Fix YAML tag serialization in frontmatter

When writing or updating frontmatter with tags, the YAML serialization
was causing tags to be malformed with improper line breaks:

Before:
tags:
- '[check-in'
- 'qr-code'
- 'architecture]'

After:
tags: [check-in, qr-code, architecture]

This fix configures yaml.dump() to use flow style (default_flow_style=None)
which ensures lists are properly serialized in a single line format.

Fixes issue where Basic Memory's write_note function consistently
malforms tags in YAML frontmatter.

Signed-off-by: Gilang Gumilar <gilangmlr@gmail.com>

Author: gilangmlr

src/basic_memory/file_utils.py

@@ -218,7 +218,8 @@ async def update_frontmatter(path: FilePath, updates: Dict[str, Any]) -> str:
         new_fm = {**current_fm, **updates}
 
         # Write new file with updated frontmatter
-        yaml_fm = yaml.dump(new_fm, sort_keys=False, allow_unicode=True)
+        # Configure YAML to use flow style for lists to prevent tag malformation
+        yaml_fm = yaml.dump(new_fm, sort_keys=False, allow_unicode=True, default_flow_style=None)
         final_content = f"---\n{yaml_fm}---\n\n{content.strip()}"
 
         logger.debug("Updating frontmatter", path=str(path_obj), update_keys=list(updates.keys()))

tests/test_yaml_tag_fix.py

@@ -0,0 +1,57 @@
+"""Test for YAML tag serialization fix."""
+
+import pytest
+from pathlib import Path
+import tempfile
+from basic_memory.file_utils import update_frontmatter, parse_frontmatter
+
+
+@pytest.mark.asyncio
+async def test_yaml_tags_serialize_correctly():
+    """Test that tags are serialized as flow style in YAML frontmatter."""
+    # Create a temporary file
+    with tempfile.NamedTemporaryFile(mode='w', suffix='.md', delete=False) as f:
+        f.write("---\ntitle: Test Note\n---\n\nTest content")
+        temp_path = Path(f.name)
+    
+    try:
+        # Update with tags
+        tags = ["check-in", "qr-code", "architecture", "system-design"]
+        await update_frontmatter(temp_path, {"tags": tags})
+        
+        # Read back the file
+        content = temp_path.read_text()
+        
+        # Parse frontmatter
+        fm = parse_frontmatter(content)
+        
+        # Verify tags are preserved correctly
+        assert fm["tags"] == tags
+        
+        # Verify the YAML format is correct (flow style)
+        assert 'tags: [' in content or 'tags: ["' in content
+        
+        # Ensure it's NOT in the malformed format
+        assert '- \'["' not in content
+        assert '- \'"' not in content
+        
+    finally:
+        # Clean up
+        temp_path.unlink()
+
+
+@pytest.mark.asyncio
+async def test_empty_tags_list():
+    """Test that empty tags list is handled correctly."""
+    with tempfile.NamedTemporaryFile(mode='w', suffix='.md', delete=False) as f:
+        f.write("---\ntitle: Test Note\n---\n\nTest content")
+        temp_path = Path(f.name)
+    
+    try:
+        await update_frontmatter(temp_path, {"tags": []})
+        content = temp_path.read_text()
+        fm = parse_frontmatter(content)
+        assert fm["tags"] == []
+        
+    finally:
+        temp_path.unlink()