What causes "failed create destination %s: %w" in BoundaryML/baml?

Create the destination directory first (os.MkdirAll(filepath.Dir(dst), 0o755)) and check ownership

failed create destination %s: %w

Error message

failed create destination %s: %w

What it means

copyFile failed to create the destination file at dst via os.Create. This indicates the destination path is unwritable, its parent directory doesn't exist, or a permission/OS-level error; the error is embedded into ErrDownloadFailed by the caller.

Source

Thrown at engine/language_client_go/baml_go/lib_common.go:644

	for _, r := range s {
		if !((r >= '0' && r <= '9') || (r >= 'a' && r <= 'f') || (r >= 'A' && r <= 'F')) {
			return false
		}
	}
	return true
}

func copyFile(src, dst string) (err error) {
	logger.Debug("Attempting to copy file", "source", src, "destination", dst)
	source, err := os.Open(src)
	if err != nil {
		return fmt.Errorf("failed open source %s: %w", src, err)
	}
	defer source.Close()

	destination, err := os.Create(dst)
	if err != nil {
		return fmt.Errorf("failed create destination %s: %w", dst, err)
	}
	defer func() {
		cerr := destination.Close()
		if err == nil && cerr != nil {
			err = fmt.Errorf("failed close destination %s: %w", dst, cerr)
		}
	}()

	_, err = io.Copy(destination, source)
	if err != nil {
		return fmt.Errorf("failed copying from %s to %s: %w", src, dst, err)
	}

	err = destination.Sync()
	if err != nil {
		return fmt.Errorf("failed syncing destination %s: %w", dst, err)
	}

View on GitHub (pinned to bd85ce9dee)

Solutions

Create the destination directory first (os.MkdirAll(filepath.Dir(dst), 0o755)) and check ownership
Run the install step with sufficient privileges or chown the install directory to the running user
Check the filesystem is writable (not mounted read-only) and has free space
Prefer same-filesystem rename so copyFile isn't needed at all

Example fix

// before
destination, err := os.Create(dst)
// after (caller ensures dir exists first)
os.MkdirAll(filepath.Dir(dst), 0o755)
destination, err := os.Create(dst)

Defensive patterns

Strategy: validation

Validate before calling

if err := os.MkdirAll(filepath.Dir(dst), 0o755); err != nil {
    return err
}
f, err := os.OpenFile(dst, os.O_WRONLY|os.O_CREATE, 0o755)
if err != nil { return err }
f.Close()

Try / catch

if err := installLibrary(dst); err != nil {
    if strings.Contains(err.Error(), "failed create destination") {
        os.MkdirAll(filepath.Dir(dst), 0o755)
        return installLibrary(dst) // retry after fixing dir
    }
    return err
}

Prevention

Always create the destination directory before writing
Run installs as a user with write access to the target dir
Check for read-only mounts and full disks before install
Prefer atomic rename within the same filesystem to skip the copy path entirely

When it happens

Trigger: os.Create(dst) fails during the copy fallback: destination directory missing, read-only filesystem, permission denied for the current user, or disk full at create time.

Common situations: Installing the BAML library into a system or plugin directory owned by root, a project cache dir deleted between steps, or read-only container filesystems.

Understand the failure class

Background: "failed to write file", "Could not save figure", "Error saving remote file" — file write failed: causes and fixes across languages and libraries — this error's family across 38 libraries.

Related errors

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/40bd337389ad9633. Report an issue: GitHub.