diff --git a/cmd/streamdata/prompt.go b/cmd/streamdata/prompt.go
new file mode 100644
index 0000000..40cf31b
--- /dev/null
+++ b/cmd/streamdata/prompt.go
@@ -0,0 +1,83 @@
+package main
+
+import (
+	"bufio"
+	"errors"
+	"io"
+	"math"
+	"os"
+	"strconv"
+	"syscall"
+
+	"hakurei.app/ext"
+)
+
+// toErr writes panic value satisfying error to the value pointed to by errP,
+// and re-panics other non-nil values.
+func toErr(errP *error) {
+	r := recover()
+	if r == nil {
+		return
+	}
+
+	if err, ok := r.(error); !ok {
+		panic(r)
+	} else if *errP == nil {
+		*errP = err
+	}
+}
+
+// isattyStd checks whether standard streams used for prompts are terminals.
+func isattyStd() {
+	if !ext.Isatty(syscall.Stdin) {
+		panic(errors.New("stdin does not appear to be a terminal"))
+	}
+	if !ext.Isatty(syscall.Stderr) {
+		panic(errors.New("stdin does not appear to be a terminal"))
+	}
+}
+
+// prompt prompts the user and reads the response as a string.
+func prompt(br *bufio.Reader, p string) (string, bool) {
+	os.Stderr.WriteString(p)
+	s, err := br.ReadString('\n')
+	if err != nil {
+		if err == io.EOF {
+			return "", false
+		}
+		panic(err)
+	}
+	return s[:len(s)-1], true
+}
+
+// noResponseUint is returned by promptUint when the user responds with a single
+// newline character.
+const noResponseUint = math.MaxUint64
+
+// promptUint prompts the user for an unsigned integer until EOF or a valid
+// response is read.
+func promptUint(br *bufio.Reader, p string) (uint64, bool) {
+invalid:
+	s, ok := prompt(br, p)
+	if !ok {
+		return 0, false
+	}
+
+	if s == "" {
+		return noResponseUint, false
+	}
+
+	v, err := strconv.ParseUint(s, 10, 64)
+	if err != nil {
+		goto invalid
+	}
+	return v, true
+}
+
+// require panics with an error if ok is false, and returns v otherwise.
+func require[T any](v T, ok bool) T {
+	if !ok {
+		panic(errors.New("cancelled"))
+	}
+	return v
+}