diff --git a/crates/xy-supervisor/src/lib.rs b/crates/xy-supervisor/src/lib.rs
index be9d9e6..1aa7124 100644
--- a/crates/xy-supervisor/src/lib.rs
+++ b/crates/xy-supervisor/src/lib.rs
@@ -2,10 +2,12 @@
 
 pub mod backoff;
 pub mod child;
+pub mod logs;
 pub mod policy;
 pub mod retry_window;
 
 pub use backoff::Backoff;
 pub use child::{ChildHandle, MockChild, MockChildController};
+pub use logs::RotatingLogWriter;
 pub use policy::{RestartDecision, decide};
 pub use retry_window::RetryWindow;
diff --git a/crates/xy-supervisor/src/logs.rs b/crates/xy-supervisor/src/logs.rs
new file mode 100644
index 0000000..0c66204
--- /dev/null
+++ b/crates/xy-supervisor/src/logs.rs
@@ -0,0 +1,104 @@
+use std::fs::{File, OpenOptions};
+use std::io::Write;
+use std::path::{Path, PathBuf};
+
+pub struct RotatingLogWriter {
+    base: PathBuf,
+    max_bytes: u64,
+    keep: usize,
+    file: File,
+    written: u64,
+}
+
+impl RotatingLogWriter {
+    pub fn open(base: &Path, max_bytes: u64, keep: usize) -> std::io::Result<Self> {
+        if let Some(parent) = base.parent() {
+            std::fs::create_dir_all(parent)?;
+        }
+        let file = OpenOptions::new().create(true).append(true).open(base)?;
+        let written = file.metadata()?.len();
+        Ok(Self {
+            base: base.to_path_buf(),
+            max_bytes,
+            keep,
+            file,
+            written,
+        })
+    }
+
+    pub fn write_line(&mut self, tag: &str, line: &str) -> std::io::Result<()> {
+        let bytes = format!("{tag} {line}\n");
+        self.file.write_all(bytes.as_bytes())?;
+        self.written += bytes.len() as u64;
+        if self.written >= self.max_bytes {
+            self.rotate()?;
+        }
+        Ok(())
+    }
+
+    fn rotate(&mut self) -> std::io::Result<()> {
+        // Drop the current handle by replacing with /dev/null briefly.
+        self.file = OpenOptions::new().read(true).open("/dev/null")?;
+        for i in (1..self.keep).rev() {
+            let src = self.gen_path(i);
+            let dst = self.gen_path(i + 1);
+            if src.exists() {
+                let _ = std::fs::rename(&src, &dst);
+            }
+        }
+        if self.base.exists() {
+            let _ = std::fs::rename(&self.base, self.gen_path(1));
+        }
+        self.file = OpenOptions::new()
+            .create(true)
+            .append(true)
+            .open(&self.base)?;
+        self.written = 0;
+        Ok(())
+    }
+
+    fn gen_path(&self, n: usize) -> PathBuf {
+        let mut s = self.base.as_os_str().to_os_string();
+        s.push(format!(".{n}"));
+        PathBuf::from(s)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use tempfile::tempdir;
+    use std::io::Read;
+
+    #[test]
+    fn writes_lines_with_tags() {
+        let dir = tempdir().unwrap();
+        let base = dir.path().join("x.log");
+        let mut w = RotatingLogWriter::open(&base, 1024, 3).unwrap();
+        w.write_line("[out]", "hello").unwrap();
+        w.write_line("[err]", "boom").unwrap();
+        let mut s = String::new();
+        File::open(&base)
+            .unwrap()
+            .read_to_string(&mut s)
+            .unwrap();
+        assert_eq!(s, "[out] hello\n[err] boom\n");
+    }
+
+    #[test]
+    fn rotates_at_threshold() {
+        let dir = tempdir().unwrap();
+        let base = dir.path().join("x.log");
+        let mut w = RotatingLogWriter::open(&base, 20, 3).unwrap();
+        for _ in 0..5 {
+            w.write_line("[out]", "0123456789").unwrap();
+        }
+        assert!(base.exists());
+        let rotated = dir.path().join("x.log.1");
+        assert!(
+            rotated.exists(),
+            "expected rotated file at {}",
+            rotated.display()
+        );
+    }
+}