diff --git a/crates/spaceshd/src/launchd.rs b/crates/spaceshd/src/launchd.rs
index 46ae93d..994e212 100644
--- a/crates/spaceshd/src/launchd.rs
+++ b/crates/spaceshd/src/launchd.rs
@@ -1,4 +1,68 @@
-use anyhow::Result;
-pub fn install_agent() -> Result<()> {
-    anyhow::bail!("install-agent implemented in Task 16")
+use anyhow::{Context, Result};
+use std::path::PathBuf;
+
+const LABEL: &str = "xyz.spacesh.daemon";
+
+fn plist_path() -> Result<PathBuf> {
+    let home = dirs::home_dir().context("no home")?;
+    Ok(home.join("Library").join("LaunchAgents").join(format!("{LABEL}.plist")))
+}
+
+/// Render the launchd plist. `run_at_load` defaults to false in this slice.
+pub fn render_plist(exe: &str, run_at_load: bool) -> String {
+    format!(
+        r#"<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<plist version="1.0">
+<dict>
+    <key>Label</key>
+    <string>{LABEL}</string>
+    <key>ProgramArguments</key>
+    <array>
+        <string>{exe}</string>
+    </array>
+    <key>KeepAlive</key>
+    <true/>
+    <key>RunAtLoad</key>
+    <{run_at_load}/>
+</dict>
+</plist>
+"#,
+        run_at_load = if run_at_load { "true" } else { "false" }
+    )
+}
+
+pub fn install_agent() -> Result<()> {
+    let exe = std::env::current_exe()?;
+    let plist = render_plist(&exe.to_string_lossy(), false);
+    let path = plist_path()?;
+    if let Some(parent) = path.parent() {
+        std::fs::create_dir_all(parent)?;
+    }
+    std::fs::write(&path, plist)?;
+    // Load it (best-effort; ignore "already loaded").
+    let _ = std::process::Command::new("launchctl")
+        .arg("load")
+        .arg(&path)
+        .status();
+    Ok(())
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn plist_has_label_keepalive_and_exe() {
+        let p = render_plist("/usr/local/bin/spaceshd", false);
+        assert!(p.contains("xyz.spacesh.daemon"));
+        assert!(p.contains("/usr/local/bin/spaceshd"));
+        assert!(p.contains("<key>KeepAlive</key>\n    <true/>"));
+        assert!(p.contains("<key>RunAtLoad</key>\n    <false/>"));
+    }
+
+    #[test]
+    fn run_at_load_toggles() {
+        assert!(render_plist("x", true).contains("<key>RunAtLoad</key>\n    <true/>"));
+    }
 }