diff --git a/crates/spacesh-pty/src/lib.rs b/crates/spacesh-pty/src/lib.rs
index 9736bfa..b2946a1 100644
--- a/crates/spacesh-pty/src/lib.rs
+++ b/crates/spacesh-pty/src/lib.rs
@@ -1 +1,148 @@
-// PtyHandle implemented in Task 3.
+use std::io::{Read, Write};
+use anyhow::Result;
+use portable_pty::{CommandBuilder, MasterPty, PtySize, native_pty_system};
+use tokio::sync::mpsc;
+
+/// A spawned PTY with its child process. Output chunks arrive on `output`.
+pub struct PtyHandle {
+    master: Box<dyn MasterPty + Send>,
+    writer: Box<dyn Write + Send>,
+    child: Box<dyn portable_pty::Child + Send + Sync>,
+    /// Raw output chunks read off the PTY master (already on the async side).
+    pub output: mpsc::Receiver<Vec<u8>>,
+}
+
+/// Parameters for spawning a surface's process.
+pub struct SpawnSpec {
+    pub command: String,
+    pub args: Vec<String>,
+    pub cwd: std::path::PathBuf,
+    pub cols: u16,
+    pub rows: u16,
+    /// Extra environment variables (e.g. SPACESH_SURFACE_ID).
+    pub env: Vec<(String, String)>,
+}
+
+impl PtyHandle {
+    pub fn spawn(spec: SpawnSpec) -> Result<Self> {
+        let pty_system = native_pty_system();
+        let pair = pty_system.openpty(PtySize {
+            rows: spec.rows,
+            cols: spec.cols,
+            pixel_width: 0,
+            pixel_height: 0,
+        })?;
+
+        let mut cmd = CommandBuilder::new(&spec.command);
+        for a in &spec.args {
+            cmd.arg(a);
+        }
+        cmd.cwd(&spec.cwd);
+        for (k, v) in &spec.env {
+            cmd.env(k, v);
+        }
+
+        let child = pair.slave.spawn_command(cmd)?;
+        // The slave handle must be dropped so the child is the only holder; otherwise
+        // EOF is never observed on the master after the child exits.
+        drop(pair.slave);
+
+        let writer = pair.master.take_writer()?;
+        let mut reader = pair.master.try_clone_reader()?;
+
+        let (tx, rx) = mpsc::channel::<Vec<u8>>(256);
+        std::thread::spawn(move || {
+            let mut buf = [0u8; 8192];
+            loop {
+                match reader.read(&mut buf) {
+                    Ok(0) => break, // EOF: child closed the pty
+                    Ok(n) => {
+                        if tx.blocking_send(buf[..n].to_vec()).is_err() {
+                            break; // receiver gone
+                        }
+                    }
+                    Err(_) => break,
+                }
+            }
+        });
+
+        Ok(Self {
+            master: pair.master,
+            writer,
+            child,
+            output: rx,
+        })
+    }
+
+    pub fn write_input(&mut self, bytes: &[u8]) -> Result<()> {
+        self.writer.write_all(bytes)?;
+        self.writer.flush()?;
+        Ok(())
+    }
+
+    pub fn resize(&self, cols: u16, rows: u16) -> Result<()> {
+        self.master.resize(PtySize { rows, cols, pixel_width: 0, pixel_height: 0 })?;
+        Ok(())
+    }
+
+    /// Best-effort wait for the child's exit code (blocking).
+    pub fn wait(&mut self) -> i32 {
+        match self.child.wait() {
+            Ok(status) => status.exit_code() as i32,
+            Err(_) => -1,
+        }
+    }
+
+    pub fn kill(&mut self) {
+        let _ = self.child.kill();
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn shell_spec(script: &str) -> SpawnSpec {
+        SpawnSpec {
+            command: "/bin/sh".into(),
+            args: vec!["-c".into(), script.into()],
+            cwd: std::env::temp_dir(),
+            cols: 80,
+            rows: 24,
+            env: vec![("SPACESH_SURFACE_ID".into(), "s_test".into())],
+        }
+    }
+
+    #[tokio::test]
+    async fn spawn_echo_produces_output() {
+        let mut handle = PtyHandle::spawn(shell_spec("printf SPACESH_OK")).unwrap();
+        let mut collected = Vec::new();
+        // Drain until EOF (channel closes when the reader thread sees EOF).
+        while let Some(chunk) = handle.output.recv().await {
+            collected.extend_from_slice(&chunk);
+        }
+        let text = String::from_utf8_lossy(&collected);
+        assert!(text.contains("SPACESH_OK"), "got: {text:?}");
+    }
+
+    #[tokio::test]
+    async fn resize_does_not_error() {
+        let handle = PtyHandle::spawn(shell_spec("sleep 0.2")).unwrap();
+        handle.resize(120, 40).unwrap();
+    }
+
+    #[tokio::test]
+    async fn input_is_echoed_back() {
+        // `cat` echoes stdin back to stdout on a pty.
+        let mut handle = PtyHandle::spawn(shell_spec("cat")).unwrap();
+        handle.write_input(b"hello\n").unwrap();
+        let mut collected = Vec::new();
+        // Read a few chunks then kill cat to end the stream.
+        if let Some(chunk) = handle.output.recv().await {
+            collected.extend_from_slice(&chunk);
+        }
+        handle.kill();
+        let text = String::from_utf8_lossy(&collected);
+        assert!(text.contains("hello"), "got: {text:?}");
+    }
+}