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/realm-load-harness/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/realm-load-harness)<a href="https://agentmods.dev/skills/cardstack/boxel/realm-load-harness"><img src="https://agentmods.dev/badge/skills/cardstack/boxel/realm-load-harness/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/realm-load-harness"><img src="https://agentmods.dev/badge/skills/cardstack/boxel/realm-load-harness.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 | $0.00000 | $0.08555 |
| Opus 5.5 | $0.00000 | $0.03422 |
| Sonnet 5.5 | $0.00000 | $0.01711 |
| Haiku 4.5 | $0.00000 | $0.00856 |
Grade A, and why
realm-load-harness 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 — 587 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Realm load harness
packages/realm-server/scripts/load-harness/ drives a non-production realm
server with the load a cohort of users produces: many sessions holding unbounded
dashboard queries open while a few sessions write cards into the same realm.
Read its README.md for the flags and the workload-file format; this skill is
about running it so the numbers mean something, and about which numbers those
are.
The harness is dependency-free by design — global fetch and node: built-ins
only — because where it runs decides what it measures.
Which path a run measures — decide this before the queries
_federated-search serves two documents from the same filter, selected by the
fields[entry] sparse fieldset, and they differ by four to six times in bytes.
Choosing wrongly measures a different code path from the one under
investigation, and no amount of care about which types get queried fixes that.
--fieldset |
on the wire | comes back | models |
|---|---|---|---|
entries (default) |
no fieldset | prerendered renderings (html + css) |
a grid, card list, search panel |
item |
fields: { entry: ['item'] } |
card serializations only | a query-backed field |
item-html |
fields: { entry: ['item','html'] } |
both | — |
Measured on a deployed realm, same filter and page size: 94 KB on the item path against 405 KB and 537 KB on the entries path for two types.
store.search sends fields: { entry: ['item'] } and instantiates cards
live from the result, so item is the path a query-backed field takes — and
the path a search-saturation investigation is usually about. Grids and the
search panel send no fieldset and get the renderings, which is right for them
because they display prerendered HTML.
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 · 587 lines · 0 tokens per session scan A 9492908e176c
realm-load-harness is a skill published in the GitHub repository cardstack/boxel (27 stars, last pushed today), licensed MIT. It costs nothing until one of its globs matches a file; then it loads 8,555 tokens. 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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