diff --git a/cli/llms_txt_detector.py b/cli/llms_txt_detector.py
new file mode 100644
index 0000000..4d702b9
--- /dev/null
+++ b/cli/llms_txt_detector.py
@@ -0,0 +1,41 @@
+# ABOUTME: Detects and validates llms.txt file availability at documentation URLs
+# ABOUTME: Supports llms-full.txt, llms.txt, and llms-small.txt variants
+
+import requests
+from typing import Optional, Dict
+
+class LlmsTxtDetector:
+    """Detect llms.txt files at documentation URLs"""
+
+    VARIANTS = [
+        ('llms-full.txt', 'full'),
+        ('llms.txt', 'standard'),
+        ('llms-small.txt', 'small')
+    ]
+
+    def __init__(self, base_url: str):
+        self.base_url = base_url.rstrip('/')
+
+    def detect(self) -> Optional[Dict[str, str]]:
+        """
+        Detect available llms.txt variant.
+
+        Returns:
+            Dict with 'url' and 'variant' keys, or None if not found
+        """
+        for filename, variant in self.VARIANTS:
+            # Try at base URL root
+            url = f"{self.base_url.split('/docs')[0]}/{filename}"
+
+            if self._check_url_exists(url):
+                return {'url': url, 'variant': variant}
+
+        return None
+
+    def _check_url_exists(self, url: str) -> bool:
+        """Check if URL returns 200 status"""
+        try:
+            response = requests.head(url, timeout=5, allow_redirects=True)
+            return response.status_code == 200
+        except requests.RequestException:
+            return False
diff --git a/tests/test_llms_txt_detector.py b/tests/test_llms_txt_detector.py
new file mode 100644
index 0000000..832a45c
--- /dev/null
+++ b/tests/test_llms_txt_detector.py
@@ -0,0 +1,13 @@
+import pytest
+from cli.llms_txt_detector import LlmsTxtDetector
+
+def test_detect_llms_txt_variants():
+    """Test detection of llms.txt file variants"""
+    detector = LlmsTxtDetector("https://hono.dev/docs")
+
+    # Mock responses
+    variants = detector.detect()
+
+    assert variants is not None
+    assert 'url' in variants
+    assert 'variant' in variants  # 'full', 'standard', 'small'