diff --git a/muxplex/cli.py b/muxplex/cli.py
index 774f03d..44a7626 100644
--- a/muxplex/cli.py
+++ b/muxplex/cli.py
@@ -2,6 +2,7 @@
 
 import argparse
 import os
+import shutil
 import sys
 from pathlib import Path
 
@@ -56,6 +57,29 @@ def serve(
     uvicorn.run(app, host=host, port=port, log_level="warning")
 
 
+def _check_dependencies() -> None:
+    """Verify required external programs are installed.
+
+    Checks for tmux and ttyd. Prints a helpful error message and exits with
+    code 1 if any are missing.
+    """
+    missing = []
+    if shutil.which("tmux") is None:
+        missing.append(("tmux", "sudo apt install tmux  /  brew install tmux"))
+    if shutil.which("ttyd") is None:
+        missing.append(("ttyd", "sudo apt install ttyd  /  brew install ttyd"))
+
+    if missing:
+        print("\n  ERROR: Required dependencies not found:\n", file=sys.stderr)
+        for name, install_hint in missing:
+            print(f"    {name}: {install_hint}", file=sys.stderr)
+        print(
+            "\n  For details: https://github.com/bkrabach/muxplex#prerequisites\n",
+            file=sys.stderr,
+        )
+        sys.exit(1)
+
+
 def _install_launchd(executable: str) -> None:
     """Install a macOS launchd agent plist to ~/Library/LaunchAgents/."""
     label = "com.muxplex"
@@ -191,6 +215,7 @@ def main() -> None:
     elif args.command == "reset-secret":
         reset_secret()
     else:
+        _check_dependencies()
         serve(
             host=args.host, port=args.port, auth=args.auth, session_ttl=args.session_ttl
         )
diff --git a/muxplex/tests/test_cli.py b/muxplex/tests/test_cli.py
index 2a5d5ec..63a8004 100644
--- a/muxplex/tests/test_cli.py
+++ b/muxplex/tests/test_cli.py
@@ -330,6 +330,85 @@ def test_install_service_help_text_mentions_background_service():
     assert "service" in help_text
 
 
+def test_check_dependencies_exits_when_ttyd_missing(monkeypatch):
+    """_check_dependencies() must sys.exit(1) when ttyd is not in PATH."""
+    import shutil
+    import pytest
+    from muxplex.cli import _check_dependencies
+
+    orig_which = shutil.which
+
+    def fake_which(name):
+        if name == "ttyd":
+            return None
+        return orig_which(name)
+
+    monkeypatch.setattr(shutil, "which", fake_which)
+
+    with pytest.raises(SystemExit) as exc_info:
+        _check_dependencies()
+    assert exc_info.value.code == 1
+
+
+def test_check_dependencies_exits_when_tmux_missing(monkeypatch):
+    """_check_dependencies() must sys.exit(1) when tmux is not in PATH."""
+    import shutil
+    import pytest
+    from muxplex.cli import _check_dependencies
+
+    orig_which = shutil.which
+
+    def fake_which(name):
+        if name == "tmux":
+            return None
+        return orig_which(name)
+
+    monkeypatch.setattr(shutil, "which", fake_which)
+
+    with pytest.raises(SystemExit) as exc_info:
+        _check_dependencies()
+    assert exc_info.value.code == 1
+
+
+def test_check_dependencies_passes_when_all_present(monkeypatch):
+    """_check_dependencies() must not raise when both tmux and ttyd are found."""
+    import shutil
+    from muxplex.cli import _check_dependencies
+
+    monkeypatch.setattr(shutil, "which", lambda name: f"/usr/bin/{name}")
+
+    # Should not raise
+    _check_dependencies()
+
+
+def test_main_check_dependencies_called_for_serve(monkeypatch):
+    """main() must call _check_dependencies() when subcommand is serve."""
+    from muxplex.cli import main
+
+    calls = []
+    monkeypatch.setattr("muxplex.cli._check_dependencies", lambda: calls.append(True))
+
+    with patch("muxplex.cli.serve"):
+        with patch("sys.argv", ["muxplex"]):
+            main()
+
+    assert len(calls) == 1, "_check_dependencies must be called once for serve"
+
+
+def test_main_check_dependencies_not_called_for_install_service(monkeypatch):
+    """main() must NOT call _check_dependencies() for install-service subcommand."""
+    from muxplex.cli import main
+
+    calls = []
+    monkeypatch.setattr("muxplex.cli._check_dependencies", lambda: calls.append(True))
+
+    with patch("muxplex.cli.install_service"):
+        with patch("sys.argv", ["muxplex", "install-service"]):
+            main()
+
+    assert len(calls) == 0, "_check_dependencies must NOT be called for install-service"
+
+
 def test_dunder_main_calls_main():
     """python -m muxplex must call cli.main()."""
     import importlib.util