Bun

Bun's value is that the runtime, the toolchain, and the batteries are one binary. Two biases decide most calls:

• Reach for what already ships. A dependency earns its place by doing what Bun.*, bun:*, and the bundled toolchain do not. Adding one that duplicates a built-in is a defect.
• Write node: only where Bun has no native equivalent, or where the code must also run under Node. Portability is a requirement to state, not a habit to keep.

Release Version

Bun ships one stable line and no long-lived support branches, so bun upgrade moves a machine to whatever release is newest. Pin the version in CI (oven-sh/setup-bun, bun-version) so a developer machine and a runner agree, and state the release a feature needs in the sentence that uses it. Never write "the latest version supports X".

The rules below assume 1.3.0 or later unless a version anchor says otherwise. Floors for the features these rules reference:

• 1.3.0 — Bun.sql for MySQL, MariaDB, and SQLite; Bun.redis; Bun.secrets; Bun.YAML; catalogs; test.concurrent, test.serial, and --concurrent; --randomize; expectTypeOf
• 1.3.2 — the isolated linker as the default for new monorepos; nativeDependencies; ignoreScripts
• 1.3.3 — test(..., { retry })
• 1.3.4 — jest.useFakeTimers(); URLPattern; --compile stops auto-loading tsconfig.json and package.json
• 1.3.5 — Bun.Terminal; Bun.stringWidth; bun:bundle feature flags; trustedDependencies limited to npm sources
• 1.3.6 — Bun.JSONC; Bun.Archive; Bun.build({ files }); metafile: true
• 1.3.7 — Bun.JSON5; Bun.JSONL; Bun.wrapAnsi
• 1.3.8 — Bun.markdown; --metafile-md
• 1.3.9 — bun run --parallel; bytecode for ES modules
• 1.3.10 — optimizeImports; standard TC39 decorators; --compile --target=browser for a single HTML file
• 1.3.11 — Bun.cron(); Bun.sliceAnsi
• 1.3.12 — Bun.WebView
• 1.3.13 — bun test --parallel, --isolate, --shard, --changed; Range and conditional requests in Bun.serve
• 1.3.14 — Bun.Image; the global virtual store; HTTP/3 in Bun.serve; HTTP/2 and HTTP/3 in fetch()
• 1.4.0 — Node 26 compatibility; Bun.XML; Bun.isStandaloneExecutable; Bun.spawn({ cgroup }); directory routes; bun audit fix, bun dedupe, bun prune, bun pm diff, bun pm licenses; bun test --timings; --asset; lockfileVersion: 2

Prefer the Built-in

Reach for the built-in, and drop the package it replaces from package.json.

• HTTP server — Bun.serve() over http.createServer(), Express, or Fastify
• Files — Bun.file() and Bun.write() over node:fs read and write helpers
• Directories — node:fs. There is no Bun.* equivalent for mkdir or readdir
• Globbing — Bun.Glob over glob and fast-glob
• Shelling out — Bun.$ over child_process.exec, zx, execa, cross-env, and rimraf
• Processes — Bun.spawn and Bun.spawnSync over child_process.spawn
• Pseudo-terminals — Bun.Terminal over node-pty (1.3.5)
• SQLite — bun:sqlite over better-sqlite3
• Postgres, MySQL, MariaDB — Bun.sql over pg and mysql2 (1.3.0)
• Redis and Valkey — Bun.redis over ioredis (1.3.0)
• S3-compatible storage — Bun.S3Client over @aws-sdk/client-s3
• Password hashing — Bun.password over bcrypt and argon2
• Testing — bun:test over Jest
• Bundling — Bun.build() and bun build over webpack, Rollup, and esbuild
• Images — Bun.Image over sharp (1.3.14)
• Headless browsing — Bun.WebView over Puppeteer (1.3.12)
• Markdown — Bun.markdown over marked (1.3.8)
• Scheduling — Bun.cron() over node-cron (1.3.11)
• Running scripts concurrently — bun run --parallel over npm-run-all and concurrently (1.3.9)
• Config formats — Bun.TOML, Bun.YAML, Bun.JSON5 (1.3.7), Bun.JSONC (1.3.6), Bun.XML (1.4.0) over @iarna/toml, js-yaml, json5, jsonc-parser, and fast-xml-parser
• Newline-delimited JSON — Bun.JSONL over ndjson (1.3.7)
• Tarballs — Bun.Archive over tar (1.3.6)
• Terminal string width — Bun.stringWidth (1.3.5), Bun.wrapAnsi (1.3.7), Bun.sliceAnsi (1.3.11) over string-width, wrap-ansi, slice-ansi, and cli-truncate
• Route patterns — URLPattern over path-to-regexp (1.3.4)
• Credential storage — Bun.secrets over a plaintext dotfile, for local development tools
• Sleeping, hashing, comparison, compression — Bun.sleep, Bun.hash, Bun.CryptoHasher, Bun.deepEquals, Bun.gzipSync, Bun.zstdCompressSync

Two boundaries hold in the other direction. Web Streams are the native currency — Bun.file().stream(), Response.body, and proc.stdout are all ReadableStream, so node:stream is for code shaped around Node, not the default. crypto.randomUUID() is the standard v4 UUID; Bun.randomUUIDv7() is the time-ordered variant, not a replacement for it.

HTTP Server

• Declare routes in the routes object, and keep fetch for genuinely unmatched requests. Precedence is exact, then parameterized, then wildcard, then the global catch-all — not source order.
• Never deploy without either an error handler or an explicit development: false. development defaults to process.env.NODE_ENV !== "production", so a server whose environment does not set NODE_ENV=production renders Bun's contextual error page to any client that can trigger a throw. On 1.3.14 that page is a ~67 KB HTML overlay carrying the thrown error's payload in an embedded __bunfallback script.
• An error handler decides the response body in both modes and replaces the error page when development is on. Without one, development: false answers 500 with the body Something went wrong! (measured on 1.3.14). The stack trace reaches stderr either way, so a missing handler costs you the response, never the log.
• idleTimeout is 10 seconds and it covers a request whose handler has not yet written a byte. A slow handler, a long poll, or a server-sent-events stream is killed mid-response and the client sees a connection reset. Raise idleTimeout, or call server.timeout(req, 0) for the one request that needs it.
• Return a Response instance as the route value for anything fixed — health checks, redirects, constant JSON. Bun dispatches it with no allocation and caches it for the server's lifetime, so server.reload() is the only way to change it.
• req.cookies writes are applied to the response only under routes. Inside a bare fetch handler nothing is tracked and you build Set-Cookie yourself.
• server.upgrade(req) must be followed by returning undefined, never a Response.
• Treat the return of send() and publish() as a status, not a byte count. -1 is backpressure and 0 is dropped; from 1.4.0 publish() also returns 0 when the topic has no subscribers.
• WebSocket handlers are declared once per server, in the websocket object, not per socket. Type ws.data through the handler object's data property.
• Response.error() and any status outside 100–999 route to error() and answer 500 (1.4.0). Earlier releases wrote an invalid status line.

Read [${CLAUDE_SKILL_DIR}/references/http-server.md] when serving files or a directory, wiring WebSocket pub/sub, tuning server lifecycle or timeouts, or enabling HTTP/2 or HTTP/3 — it carries the five route value shapes and their caching and 404 behavior, the directory-route path rules, the full websocket option set with its defaults, and the stop, reload, and closeIdleConnections semantics.

Files

• Bun.file(path) is lazy. Constructing it touches no disk; size is 0 and exists() is false for a missing path rather than throwing.
• Bun.write(dest, data) takes anything: string, Blob, BunFile, ArrayBuffer, TypedArray, or Response. It picks the fastest syscall for the pair, so copying a file is Bun.write(Bun.file(dst), Bun.file(src)), not a manual read-then-write.
• file.writer() returns a FileSink for incremental writes, and the process stays alive until .end() is called. .unref() opts out.
• Bun.stdin, Bun.stdout, and Bun.stderr are BunFiles, so streaming a file to stdout is one Bun.write call.

Shell and Processes

• Interpolated values in Bun.$ are single literal strings, so command injection is closed by construction. Two holes remain and neither is Bun's to close: spawning bash -c "...${input}" hands control to a real shell, and an external command may read attacker-supplied text as its own flag (--upload-pack=). Validate arguments you pass on.
• Only glob patterns written in the template expand (1.4.0). A * arriving through ${...}, a shell variable, command substitution, or quoted text is literal, and ?, [...], and a leading ! are literal everywhere.
• A non-zero exit throws ShellError. Use .nothrow() where you check exitCode yourself.
• Bun.$ is not /bin/sh. It is an in-process interpreter, so .sh files run identically on Windows and the builtin set is fixed.
• stderr defaults to "inherit" in Bun.spawn, so proc.stderr is undefined. Pass stderr: "pipe" to read it. stdout defaults to "pipe" and stdin to null.
• The parent process does not exit while a child is alive. Call proc.unref() for a child that must not hold it open.
• Set serialization: "json" for IPC with a Node process. The default "advanced" is JavaScriptCore's format and Node cannot read it.

Read [${CLAUDE_SKILL_DIR}/references/shell-and-processes.md] when redirecting shell I/O into JavaScript objects, driving an interactive program through a PTY, applying cgroup limits, or spawning a Worker — it carries the redirection and builtin tables, the stdio option matrix, the Bun.Terminal platform differences, and the worker options.

Testing

• Import from bun:test, never from jest or vitest. jest.* and vi.* exist inside bun:test for ported suites.
• mock.module() is not hoisted. Unlike jest.mock, it runs where it is written, so a module imported above it has already been evaluated and its side effects have already happened. Put the override in a --preload script when the original must never run.
• Pick the right global reset. mock.clearAllMocks() clears history and keeps implementations; jest.resetAllMocks() also drops them (1.4.0 — before, it behaved like clearAllMocks); mock.restore() restores spied originals and undoes no mock.module() override.
• expectTypeOf is a no-op at runtime. A green bun test proves nothing about types — run bunx tsc --noEmit separately.
• bun test sets NODE_ENV=test unless it is already set, and runs in UTC. process.env.TZ stays unset — the runtime zone is UTC, not an environment variable a preload can read.
• An unhandled rejection between tests fails the run. When it happens while a file loads, none of that file's tests run at all.
• Three concurrency knobs are independent: --parallel spreads files across processes, --concurrent and test.concurrent overlap async tests inside one file, --shard=i/n splits files across machines (--parallel and --shard from 1.3.13). --parallel implies --isolate.
• Reach for --parallel --no-isolate on a suite of many small files sharing a large import graph. Isolation re-evaluates every import per file, and on that shape plain bun test beats isolated workers.
• Key per-worker resources off BUN_TEST_WORKER_ID (or JEST_WORKER_ID) so parallel workers do not share a database or a port.
• The default per-test timeout is 5000 ms.

Read [${CLAUDE_SKILL_DIR}/references/testing.md] when writing the first test in a project, configuring CI sharding, reaching for a matcher or a snapshot form, or debugging a mock that stopped returning values — it carries the discovery patterns, the matcher inventory, the mock-reset semantics in full, the --timings sharding workflow, and the bunfig.toml test keys.

Package Manager

• trustedDependencies replaces Bun's built-in allow list; it never extends it. Adding one package silently disables postinstall for every package the built-in list covered, so re-list the ones you still need. [] opts out of lifecycle scripts entirely.
• The built-in list applies only to npm-registry sources (1.3.5). A file:, link:, git:, or github: dependency named esbuild inherits nothing from the real esbuild.
• The linker default comes from the lockfile's configVersion, not from a global setting. A new monorepo gets the isolated linker (1.3.2); a new single-package project and every project whose lockfile predates 1.3.2 get the hoisted one. Two machines therefore lay out node_modules differently unless bun.lock is committed.
• The isolated linker still permits phantom dependencies by default. Set [install] hoist = false to stop node_modules/.bun/node_modules from being created.
• Run bun ci in CI, not bun install. Bun does not turn on the frozen lockfile automatically; bun ci is bun install --frozen-lockfile.
• --production implies --frozen-lockfile and skips devDependencies, but removes nothing already installed. bun prune --production (1.4.0) is what shrinks a build image.

Read [${CLAUDE_SKILL_DIR}/references/package-manager.md] when configuring a monorepo, choosing a linker, hardening a supply chain, or diagnosing an install that differs between machines — it carries the configVersion table, the trustedDependencies modes, workspaces and catalogs, the --filter grammar, minimumReleaseAge and the security scanner, and the bunfig.toml install keys.

Building and Compiling

• Bun.build() rejects with an AggregateError on failure. throw defaults to true, so result.success is meaningful only when you pass throw: false. Checking success on a default build reads a value that never arrives.
• target defaults to "browser". Set target: "bun" for server code, which also emits the // @bun pragma so the runtime skips re-transpiling.
• Ship --compile --minify --sourcemap --bytecode for a production binary. Bytecode needs target: "bun", and for ES modules it needs --compile as well (1.3.9).
• A compiled binary does not auto-load tsconfig.json or package.json (1.3.4), but it does auto-load .env and bunfig.toml. Pass --no-compile-autoload-dotenv and --no-compile-autoload-bunfig where a deployment must not read files from its working directory.
• Cross-compile with --target bun-<os>-<arch>[-musl]. From 1.4.0 x64 ships only the baseline build and the -march=haswell build is gone, so -baseline and -modern resolve to the same binary and neither is a choice.
• A macro's return value must be serializable and its arguments statically known. Code inside node_modules cannot invoke a macro, though your own code may import one from a package.
• Bun.markdown.html() does not sanitize. Raw HTML, event-handler attributes, and javascript: hrefs pass through verbatim, so untrusted Markdown needs a sanitizer before it reaches a browser.

Read [${CLAUDE_SKILL_DIR}/references/build-and-compile.md] when configuring a bundle, embedding assets in an executable, writing a plugin, or reaching for a macro — it carries the full Bun.build option set, code-splitting and optimizeImports behavior, the cross-compilation targets, the embedded-file and /$bunfs rules, the plugin hooks, and the macro serialization constraints.

Data Clients

• Open bun:sqlite with { strict: true }. Without it a missing bind parameter is not an error — a typo binds nothing and the query returns a wrong result silently.
• db.query() caches the compiled statement (20 most recent); db.prepare() does not. Use prepare() for SQL generated at runtime so one-off queries do not evict the statements that repeat.
• Enable WAL with PRAGMA journal_mode = WAL for anything with concurrent readers.
• Set safeIntegers: true when a column can exceed 2^53. The default returns number and rounds silently.
• Every interpolation in a Bun.sql template is a parameter. Use the sql() helper for identifiers, column subsets, and value lists; sql.unsafe escapes nothing.
• Bun.sql queries are lazy — they start on await or .execute(), and .cancel() stops a running one.
• Release a reserved connection. sql.reserve() takes one out of the pool until release(), or use using.
• Set prepare: false behind PgBouncer in transaction mode.

Read [${CLAUDE_SKILL_DIR}/references/data-clients.md] when choosing a client, closing a SQLite database cleanly, building dynamic SQL, or configuring pooling and TLS — it carries the bun:sqlite close and WAL-sidecar semantics, the Bun.sql fragment helpers and query modes, and the Bun.redis and Bun.S3Client configuration.

Configuration and Environment

• bunfig.toml configures the runtime, bun test, bun install, and bun run. A CLI flag overrides it; a project's bunfig.toml overrides .npmrc for the same key (1.4.0).
• .env files load automatically, in order: .env, then .env.{development,production,test} by NODE_ENV, then .env.local (skipped when NODE_ENV=test), then .env.{mode}.local. Values expand $VAR references unless the $ is escaped.
• Bun invoked as node does not load .env (1.4.0) — under bun --bun, bunx --bun, or a node symlink. A package.json script calling node under bun --bun run therefore sees those variables as undefined. Pass --env-file to keep them.
• Disable automatic loading in production with --no-env-file or env = false. Files named by --env-file still load.
• bunfig.toml parsing is strict (1.4.0): an unquoted string value, a missing newline between pairs, or an integer past Number.MAX_SAFE_INTEGER is a SyntaxError at startup.
• Bun.YAML follows YAML 1.2 (1.3.5), so yes, no, on, and off parse as strings. An on: key in a GitHub Actions workflow is the string "on".

Node Compatibility

From 1.4.0 Bun targets Node 26 and reports itself as such; 1.3.x reports Node 24. Most packages run unchanged. Reach for a node: module deliberately in these cases:

• node:sea — not implemented. bun build --compile is the replacement and a different API.
• node:test — partial. Write bun:test.
• node:crypto beyond BoringSSL — no ed448, x448, rsa-pss, dsa, dh, secp256k1, or the CCM, OCB, XTS, and chacha20-poly1305 ciphers.
• node:async_hooks — AsyncLocalStorage works but does not propagate into Worker, MessagePort, or BroadcastChannel. createHook and the async-id APIs are stubs.
• HTTP load balancing across processes — node:cluster shares an HTTP socket only on Linux, through SO_REUSEPORT.
• node:v8 serialize/deserialize — JavaScriptCore's wire format, not V8's, so a buffer does not travel between Bun and Node.
• Loader hooks — module.register is a no-op. Use Bun.plugin.

Read [${CLAUDE_SKILL_DIR}/references/node-compat.md] when porting a Node codebase, when a node: API behaves differently under Bun, or when a package that works under Node fails — it carries the per-module gap list and the fetch, Web API, and module-resolution behavior changes in 1.4.0 that break working code.

Application

When writing Bun, apply these conventions silently — do not narrate a rule while following it. Where existing code uses a node: API that has a Bun-native equivalent, follow the codebase. Flag the alternative once.

When reviewing Bun, cite the violation and show the fix inline. Do not lecture.

Bad:  "Bun's build API generally prefers rejection over a status flag, so you may want to..."
Good: if (!result.success) -> Bun.build() rejects; catch the AggregateError, or pass throw: false

Integration

The javascript and typescript skills own the language and the type system; this skill states only where Bun departs from them. The nodejs skill owns what a node: module does — this skill owns only where Bun's implementation is absent, partial, or behaves differently, and never teaches Node itself.

bun:test and bun install are this skill's, and the vitest skill owns Vitest. Which of them a project adopts is a project decision, not a rule either skill states. The coding skill governs workflow.