diff --git a/src/revenueholdings/cli.py b/src/revenueholdings/cli.py
index 6decad1..dc07416 100644
--- a/src/revenueholdings/cli.py
+++ b/src/revenueholdings/cli.py
@@ -120,7 +120,11 @@ def show_versions(
     tool: str | None = typer.Argument(None, help="Check version of a specific tool."),
 ):
     """Show installed tool versions."""
-    targets = ([tool] if tool in TOOLS else []) if tool else _builtins.list(TOOLS.keys())
+    if tool is not None and tool not in TOOLS:
+        console.print(f"[red]Unknown tool: {tool}[/red]")
+        console.print(f"Available: {', '.join(TOOLS.keys())}")
+        raise typer.Exit(code=1)
+    targets = [tool] if tool else _builtins.list(TOOLS.keys())
 
     for t in targets:
         info = TOOLS[t]
diff --git a/tests/test_cli.py b/tests/test_cli.py
index 9a45833..6d76c55 100644
--- a/tests/test_cli.py
+++ b/tests/test_cli.py
@@ -4,6 +4,7 @@
 from revenueholdings import TOOLS, __version__
 from revenueholdings.cli import app
 from typer.testing import CliRunner
+from unittest import mock
 
 runner = CliRunner()
 
@@ -35,12 +36,73 @@ def test_unknown_tool(self):
         assert "Unknown" in result.stdout
 
 
+class TestInstallCommand:
+    @mock.patch("revenueholdings.cli.subprocess.run")
+    def test_install_specific_tool(self, mock_run):
+        """Install a specific tool by name."""
+        mock_run.return_value = mock.MagicMock(returncode=0, stdout="", stderr="")
+        result = runner.invoke(app, ["install", "guard"])
+        assert result.exit_code == 0
+        assert "Successfully" in result.stdout
+        mock_run.assert_called_once()
+
+    @mock.patch("revenueholdings.cli.subprocess.run")
+    def test_install_all(self, mock_run):
+        """Install all tools via the 'all' alias."""
+        mock_run.return_value = mock.MagicMock(returncode=0, stdout="", stderr="")
+        result = runner.invoke(app, ["install", "all"])
+        assert result.exit_code == 0
+        assert "Successfully" in result.stdout
+        mock_run.assert_called_once()
+
+    def test_install_unknown_tool(self):
+        """Error on unknown tool name."""
+        result = runner.invoke(app, ["install", "nonexistent"])
+        assert result.exit_code == 1
+        assert "Unknown" in result.stdout
+        assert "Available:" in result.stdout
+
+    @mock.patch("revenueholdings.cli.subprocess.run")
+    def test_install_failure(self, mock_run):
+        """Handle pip install failure gracefully."""
+        mock_run.return_value = mock.MagicMock(returncode=1, stdout="", stderr="Error message")
+        result = runner.invoke(app, ["install", "guard"])
+        assert result.exit_code == 1
+        assert "failed" in result.stdout.lower()
+
+
 class TestVersionsCommand:
     def test_versions_runs(self):
+        """List all tool versions without error."""
         result = runner.invoke(app, ["versions"])
-        # Should succeed even if tools aren't installed
         assert result.exit_code == 0
 
+    def test_versions_unknown_tool_fails(self):
+        """Error on unknown tool name."""
+        result = runner.invoke(app, ["versions", "nonexistent"])
+        assert result.exit_code == 1
+        assert "Unknown" in result.stdout
+
+    @mock.patch("revenueholdings.cli.subprocess.run")
+    def test_versions_specific_tool_not_installed(self, mock_run):
+        """Show 'not installed' for a tool that isn't installed."""
+        mock_run.return_value = mock.MagicMock(
+            returncode=1, stdout="", stderr=""
+        )
+        result = runner.invoke(app, ["versions", "guard"])
+        assert result.exit_code == 0
+        assert "guard" in result.stdout
+        assert "not installed" in result.stdout
+
+
+class TestDispatchCommands:
+    def test_invalid_tool_subcommand(self):
+        """Reject dispatch to an unknown tool subcommand."""
+        result = runner.invoke(app, ["nonexistent"])
+        # typer outputs error to stderr, not stdout
+        assert result.exit_code != 0
+        assert "No such command" in result.stdout or "No such command" in result.stderr
+
 
 class TestHelp:
     def test_help(self):