Borrowing it
Nothing to install: this file belongs to cardstack/boxel. 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/cardstack/boxel/main/.claude/skills/filedef-authoring/SKILL.mdgit clone --depth 1 https://github.com/cardstack/boxelWrote 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/cardstack/boxel/filedef-authoring)<a href="https://agentmods.dev/skills/cardstack/boxel/filedef-authoring"><img src="https://agentmods.dev/badge/skills/cardstack/boxel/filedef-authoring/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/cardstack/boxel/filedef-authoring"><img src="https://agentmods.dev/badge/skills/cardstack/boxel/filedef-authoring.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 | $0.00212 | $0.02317 |
| Opus 5.5 | $0.00085 | $0.00927 |
| Sonnet 5.5 | $0.00042 | $0.00463 |
| Haiku 4.5 | $0.00021 | $0.00232 |
Grade A, and why
filedef-authoring 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 — 82 lines — stays where its author put it; the contents beside it link to each section on GitHub.
FileDef authoring & content-only rendering
FileDef families (markdown, image, audio, video, code, data, archive, font, 3D, …) live in packages/base/, with the shared machinery in packages/base/file-formats/. A family is a FileDef subclass that adds its @fields and, when a custom render beats the fallback, one renderer; it does not re-implement identity, facts, budgets, or state handling.
Family architecture
- Four shared shells —
FileAtomShell,FileEmbeddedShell,FileFittedShell,FileIsolatedShell(infile-formats/). Every family inherits these fromFileDefand supplies only the renderer they mount, rather than shipping its ownisolated/embedded/fitted/atomtemplates. The shells draw the chrome: the isolated shell's file bar (Download / Copy link) and the GENERAL / PARSER OUTPUT / DERIVED metadata inspector, which is also where a file's provenance is shown. FilePreviewStageis the slot the shells project into. It owns the render concerns every family shares — the current-render cache, the loading / failure / staleness panes, and the generic fallback pane for a family with no renderer. Provenance is not a stage concern: the stage deliberately floats no provenance overlay (it would land on whatever the family draws in that corner) and leaves it to the isolated shell's metadata chrome.static previewComponentis the family hook, and it is optional. AFileDefsubclass may setstatic previewComponent = XPreviewandFilePreviewStagemounts it; a family that declares none falls through to the generic fallback pane.filePreviewComponentFor(instance)resolves the renderer a file's class declares (read off the instance's constructor, since alinksTo(FileDef)is routinely a subclass instance).
So a new family = add @fields, plus a previewComponent only when a custom render beats the fallback. It should not touch the shells.
The wrapped-default decision (do not "fix" it)
The default FileDef format templates are wrapped by design: the isolated view is the inspector (file bar + metadata groups). Navigating to a .md URL landing on that inspector is intentional, not a bug. Do not strip the chrome from the default templates.
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 · 82 lines · 212 tokens per session scan A 7324debb7a5e
filedef-authoring is a skill published in the GitHub repository cardstack/boxel (27 stars, last pushed today), licensed MIT. It adds 212 tokens to every session and 2,317 once invoked, about $0.0008 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 2026-10-21.
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