diff --git a/cmd/streamdata/progress.go b/cmd/streamdata/progress.go
new file mode 100644
index 0000000..16f6d90
--- /dev/null
+++ b/cmd/streamdata/progress.go
@@ -0,0 +1,90 @@
+package main
+
+import (
+	"io"
+	"os"
+	"strconv"
+	"sync/atomic"
+	"syscall"
+	"time"
+
+	"hakurei.app/ext"
+)
+
+// pendingMessage is written to the current line every tick if non-nil.
+var pendingMessage atomic.Pointer[string]
+
+func init() {
+	ticker := time.NewTicker(time.Second / 100)
+	go func() {
+		for {
+			<-ticker.C
+
+			p := pendingMessage.Swap(nil)
+			if p == nil {
+				continue
+			}
+			os.Stderr.WriteString("\x1b[2K" + *p + "\r")
+		}
+	}()
+}
+
+// progressWriter writes a progress bar via writeCurrent.
+type progressWriter struct {
+	// For progress indication. Has no effect unless greater than zero.
+	contentLength int64
+	// Accumulated bytes.
+	n int64
+
+	// Increments with n, but resets periodically.
+	segment int64
+	// Reference time since segment was reset.
+	ref time.Time
+}
+
+// measureInterval is the interval at which progressWriter.segment is reset.
+const measureInterval = 5 * time.Second
+
+func (pw *progressWriter) Write(p []byte) (n int, err error) {
+	n = len(p)
+	pw.n += int64(n)
+	pw.segment += int64(n)
+	d := time.Now().Sub(pw.ref)
+
+	var s string
+	if pw.contentLength > 0 {
+		s += " " + strconv.FormatInt(pw.n/(1<<20), 10) + " / " +
+			strconv.FormatInt(pw.contentLength/(1<<20), 10) + " MiB |"
+	}
+	s += " " + strconv.FormatFloat(
+		float64(pw.segment)/d.Seconds()/float64(1<<20),
+		'f', 2, 64,
+	) + " MiB/s"
+	pendingMessage.Store(&s)
+
+	if d > measureInterval {
+		pw.segment = 0
+		pw.ref = time.Now()
+	}
+
+	return
+}
+
+// copyProgress is like [io.Copy], but prints progress to [os.Stderr] if it is
+// attached to a terminal.
+//
+// Percentage is emitted if contentLength is greater than zero.
+func copyProgress(dst io.Writer, src io.Reader, contentLength int64) (
+	written int64,
+	err error,
+) {
+	if !ext.Isatty(syscall.Stderr) {
+		return io.Copy(dst, src)
+	}
+	defer os.Stderr.WriteString("\n")
+	return io.Copy(io.MultiWriter(dst, &progressWriter{
+		contentLength: contentLength,
+
+		ref: time.Now(),
+	}), src)
+}
diff --git a/go.mod b/go.mod
index 055db0f..06781e3 100644
--- a/go.mod
+++ b/go.mod
@@ -1,3 +1,5 @@
 module git.gensokyo.uk/yonah/streamdata
 
-go 1.25.7
+go 1.26
+
+require hakurei.app v0.3.8-0.20260317115839-6cdb6a652b63
diff --git a/go.sum b/go.sum
new file mode 100644
index 0000000..5de2cc6
--- /dev/null
+++ b/go.sum
@@ -0,0 +1,2 @@
+hakurei.app v0.3.8-0.20260317115839-6cdb6a652b63 h1:Nx5eaHEbWUx6pwA4A+EdmHXvCvO3Xbw7PPcT1QbsVcA=
+hakurei.app v0.3.8-0.20260317115839-6cdb6a652b63/go.mod h1:rIWpoHYiZtIPue49bQLkROpQHYh/j2VjJGFrzNrvixU=