Bun Runtime

Sub-skill of bun. Covers the Bun 1.3.13 runtime surface as it shows up in build scripts, codegen, and one-off tooling. Browser app code is built by Vite — Bun does not execute the React app at runtime.

When to invoke

• Writing a maintenance script in scripts/*.ts that touches the filesystem or shells out.
• Running a tool through bunx <package> (the npx equivalent).
• Authoring a tiny local-only HTTP helper for tests/fixtures (never the production server — there is no production server).

Owns

Bun runtime APIs (Bun.serve, Bun.file, Bun.$, FFI), runtime flags, and the rule that browser code is bundled by Vite — Bun is the script/tooling runtime.

Defers to

• bun (parent) — for Bun version pinning and overall routing.
• bun-package-manager — for installing the script's dependencies.
• bun-test — when the script is actually a test file.
• nitro — for the static build output; Bun does not serve the prod app.

Dean-stack rules

• Pillar 4 (CLI-gate-first) means: any script you write to support bun run check must exit non-zero on any warning, mirroring the gate's contract.
• The runtime is tooling-only. The kid loads the app from GitHub Pages — there is no Bun on the server because there is no server.
• Use Bun.$ over child_process.spawn for cross-platform shell snippets (Bun escapes interpolation safely; Node's exec does not).

Patterns

Read/write files in scripts

// scripts/sync-fixtures.ts — runs via `bun run scripts/sync-fixtures.ts`
const manifest = await Bun.file("./fixtures/manifest.json").json();
await Bun.write("./apps/web/public/fixtures.json", JSON.stringify(manifest, null, 2));

Bun.file is lazy; Bun.write accepts strings, bytes, or another BunFile.

Shell out safely with Bun.$

import { $ } from "bun";

const sha = (await $`git rev-parse --short HEAD`.text()).trim();
const { exitCode } = await $`bun run check`.nothrow();
if (exitCode !== 0) process.exit(exitCode);

Interpolation is auto-escaped. nothrow() returns the exit code instead of throwing — needed when chaining gate steps.

Run third-party CLIs with bunx

bunx playwright install chromium
bunx @tanstack/router-cli generate

bunx is the only correct invocation form in this repo. Never npx.

Local-only helper server (rare)

// dev fixture server — never runs in production
const server = Bun.serve({
  port: 0, // OS-assigned
  routes: {
    "/health": new Response("ok"),
    "/fixture/:id": (req) => Response.json({ id: req.params.id }),
  },
});
console.log(`fixtures on ${server.url}`);

Use only for Playwright fixtures or local dev tooling. The dean-stack app itself ships as static files — see nitro.

Anti-patterns

• Don't run app code through bun run apps/web/... — Vite builds the SPA; Bun's runtime never executes React in this stack.
• Don't use node:child_process for shell commands — Bun.$ is cross-platform and escape-safe.
• Don't reach for dotenv/nodemon/tsx — Bun loads .env* automatically and runs .ts natively.
• Don't write a long-lived Bun.serve for production — the deploy target is GitHub Pages (static); a server contradicts that.

Triggers on

bun runtime, Bun.serve, Bun.file, Bun.$, bun shell, bunx, bun script