# Auto-install (/docs/runtime/auto-install)

<!-- agent-signals: reading_time_min: 3 · est_tokens: 1089 · updated: 2026-07-28 -->
Related: [Watch Mode](/docs/runtime/watch-mode.md), [Debugging](/docs/runtime/debugger.md), [REPL](/docs/runtime/repl.md), [bunfig.toml](/docs/runtime/bunfig.md), [File Types](/docs/runtime/file-types.md), [Module Resolution](/docs/runtime/module-resolution.md)

If Bun finds no `node_modules` directory in the working directory or higher, it abandons Node.js-style module resolution in favor of the **Bun module resolution algorithm**.

Under Bun-style module resolution, Bun auto-installs every imported package on the fly into a [global module cache](/pm/global-cache) during execution (the same cache used by [`bun install`](/pm/cli/install)).

```ts index.ts icon="/icons/typescript.svg"
import { foo } from "foo"; // install `latest` version

foo();
```

The first time you run this script, Bun auto-installs `"foo"` and caches it. Later runs use the cached version.

***

## Version resolution [#version-resolution]

Bun determines which version to install as follows:

1. Check for a `bun.lock` file in the project root. If it exists, use the version specified in the lockfile.
2. Otherwise, scan up the tree for a `package.json` that includes `"foo"` as a dependency. If found, use the specified semver version or version range.
3. Otherwise, use `latest`.

***

## Cache behavior [#cache-behavior]

Once Bun determines a version or version range, it:

1. Checks the module cache for a compatible version. If one exists, uses it.
2. When resolving `latest`, checks if `package@latest` was downloaded and cached in the last *24 hours*. If so, uses it.
3. Otherwise, downloads and installs the appropriate version from the `npm` registry.

***

## Installation [#installation]

Bun installs and caches packages into `<cache>/<pkg>@<version>`, so multiple versions of the same package can be cached at once. It also creates a symlink under `<cache>/<pkg>/<version>` to speed up looking up all cached versions of a package.

***

## Version specifiers [#version-specifiers]

To bypass version resolution entirely, specify a version or version range directly in your import statement.

```ts index.ts icon="/icons/typescript.svg"
import { z } from "zod@3.0.0"; // specific version
import { z } from "zod@next"; // npm tag
import { z } from "zod@^3.20.0"; // semver range
```

***

## Benefits [#benefits]

* **Space efficiency** — Each version of a dependency exists in only one place on disk. This saves space and time compared to redundant per-project installations.
* **Portability** — Your source file is *self-contained*, so sharing scripts and gists doesn't mean zipping up a directory of code and config files. With version specifiers in `import` statements, even a `package.json` isn't necessary.
* **Convenience** — You don't need to run `npm install` or `bun install` before running a file or script with `bun run`.
* **Backwards compatibility** — Because Bun still respects the versions specified in `package.json` if one exists, you can switch to Bun-style resolution with a single command: `rm -rf node_modules`.

***

## Limitations [#limitations]

* No Intellisense. TypeScript auto-completion in IDEs relies on type declaration files inside `node_modules`. We are investigating solutions to this.
* No [patch-package](https://github.com/ds300/patch-package) support

***

## FAQ [#faq]

<AccordionGroup>
  <Accordion title="How is this different from what pnpm does?">
    With pnpm, you have to run `pnpm install`, which creates a `node_modules` folder of symlinks for the runtime to resolve. By contrast, Bun resolves dependencies on the fly when you run a file; there's no need to run any `install` command ahead of time. Bun also doesn't create a `node_modules` folder.
  </Accordion>

  <Accordion title="How is this different from Yarn Plug'N'Play does?">
    With Yarn, you must run `yarn install` before you run a script. By contrast, Bun resolves dependencies on the fly when you run a file; there's no need to run any `install` command ahead of time.

    Yarn Plug'N'Play also uses zip files to store dependencies. This makes dependency loading [slower at runtime](https://twitter.com/jarredsumner/status/1458207919636287490), as random access reads on zip files tend to be slower than the equivalent disk lookup.
  </Accordion>

  <Accordion title="How is this different from what Deno does?">
    Deno requires an `npm:` specifier before each npm `import`, lacks support for import maps through `compilerOptions.paths` in `tsconfig.json`, and has incomplete support for `package.json` settings. Unlike Deno, Bun does not currently support URL imports.
  </Accordion>
</AccordionGroup>