diff --git a/CODE_OF_CONDUCT.md b/CODE_OF_CONDUCT.md
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index 0000000..6738dd7
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+++ b/CODE_OF_CONDUCT.md
@@ -0,0 +1,4 @@
+# Contributor Covenant Code of Conduct
+
+## Our Pledge
+We as members, contributors, and leaders pledge to make participation in our community a harassment-free experience for everyone.
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
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+++ b/CONTRIBUTING.md
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+# Contributing to Sublist3r
+
+## Getting Started
+1. Fork the repository
+2. Clone your fork
+3. Install dependencies: `pip install -r requirements.txt`
+4. Install test deps: `pip install pytest`
+
+## Development
+- Run tests: `python -m pytest tests/ -v`
+- Sublist3r supports Python 3.8+
+
+## Pull Request Process
+1. Ensure all tests pass
+2. Add tests for new functionality
+3. Create a PR with a clear description
diff --git a/SECURITY.md b/SECURITY.md
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+++ b/SECURITY.md
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+# Security Policy
+
+## Reporting a Vulnerability
+Please open an issue to report security vulnerabilities.
diff --git a/tests/test_sublist3r.py b/tests/test_sublist3r.py
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+++ b/tests/test_sublist3r.py
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+# -*- coding: utf-8 -*-
+import os
+import sys
+import json
+import tempfile
+from unittest.mock import patch, MagicMock
+
+import pytest
+
+# Add parent to path
+sys.path.insert(0, os.path.join(os.path.dirname(__file__), '..'))
+
+from sublist3r import (
+    main, parse_args, write_file, subdomain_sorting_key,
+    GoogleEnum, YahooEnum, BingEnum, BaiduEnum, AskEnum,
+    NetcraftEnum, DNSdumpster, Virustotal, ThreatCrowd,
+    CrtSearch, PassiveDNS, portscan, no_color, banner,
+    parser_error, G, Y, B, R, W,
+)
+
+
+class TestUtilities:
+    """Test utility functions."""
+
+    def test_no_color(self):
+        import sublist3r as s
+        s.no_color()
+        assert s.G == ''
+        assert s.Y == ''
+        assert s.B == ''
+        assert s.R == ''
+        assert s.W == ''
+
+    def test_parser_error(self):
+        with pytest.raises(SystemExit):
+            parser_error("test error")
+
+    def test_write_file(self):
+        subdomains = ["test1.example.com", "test2.example.com"]
+        f = tempfile.NamedTemporaryFile(mode='w', delete=False)
+        f.close()
+        write_file(f.name, subdomains)
+        with open(f.name) as fh:
+            content = fh.read()
+        assert "test1.example.com" in content
+        assert "test2.example.com" in content
+        os.unlink(f.name)
+
+    def test_write_file_empty(self):
+        f = tempfile.NamedTemporaryFile(mode='w', delete=False)
+        f.close()
+        write_file(f.name, [])
+        with open(f.name) as fh:
+            content = fh.read()
+        assert content == ''
+        os.unlink(f.name)
+
+    def test_subdomain_sorting_key(self):
+        key = subdomain_sorting_key("www.example.com")
+        assert isinstance(key, tuple)
+        assert len(key) == 2
+
+    def test_subdomain_sorting_key_reversed(self):
+        key = subdomain_sorting_key("test.www.example.com")
+        assert isinstance(key, tuple)
+        assert len(key) == 2
+
+    def test_banner_output(self, capsys):
+        banner()
+        captured = capsys.readouterr()
+        assert "ahmed" in captured.out.lower() or "aboul" in captured.out.lower()
+
+
+class TestMainFunction:
+    """Test the main function with mocked engines."""
+
+    def test_main_invalid_domain(self):
+        """Test main with invalid domain."""
+        result = main(
+            domain="invalid domain!!!",
+            threads=2,
+            savefile=None,
+            ports=None,
+            silent=True,
+            verbose=False,
+            enable_bruteforce=False,
+            engines=None
+        )
+        assert isinstance(result, list) or result is None
+
+
+class TestEngineBase:
+    """Test base engine class."""
+
+    def test_engine_init(self):
+        """Test engine initialization."""
+        engine = GoogleEnum("http://example.com", silent=True, verbose=False)
+        assert engine.domain == "example.com"
+        assert hasattr(engine, 'subdomains')
+
+    def test_engine_max_subdomains(self):
+        """Test max subdomains check."""
+        engine = GoogleEnum("http://example.com", silent=True, verbose=False)
+        # Should return True if count is below max
+        result = engine.check_max_subdomains(10)
+        assert result is True or result is False
+
+
+class TestPortScan:
+    """Test port scan class."""
+
+    def test_portscan_init(self):
+        """Test portscan initialization."""
+        hostnames = ["www.example.com", "api.example.com"]
+        ps = portscan(hostnames, [80, 443])
+        assert len(ps.subdomains) == 2
+        assert len(ps.ports) == 2
+
+    def test_portscan_empty_hostnames(self):
+        """Test portscan with empty hostnames."""
+        ps = portscan([], [80])
+        assert len(ps.subdomains) == 0