What is modern-js-runtimes?

A set of rules for developing JavaScript and TypeScript projects with Bun, a runtime, package manager, and test runner. It also distinguishes Ethereum's Keccak-256 hashing from the similar but incompatible SHA3-256 standard.

Running TypeScript directly, installing dependencies, testing with Bun, using Bun APIs, and checking Ethereum-related code for the wrong hashing algorithm.

It prevents mixed package managers from breaking lockfiles and avoids incorrect hashes that can silently invalidate Ethereum selectors, signatures, or storage slots.

modern-js-runtimes — Skill for Claude Code and Codex by jxoesneon

Install

Getting it into your agent

One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.

Any agent

npx skills add jxoesneon/Ciel --skill modern-js-runtimes

Clone the repo

git clone --depth 1 https://github.com/jxoesneon/Ciel

Made for: Claude Code, Codex.

Wrote 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.

README.md

[![agentmods](https://agentmods.dev/badge/skills/jxoesneon/ciel/modern-js-runtimes/github.svg)](https://agentmods.dev/skills/jxoesneon/ciel/modern-js-runtimes)

Your own site

<a href="https://agentmods.dev/skills/jxoesneon/ciel/modern-js-runtimes"><img src="https://agentmods.dev/badge/skills/jxoesneon/ciel/modern-js-runtimes/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.

Your own site · 80×15

<a href="https://agentmods.dev/skills/jxoesneon/ciel/modern-js-runtimes"><img src="https://agentmods.dev/badge/skills/jxoesneon/ciel/modern-js-runtimes.svg" alt="Reviewed on agentmods" width="80" height="20"></a>

Per session 20 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.

When invoked 529 The whole file, excluding the scripts and references it only reads on demand.

Security scan A 0 findings. A grade says what 26 rules found in the file — not that it is safe.

Origin original No closer match found in the catalogue.

Token cost

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	$0.00020	$0.00529
Opus 5	$0.00010	$0.00264
Sonnet 5	$0.00004	$0.00106
Haiku 4.5	$0.00002	$0.00053

Measured 8d ago against content hash 21bdf31102bd, method: parsed. Prices are Anthropic first-party input rates as of 2026-09-11, from the pricing page.

Security

Grade A, and why

modern-js-runtimes 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 8d 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.

skills/modern-js-runtimes/SKILL.md · 49 lines

What it actually says

CIEL ADAPTATION: Modern JS Runtimes (Bun & Keccak)

This skill manages high-speed JS/TS development using Bun and enforces cryptographic correctness for specialized domains (Ethereum).

The Bun Stack

All-in-One: Use Bun as the Runtime, Package Manager (bun install), and Test Runner (bun test).
Natiive TS: Run .ts files directly without a separate transpilation step.
Performance: Prefer Bun's native APIs (e.g., Bun.file(), Bun.serve()) over Node equivalents for hot paths.

Hashing Integrity (Ethereum)

Hard Gate: For Ethereum contexts (selectors, signatures, storage slots), NEVER use Node's crypto.createHash('sha3-256') (NIST SHA3).
Mandate: Use keccak256 from ethers, viem, or web3.
Audit: Grep for createHash.*sha3 in Ethereum-related packages to find silent bugs.

Operational Standards

Lockfiles: ALWAYS commit bun.lock (text) or bun.lockb (binary) for reproducible installs.
Vercel: Explicitly set the runtime to Bun in project settings for 2x deployment speed.

Anti-Patterns

Mixing Managers: Using npm or yarn in a Bun-initialized project (corrupts lockfiles).
Amnesiac Hashing: Assuming "SHA3" means "Keccak" in JS (NIST SHA3 != Keccak-256).
Silent Bun Failure: Ignoring Bun-specific dependency issues; fallback to Node if compatibility is broken.

Changes

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.

8d ago First seen · 49 lines · 20 tokens per session scan A 21bdf31102bd

Subscribe to this mod's changes

modern-js-runtimes is a skill published in the GitHub repository jxoesneon/Ciel (1 stars, last pushed 4d ago), licensed Apache-2.0. It adds 20 tokens to every session and 529 once invoked, about $0.0001 per session on Opus 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 2026-09-03.

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