Bun's behavior can be configured using its configuration file, bunfig.toml.

In general, Bun relies on pre-existing configuration files like package.json and tsconfig.json to configure its behavior. bunfig.toml is only necessary for configuring Bun-specific things. This file is optional, and Bun will work out of the box without it.

Global vs. local

In general, it's recommended to add a bunfig.toml file to your project root, alongside your package.json.

To configure Bun globally, you can also create a .bunfig.toml file at one of the following paths:

  • $HOME/.bunfig.toml
  • $XDG_CONFIG_HOME/.bunfig.toml

If both a global and local bunfig are detected, the results are shallow-merged, with local overriding global. CLI flags will override bunfig setting where applicable.


Bun's runtime behavior is configured using top-level fields in the bunfig.toml file.


An array of scripts/plugins to execute before running a file or script.

# scripts to run before `bun run`-ing a file or script
# register plugins by adding them to this list
preload = ["./preload.ts"]


Configure how Bun handles JSX. You can also set these fields in the compilerOptions of your tsconfig.json, but they are supported here as well for non-TypeScript projects.

jsx = "react"
jsxFactory = "h"
jsxFragment = "Fragment"
jsxImportSource = "react"

Refer to the tsconfig docs for more information on these fields.


Enable smol mode. This reduces memory usage at the cost of performance.

# Reduce memory usage at the cost of performance
smol = true


Set the log level. This can be one of "debug", "warn", or "error".

logLevel = "debug" # "debug" | "warn" | "error"


The define field allows you to replace certain global identifiers with constant expressions. Bun will replace any usage of the identifier with the expression. The expression should be a JSON string.

# Replace any usage of "process.env.bagel" with the string `lox`.
# The values are parsed as JSON, except single-quoted strings are supported and `'undefined'` becomes `undefined` in JS.
# This will probably change in a future release to be just regular TOML instead. It is a holdover from the CLI argument parsing.
"process.env.bagel" = "'lox'"


Configure how Bun maps file extensions to loaders. This is useful for loading files that aren't natively supported by Bun. If

# when a .bagel file is imported, treat it like a tsx file
".bagel" = "tsx"

Bun supports the following loaders:

  • jsx
  • js
  • ts
  • tsx
  • css
  • file
  • json
  • toml
  • wasm
  • napi
  • base64
  • dataurl
  • text


The telemetry field permit to enable/disable the analytics records. Bun records bundle timings (so we can answer with data, "is Bun getting faster?") and feature usage (e.g., "are people actually using macros?"). The request body size is about 60 bytes, so it's not a lot of data. By default the telemetry is enabled. Equivalent of DO_NOT_TRACK env variable.

telemetry = false

Test runner

The test runner is configured under the [test] section of your bunfig.toml.

# configuration goes here


The root directory to run tests from. Default ..

root = "./__tests__"


Same as the top-level preload field, but only applies to bun test.

preload = ["./setup.ts"]


Same as the top-level smol field, but only applies to bun test.

smol = true


Enables coverage reporting. Default false. Use --coverage to override.

coverage = false


To specify a coverage threshold. By default, no threshold is set. If your test suite does not meet or exceed this threshold, bun test will exit with a non-zero exit code to indicate the failure.


# to require 90% line-level and function-level coverage
coverageThreshold = 0.9

Different thresholds can be specified for line-wise, function-wise, and statement-wise coverage.

coverageThreshold = { line = 0.7, function = 0.8, statement = 0.9 }


Whether to skip test files when computing coverage statistics. Default false.

coverageSkipTestFiles = false

Package manager

Package management is a complex issue; to support a range of use cases, the behavior of bun install can be configured under the [install] section.

# configuration here


Whether to install optional dependencies. Default true.

optional = true


Whether to install development dependencies. Default true.

dev = true


Whether to install peer dependencies. Default true.

peer = true


Whether bun install will run in "production mode". Default false.

In production mode, "devDependencies" are not installed. You can use --production in the CLI to override this setting.

production = false


Whether to set an exact version in package.json. Default false.

By default Bun uses caret ranges; if the latest version of a package is 2.4.1, the version range in your package.json will be ^2.4.1. This indicates that any version from 2.4.1 up to (but not including) 3.0.0 is acceptable.

exact = false


To configure Bun's package auto-install behavior. Default "auto" — when no node_modules folder is found, Bun will automatically install dependencies on the fly during execution.

auto = "auto"

Valid values are:

"auto"Resolve modules from local node_modules if it exists. Otherwise, auto-install dependencies on the fly.
"force"Always auto-install dependencies, even if node_modules exists.
"disable"Never auto-install dependencies.
"fallback"Check local node_modules first, then auto-install any packages that aren't found. You can enable this from the CLI with bun -i.


When true, bun install will not update bun.lockb. Default false. If package.json and the existing bun.lockb are not in agreement, this will error.

frozenLockfile = false


Whether bun install will actually install dependencies. Default false. When true, it's equivalent to setting --dry-run on all bun install commands.

dryRun = false


To configure the directory where Bun puts globally installed packages.

# where `bun install --global` installs packages
globalDir = "~/.bun/install/global"


To configure the directory where Bun installs globally installed binaries and CLIs.

# where globally-installed package bins are linked
globalBinDir = "~/.bun/bin"


The default registry is https://registry.npmjs.org/. This can be globally configured in bunfig.toml:

# set default registry as a string
registry = "https://registry.npmjs.org"
# set a token
registry = { url = "https://registry.npmjs.org", token = "123456" }
# set a username/password
registry = "https://username:password@registry.npmjs.org"


To configure a registry for a particular scope (e.g. @myorg/<package>) use install.scopes. You can reference environment variables with $variable notation.

# registry as string
myorg = "https://username:password@registry.myorg.com/"

# registry with username/password
# you can reference environment variables
myorg = { username = "myusername", password = "$npm_password", url = "https://registry.myorg.com/" }

# registry with token
myorg = { token = "$npm_token", url = "https://registry.myorg.com/" }


To configure the cache behavior:


# the directory to use for the cache
dir = "~/.bun/install/cache"

# when true, don't load from the global cache.
# Bun may still write to node_modules/.cache
disable = false

# when true, always resolve the latest versions from the registry
disableManifest = false


To configure lockfile behavior, use the install.lockfile section.

Whether to generate a lockfile on bun install. Default true.

save = true

Whether to generate a non-Bun lockfile alongside bun.lockb. (A bun.lockb will always be created.) Currently "yarn" is the only supported value.

print = "yarn"

bun run

The bun run command can be configured under the [run] section. These apply to the bun run command and the bun command when running a file or executable or script.

Currently, bunfig.toml isn't always automatically loaded for bun run in a local project (it does check for a global bunfig.toml), so you might still need to pass -c or -c=bunfig.toml to use these settings.

run.shell - use the system shell or Bun's shell

The shell to use when running package.json scripts via bun run or bun. On Windows, this defaults to "bun" and on other platforms it defaults to "system".

To always use the system shell instead of Bun's shell (default behavior unless Windows):

# default outside of Windows
shell = "system"

To always use Bun's shell instead of the system shell:

# default on Windows
shell = "bun"

run.bun - auto alias node to bun

When true, this prepends $PATH with a node symlink that points to the bun binary for all scripts or executables invoked by bun run or bun.

This means that if you have a script that runs node, it will actually run bun instead, without needing to change your script. This works recursively, so if your script runs another script that runs node, it will also run bun instead. This applies to shebangs as well, so if you have a script with a shebang that points to node, it will actually run bun instead.

By default, this is enabled if node is not already in your $PATH.

# equivalent to `bun --bun` for all `bun run` commands
bun = true

You can test this by running:

bun --bun which node # /path/to/bun
bun which node # /path/to/node

This option is equivalent to prefixing all bun run commands with --bun:

bun --bun run dev
bun --bun dev
bun run --bun dev

If set to false, this will disable the node symlink.

run.silent - suppress reporting the command being run

When true, suppresses the output of the command being run by bun run or bun.

silent = true

Without this option, the command being run will be printed to the console:

bun run dev
$ echo "Running \"dev\"..."
Running "dev"...

With this option, the command being run will not be printed to the console:

bun run dev
Running "dev"...

This is equivalent to passing --silent to all bun run commands:

bun --silent run dev
bun --silent dev
bun run --silent dev