What causes "open() can't be used with directories, path: %q" in grafana/k6?

Point open() at the concrete file, e.g. './data/users.csv'

open() can't be used with directories, path: %q

Error message

open() can't be used with directories, path: %q

What it means

readFile in internal/js/initcontext.go stats the requested path (a workaround for spf13/fsext#201) and refuses when it resolves to a directory. k6's open() reads one file fully into memory; there is no directory listing API, so pointing it at a folder is rejected with the offending path in the message.

Source

Thrown at internal/js/initcontext.go:53

}

func readFile(fileSystem fsext.Fs, filename string) (data []byte, err error) {
	defer func() {
		if errors.Is(err, fsext.ErrPathNeverRequestedBefore) {
			// loading different files per VU is not supported, so all files should are going
			// to be used inside the scenario should be opened during the init step (without any conditions)
			err = fmt.Errorf(
				"open() can't be used with files that weren't previously opened during initialization (__VU==0), path: %q",
				filename,
			)
		}
	}()

	// Workaround for https://github.com/spf13/fsext/issues/201
	if isDir, err := fsext.IsDir(fileSystem, filename); err != nil {
		return nil, err
	} else if isDir {
		return nil, fmt.Errorf("open() can't be used with directories, path: %q", filename)
	}

	return fsext.ReadFile(fileSystem, filename)
}

// allowOnlyOpenedFiles enables seen only files
func allowOnlyOpenedFiles(fs fsext.Fs) {
	alreadyOpenedFS, ok := fs.(fsext.OnlyCachedEnabler)
	if !ok {
		return
	}

	alreadyOpenedFS.AllowOnlyCached()
}

func generateSourceMapLoader(logger logrus.FieldLogger, filesystems map[string]fsext.Fs,
) func(path string) ([]byte, error) {
	return func(path string) ([]byte, error) {

View on GitHub (pinned to 93accf6570)

Solutions

Point open() at the concrete file, e.g. './data/users.csv'
Open each file in the folder explicitly during init
Check for trailing slashes and for './' vs '/' when building relative paths

Example fix

// before: path points at a directory
const data = open('./data/');

// after: path points at a file
const data = open('./data/users.csv');

Defensive patterns

Strategy: validation

Prevention

Always pass a concrete file path ending in a filename, not a directory
Watch for trailing slashes and for './' vs absolute paths when composing paths
List the files you plan to open in a comment so path typos are caught in review

When it happens

Trigger: open('data/') with a trailing slash or any path that resolves to a directory; a path where the filename was forgotten; a joined path that ends at a folder instead of a file.

Common situations: Typos in relative paths; assuming k6 can read a folder of fixtures like Node's fs; archives where a directory shadows the intended file.

Related errors

AI-assisted analysis of grafana/k6@93accf6570 (2026-08-15). Data as JSON: /api/errors/0bfb9f2b8b578803. Report an issue: GitHub.