diff --git a/muxplex/cli.py b/muxplex/cli.py
index 26ff24c..774f03d 100644
--- a/muxplex/cli.py
+++ b/muxplex/cli.py
@@ -56,10 +56,44 @@ def serve(
     uvicorn.run(app, host=host, port=port, log_level="warning")
 
 
-def install_service(*, system: bool = False) -> None:
-    """Install muxplex as a systemd service."""
-    executable = sys.executable
+def _install_launchd(executable: str) -> None:
+    """Install a macOS launchd agent plist to ~/Library/LaunchAgents/."""
+    label = "com.muxplex"
+    plist = f"""<?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>{executable}</string>
+        <string>-m</string>
+        <string>muxplex</string>
+    </array>
+    <key>RunAtLoad</key>
+    <true/>
+    <key>KeepAlive</key>
+    <true/>
+    <key>StandardOutPath</key>
+    <string>/tmp/muxplex.log</string>
+    <key>StandardErrorPath</key>
+    <string>/tmp/muxplex.err</string>
+</dict>
+</plist>
+"""
+    path = Path.home() / "Library" / "LaunchAgents" / f"{label}.plist"
+    path.parent.mkdir(parents=True, exist_ok=True)
+    path.write_text(plist)
+    print(f"Launch agent written to {path}")
+    print("Enable with:")
+    print(f"  launchctl load {path}")
+    print("Disable with:")
+    print(f"  launchctl unload {path}")
 
+
+def _install_systemd(executable: str, *, system: bool = False) -> None:
+    """Install a Linux systemd service unit file."""
     _raw_path = os.environ.get("PATH", "/usr/local/bin:/usr/bin:/bin")
     _safe_path = ":".join(p for p in _raw_path.split(":") if not p.startswith("/mnt/"))
     _safe_path = _safe_path or "/usr/local/bin:/usr/bin:/bin"
@@ -97,6 +131,16 @@ WantedBy={"multi-user.target" if system else "default.target"}
     print(f"Enable with:\n  {reload_cmd}")
 
 
+def install_service(*, system: bool = False) -> None:
+    """Install muxplex as a background service (launchd on macOS, systemd on Linux)."""
+    executable = sys.executable
+
+    if sys.platform == "darwin":
+        _install_launchd(executable)
+    else:
+        _install_systemd(executable, system=system)
+
+
 def main() -> None:
     """CLI entry point."""
     parser = argparse.ArgumentParser(
@@ -124,7 +168,10 @@ def main() -> None:
     sub = parser.add_subparsers(dest="command")
     sub.add_parser("serve", help="Start the server (default)")
 
-    svc = sub.add_parser("install-service", help="Install systemd service unit")
+    svc = sub.add_parser(
+        "install-service",
+        help="Install as a background service (systemd on Linux, launchd on macOS)",
+    )
     svc.add_argument(
         "--system", action="store_true", help="System-wide (requires sudo)"
     )
diff --git a/muxplex/tests/test_cli.py b/muxplex/tests/test_cli.py
index 76247f6..2a5d5ec 100644
--- a/muxplex/tests/test_cli.py
+++ b/muxplex/tests/test_cli.py
@@ -260,6 +260,76 @@ def test_reset_secret_prints_warning(tmp_path, monkeypatch, capsys):
     )
 
 
+def test_install_service_writes_launchd_plist_on_macos(tmp_path, monkeypatch):
+    """install_service() on macOS writes a launchd plist to ~/Library/LaunchAgents/."""
+    from muxplex.cli import install_service
+
+    fake_home = tmp_path / "home"
+    fake_home.mkdir()
+    monkeypatch.setattr(Path, "home", staticmethod(lambda: fake_home))
+    monkeypatch.setattr("sys.platform", "darwin")
+
+    install_service(system=False)
+
+    plist_path = fake_home / "Library" / "LaunchAgents" / "com.muxplex.plist"
+    assert plist_path.exists(), "Plist file must be created on macOS"
+    content = plist_path.read_text()
+    assert "com.muxplex" in content
+    assert "RunAtLoad" in content
+    assert "ProgramArguments" in content
+    assert "LaunchAgents" in str(plist_path)
+
+
+def test_install_service_does_not_write_systemd_on_macos(tmp_path, monkeypatch):
+    """install_service() on macOS must NOT write a systemd unit file."""
+    from muxplex.cli import install_service
+
+    fake_home = tmp_path / "home"
+    fake_home.mkdir()
+    monkeypatch.setattr(Path, "home", staticmethod(lambda: fake_home))
+    monkeypatch.setattr("sys.platform", "darwin")
+
+    install_service(system=False)
+
+    systemd_path = fake_home / ".config" / "systemd" / "user" / "muxplex.service"
+    assert not systemd_path.exists(), "No systemd unit file should be written on macOS"
+
+
+def test_install_service_writes_systemd_on_linux(tmp_path, monkeypatch):
+    """install_service() on Linux writes a systemd unit to ~/.config/systemd/user/."""
+    from muxplex.cli import install_service
+
+    fake_home = tmp_path / "home"
+    fake_home.mkdir()
+    monkeypatch.setattr(Path, "home", staticmethod(lambda: fake_home))
+    monkeypatch.setattr("sys.platform", "linux")
+
+    install_service(system=False)
+
+    unit_path = fake_home / ".config" / "systemd" / "user" / "muxplex.service"
+    assert unit_path.exists(), "Systemd unit file must be created on Linux"
+    content = unit_path.read_text()
+    assert "[Unit]" in content
+    assert "[Service]" in content
+
+
+def test_install_service_help_text_mentions_background_service():
+    """install-service help must mention 'service', not just 'systemd'."""
+    import io
+    from muxplex.cli import main
+
+    buf = io.StringIO()
+    with patch("sys.argv", ["muxplex", "install-service", "--help"]):
+        try:
+            with patch("sys.stdout", buf):
+                main()
+        except SystemExit:
+            pass
+
+    help_text = buf.getvalue().lower()
+    assert "service" in help_text
+
+
 def test_dunder_main_calls_main():
     """python -m muxplex must call cli.main()."""
     import importlib.util