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

Check free disk space on the destination volume and retry

failed close destination

Error message

failed close destination %s: %w

What it means

This error is produced by copyFile in baml_go/lib_common.go when closing the destination file (via defer) fails after the copy itself succeeded. The library wraps Close errors so callers learn the copied file may not be fully persisted/released. It only surfaces if no earlier error occurred (err == nil at defer time).

Solutions

Check free disk space on the destination volume and retry
Verify the destination directory/filesystem is writable and healthy (not read-only, not NFS with stale locks)
Re-run the download; the library re-downloads the native client artifact
If persistent, clear the partially written destination file before retrying

Example fix

// before (disk full)
cerr := destination.Close() // "no space left on device" -> failed close destination
// after (ensure space before copy)
if stat, err := os.Stat(filepath.Dir(dst)); err == nil {
    if usage := checkFreeSpace(stat); usage < requiredBytes {
        return fmt.Errorf("insufficient disk space at %s", filepath.Dir(dst))
    }
}

Defensive patterns

Strategy: try-catch

Validate before calling

if usage, err := diskUsage(filepath.Dir(dst)); err == nil && usage < requiredBytes {
    return fmt.Errorf("insufficient disk space for %s", dst)
}

Try / catch

if err := baml.DownloadLibrary(dst); err != nil {
    if strings.Contains(err.Error(), "failed close destination") {
        // recover: free space / remount, delete partial dst, retry once
        os.Remove(dst)
    }
    return err
}

Prevention

Monitor free disk space before running first-time native downloads
Keep the library cache on a healthy local (non-network, non-noexec) filesystem
Treat close/sync errors as data-integrity signals — delete partial files on failure

When it happens

Trigger: downloadBamlLibrary calls copyFile; io.Copy succeeds but destination.Close() returns an error (e.g. disk full detected on close, NFS/overlayfs flush failure, file lock issues).

Common situations: Full or flaky disk when downloading the BAML native library on first run; containers with read-only or nearly-full writable layers; network filesystems where Close can fail after writes appear successful.

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/e4ab541d4cec83b7. Report an issue: GitHub.

Appendix: source

Thrown at engine/language_client_go/baml_go/lib_common.go:649

	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)
	}

	logger.Debug("File copied successfully", "source", src, "destination", dst)
	return nil
}

func isSupportedPlatform() bool {

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