feat(cli): add --check validation-only mode

README.md

@@ -51,9 +51,10 @@ echo '{"x": 1}' | toon -            # Stdin/stdout
 # Options
 toon data.json --encode --delimiter "\t" --length-marker
 toon data.toon --decode --no-strict --indent 4
+toon data.toon --check                  # Validate only (exit code)
 ```
 
-**Options:** `-e/--encode` `-d/--decode` `-o/--output` `--delimiter` `--indent` `--length-marker` `--no-strict`
+**Options:** `-e/--encode` `-d/--decode` `-o/--output` `--delimiter` `--indent` `--length-marker` `--no-strict` `--check`
 
 ## API Reference
 

src/toon_format/cli.py

@@ -76,6 +76,11 @@ def main() -> int:
         action="store_true",
         help="Disable strict validation when decoding",
     )
+    parser.add_argument(
+        "--check",
+        action="store_true",
+        help="Validate input only and return exit status without writing output",
+    )
 
     args = parser.parse_args()
 
@@ -98,6 +103,9 @@ def main() -> int:
     if args.encode and args.decode:
         print("Error: Cannot specify both --encode and --decode", file=sys.stderr)
         return 1
+    if args.check and args.output:
+        print("Error: Cannot specify both --check and --output", file=sys.stderr)
+        return 1
 
     if args.encode:
         mode = "encode"
@@ -145,6 +153,9 @@ def main() -> int:
         return 1
 
     # Write output
+    if args.check:
+        return 0
+
     try:
         if args.output:
             output_path = Path(args.output)

tests/test_cli.py

@@ -272,6 +272,70 @@ def test_no_strict_option(self, tmp_path):
                 result = main()
                 assert result == 0
 
+    def test_check_valid_json_returns_zero_without_output(self, tmp_path):
+        """Check mode should validate JSON input and not emit output."""
+        input_file = tmp_path / "input.json"
+        input_file.write_text('{"ok": true}')
+
+        with patch("sys.stdout", new_callable=StringIO) as mock_stdout:
+            with patch("sys.stderr", new_callable=StringIO) as mock_stderr:
+                with patch("sys.argv", ["toon", str(input_file), "--check"]):
+                    result = main()
+                    assert result == 0
+                    assert mock_stdout.getvalue() == ""
+                    assert mock_stderr.getvalue() == ""
+
+    def test_check_invalid_json_returns_error(self, tmp_path):
+        """Check mode should fail for invalid JSON when encoding."""
+        input_file = tmp_path / "input.json"
+        input_file.write_text('{"broken": invalid}')
+
+        with patch("sys.stdout", new_callable=StringIO) as mock_stdout:
+            with patch("sys.stderr", new_callable=StringIO) as mock_stderr:
+                with patch("sys.argv", ["toon", str(input_file), "--check"]):
+                    result = main()
+                    assert result == 1
+                    assert mock_stdout.getvalue() == ""
+                    assert "Error during encode" in mock_stderr.getvalue()
+
+    def test_check_valid_toon_returns_zero_without_output(self, tmp_path):
+        """Check mode should validate TOON input and not emit output."""
+        input_file = tmp_path / "input.toon"
+        input_file.write_text("name: Alice\nage: 30")
+
+        with patch("sys.stdout", new_callable=StringIO) as mock_stdout:
+            with patch("sys.stderr", new_callable=StringIO) as mock_stderr:
+                with patch("sys.argv", ["toon", str(input_file), "--check"]):
+                    result = main()
+                    assert result == 0
+                    assert mock_stdout.getvalue() == ""
+                    assert mock_stderr.getvalue() == ""
+
+    def test_check_invalid_toon_returns_error(self, tmp_path):
+        """Check mode should fail for invalid TOON when decoding."""
+        input_file = tmp_path / "input.toon"
+        input_file.write_text("items[2]: 1")
+
+        with patch("sys.stdout", new_callable=StringIO) as mock_stdout:
+            with patch("sys.stderr", new_callable=StringIO) as mock_stderr:
+                with patch("sys.argv", ["toon", str(input_file), "--check"]):
+                    result = main()
+                    assert result == 1
+                    assert mock_stdout.getvalue() == ""
+                    assert "Error during decode" in mock_stderr.getvalue()
+
+    def test_error_check_and_output_together(self, tmp_path):
+        """Check mode cannot be combined with output path."""
+        input_file = tmp_path / "input.json"
+        input_file.write_text('{"test": true}')
+        output_file = tmp_path / "output.toon"
+
+        with patch("sys.stderr", new_callable=StringIO) as mock_stderr:
+            with patch("sys.argv", ["toon", str(input_file), "--check", "-o", str(output_file)]):
+                result = main()
+                assert result == 1
+                assert "Cannot specify both --check and --output" in mock_stderr.getvalue()
+
     def test_decode_indent_option_affects_output(self, tmp_path):
         """Ensure --indent controls the JSON formatting."""
         input_file = tmp_path / "input.toon"