diff --git a/atroposlib/frontend/jsonl2html.py b/atroposlib/frontend/jsonl2html.py
index 0dcbad7a..45d7f322 100644
--- a/atroposlib/frontend/jsonl2html.py
+++ b/atroposlib/frontend/jsonl2html.py
@@ -60,9 +60,31 @@ def create_html_for_group(group_data, index):
 
     items_html = ""
     for i, (msg, score) in enumerate(zip(messages, scores)):
+        # Handle both string and nested message formats
+        if isinstance(msg, str):
+            # Simple string format (original behavior)
+            content = msg
+        elif isinstance(msg, list):
+            # Nested conversation format: List[Message] where Message is a dict
+            # Format each message with its role
+            parts = []
+            for m in msg:
+                if isinstance(m, dict):
+                    role = m.get("role", "unknown")
+                    content_text = m.get("content", "")
+                    parts.append(f"**{role.capitalize()}**: {content_text}")
+                else:
+                    # Fallback for unexpected format
+                    parts.append(str(m))
+            content = "\n\n".join(parts)
+        else:
+            # Fallback for any other type
+            content = str(msg)
+
         rendered_markdown = markdown.markdown(
-            msg, extensions=["fenced_code", "tables", "nl2br"]
+            content, extensions=["fenced_code", "tables", "nl2br"]
         )
+
         score_class = get_score_class(score)
         item_id = f"group-{index}-item-{i}"
         items_html += textwrap.dedent(
diff --git a/atroposlib/tests/test_jsonl2html.py b/atroposlib/tests/test_jsonl2html.py
new file mode 100644
index 00000000..61032fca
--- /dev/null
+++ b/atroposlib/tests/test_jsonl2html.py
@@ -0,0 +1,133 @@
+"""Tests for jsonl2html.py message format handling."""
+
+import json
+
+from atroposlib.frontend.jsonl2html import create_html_for_group, generate_html
+
+
+class TestCreateHtmlForGroup:
+    """Test create_html_for_group with different message formats."""
+
+    def test_string_messages_format(self):
+        """Test with original string messages format (backward compatibility)."""
+        group_data = {
+            "messages": ["Hello, world!", "This is a test message."],
+            "scores": [0.5, 0.8],
+        }
+
+        html = create_html_for_group(group_data, 0)
+
+        assert html is not None
+        assert "Hello, world!" in html
+        assert "This is a test message." in html
+        assert "0.5" in html
+        assert "0.8" in html
+
+    def test_nested_messages_format(self):
+        """Test with nested conversation format: List[List[Message]]."""
+        group_data = {
+            "messages": [
+                [
+                    {"role": "system", "content": "You are a helpful assistant."},
+                    {"role": "user", "content": "What is 2+2?"},
+                    {"role": "assistant", "content": "2+2 equals 4."},
+                ],
+                [
+                    {"role": "user", "content": "Hello!"},
+                    {"role": "assistant", "content": "Hi there!"},
+                ],
+            ],
+            "scores": [1.0, 0.9],
+        }
+
+        html = create_html_for_group(group_data, 0)
+
+        assert html is not None
+        # Check that content is rendered
+        assert "You are a helpful assistant" in html
+        assert "What is 2+2?" in html
+        assert "2+2 equals 4" in html
+        assert "Hello!" in html
+        assert "Hi there!" in html
+        # Check that roles are rendered
+        assert "System" in html
+        assert "User" in html
+        assert "Assistant" in html
+
+    def test_empty_messages(self):
+        """Test with empty messages list."""
+        group_data = {
+            "messages": [],
+            "scores": [],
+        }
+
+        html = create_html_for_group(group_data, 0)
+
+        # Should return empty string for no items
+        assert html == ""
+
+    def test_mismatched_lengths(self):
+        """Test with mismatched messages and scores lengths."""
+        group_data = {
+            "messages": ["Message 1", "Message 2", "Message 3"],
+            "scores": [0.5],  # Only one score
+        }
+
+        # Should handle gracefully by using minimum length
+        html = create_html_for_group(group_data, 0)
+
+        assert "Message 1" in html
+        assert "Message 2" not in html  # Should be skipped
+        assert "Message 3" not in html  # Should be skipped
+
+
+class TestGenerateHtml:
+    """Test the full generate_html function."""
+
+    def test_generate_html_with_nested_messages(self, tmp_path):
+        """Test full HTML generation with nested message format."""
+        # Create test JSONL file
+        jsonl_file = tmp_path / "test.jsonl"
+
+        test_data = {
+            "messages": [
+                [
+                    {"role": "system", "content": "Be helpful."},
+                    {"role": "user", "content": "Test question"},
+                    {"role": "assistant", "content": "Test answer"},
+                ]
+            ],
+            "scores": [0.75],
+        }
+
+        with open(jsonl_file, "w") as f:
+            f.write(json.dumps(test_data) + "\n")
+
+        # Generate HTML
+        output_file = tmp_path / "test.html"
+        generate_html(str(jsonl_file), str(output_file))
+
+        # Verify output exists and contains expected content
+        assert output_file.exists()
+        html_content = output_file.read_text()
+        assert "Test question" in html_content
+        assert "Test answer" in html_content
+
+    def test_generate_html_with_string_messages(self, tmp_path):
+        """Test full HTML generation with string message format (backward compat)."""
+        jsonl_file = tmp_path / "test_strings.jsonl"
+
+        test_data = {
+            "messages": ["Simple string message"],
+            "scores": [0.5],
+        }
+
+        with open(jsonl_file, "w") as f:
+            f.write(json.dumps(test_data) + "\n")
+
+        output_file = tmp_path / "test_strings.html"
+        generate_html(str(jsonl_file), str(output_file))
+
+        assert output_file.exists()
+        html_content = output_file.read_text()
+        assert "Simple string message" in html_content