Borrowing it
Nothing to install: this file belongs to webern/mx. 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/webern/mx/main/.agents/skills/mx-api-add-feature/SKILL.mdgit clone --depth 1 https://github.com/webern/mxWrote 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/webern/mx/mx-api-add-feature)<a href="https://agentmods.dev/skills/webern/mx/mx-api-add-feature"><img src="https://agentmods.dev/badge/skills/webern/mx/mx-api-add-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/webern/mx/mx-api-add-feature"><img src="https://agentmods.dev/badge/skills/webern/mx/mx-api-add-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 | $0.00037 | $0.00558 |
| Opus 5.5 | $0.00015 | $0.00223 |
| Sonnet 5.5 | $0.00007 | $0.00112 |
| Haiku 4.5 | $0.00004 | $0.00056 |
Grade A, and why
mx-api-add-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 2mo 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 — 55 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Add an mx::api feature
Read the mx-api-doctrine skill before designing, implementing, or reviewing the change.
Investigate
Establish the requested user-facing behavior before choosing an implementation:
- Find the relevant MusicXML elements and attributes in the schema and corpus.
- Check whether
mx::apialready represents the feature under a different shape or name. - Inspect the generated
mx::coretypes, which always implement the requested features. - Find analogous api types, reader/writer code, and tests to use as patterns.
Design and implement
Choose an api shape using the doctrine and precedent rather than mirroring MusicXML's shape. Keep the change focused on the requested behavior, but include whatever representation is needed to make that behavior sound.
Feature work commonly touches:
- public data types in
src/include/mx/api/ - core-to-api translation in
src/private/mx/impl/*Reader*or*Functions* - api-to-core translation in
src/private/mx/impl/*Writer*or*Functions* - enum conversion in
src/private/mx/impl/Converter.* - tests in
src/private/mxtest/api/
These are landmarks, not a checklist. Follow the closest existing feature through both translation directions and change only the files the design requires. Do not edit generated core files by hand.
Verify
- Check API round-trip discovery with
make api-roundtrip-discoverto see if you have unlocked a file. If so, add it to the round-trip baselinesrc/private/mxtest/api/roundtrip-baseline.txt. If your changes did not unlock a file, still add it and note that it was discovered already passing.
If your reader and writer functions seem complex, add testing. Here are some ideas:
- an in-memory
ScoreDataround trip withmxtest::roundTrip. Probe theScoreDatavalues in the unit test. - XML output/input checks with
mxtest::toXmlandmxtest::fromXml
Run the applicable gates from AGENTS.md; api feature work normally includes make api-test,
make api-roundtrip, and make fmt.
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.
- 2mo ago First seen · 55 lines · 37 tokens per session scan A 469c9b4d6857
mx-api-add-feature is a skill published in the GitHub repository webern/mx (93 stars, last pushed yesterday), licensed MIT. It adds 37 tokens to every session and 558 once invoked, about $0.0001 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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