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/.github/instructions/api-headers.instructions.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/instructions/webern/mx/api-headers)<a href="https://agentmods.dev/instructions/webern/mx/api-headers"><img src="https://agentmods.dev/badge/instructions/webern/mx/api-headers/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/instructions/webern/mx/api-headers"><img src="https://agentmods.dev/badge/instructions/webern/mx/api-headers.svg" alt="Reviewed on agentmods" width="80" 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 5.1 | $0.00426 | $0.00426 |
| Opus 5.5 | $0.00170 | $0.00170 |
| Sonnet 5.5 | $0.00085 | $0.00085 |
| Haiku 4.5 | $0.00043 | $0.00043 |
Grade A, and why
mx api-headers.instructions.md 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.
What it actually says
These are the public mx::api headers: plain-old-data structs that re-shape MusicXML into a
simpler model (doctrine: .claude/skills/mx-api-doctrine/SKILL.md). Review for:
- No
mx::coretype may appear in a public header. - New absent-able fields must be
std::optional<T>. New absent-able enums must instead have anunspecifiedfirst enumerator (including uses of the ternaryBool). Reject new-1sentinels and newbool is...Specified/has...flags; do not ask for migration of existing ones (tracked in issue #249). - Every new field needs a default value and a matching
MXAPI_EQUALS_MEMBER(field)line (MXAPI_DOUBLES_EQUALS_MEMBERfor doubles) in the type's equality block. A missing line silently exempts the field from equality and round-trip checks -- flag it as a bug. - New data must not mirror MusicXML's raw shape when the element is stateful, flat, duplicated, or id-linked; check the change against the principles doc.
- A new positioned-in-a-measure type needs
int tickTimePosition; durations are in ticks. - No UB or exceptions reachable through the public interface; a failed precondition must never
throw. Flag new
Resultusage outsideMusicXml, uncheckedstd::get, or accessors that return references guarded only by a precondition. - Kind-specific payloads use the choice-class pattern (
TimeChoice.h,MarkDataChoice.h), not loose fields that apply only to some kinds. - Comments must serve a user authoring or reading notation: crisp, semantics-first, explaining how MusicXML encodes the feature. Flag round-trip framing ("the source", "preserved") except on true fidelity knobs, notes about what is not modeled, and separator/banner comments.
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 3a6fc543c505
- 2mo ago First seen · 27 lines · 426 tokens per session scan A 3db80f52514e
mx api-headers.instructions.md is an instructions file published in the GitHub repository webern/mx (93 stars, last pushed yesterday), licensed MIT. It adds 426 tokens to every session, about $0.0017 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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