/add-language-feature"><img src="https://agentmods.dev/badge/skills/ymm-oss/fsl/add-language-feature/github.svg" alt="Measured on agentmods" height="20"></a>Or the 80×15 button, for a site that already has a row of RSS and ATOM ones. Only the verdict fits; the numbers stay here.
<a href="https://agentmods.dev/skills/ymm-oss/fsl/add-language-feature"><img src="https://agentmods.dev/badge/skills/ymm-oss/fsl/add-language-feature.svg" alt="Reviewed on agentmods" width="80" height="20"></a>- NVIDIA SkillSpector pass
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 5.1 | $0.00039 | $0.00537 |
| Opus 5.5 | $0.00016 | $0.00215 |
| Sonnet 5.5 | $0.00008 | $0.00107 |
| Haiku 4.5 | $0.00004 | $0.00054 |
Grade A, and why
add-language-feature 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 1mo 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 — 43 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Add a native FSL language feature
The Rust workspace is authoritative. Before editing, classify the feature as surface-only lowering, typed Kernel behavior, evaluator semantics, CLI/report behavior, or public Kernel contract.
Coupled checklist
rust/fsl-syntax: tokens, parsing, AST, and locations when syntax changes.rust/fsl-core: validation, types, name resolution, dialect lowering, and Kernel projection.rust/fsl-runtime: concrete Monitor/BFS behavior for evaluable semantics.rust/fsl-verifier: symbolic BMC/induction/refinement/liveness behavior.rust/fsl-toolsorrust/fslc: report/command/envelope behavior when applicable.rust/fsl-wasm: only when Worker-visible behavior or shared envelopes require it.- Focused positive, negative, and boundary regression evidence.
docs/manual/LANGUAGE.md,docs/manual/LANGUAGE.ja.md,skills/fsl/references/, relevantdocs/design/DESIGN-*.md, docs map, and a newchangelog.d/<id>-<slug>.<category>.mdfragment (seechangelog.d/README.md) -- not a directCHANGELOG.mdedit.- A new declaration, binder, or reference form:
rust/fsl-lsp/src/index.rsand a targeted role/scope test. - Schemas/conformance vectors when the public Kernel contract changes.
Do not update src/fslc by default. Record the explicit compatibility or LSP contract before changing
the frozen Python reference.
Verification
- Iterate with the native CLI:
cargo run --manifest-path rust/Cargo.toml -p fslc-rust --bin fslc -- check <case>and the smallest relevantverifycommand. - Run focused syntax/core/runtime/verifier tests for the changed seam.
- Check symbolic/concrete/BFS agreement and a false-negative case.
- Run induction, mutation, vacuity, refinement, replay, or browser evidence when the contract calls for it.
- Run formatting and Clippy, then wider workspace and compatibility gates in proportion to impact.
- Invoke the soundness and coupled-change reviewers before completion.
Never widen an allowlist, hollow a spec, or hand-edit a generated artifact to make the gate pass.
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.
- 1mo ago Changed 1d5355062dc7
- 1mo ago Changed · +2 lines 51efcf3e9244
- 2mo ago First seen · 41 lines · 39 tokens per session scan A e4eb91e40554
add-language-feature is a skill published in the GitHub repository ymm-oss/fsl (35 stars, last pushed today), licensed Apache-2.0. It adds 39 tokens to every session and 537 once invoked, about $0.0002 per session on Opus 5.5. 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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