bun-runtime

A guide to Bun, a JavaScript toolkit that can run programs, install packages, bundle code, and run tests. It explains when Bun fits better than Node.js and how to move between them.

In plain words
What is it for?
It is for creating or migrating JavaScript and TypeScript projects, running scripts and tests, bundling applications, managing packages, and configuring Bun deployments.
Why use it?
It helps choose a JavaScript toolchain and avoid compatibility problems when adopting Bun or moving an existing project from Node.js.

Skill for Claude CodeCodex

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.

agentmods
npx agentmods add skills/junmystery/agent-guidance-python/bun-runtime
Any agent
npx skills add JunMystery/Agent-Guidance-Python --skill bun-runtime
Clone the repo
git clone --depth 1 https://github.com/JunMystery/Agent-Guidance-Python

Made for: Claude Code, Codex.

Per session 34 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.
When invoked 666 The whole file, excluding the scripts and references it only reads on demand.
Security scan A 1 finding. Scan, not verified.
Origin 100% copy Near-identical to another mod 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.

ModelPer sessionOnce invoked
Fable 5 $0.00034 $0.00666
Opus 5 $0.00017 $0.00333
Sonnet 5 $0.00007 $0.00133
Haiku 4.5 $0.00003 $0.00067

Measured 2d ago against content hash 8015a977827d, method: parsed. Prices are Anthropic first-party input rates as of 2026-08-30, from the pricing page.

Security

Grade A, and why

bun-runtime scanned grade A with 1 finding 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 2d 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.

Makes network callslowCapability

Not a fault in itself. Listed so you know the mod talks to something, and to what.

fetch(req) {
Origin

This is a copy

100% identical to bun-runtime — 2 lines differ, which has more behind it and is treated as the original. This page carries a canonical link to it rather than competing with it.

skills/bun-runtime/SKILL.md · 86 lines

How it starts

The opening of the file, as written. The whole thing — 86 lines — stays where its author put it; the contents beside it link to each section on GitHub.

Bun Runtime

Bun is a fast all-in-one JavaScript runtime and toolkit: runtime, package manager, bundler, and test runner.

When to Use

  • Prefer Bun for: new JS/TS projects, scripts where install/run speed matters, Vercel deployments with Bun runtime, and when you want a single toolchain (run + install + test + build).
  • Prefer Node for: maximum ecosystem compatibility, legacy tooling that assumes Node, or when a dependency has known Bun issues.

Use when: adopting Bun, migrating from Node, writing or debugging Bun scripts/tests, or configuring Bun on Vercel or other platforms.

How It Works

  • Runtime: Drop-in Node-compatible runtime (built on JavaScriptCore, implemented in Zig).
  • Package manager: bun install is significantly faster than npm/yarn. Lockfile is bun.lock (text) by default in current Bun; older versions used bun.lockb (binary).
  • Bundler: Built-in bundler and transpiler for apps and libraries.
  • Test runner: Built-in bun test with Jest-like API.

Migration from Node: Replace node script.js with bun run script.js or bun script.js. Run bun install in place of npm install; most packages work. Use bun run for npm scripts; bun x for npx-style one-off runs. Node built-ins are supported; prefer Bun APIs where they exist for better performance.

Vercel: Set runtime to Bun in project settings. Build: bun run build or bun build ./src/index.ts --outdir=dist. Install: bun install --frozen-lockfile for reproducible deploys.

Examples

Run and install

# Install dependencies (creates/updates bun.lock or bun.lockb)
bun install

# Run a script or file
bun run dev
bun run src/index.ts
bun src/index.ts

Scripts and env

bun run --env-file=.env dev
FOO=bar bun run script.ts

Testing

bun test
bun test --watch
// test/example.test.ts
import { expect, test } from "bun:test";

test("add", () => {
  expect(1 + 2).toBe(3);
});

Runtime API

Read the full file on GitHub · 86 lines

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.

  1. 2d ago First seen · 86 lines · 34 tokens per session scan A 8015a977827d

Subscribe to this mod's changes

bun-runtime is a skill published in the GitHub repository JunMystery/Agent-Guidance-Python (2 stars, last pushed 1mo ago), licensed MIT. It adds 34 tokens to every session and 666 once invoked, about $0.0002 per session on Opus 5. A static security scan graded it A with 1 finding (makes network calls). It is 100% identical to bun-runtime, differing in 2 lines, and is treated as a copy.

Related

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common-session-retrospective

Analyze conversation corrections to detect skill gaps and prepare targeted skill-library maintenance tasks. Use after any session with user corrections, rework, or retrospective requests. After finding correction loops, also load +common/common-learning-log to persist mistake entries to AGENTSLEARNING.md.

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common-store-changelog

Generate user-facing release notes for the App Store and Google Play from git history (App Store <=4000 chars, Google Play <=500). Use when generating release notes, app store changelog, play store release, or "what's new" text for a mobile app.

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typescript-language

Apply modern TypeScript standards for type safety and maintainability. Use when working with types, interfaces, generics, enums, unions, or tsconfig settings.

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Conduct high-quality, persona-driven code reviews. Use when reviewing PRs, critiquing code quality, or analyzing changes for team feedback.

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