diff --git a/tests/test_cli.py b/tests/test_cli.py
new file mode 100644
index 0000000..2fbfae6
--- /dev/null
+++ b/tests/test_cli.py
@@ -0,0 +1,141 @@
+"""Tests for the CLI interface."""
+
+import pytest
+import sys
+from unittest.mock import patch, MagicMock
+from typer.testing import CliRunner
+
+from cli_explain_fix.cli import app
+
+
+class TestCLI:
+    """Test cases for CLI interface."""
+
+    def setup_method(self):
+        """Set up CLI runner for each test."""
+        self.runner = CliRunner()
+
+    def test_main_no_input(self):
+        """Test CLI shows help when no input provided."""
+        result = self.runner.invoke(app, ["main"])
+        assert result.exit_code == 0
+
+    def test_main_with_input(self):
+        """Test CLI with direct input."""
+        result = self.runner.invoke(app, ["main", "ValueError: test error"])
+        assert result.exit_code == 0
+
+    def test_main_with_json_flag(self):
+        """Test CLI with JSON output flag."""
+        result = self.runner.invoke(app, ["main", "--json", "ValueError: test error"])
+        assert result.exit_code == 0
+
+    def test_main_with_plain_flag(self):
+        """Test CLI with plain text output."""
+        result = self.runner.invoke(app, ["main", "--plain", "ValueError: test error"])
+        assert result.exit_code == 0
+
+    def test_main_with_language_flag(self):
+        """Test CLI with explicit language flag."""
+        result = self.runner.invoke(app, ["main", "--lang", "python", "ImportError: No module named foo"])
+        assert result.exit_code == 0
+
+    def test_main_with_verbose_flag(self):
+        """Test CLI with verbose flag."""
+        result = self.runner.invoke(app, ["main", "--verbose", "ValueError: test error"])
+        assert result.exit_code == 0
+
+    def test_main_with_no_color_flag(self):
+        """Test CLI with no-color flag."""
+        result = self.runner.invoke(app, ["main", "--no-color", "ValueError: test error"])
+        assert result.exit_code == 0
+
+    def test_list_languages_command(self):
+        """Test list-languages command."""
+        result = self.runner.invoke(app, ["list-languages"])
+        assert result.exit_code == 0
+        assert "python" in result.output.lower() or "Supported" in result.output
+
+    def test_list_errors_command(self):
+        """Test list-errors command."""
+        result = self.runner.invoke(app, ["list-errors"])
+        assert result.exit_code == 0
+
+    def test_list_errors_with_language_filter(self):
+        """Test list-errors with language filter."""
+        result = self.runner.invoke(app, ["list-errors", "--lang", "python"])
+        assert result.exit_code == 0
+
+    def test_show_config_command(self):
+        """Test show-config command."""
+        result = self.runner.invoke(app, ["show-config"])
+        assert result.exit_code == 0
+
+    def test_main_with_file_input(self, tmp_path):
+        """Test CLI with file input."""
+        test_file = tmp_path / "error.txt"
+        test_file.write_text("ValueError: test error from file")
+
+        result = self.runner.invoke(app, ["main", "--file", str(test_file)])
+        assert result.exit_code == 0
+
+    def test_main_with_nonexistent_file(self):
+        """Test CLI with nonexistent file."""
+        result = self.runner.invoke(app, ["main", "--file", "/nonexistent/file.txt"])
+        assert result.exit_code != 0
+        assert "error" in result.output.lower() or "Error" in result.output
+
+    def test_main_with_stdin_input(self):
+        """Test CLI with stdin input."""
+        result = self.runner.invoke(app, ["main"], input="ValueError: stdin error")
+        assert result.exit_code == 0
+
+    def test_main_with_theme_option(self):
+        """Test CLI with theme option."""
+        result = self.runner.invoke(app, ["main", "--theme", "dark", "ValueError: test error"])
+        assert result.exit_code == 0
+
+    def test_stdin_takes_precedence_over_args(self):
+        """Test that stdin is read when provided with args."""
+        result = self.runner.invoke(app, ["main", "ValueError: arg input"], input="ValueError: stdin input")
+        assert result.exit_code == 0
+
+
+class TestCLIOutputFormats:
+    """Test CLI output format options."""
+
+    def setup_method(self):
+        """Set up CLI runner for each test."""
+        self.runner = CliRunner()
+
+    def test_json_output_format(self):
+        """Test JSON output contains expected fields."""
+        result = self.runner.invoke(app, ["main", "--json", "ImportError: No module named foo"])
+        assert result.exit_code == 0
+
+    def test_plain_output_contains_fix_steps(self):
+        """Test plain output contains fix instructions."""
+        result = self.runner.invoke(app, ["main", "--plain", "ValueError: invalid value"])
+        assert result.exit_code == 0
+        assert "How to fix" in result.output or "fix" in result.output.lower()
+
+
+class TestCLIErrorHandling:
+    """Test CLI error handling."""
+
+    def setup_method(self):
+        """Set up CLI runner for each test."""
+        self.runner = CliRunner()
+
+    def test_invalid_file_path(self):
+        """Test handling of invalid file path."""
+        result = self.runner.invoke(app, ["main", "--file", "/path/that/does/not/exist/error.txt"])
+        assert result.exit_code != 0
+
+    def test_empty_file(self, tmp_path):
+        """Test handling of empty file."""
+        test_file = tmp_path / "empty.txt"
+        test_file.write_text("")
+
+        result = self.runner.invoke(app, ["main", "--file", str(test_file)])
+        assert result.exit_code == 0