diff --git a/crates/xy-supervisor/src/logs.rs b/crates/xy-supervisor/src/logs.rs
index f80276a..40427a0 100644
--- a/crates/xy-supervisor/src/logs.rs
+++ b/crates/xy-supervisor/src/logs.rs
@@ -69,6 +69,24 @@ impl RotatingLogWriter {
     }
 }
 
+impl std::io::Write for RotatingLogWriter {
+    fn write(&mut self, buf: &[u8]) -> std::io::Result<usize> {
+        self.file.write_all(buf)?;
+
+        self.written += buf.len() as u64;
+
+        if self.written >= self.max_bytes {
+            self.rotate()?;
+        }
+
+        Ok(buf.len())
+    }
+
+    fn flush(&mut self) -> std::io::Result<()> {
+        self.file.flush()
+    }
+}
+
 #[derive(Clone)]
 pub struct RingBuffer {
     inner: Arc<Mutex<RingBufferInner>>,
@@ -260,4 +278,39 @@ mod tests {
         assert_eq!(got.stream, LogStream::Stdout);
         assert_eq!(sink.ring.snapshot_tail(None).len(), 1);
     }
+
+    #[test]
+    fn write_trait_appends_bytes() {
+        use std::io::Write;
+
+        let tmp = tempfile::tempdir().unwrap();
+        let base = tmp.path().join("daemon.log");
+
+        let mut writer = RotatingLogWriter::open(&base, 1024, 3).unwrap();
+
+        writer.write_all(b"hello\n").unwrap();
+        writer.flush().unwrap();
+
+        let contents = std::fs::read_to_string(&base).unwrap();
+        assert_eq!(contents, "hello\n");
+    }
+
+    #[test]
+    fn write_trait_rotates_at_threshold() {
+        use std::io::Write;
+
+        let tmp = tempfile::tempdir().unwrap();
+        let base = tmp.path().join("daemon.log");
+
+        let mut writer = RotatingLogWriter::open(&base, 8, 3).unwrap();
+
+        writer.write_all(b"0123456789").unwrap();
+        writer.write_all(b"after\n").unwrap();
+        writer.flush().unwrap();
+
+        let rotated = tmp.path().join("daemon.log.1");
+        assert!(rotated.exists());
+        assert_eq!(std::fs::read_to_string(&rotated).unwrap(), "0123456789");
+        assert_eq!(std::fs::read_to_string(&base).unwrap(), "after\n");
+    }
 }