What causes "error while Copy (%s): %w" in docker/cli?

Re-establish the operation once the daemon is stable ('docker version').

error while Copy ( )

Error message

error while Copy (%s): %w

What it means

Wrapped I/O error from io.Copy while shuttling bytes between stdio and the daemon raw stream connection in 'docker system dial-stdio'. The debugDescription distinguishes the 'stdin to stream' vs 'stream to stdout' direction.

Solutions

Re-establish the operation once the daemon is stable ('docker version').
Ensure the downstream consumer of stdout is alive for the whole transfer.
For tcp endpoints, confirm network stability / reduce MTU issues.

Example fix

// before: copy fails mid-transfer due to daemon reset
docker system dial-stdio

// after: stabilize daemon then retry
systemctl restart docker
sleep 2
docker version && docker system dial-stdio

Defensive patterns

Strategy: try-catch

Type guard

func isCopyErr(err error) bool {
	return err != nil && strings.Contains(err.Error(), "error while Copy")
}

Try / catch

if _, err := io.Copy(to, from); err != nil {
    if errors.Is(err, io.EOF) || isClosedConn(err) {
        // peer closed cleanly
        return nil
    }
    return fmt.Errorf("error while Copy (%s): %w", dir, err)
}

Prevention

Treat clean peer-close as EOF, not error, where appropriate.
Keep the daemon stable across the proxied operation.
Ensure the stdout consumer stays alive for the full transfer.

When it happens

Trigger: Either half of the bidirectional copy fails: the daemon closes the stream mid-transfer, the peer resets the connection, stdin/stdout hits an error, or the underlying connection breaks. Returned by copier at dial_stdio.go:84-86.

Common situations: Daemon restart during a proxied operation; peer disconnect; broken pipe on stdout (e.g. piping to a closed process); network interruption over a tcp daemon endpoint.

Related errors

AI-assisted analysis of docker/cli@4f84911bfe (2026-08-07). Data as JSON: /api/errors/1343fbce53aecbc5. Report an issue: GitHub.

Appendix: source

Thrown at cli/command/system/dial_stdio.go:85

		err = <-conn2stdout
	case err = <-conn2stdout:
		// return immediately without waiting for stdin to be closed.
		// (stdin is never closed when tty)
	}
	return err
}

func copier(to halfWriteCloser, from halfReadCloser, debugDescription string) error {
	defer func() {
		if err := from.CloseRead(); err != nil {
			logrus.Errorf("error while CloseRead (%s): %v", debugDescription, err)
		}
		if err := to.CloseWrite(); err != nil {
			logrus.Errorf("error while CloseWrite (%s): %v", debugDescription, err)
		}
	}()
	if _, err := io.Copy(to, from); err != nil {
		return fmt.Errorf("error while Copy (%s): %w", debugDescription, err)
	}
	return nil
}

type halfReadCloser interface {
	io.Reader
	CloseRead() error
}

type halfWriteCloser interface {
	io.Writer
	CloseWrite() error
}

type halfCloser interface {
	halfReadCloser
	halfWriteCloser
}

View on GitHub (pinned to 4f84911bfe)