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/prerender-sizing/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/prerender-sizing)<a href="https://agentmods.dev/skills/cardstack/boxel/prerender-sizing"><img src="https://agentmods.dev/badge/skills/cardstack/boxel/prerender-sizing/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/prerender-sizing"><img src="https://agentmods.dev/badge/skills/cardstack/boxel/prerender-sizing.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 | $0.00138 | $0.05346 |
| Opus 5.5 | $0.00055 | $0.02138 |
| Sonnet 5.5 | $0.00028 | $0.01069 |
| Haiku 4.5 | $0.00014 | $0.00535 |
Grade A, and why
prerender-sizing 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 — 337 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Prerender server sizing
The prerender pool's tab capacity is governed by a small set of SSM-driven knobs:
| Env var | What it controls |
|---|---|
PRERENDER_PAGE_POOL_MIN |
Idle floor — pool never contracts below this. |
PRERENDER_PAGE_POOL_MAX |
Burst ceiling reachable by any priority. |
PRERENDER_PAGE_POOL_HIGH_PRIORITY_MAX |
Extra ceiling, reachable only when caller priority >= HIGH_PRIORITY_THRESHOLD. |
PRERENDER_HIGH_PRIORITY_THRESHOLD |
Priority bar that unlocks the upper tier. |
PRERENDER_PAGE_POOL_IDLE_CONTRACTION_MS |
Hysteresis window before each contraction tick. |
PRERENDER_SHARED_CONTEXT_CAP |
Absolute LRU cap for cached BrowserContexts. |
Plus the ECS task definition's cpu and memory. All these together form the memory envelope that bounds how many warmed BrowserContexts the system can hold and how much burst headroom it has.
The right values aren't fixed once. As the workload shifts — more realms, larger card fan-outs, busier user traffic — the envelope needs re-tuning. This skill is the methodology to derive new values from telemetry rather than guessing.
When to use this skill
Trigger on any of:
- "Are we sized right for prerender?" / "should we resize the prerender ECS task?"
- "We're hitting memory pressure / OOM on the prerender service."
- "We added more realms / a bigger card schema, do we need more prerender capacity?"
- "Why does the dashboard show prerender memory peak at X%?"
- "Should I bump
PRERENDER_PAGE_POOL_MAXfrom N to M?"
If the user is asking "why did this single render time out", that's the indexing-diagnostics skill, not this one. This skill is for capacity planning.
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 · 337 lines · 138 tokens per session scan A dbde7ed4fc72
prerender-sizing is a skill published in the GitHub repository cardstack/boxel (27 stars, last pushed today), licensed MIT. It adds 138 tokens to every session and 5,346 once invoked, about $0.0006 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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