diff --git a/metropolis/test/launch/prefixed_stdio.go b/metropolis/test/launch/prefixed_stdio.go
new file mode 100644
index 0000000..c851c44
--- /dev/null
+++ b/metropolis/test/launch/prefixed_stdio.go
@@ -0,0 +1,40 @@
+package launch
+
+import (
+	"fmt"
+	"io"
+	"strings"
+
+	"source.monogon.dev/osbase/logbuffer"
+)
+
+// prefixedStdio is a io.ReadWriter which splits written bytes into lines,
+// prefixes them with some known prefix, and spits them to os.Stdout.
+//
+// io.Reader is implemented for compatibility with code which expects an
+// io.ReadWriter, but always returns EOF.
+type prefixedStdio struct {
+	*logbuffer.LineBuffer
+}
+
+// newPrefixedStdio returns a prefixedStdio that prefixes all lines with <num>|,
+// used to distinguish different VMs used within the launch codebase.
+func newPrefixedStdio(num int) prefixedStdio {
+	return prefixedStdio{
+		logbuffer.NewLineBuffer(1024, func(l *logbuffer.Line) {
+			s := strings.TrimRight(l.String(), " \t\n\r")
+			s = strings.TrimLeft(s, "\n\r\t")
+			// TODO(q3k): don't just skip lines containing escape sequences, strip the
+			// sequences out. Or stop parsing qemu logs and instead dial log endpoint in
+			// spawned nodes.
+			if strings.Contains(s, "\u001b") {
+				return
+			}
+			fmt.Printf("%02d| %s\n", num, s)
+		}),
+	}
+}
+
+func (p prefixedStdio) Read(_ []byte) (int, error) {
+	return 0, io.EOF
+}