diff --git a/tests/test_cli.py b/tests/test_cli.py
new file mode 100644
index 0000000..f2953b0
--- /dev/null
+++ b/tests/test_cli.py
@@ -0,0 +1,134 @@
+"""Tests for the CLI module."""
+
+import sys
+from pathlib import Path
+
+sys.path.insert(0, str(Path(__file__).parent.parent / 'src'))
+
+from click.testing import CliRunner
+from gdiffer.cli import main
+
+
+class TestCLIMain:
+    def test_main_help(self):
+        runner = CliRunner()
+        result = runner.invoke(main, ["--help"])
+        assert result.exit_code == 0
+
+    def test_main_version(self):
+        runner = CliRunner()
+        result = runner.invoke(main, ["--version"])
+        assert result.exit_code == 0
+        assert "0.1.0" in result.output
+
+
+class TestExplainCommand:
+    def test_explain_simple_diff(self):
+        diff = """diff --git a/test.py b/test.py
+index 123..456 100644
+--- a/test.py
++++ b/test.py
+@@ -1,3 +1,3 @@
+-def hello():
+-    print("Hello")
++def hello():
++    print("Hello, World!")
+"""
+        runner = CliRunner()
+        result = runner.invoke(main, ["explain", diff])
+        assert result.exit_code == 0
+        assert "test.py" in result.output or "Files" in result.output
+
+    def test_explain_no_input(self):
+        runner = CliRunner()
+        result = runner.invoke(main, ["explain"])
+        assert result.exit_code != 0
+
+    def test_explain_json_format(self):
+        diff = """diff --git a/test.py b/test.py
+new file mode 100644
+--- /dev/null
++++ b/test.py
+@@ -0,0 +1 @@
++print("hello")
+"""
+        runner = CliRunner()
+        result = runner.invoke(main, ["--output", "json", "explain", diff])
+        assert result.exit_code == 0
+        assert "{" in result.output
+
+
+class TestIssuesCommand:
+    def test_issues_with_security_issue(self):
+        diff = """diff --git a/db.py b/db.py
+--- a/db.py
++++ b/db.py
+@@ -1,2 +1,3 @@
+ def get_user(username):
+     query = "SELECT * FROM users WHERE name = '" + username + "'"
++    query = "SELECT * FROM users WHERE id = " + user_id
+"""
+        runner = CliRunner()
+        result = runner.invoke(main, ["issues"], input=diff)
+        assert result.exit_code == 0
+
+
+class TestSummarizeCommand:
+    def test_summarize_simple_diff(self):
+        diff = """diff --git a/test.py b/test.py
+--- a/test.py
++++ b/test.py
+@@ -1 +1 @@
+-old
++new
+"""
+        runner = CliRunner()
+        result = runner.invoke(main, ["summarize"], input=diff)
+        assert result.exit_code == 0
+        assert "Files" in result.output or "changed" in result.output.lower()
+
+    def test_summarize_multi_file(self):
+        diff = """diff --git a/file1.py b/file1.py
+new file mode 100644
+--- /dev/null
++++ b/file1.py
+@@ -0,0 +1 @@
++print(1)
+
+diff --git a/file2.py b/file2.py
+deleted file mode 100644
+--- a/file2.py
++++ /dev/null
+@@ -1 +0,0 @@
+-print(2)
+"""
+        runner = CliRunner()
+        result = runner.invoke(main, ["summarize"], input=diff)
+        assert result.exit_code == 0
+
+
+class TestCLIOptions:
+    def test_verbose_option(self):
+        diff = """diff --git a/test.py b/test.py
+--- a/test.py
++++ b/test.py
+@@ -1 +1 @@
+-old
++new
+"""
+        runner = CliRunner()
+        result = runner.invoke(main, ["--verbose", "explain", diff])
+        assert result.exit_code == 0
+
+    def test_output_format_option(self):
+        diff = """diff --git a/test.py b/test.py
+--- a/test.py
++++ b/test.py
+@@ -1 +1 @@
+-old
++new
+"""
+        for fmt in ["terminal", "json", "plain"]:
+            runner = CliRunner()
+            result = runner.invoke(main, ["--output", fmt, "explain", diff])
+            assert result.exit_code == 0