Runtime APIs

Two rules, in order:

1. Bun-first. When the file runs under Bun and a native equivalent exists, use it: Bun.file, Bun.write, Bun.spawn/Bun.spawnSync, Bun.Glob, Bun.YAML, Bun.serve.
2. Else the node: prefix. Reach for the Node built-in with its protocol prefix: import { join } from "node:path", require("node:fs"). Never the bare specifier: write node:path, not path.

The prefix is mandatory even in Bun-only code: it marks the import as a built-in rather than an npm package, so resolution is identical under every runtime.

Not everything runs under Bun

Bun.* exists only on the Bun runtime. These areas run on Node in at least one of their environments, so node: is load-bearing and Bun.* is a crash, not a style choice:

Area	Runtime	Rule
packages/cli/**	Node	node: only. Published to npm, launched via npx (bin shebang #!/usr/bin/env node); Bun.* breaks every npx user.
web/next/**	Node + Bun	node: only. next dev runs under the system Node locally; Docker (bun server.js) and Vercel (bunVersion, built via bun run --bun) run it under Bun. Stays portable.
packages/env/**	Node + Bun	node: only. Imported by web (Node and Bun) and api (Bun); stays portable. Stricter for web-next.ts and the package index: client components import them, so they carry no node:* at all, prefixed or not. The .env load (node:path + dotenv) lives in load-dotenv.ts, which only the server targets import; a new server target imports @/load-dotenv first, and a script that reaches env through web-next or the index imports @packages/env/load-dotenv itself.
.github/workflows/* (actions/github-script)	Node	require("node:...").

Bun-first therefore applies to the Bun-only files: .github/scripts/*.ts, packages/scripts/src/*.ts (bun x.ts) and the end-to-end helper tests/stack.ts (bun test). The CLI test files run under bun test but mirror the CLI's node: style on purpose.

Bun equivalents

Need	Bun (preferred)	Node fallback
read file text	await Bun.file(p).text()	node:fs readFileSync
read file bytes	await Bun.file(p).arrayBuffer()	node:fs readFileSync
write file	await Bun.write(p, data)	node:fs writeFileSync
file exists	await Bun.file(p).exists()	node:fs existsSync
spawn a process	Bun.spawn / Bun.spawnSync	node:child_process
glob	new Bun.Glob(pattern)	(none)
parse YAML	Bun.YAML.parse	(none)
serve HTTP/WS	Bun.serve	@hono/node-server (Vercel only)

Built-ins with no Bun equivalent (always node:)

node:path, node:os, node:util, node:crypto, node:readline/promises, and the directory/sync parts of node:fs (existsSync in sync code, mkdirSync, mkdtempSync, readdirSync, rmSync). Bun ships no Bun.path/Bun.os, so node: is the idiomatic API for these even in pure-Bun files.

Audit

No bare (unprefixed) built-in specifier may exist anywhere. The $B alternation is the full Node built-in list, so a new bare import cannot slip through; extend it only when Node ships a new module. This prints nothing when clean:

B='assert|async_hooks|buffer|child_process|cluster|console|constants|crypto|dgram|diagnostics_channel|dns|domain|events|fs|http|http2|https|inspector|module|net|os|path|perf_hooks|process|punycode|querystring|readline|repl|stream|string_decoder|sys|timers|tls|trace_events|tty|url|util|v8|vm|wasi|worker_threads|zlib'
rg -n "from ['\"]($B)(/[a-z_]+)?['\"]|require\(['\"]($B)(/[a-z_]+)?['\"]\)" -g '!**/dist/**' -g '!.claude/worktrees/**' --hidden

The full per-file inventory lives in index.md: every file, its runtime, and the node: modules it imports. Regenerate it when the surface changes:

rg -n "node:[a-z/_]+" -g '!**/dist/**' -g '!.claude/worktrees/**' --hidden