diff --git a/logtail/logtail.go b/logtail/logtail.go
index 4544af9d7..f17fd11d5 100644
--- a/logtail/logtail.go
+++ b/logtail/logtail.go
@@ -735,6 +735,8 @@ func (l *Logger) Write(buf []byte) (int, error) {
 	if len(buf) == 0 {
 		return 0, nil
 	}
+	inLen := len(buf) // length as provided to us, before modifications to downstream writers
+
 	level, buf := parseAndRemoveLogLevel(buf)
 	if l.stderr != nil && l.stderr != io.Discard && int64(level) <= atomic.LoadInt64(&l.stderrLevel) {
 		if buf[len(buf)-1] == '\n' {
@@ -752,7 +754,7 @@ func (l *Logger) Write(buf []byte) (int, error) {
 
 	b := l.encodeLocked(buf, level)
 	_, err := l.sendLocked(b)
-	return len(buf), err
+	return inLen, err
 }
 
 var (
diff --git a/logtail/logtail_test.go b/logtail/logtail_test.go
index f145d7a35..f0d3f36b2 100644
--- a/logtail/logtail_test.go
+++ b/logtail/logtail_test.go
@@ -379,3 +379,30 @@ func TestEncode(t *testing.T) {
 		}
 	}
 }
+
+// Test that even if Logger.Write modifies the input buffer, we still return the
+// length of the input buffer, not what we shrank it down to. Otherwise the
+// caller will think we did a short write, violating the io.Writer contract.
+func TestLoggerWriteResult(t *testing.T) {
+	buf := NewMemoryBuffer(100)
+	lg := &Logger{
+		clock:  tstest.NewClock(tstest.ClockOpts{Start: time.Unix(123, 0)}),
+		buffer: buf,
+	}
+
+	const in = "[v1] foo"
+	n, err := lg.Write([]byte(in))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got, want := n, len(in); got != want {
+		t.Errorf("Write = %v; want %v", got, want)
+	}
+	back, err := buf.TryReadLine()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got, want := string(back), `{"logtail": {"client_time": "1970-01-01T00:02:03Z"}, "v":1,"text": "foo"}`+"\n"; got != want {
+		t.Errorf("mismatch.\n got: %#q\nwant: %#q", back, want)
+	}
+}